Aerosol Acidity Controls Methanesulfonic Acid Evaporation From Aerosols During Antarctic Katabatic Outflow

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Osuagwu, Z.D. Ristovski, R. S. Humphries, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6905825/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 28 Nov, 2025 Read the published version in Communications Earth & Environment → Version 1 posted You are reading this latest preprint version Abstract Methanesulfonic acid (MSA), a key oxidation product of dimethyl sulfide (DMS), plays a crucial role in the atmospheric sulfur cycle and in the formation of cloud condensation nuclei (CCN). MSA contributes significantly to aerosol growth and, potentially, the modulation of cloud microphysical properties, particularly in remote marine and polar regions where CCN concentrations are relatively low. Here we focus on an eight-day period of elevated gaseous MSA observed along the coastal region of East Antarctica that coincided with persistent katabatic outflow. We show that this outflow brings biogenically dominated, highly acidic aerosols with elevated gaseous MSA resulting from evaporation off the surface of these aerosol particles. While MSA evaporation is promoted by a decrease in relative humidity, we show that aerosol acidity is the primary driver of this process. These results provide new insights into processes involved in the marine sulfur cycle, which should be included when using observations of DMS oxidation products to guide model evaluation and development. Furthermore, they reveal the highly acidic nature of Southern Ocean aerosols and highlight the importance of aerosol acidity on atmospheric processes. Physical sciences/Chemistry/Environmental chemistry/Atmospheric chemistry Earth and environmental sciences/Climate sciences/Atmospheric science/Atmospheric chemistry Earth and environmental sciences/Climate sciences/Atmospheric science/Atmospheric dynamics Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The connection between ocean biogeochemistry and the atmosphere is critical for understanding Southern Ocean and Antarctic aerosols, their interaction with clouds and how they might be impacted by a changing climate [ 1 ]. Recent work has shown that biogenic aerosols increase cloud condensation nuclei and modulate cloud properties over the coastal East Antarctica [ 2 ]. These biogenic aerosols are produced primarily from dimethyl sulfide (DMS; CH 3 SCH 3 ) [ 3 ] and potentially other volatile sulfur-based compounds [ 4 ] emitted by ocean microbiota. Once emitted in the atmosphere, DMS is chemically transformed through complex oxidation pathways, ultimately leading to the production of sulfuric acid (SA; H 2 SO 4 ) and methanesulfonic acid (MSA; CH 3 SO 3 H). These are compounds that, due to their low volatilities, can nucleate to form new particles and/or condense onto existing aerosol particles, thereby promoting growth into sizes capable of acting as Cloud Condensation Nuclei (CCN). Gaseous DMS is oxidised in the atmosphere via two main pathways. The first involves hydrogen atom abstraction, primarily by hydroxyl radical (.OH), but also by halogen radicals, such as chlorine (Cl.), and bromine oxide (BrO), and the nitrate radical (NO 3 .) [ 5 ]. This pathway is favoured at higher temperatures (e.g. lower latitudes) and predominantly produces sulfur dioxide (SO 2 ), which is efficiently converted to H 2 SO 4 [ 6 ]. The second pathway involves hydroxyl radical addition, which is favoured at lower temperatures (e.g. polar environments and/or the free troposphere) and the dominant product is MSA [ 5 , 7 ]. This temperature dependence has been demonstrated in the field through an increase in the ratio of particulate MSA to non-sea-salt sulfate (nss-SO 4 2− ) with latitude, in regions where nss-SO 4 2− is minimally influenced by anthropogenic activities in the Southern Hemisphere [ 8 – 11 ]. In addition, recent chamber experiments have shown an order of magnitude increase in the gas phase MSA/H 2 SO 4 ratio when temperature is decreased from 25 to -10 o C [ 12 ]. MSA is also the dominant product of DMS aqueous phase oxidation by O 3 and OH radicals, which occurs in deliquesced particles or cloud droplets [ 13 , 14 ]. Modelling studies have proposed this to be an important MSA formation mechanism [ 13 – 16 ] and Kecorius et al have suggested that upon droplet evaporation, particulate MSA can partition into the gas phase [ 15 ]. Consistent with this, previous field studies have observed a negative correlation between gaseous MSA and relative humidity (RH) and, based on that, hypothesized that MSA may evaporate from particles as RH and aerosol liquid water content (ALWC) decreases [ 17 – 20 ]. This process will be further enhanced by aerosol acidity, with more acidic aerosol promoting MSA partitioning into the gas phase [ 21 , 22 ]. Overall, the prevalent DMS oxidation mechanism – and the resulting oxidation products - will depend on environmental conditions, such as temperature and humidity, availability of different oxidants, and cloud presence. Despite this, large-scale chemical transport and climate models use fixed yields of SO 2 and MSA to simulate sulfate aerosol formation and most consider only gas phase reactions. Although DMS chemistry has been studied for decades, there are still large uncertainties in the amount and spatial distribution of aerosols resulting from its oxidation [ 5 , 13 , 23 , 24 ]. During austral summer the polar sector of the Southern Ocean has the highest concentrations of aqueous DMS on the planet [ 25 ]. However, studies investigating DMS fluxes into the Southern Ocean atmosphere, as well as the yields of its terminal oxidation products (MSA and SA), are scarce. In this study we focus on simultaneous measurements of gaseous MSA and SA in the Antarctic West Pacific Ocean sector (90–160°E), highlighting a unique eight-day period during the CAPRICORN-2 (2018) Southern Ocean voyage characterised by elevated gaseous MSA concentrations. We show that this period coincides with katabatic outflow from the Antarctic continent and, by combining observations with thermodynamic modelling of MSA gas – particle partitioning, we provide evidence for MSA evaporation from aerosols being the source of enhanced gas-phase MSA. We demonstrate that katabatic outflow results in highly acidic and biogenically dominated aerosols and identify aerosol pH to be the primary driver of MSA evaporation. Results Gaseous MSA and SA during CAPRICORN-2 The CAPRICORN-2 (Clouds, Aerosols , Precipitation, Radiation, and Atmospheric Composition over the Southern Ocean) voyage took place between 10 Jan and 22 Feb 2018 on board the Australian Research Vessel (RV) Investigator. The RV Investigator travelled southward from Hobart, Tasmania and reached ~ 65 o S where it transected the longitudinal area between 130 o and 150 o E. A prominent feature of the CAPRICORN-2 voyage was an eight-day long period (29 Jan – 6 Feb) of persistently high levels of gaseous MSA (MSA g 2 (± 1) × 10 7 molecules cm − 3 ), roughly an order of magnitude higher than the average of the rest of the data (Fig. 1 a). Gaseous sulfuric acid (SA g ), on the other hand, did not exhibit any substantial increase during the “high MSA” period. SA g concentrations were on average an order of magnitude lower than those of MSA g during the campaign and two orders of magnitude lower than MSA g during the “high MSA” period (Supplementary Fig. 1). Our observed gaseous MSA g /SA g ratio during the “high MSA” period is remarkably high (~ 100 during the “high MSA” period) and substantially higher than previously reported for the Southern Ocean and Antarctica [ 17 , 26 – 29 ]. As this is the first study focusing on the atmospheric MSA g and SA g in the Antarctic West Pacific Ocean sector (90–160°E), these unique observations might reflect large-scale compositional differences between oceanic sectors of coastal Antarctica. Throughout the whole voyage SA g exhibits a typical diurnal pattern (Fig. 1 b) dictated by solar radiation intensity, reflecting the diurnal pattern of OH radicals needed for the oxidation of SO 2 into H 2 SO 4 . On average, intensities of SA g increase by a factor of 2 around midday. This pattern has been previously observed in different marine environments, including polar regions [ 17 , 30 – 32 ]. This indicates that local photooxidation of DMS and possibly other volatile sulfur compounds is the dominant pathway of the SA g production in the sampled region. MSA g diurnal behaviour (Fig. 1 c) is presented separately for the “high MSA” period and the rest of the voyage due to an order of magnitude difference in MSA g concentrations between those two periods. While the 8-day “high MSA” period shows up to 70% increase in median MSA g intensity around midday, attributing this to photochemical production might be unwarranted, as MSA g day-to-day variability during this short period does not consistently align with the solar radiation cycle (Supplementary Fig. 2). Moreover, data from the remainder of the sampling period show that MSA g shows no diurnal pattern and, on average, has no significant difference in concentrations between day and night. (Fig. 1 c). Such absence of any particular diurnal trend for MSA g has been previously observed in studies focusing on the Southern Ocean and Antarctic region [ 17 , 32 ], while marine atmosphere studies outside of polar regions reported diurnal behaviour of MSA g similar to that of SA g [ 30 , 31 , 33 ]. Modelling studies have proposed that a large portion of MSA is formed via aqueous phase oxidation of DMS and its intermediate products [ 13 , 14 ]. In addition, it has been previously suggested that particulate MSA could partition into the gas phase depending on temperature, humidity and particle acidity [ 17 – 22 ]. This supports our observed MSA g trends. A more detailed analysis of MSA g behaviour in relation to RH and particle acidity will be presented in subsequent sections. Air masses origin during the “high MSA” period The period of high gaseous MSA occurred at high latitudes (> 63 o S), coinciding with the ship’s southward progression into latitudes typically influenced by the polar cell (Fig. 1 c). Elevated MSA g concentrations persisted during the ship’s eastward travel (140 o to 150 o E at ~ 64 o S), and declined after the ship moved northward (< 63 o S). This compositional change is consistent with previous observations [ 34 – 37 ] and Humphries et al [ 35 ] coined a term “Atmospheric Compositional Front of Antarctica” to describe a distinct change in aerosol composition at a boundary that varies spatially and temporally between 60 and 65 o S in the East Antarctic region. However, a later southward transect through similarly high latitudes near ~ 132 o E cell did not lead to a comparable increase in gaseous MSA, suggesting compositional variability not wholly dependent on latitude. The majority of air masses during the “high MSA” period have recent influence from the Antarctic continent (Fig. 1 d). While a few Antarctic and Southern Ocean studies have reported gaseous MSA concentrations [ 17 , 23 , 26 – 29 , 38 ], no other campaigns have documented such a pronounced increase in gaseous MSA associated with continental air mass influence. In contrast, during a subsequent period (9–10 February), despite air masses also originating from Antarctica, there was no significant increase in MSA g . This demonstrates that Antarctic air masses do not uniformly result in atmospheric compositional changes. This variability led us to explore additional factors that may have influenced our observations. We found that the tenfold increase in MSA g that we observed on 29 Jan coincided with a sharp and sustained drop in both absolute humidity and temperature throughout the entire “high MSA” period (Supplementary Fig. 3a). Such dry, cold air from the Antarctic continent is often linked to katabatic outflow [ 39 ]. Katabatic winds are an important climatological feature of Antarctica, occurring when cold, dense air descends from the Antarctic plateau along the slope of the continent under the influence of gravity. Previous studies have shown that there is a spatial variability in katabatic drainage patterns, driven primarily by topography, and have identified the zones of drainage convergence or “confluence zones”, where katabatic winds are intensified and more persistent [ 40 , 41 ]. One such zone is the coastal region of Adelie land around 142 o E, a region of the strongest observed surface winds on Earth [ 40 – 42 ]. The 8-day-long “high MSA” event occurred while the ship was in that region (140 o – 150 o E). Katabatic winds are known to promote vertical mixing and disrupt shallow low-level inversions, facilitating the entrainment of air from the free troposphere [ 43 ]. These winds are also associated with snowfall sublimation and in Adelie land have been shown to decrease the amount of precipitation reaching the ground by up to 35% [ 44 ]. This suggests that katabatic winds played a key role in modulating MSA levels during CAPRICORN-2. Aerosol properties during the “high MSA” period To explore aerosol properties during the “high MSA” period, we investigated the aerosol number size distribution and PM1 chemical composition (Fig. 2 ). A notable feature in the number size distribution is a prominent and persistent accumulation mode throughout the entire “high MSA” period. This period was also marked by persistently increased CCN concentrations (Fig. 2 b), averaging at 238 ± 45 cm − 3 compared to 121 ± 68 cm − 3 for the rest of the CAPRICORN-2 voyage (values given for 0.35% supersaturation as a good proxy for marine cloud-effective supersaturations [ 45 ]). This is consistent with the latitudinal distribution of aerosol and CCN concentrations across the Southern Ocean reported in Humphries ae al [ 35 ] that shows an increase in these aerosol parameters at the southernmost latitude bin (65-70 o S). PM1 composition data reveal that particulate SO 4 2− and MSA increase during the “high MSA” period in the same manner as gaseous MSA (Fig. 2 a). There is a strong correlation between MSA g and particulate SO 4 2− (R = 0.71), and MSA g and particulate MSA (R = 0.62) during the “high MSA” period (Supplementary Fig. 4a and b). A very similar relationship was found between MSA g and CCN number concentration (R = 0.68; Supplementary Fig. 4c). Previous studies have shown the same trends for particulate MSA and SO 4 2− , and CCN [ 35 , 46 ], but this is the first time that such a strong alignment between gaseous MSA and CCN has been observed. During the “high MSA” period, particulate SO 4 2− increases approximately 4 times compared to the rest of the data, and particulate MSA increases approximately 6 times compared to the rest of the data. This results in an increase of the MSA/SO 4 2− ratio from 0.23 ± 0.08 to 0.41 ± 0.06 (Supplementary Fig. 5). NH 4 + also increases almost 3 times during the “high MSA” period. At the same time, Na + and Cl − ions, as markers for sea spray aerosols, drop down substantially (approximately 2.5 and 20 times, respectively) (Supplementary Fig. 6a and b). During the “high MSA” period, SO 4 2− and MSA represented on average 88% of all measured species, whereas during the rest of the voyage, these species made up on average 21% of all measured species (Supplementary Fig. 6c and d). Supplementary Fig. 6 illustrates a significant difference between average PM1 ionic composition during the “high MSA” period compared to the rest of the measuring period. Overall, the observed increase in gaseous MSA was clearly reflected in the particle phase and demonstrates that during the “high MSA” period PM1 was strongly dominated by species of biogenic origin. Further investigation of the PM1 composition points to two important observations - summarised in Fig. 2 c and d, and in the text below - that further underscores the distinct characteristics of the “high MSA” period: Cl - /Na + ratio : Another notable feature of the “high MSA” period is a strong, almost complete chloride depletion (Fig. 2 c). Cl⁻/Na⁺ molar ratio of 1.18 is typical of seawater and expected to be the same or similar for freshly produced sea salt particles. Values lower than this suggest Cl⁻ displacement as HCl by less volatile acids (H 2 SO 4 , MSA, HNO 3 ) or heterogeneous oxidation by O 3 and OH radicals. However, given the low reaction rate constants for sea spray aerosol oxidation by O₃ and OH radicals, displacement by acidic species is considered to be the dominant pathway [ 47 ]. Cl - depletion in marine aerosols has been previously observed all around the world, including polar regions [ 47 – 50 ] Here, we observe up to 98% Cl⁻ loss during the "high MSA" period (assuming all Na⁺ originates from sea spray aerosol). This near complete Cl - depletion suggests significant chemical ageing of sea spray aerosols, with Cl - being replaced by H 2 SO 4 and MSA. This is supported by 4–6 times increase in PM1 SO 4 2- and MSA during the “high MSA” period (Fig. 2 a). Ion balance : Fig. 2 d shows the concentration of H⁺ in PM1, calculated from ionic charge balance, where a value of 0 indicates that all anions are fully neutralised by cations and values > 0 (excess anions) suggest the presence of an unmeasured cation, assumed to be H + . Conversely, values below 0 (deficit of anions) imply a missing anion, likely OH - . For most days during CAPRICORN-2, the charge balance was > 0 indicating an excess of anions and hence an acidic nature of aerosols. During the “high MSA” period, the excess of anions increased approximately fivefold, suggesting a substantial rise in aerosol acidity. To further explore aerosol acidity, we used the Extended Aerosol Inorganic Model IV (E-AIM IV) that predicts aerosol pH based on bulk ionic composition and environmental conditions (temperature and RH). Details on input parameters are given in the Supplementary Information. The modelled pH values (Fig. 2 d) closely mirror the trends in ionic charge balance, supporting the evidence of more acidic aerosol conditions during the “high MSA” period. Specifically, the aerosol was 1.2 pH units more acidic during this period (-0.7 ± 0.4) compared to the average across the rest of the campaign (0.5 ± 0.7). Supplementary Fig. 8 shows that the pH output for two different E-AIM model settings (II and IV) has the same trend for the majority of the measuring period and an excellent agreement for the “high MSA” period (on average, not more than 0.2 pH units difference). These are the first observationally constrained estimates of aerosol acidity over the high latitude Southern Ocean, specifically sea ice region of East Antarctica. Our results indicate consistently acidic aerosols throughout the sampling period, with pH values ranging from − 1.4 to 2. This aligns with recent global estimates based on different aircraft campaigns around the globe showing that aerosol pH decreases with remoteness– from pH values around 3 near polluted continental regions to -1 in remote marine environments [ 51 ]. Gas–Particle Partitioning of MSA in Antarctic Outflow: Influence of RH and Aerosol Acidity Given the strong positive correlation between MSA g and particulate SO₄²⁻ and MSA, as well as CCN concentrations during the “high MSA” period, along with the highly acidic aerosols and dry air masses, we investigated the potential evaporation of MSA from the surface of particles. We used the E-AIM to simulate MSA gas – particle partitioning (see Methods for details), where Model II represents marine secondary aerosols and Model IV represents an internally mixed aerosol of primary sea spray and secondary species. As shown in Fig. 3 a both E-AIM configurations successfully simulate the observed MSA g pattern during the “high MSA” period, with estimated concentrations within the same order of magnitude as observations. Our observations combined with E-AIM model results provide direct evidence that MSA undergoes evaporation from the surface of aerosols under ambient marine boundary layer conditions. Several studies have suggested that MSA may evaporate from particles as RH decreases [ 17 – 20 ]. However, gas – particle partitioning of semivolatile acids, such as MSA, can be affected by both RH, which governs aerosol liquid water content (ALWC), and aerosol acidity [ 43 ]. A reduction in ALWC promotes evaporation by decreasing the particle's capacity to retain dissolved species, while simultaneously contributing to a drop in aerosol pH. Very low aerosol pH (< 0) reflects a substantial shortage of neutralizing bases (e.g., NH₃), promoting the formation of the protonated form of strong acids like MSA, which increases their tendency to partition into the gas phase, particularly at low RH. The “high MSA” period coincided with low RH (Supplementary Fig. 3b) and highly acidic aerosols (pH < 0) and we show there is a strong inverse linear relationship between logMSA g and aerosol pH (R = -0.73), with the highest MSA g concentrations also coinciding with the lowest RHs (Fig. 3 b). To investigate which of these two factors MSA partitioning is more sensitive to, we performed an E-AIM IV simulation at a constant temperature of 273 K, varying RH between 60% and 100%, using PM1 composition from the “high MSA” period and adjusting only NH 4 + concentrations to obtain different pH values. The results indicate that under the pH and RH conditions characteristic for CAPRICORN-2, MSA partitioning is more sensitive to changes in pH (Fig. 3 c). For example, when RH changes from 60 to 90% MSA g stays within the same order of magnitude and for a change in p Therefore, aerosol acidity and pH changes to values < 0 seem to be the main driver of MSA evaporation. It is important to note is that despite the ten-fold increase in MSA g during the “high MSA” period, these gas phase concentrations represent a small fraction of the total MSA (< 10%). Figure 3 d shows the theoretical pH dependence of particulate fraction of MSA (MSA p /MSA p +MSA g ) calculated for several different ALWCs [ 52 ], along with observational and E-AIM IV modelled data for CAPRICORN-2 and demonstrates excellent agreement of our observed data with theoretical predictions of MSA partitioning. This provides further evidence of pH-driven partitioning of MSA. Discussion and atmospheric implications Antarctic coastal waters are known to be one of the most biologically productive regions of the world during austral summer, resulting in high DMS and other volatile sulfur compounds production [ 25 ]. Antarctica is also surrounded by a band of low pressure between 60 o and 65 o S known as the circumpolar trough [ 41 ], which is further enhanced by frequent cyclones and mesocyclones around its coast [ 53 ]. These low pressure systems drive a net upward transport of Antarctic coastal air masses, which can have two important consequences for the atmospheric processing of DMS: (1) aqueous phase oxidation of DMS in clouds formed due to atmospheric uplift, with the subsequent free-tropospheric evaporation of cloud droplets leading to biogenically enriched particles; (2) free-tropospheric oxidation of DMS, predominantly via OH addition pathway, which is preferred at low temperature conditions [ 5 ], with the free troposphere providing favourable environmental conditions (low temperature, low condensation sink) for H 2 SO 4 and/or MSA-driven new particle formation. Both scenarios will result in increased MSA production and overall increased particulate MSA/SO 4 2− ratio. This agrees with our CAPRICORN-2 observations (Supplementary Fig. 5). Subsidence of free-tropospheric air over Antarctica and its entrainment into the boundary layer is promoted by the continental outflow and katabatic drainage of air masses over the slopes of the continent. Antarctic surface winds have a strong directional constancy of around 90% [ 43 ] and are frequently attributed to katabatic outflow, downslope winds driven by gravity due to radiative cooling and densification of air over the sloping surface. Northward air flow (Fig. 2 d) in combination with a decrease in temperature and absolute humidity (Supplementary Fig. 3a) provides strong evidence for katabatic outflow during the “high MSA” period. We show that these air masses carry highly acidic particles (reaching pH of up to -1.5) dominated by DMS oxidation products (~ 90% of anions). In addition, these particles are almost completely depleted in Cl − , which is driven by the accumulation of low vapour pressure acids (i.e. H 2 SO 4 and MSA) and insufficient availability of ammonia to neutralise them. This is particularly important as most chemical transport models tend to underestimate aerosol acidity in remote marine regions, which leads to underestimation of water uptake and underestimation of aerosol direct radiative effect [ 51 ]. In summary, we suggest that these highly acidic, biogenically dominated aerosol particles are the result of long-range transport that is governed by the dominant large-scale circulation pattern within the Antarctic polar cell. Furthermore, we demonstrate that such compositional make-up, in combination with low RH characteristic of katabatic winds, leads to pH-driven evaporation of MSA, revealing a previously unconfirmed process in sulfur cycling. Many aerosol transformation processes are driven by acidity, including gas – particle partitioning of acids and bases [ 57 ]. Previous studies have reported a positive correlation between the decrease in RH and an increase in gaseous MSA, suggesting MSA evaporation at lower RH. While RH and aerosol pH are closely linked through aerosol liquid water content, we show that this process is more sensitive to pH. Processes outlined here are schematically illustrated in Fig. 4 . The partitioning of MSA into the gas phase can extend its atmospheric lifetime by reducing its removal through wet deposition. In addition, the evaporation of MSA could potentially have implications for CCN number concentrations. Evaporation of MSA from the surface of highly acidic particles will be more pronounced for smaller particles due to the Kelvin effect. As the observed MSA partitioning is pH-driven, particulate MSA in externally mixed aerosols could potentially partition from more acidic particles to less acidic particles. The partitioning of MSA and both its dependence and influence on the aerosol size distribution and potential implication for CCN concentrations needs further exploration. DMS oxidation represents a key process influencing CCN and cloud droplet number in the Southern Ocean atmosphere. It is expected that DMS emissions will increase in a warming climate due to increase in near-surface wind speeds and sea surface temperatures [ 54 ]. MSA is a non-negligible DMS oxidation product and yet it is not well represented in models [ 13 , 24 ]. It is still an open question of whether the partitioning of MSA into the gas phase from the aerosol surface is a significant source of boundary layer gaseous MSA compared to direct gas-phase production from boundary layer DMS oxidation. If it is, as our study suggests, then models must account for this process when simulating gas phase MSA and comparing it with observations. Capturing DMS chemistry along the entire transport pathways, including the free troposphere and over the continent, is essential for improving the accuracy of chemical transport models and global climate models. Methodology Gaseous MSA and SA measurements MSA and SA were measured using a high-resolution time-of-flight Chemical Ionisation Mass Spectrometer with nitrate reagent ions (NO 3 -CIMS) [ 55 , 56 ]. Nitrate ions ((HNO 3 ) x NO 3 − ; x = 0–2) were generated by introducing HNO 3 vapour into 20 L min − 1 HEPA-filtered sheath air flow and exposing it to X-ray radiation within the chemical ionisation inlet. The HNO 3 vapour was generated by passing 3 mL min − 1 of N 2 over a concentrated (70% wt) HNO 3 reservoir. Outdoor air was sampled through a 2 m, ½” stainless-steel, non-heated tube at 30 L min − 1 ,with 10 L min − 1 directed to the NO 3 -CIMS and 20 L min − 1 as auxiliary flow to reduce residence time in the tubing. Diffusional losses in the sampling line were calculated based on the diffusion coefficient of sulfuric acid [ 57 ] and applied to both SA and MSA. The intensity of SA and MSA was calculated by summing ions originating from each of these two species, normalising them to the sum of the reagent ions and multiplying by a calibration factor. While the nitrate-CIMS was not directly calibrated for sulfuric acid in this study, literature-reported calibration factors are broadly consistent across studies and a median calibration factor derived from 21 literature values was used (details in SI). The same calibration factor was used for MSA based on the assumption that ionisation of these species is equal and proceeds at the kinetic limit due to having a lower proton affinity than nitric acid, and transmission losses in the instrument are comparable. Aerosol measurements Aerosols were sampled using a common sampling inlet located at the bow of the ship, 18 m above sea level, with the flowrate of the common sampling line set to 180 L min − 1 . The details of the sampling inlet and standard atmospheric measurement capabilities onboard RV Investigator are given in [ 58 ]. For all aerosol data, as well as gaseous sulfuric acid, periods influenced by ship exhaust were removed based on the method developed by Humphries et al [ 58 ]. This resulted in removing 14% of the data. The aerosol number size distribution was measured over the mobility diameter range 4–661 nm with two scanning mobility particle sizers (SMPS) - GRIMM Nano-SMPS (4-409 nm) and TSI long-column SMPS (15–661 nm). A merged size distribution was calculated at 5 min time resolution, combining the GRIMM Nano-SMPS size bins up to 35.4 nm and the TSI long-column SMPS size bins up to 661 nm. CCN number concentrations were measured with a DMT CCN-100 counter, cycling between 1.05%, 0.65%, 0.55%, 0.45%, 0.35% and 0.25% supersaturation (SS). This sequence was repeated hourly, giving 10 min of data at each supersaturation setting [ 59 ]. In this study, only 0.35% SS data is used. The chemical composition of non-refractory submicron aerosol was measured using an Aerodyne time-of-flight aerosol chemical speciation monitor (ToF-ACSM) equipped with a PM1 aerodynamic lens and the standard vaporiser. 10-minute averages of particulate sulfate and MSA are presented here. MSA concentrations were estimated from the m/z 79 peak (CH 3 SO 2 + fragment) as described in [ 60 ], but were largely below the detection limit outside of the “high MSA” period. In remote marine environments chloride originates primarily from sea salt aerosol; however, sea salt particles are refractory and as such are not efficiently vaporised in the ACSM. NH 4 + measured by the ACSM was below the detection limit during the whole measurement campaign. Alongside the ToF-ACSM measurements, PM 1 aerosol was collected on 47 mm quartz filters, sampling between 20 and 48 hr. To avoid contamination by exhaust aerosol from the Research Vessel itself, a switching controller was employed, which paused sampling when relative wind direction was between 90 and 270◦ and the concentrations of particles exceeded a threshold value 1000 Anion and cation concentrations are determined with a Dionex ICS-3000 reagent-free ion chromatograph as described in [ 59 ]. Measured inorganic ions included Na + , NH 4 + , K + , Ca 2+ , Mg 2+ , SO 4 2− , NO 3 − , Br − and Cl − , and the organic ions included methane sulphonate (MS − ), oxalate (C 2 O 4 2− ), acetate (CH 3 COO − ) and formate (HCOO − ). All values reported in this manuscript are blank corrected. RV Investigator underway data Standard meteorological parameters (temperature, relative humidity, pressure, wind speed and direction) were measured from two locations on the ship (port and starboard) at ~ 25 m above sea level and they are a part of continuously collected “underway data”. Back trajectory analyses To estimate the origin of sampled airmasses, we performed backward simulation of airmass trajectories using NOAA’s Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model [ 61 ]. The meteorological data from the Global Data Assimilation System (GDAS) at 0.5 degrees resolution was used for the HYSPLIT trajectory calculations. 5-day-long backtrajectories were initiated at ship’s location for each hour of the voyage and an arrival height of 100 m above sea level. Extended Aerosol Inorganics Model (E-AIM) E-AIM ( http://www.aim.env.uea.ac.uk/aim/aim.php , last access: 23.12.2024) is a widely used thermodynamic model utilised here to model gas-particle partitioning of MSA and to estimate aerosol acidity based on aerosol bulk chemical composition and environmental conditions (T, RH) [ 62 – 64 ]. E-AIM Models II and IV were used. Model II is a thermodynamic equilibrium model for the H + - NH 4 + - nssSO 4 2− - NO 3 − - H 2 O mixture system, here used as a representative of secondary aerosol. Model IV includes H + -NH 4 + -Na + -SO 4 2− -NO 3 − -Cl − -H 2 O, here used to represent an internal mixture of primary sea spray aerosol and secondary inorganic species. In addition to concentrations of ionic species (further details in SI), including H + estimated from charge balance, both models had total MSA (MSA p and MSA g ) included. The MSA thermodynamic properties used in the model are the same as the ones reported in Baccarini et al [ 13 ]. As E-AIM can use only Na + , other metal ions (Ca 2+ , K + , Mg 2+ ) were accounted for by converting them to charge equivalent amounts of Na + . E-AIM provides molar fraction based H + activity coefficients, which were converted to molality-based coefficients following IUPAC recommendations for pH calculations ( https://goldbook.iupac.org/terms/view/P04524 ). Declarations Acknowledgements This research was supported by a grant of sea time on RV Investigator from the CSIRO Marine National Facility ( https://ror.org/01mae9353 ). The Authors wish to thank the CSIRO Marine National Facility (MNF) for its support in the form of sea time on RV Investigator, support personnel, scientific equipment and data management. In particular, we thank the technical and IT support personnel on board the voyage, with particular thanks to the Seagoing Instrumentation Team ashore. References Mallet M et al (2023) Untangling the influence of Antarctic and Southern Ocean life on clouds. Elementa: Science of the Anthropocene., 11(1) Mallet MD et al (2025) Biological enhancement of cloud droplet concentrations observed off East Antarctica. npj Clim Atmospheric Sci 8(1):113 Charlson RJ et al (1987) Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature 326(6114):655–661 Wohl C et al (2024) Marine emissions of methanethiol increase aerosol cooling in the Southern Ocean. Sci Adv 10(48):eadq2465 Barnes I et al (2006) Dimethyl Sulfide and Dimethyl Sulfoxide and Their Oxidation in the Atmosphere. Chem Rev 106(3):940–975 Faloona I (2009) Sulfur processing in the marine atmospheric boundary layer: A review and critical assessment of modeling uncertainties. Atmos Environ 43(18):2841–2854 Chen J et al (2023) Atmospheric Gas-Phase Formation of Methanesulfonic Acid, vol 57. Environmental Science & Technology, pp 21168–21177. 50 Bates TS et al (1992) Variations in the methanesulfonate to sulfate molar ratio in submicrometer marine aerosol particles over the south Pacific Ocean. J Geophys Research: Atmos 97(D9):9859–9865 Chen L et al (2012) Latitudinal distributions of atmospheric MSA and MSA/nss-SO42 – ratios in summer over the high latitude regions of the Southern and Northern Hemispheres. J Geophys Research: Atmos, 117(D10). Yan J et al (2019) Significant Underestimation of Gaseous Methanesulfonic Acid (MSA) over Southern Ocean, vol 53. Environmental Science & Technology, pp 13064–13070. 22 Davison B et al (1996) Dimethyl sulfide and its oxidation products in the atmosphere of the Atlantic and Southern Oceans. Atmos Environ 30(10):1895–1906 Shen J et al (2022) High Gas-Phase Methanesulfonic Acid Production in the OH-Initiated Oxidation of Dimethyl Sulfide at Low Temperatures, vol 56. Environmental Science & Technology, pp 13931–13944. 19 Hoffmann EH et al (2016) An advanced modeling study on the impacts and atmospheric implications of multiphase dimethyl sulfide chemistry. Proceedings of the National Academy of Sciences, 113(42): pp. 11776–11781 Chen Q et al (2018) DMS oxidation and sulfur aerosol formation in the marine troposphere: a focus on reactive halogen and multiphase chemistry. Atmos Chem Phys 18(18):13617–13637 Kecorius S et al (2023) Rapid growth of Aitken-mode particles during Arctic summer by fog chemical processing and its implication. PNAS Nexus, 2(5) von Glasow R et al (2004) Model study of multiphase DMS oxidation with a focus on halogens. Atmos Chem Phys 4(3):589–608 Baccarini A et al (2021) Low-Volatility Vapors and New Particle Formation Over the Southern Ocean During the Antarctic Circumnavigation Expedition. J Geophys Research: Atmos, 126(22): p. e2021JD035126. Mauldin III (1999) Observations of H2SO4 and MSA during PEM-Tropics-A. J Geophys Research: Atmos 104(D5):5801–5816 Scholz W et al (2023) Measurement report: Long-range transport and the fate of dimethyl sulfide oxidation products in the free troposphere derived from observations at the high-altitude research station Chacaltaya (5240 m a.s.l.) in the Bolivian Andes. Atmos. Chem. Phys., 23(2): pp. 895–920 Zhang Y et al (2014) Surface and free tropospheric sources of methanesulfonic acid over the tropical Pacific Ocean. Geophys Res Lett 41(14):5239–5245 Dada L et al (2022) A central arctic extreme aerosol event triggered by a warm air-mass intrusion. Nat Commun 13(1):5290 Hodshire AL et al (2019) The potential role of methanesulfonic acid (MSA) in aerosol formation and growth and the associated radiative forcings. Atmos Chem Phys 19(5):3137–3160 Davis DD et al (1998) DMS oxidation in the Antarctic marine boundary layer: Comparison of model simulations and held observations of DMS, DMSO, DMSO2, H2SO4(g), MSA(g), and MSA(p). J Phys Res 103:1657–1678 Fung KM et al (2022) Exploring dimethyl sulfide (DMS) oxidation and implications for global aerosol radiative forcing. Atmos Chem Phys 22(2):1549–1573 Hulswar S et al (2022) Third revision of the global surface seawater dimethyl sulfide climatology (DMS-Rev3). Earth Syst Sci Data 14(7):2963–2987 Brean J et al (2021) Open ocean and coastal new particle formation from sulfuric acid and amines around the Antarctic Peninsula. Nat Geosci 14(6):383–388 Jokinen T et al (2018) Ion-induced sulfuric acid–ammonia nucleation drives particle formation in coastal Antarctica. Sci Adv 4(11):eaat9744 Mauldin RL et al (2004) Measurements of OH, HO2 + RO2, H2SO4, and MSA at the South Pole during ISCAT 2000. Atmos Environ 38(32):5423–5437 Quéléver LLJ et al (2022) Investigation of new particle formation mechanisms and aerosol processes at Marambio Station, Antarctic Peninsula. Atmos Chem Phys 22(12):8417–8437 Bardouki H et al (2003) Gaseous (DMS, MSA, SO 2, H 2 SO 4<!-- sub > and DMSO) and particulate (sulfate and methanesulfonate) sulfur species over the northeastern coast of Crete. Atmos. Chem. Phys., 3(5): pp. 1871–1886 Berresheim H et al (2002) Gas-aerosol relationships of H2SO4, MSA, and OH: Observations in the coastal marine boundary layer at Mace Head, Ireland. Journal of Geophysical Research: Atmospheres, 107(D19): p. PAR 5-1-PAR 5–12 Peltola M et al (2023) Chemical precursors of new particle formation in coastal New Zealand. Atmos Chem Phys 23(7):3955–3983 Zhang Y et al (2024) Measurements of particulate methanesulfonic acid above the remote Arctic Ocean using a high resolution aerosol mass spectrometer. Atmos Environ 331:120538 Alroe J et al (2020) Marine productivity and synoptic meteorology drive summer-time variability in Southern Ocean aerosols. Atmos Chem Phys 20(13):8047–8062 Humphries RS et al (2021) Southern Ocean latitudinal gradients of cloud condensation nuclei. Atmos Chem Phys 21(16):12757–12782 Humphries RS et al (2016) Unexpectedly high ultrafine aerosol concentrations above East Antarctic sea ice. Atmos Chem Phys 16(4):2185–2206 Simmons JB et al (2021) Summer aerosol measurements over the East Antarctic seasonal ice zone. Atmos Chem Phys 21(12):9497–9513 Jefferson AJ et al (1998) OH photochemistry and methane sulfonic acid formation in the coastal Antarctic boundary layer. Oceanogr Literature Rev 6:922 Chambers SD et al (2018) Characterizing Atmospheric Transport Pathways to Antarctica and the Remote Southern Ocean Using Radon-222. Front Earth Sci, Volume 6–2018 Parish TR et al (1987) The surface windfield over the Antarctic ice sheets. Nature 328(6125):51–54 Parish TR et al (2007) Reexamination of the Near-Surface Airflow over the Antarctic Continent and Implications on Atmospheric Circulations at High Southern Latitudes. Mon Weather Rev 135(5):1961–1973 Wendler G et al (1997) On the extraordinary katabatic winds of Adélie Land. J Geophys Research: Atmos 102(D4):4463–4474 Parish TR et al (2003) The Role of Katabatic Winds on the Antarctic Surface Wind Regime. Mon Weather Rev 131(2):317–333 Grazioli J et al (2017) Katabatic winds diminish precipitation contribution to the Antarctic ice mass balance. Proceedings of the National Academy of Sciences, 114(41): pp. 10858–10863 Sanchez KJ et al (2021) Measurement report: Cloud processes and the transport of biological emissions affect southern ocean particle and cloud condensation nuclei concentrations. Atmos Chem Phys 21(5):3427–3446 Ayers GP et al (1991) Seasonal relationship between cloud condensation nuclei and aerosol methanesulphonate in marine air. Nature 353(6347):834–835 Su B et al (2022) A review of atmospheric aging of sea spray aerosols: Potential factors affecting chloride depletion. Atmos Environ 290:119365 Gonçalves SJ et al (2021) Photochemical reactions on aerosols at West Antarctica: A molecular case-study of nitrate formation among sea salt aerosols. Sci Total Environ 758:143586 Legrand M et al (2017) Year-round records of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) – Part 1: Fractionation of sea-salt particles. Atmos Chem Phys 17(22):14039–14054 Kerminen V-M et al (2000) Chemistry of sea-salt particles in the summer Antarctic atmosphere. Atmos Environ 34(17):2817–2825 Nault BA et al (2021) Chemical transport models often underestimate inorganic aerosol acidity in remote regions of the atmosphere. Commun Earth Environ 2(1):93 Nenes A et al (2020) Aerosol pH and liquid water content determine when particulate matter is sensitive to ammonia and nitrate availability. Atmos Chem Phys 20(5):3249–3258 Simmonds I et al (2003) Synoptic Activity in the Seas around Antarctica. Mon Weather Rev 131(2):272–288 Joge SD et al (2025) Climate warming increases global oceanic dimethyl sulfide emissions. Proceedings of the National Academy of Sciences, 122(23): p. e2502077122 Jokinen T et al (2012) Atmospheric sulphuric acid and neutral cluster measurements using CI-APi-TOF. Atmos Chem Phys 12(9):4117–4125 Kurtén, T., et al., The effect of H 2 SO 4 – amine clustering on chemical ionization mass spectrometry (CIMS) measurements of gas-phase sulfuric acid. Atmos. Chem. Phys., 2011. 11(6): pp. 3007–3019 Hanson DR et al (2000) Diffusion of H2SO4 in Humidified Nitrogen: Hydrated H2SO4. J Phys Chem A 104(8):1715–1719 Humphries RS et al (2019) Identification of platform exhaust on the RV Investigator. Atmos Meas Tech 12(6):3019–3038 Humphries R et al (2021) CAPRICORN2 - Atmospheric aerosol measurements from the RV Investigator voyage IN2018_V01. Commonwealth Scientific and Industrial Research Organisation Zorn SR et al (2008) Characterization of the South Atlantic marine boundary layer aerosol using an aerodyne aerosol mass spectrometer. Atmos Chem Phys 8(16):4711–4728 Draxler R et al (1998) An overview of the HYSPLIT_4 modeling system for trajectories, dispersion, and deposition. Aust Meteorol Mag 47:295–308 Clegg SL et al (2006) Thermodynamic Models of Aqueous Solutions Containing Inorganic Electrolytes and Dicarboxylic Acids at 298.15 K. 2. Systems Including Dissociation Equilibria. J Phys Chem A 110(17):5718–5734 Clegg SL et al (2006) Thermodynamic Models of Aqueous Solutions Containing Inorganic Electrolytes and Dicarboxylic Acids at 298.15 K. 1. The Acids as Nondissociating Components. J Phys Chem A 110(17):5692–5717 Wexler AS et al (2002) Atmospheric aerosol models for systems including the ions H+, NH4+, Na+, SO42–, NO3–, Cl–, Br–, and H2O. Journal of Geophysical Research: Atmospheres, 107(D14): p. ACH 14-1-ACH 14–14 Additional Declarations There is NO Competing Interest. Supplementary Files CAPRICORN2papersupplementMiljevicetal.docx Supplementary Information Cite Share Download PDF Status: Published Journal Publication published 28 Nov, 2025 Read the published version in Communications Earth & Environment → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6905825","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":473634357,"identity":"49a159ae-6620-4bb5-9310-20c2e9966aa1","order_by":0,"name":"B.Miljevic","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA5klEQVRIie3OMQrCMBSA4VcK6WDqXKnUK1Q6lGLxLK8UunkGBUEXdRc8REFwDnRw8QABF7s4OSgugoPGKC5CyOiQnzwIIR8JgMn0h4Vi2IECELDZ+4jpEJSEIABqEnkRgIZ6JIZGxrCdZgtnd9m7NwiaHK0rVZBk5JbiY0U2oYN1jyJELY62ryIhk6SKiOdufEGykiPokIcg9PgiQ0HsuwZhgSDkRTDkSJSvJGNJ8oDQIkpWhddd7upJslKQ2Jmvz5dZn3amVc1PadppbvOKnxQEbDHW7L335AJrpAKfbl9iMplMpt+eTwNHuU4DpkEAAAAASUVORK5CYII=","orcid":"","institution":"School of Earth and Atmospheric Sciences, Queensland University of Technology","correspondingAuthor":true,"prefix":"","firstName":"","middleName":"","lastName":"B.Miljevic","suffix":""},{"id":492208158,"identity":"83645fd8-6028-497e-b04b-74cb153ed607","order_by":1,"name":"M.D. Mallet","email":"","orcid":"","institution":"Australian Antarctic Program Partnership, Institute for Marine and Antarctic Studies (IMAS), University of Tasmania, Hobart, Australia","correspondingAuthor":false,"prefix":"","firstName":"M.D.","middleName":"","lastName":"Mallet","suffix":""},{"id":492208458,"identity":"4f935dd5-4270-4dfa-97e1-c26ee5b9b544","order_by":2,"name":"C.G. Osuagwu","email":"","orcid":"","institution":"School of Earth and Atmospheric Sciences, Queensland University of Technology, Brisbane, Australia","correspondingAuthor":false,"prefix":"","firstName":"C.G.","middleName":"","lastName":"Osuagwu","suffix":""},{"id":492208570,"identity":"25a3f054-e29a-48b2-b0fe-ef327d4bc6d7","order_by":3,"name":"Z.D. Ristovski","email":"","orcid":"","institution":"School of Earth and Atmospheric Sciences, Queensland University of Technology, Brisbane, Australia","correspondingAuthor":false,"prefix":"","firstName":"Z.D.","middleName":"","lastName":"Ristovski","suffix":""},{"id":492208684,"identity":"377395ac-a15e-4959-ba62-cad1a38b0ae2","order_by":4,"name":"R. S. Humphries","email":"","orcid":"","institution":"Australian Antarctic Program Partnership, Institute for Marine and Antarctic Studies (IMAS), University of Tasmania, Hobart, Australia","correspondingAuthor":false,"prefix":"","firstName":"R.","middleName":"S.","lastName":"Humphries","suffix":""},{"id":492208706,"identity":"fa05a8e3-94de-4f67-9f1b-512d11ba7db3","order_by":5,"name":"P. 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Traces represent median values and shaded areas 75\u003csup\u003eth\u003c/sup\u003e and 25\u003csup\u003eth\u003c/sup\u003e percentiles. The black dashed line is the solar radiation median values. \u0026nbsp;MSA\u003csub\u003eg\u003c/sub\u003e diurnal profile is presented separately for the “high MSA” period (8 days) and the rest of the data (29 days) due to MSA\u003csub\u003eg\u003c/sub\u003e being an order of magnitude more intense during that period. \u0026nbsp;The Time of Day is adjusted so 12:00 represents local noon. (d) Ship track coloured by gaseous MSA concentrations. (e) five-day long HYSPLIT backtrajectories along each hour of ship track coloured by gaseous MSA concentrations.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-6905825/v1/fd1bac8c202939ab80fae563.png"},{"id":85694169,"identity":"2f33c401-1946-45d5-b95c-38a5c6d7b606","added_by":"auto","created_at":"2025-06-30 17:43:07","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":671422,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePM1 composition and number size distribution during CAPRICORN-2.\u003c/strong\u003e (a)\u0026nbsp; SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e, MSA, and NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e in submicron aerosol and based on ~24 hr PM1 filters analysis and as measured by ACSM (only SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e and MSA). MSA concentrations estimated from the ACSM were corrected based on the calibration factor obtained from comparison with the PM1 filter data. ACSM MSA values outside of the “high MSA” period were below the limit of detection. The gray shaded area represents MSA\u003csub\u003eg\u003c/sub\u003e ; (b) Aerosol number size distribution and CCN number concentration at 0.35% supersaturation; \u0026nbsp;(c) Cl\u003csup\u003e-\u003c/sup\u003e/Na\u003csup\u003e+ \u003c/sup\u003emolar ratio \u0026nbsp;with black dashed line representing Cl\u003csup\u003e-\u003c/sup\u003e/Na\u003csup\u003e+ \u003c/sup\u003emolar ratio in seawater and is assumed to be the same for freshly emitted sea salt particles; \u0026nbsp;b) H\u003csup\u003e+ \u003c/sup\u003econcentration (red markers) in PM1 calculated from ionic charge balance (Σ|z\u003csub\u003ean\u003c/sub\u003e|n\u003csub\u003ean \u003c/sub\u003e- Σz\u003csub\u003ecat\u003c/sub\u003en\u003csub\u003ecat\u003c/sub\u003e; z = charge) \u0026nbsp;and aerosol pH estimated from E-AIM Model IV (purple line). The size of markers in both graphs is shown as a function of particulate MSA molar concentration.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-6905825/v1/b5427adcf3be6b8719ccabbd.png"},{"id":85694378,"identity":"643274d8-a96e-4c54-b16c-3c955ba4ee11","added_by":"auto","created_at":"2025-06-30 17:51:07","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":425057,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMSA partitioning and influence of pH on this process. \u003c/strong\u003e(a) Comparison of MSA\u003csub\u003eg\u003c/sub\u003e observations with MSA\u003csub\u003eg\u003c/sub\u003e E-AIM simulations. E-AIM IV models MSA partitioning for H\u003csup\u003e+\u003c/sup\u003e-NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-Na\u003csup\u003e+\u003c/sup\u003e-SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2−\u003c/sup\u003e-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e-Cl\u003csup\u003e−\u003c/sup\u003e-H\u003csub\u003e2\u003c/sub\u003eO (+total MSA) system and assumes aerosol to be an internal mixture; E-AIM II models MSA partitioning for H\u003csup\u003e+\u003c/sup\u003e - NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e - nssSO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2−\u003c/sup\u003e - NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e - H\u003csub\u003e2\u003c/sub\u003eO (+total MSA) system and assumes these components are a part of the secondarily formed aerosol. Shaded areas represent periods influenced by Antarctic air masses. (b) Relationship between logarithm of observed MSA\u003csub\u003eg\u003c/sub\u003e and E-AIM modelled pH, coloured by RH. (c) E-AIM IV modelled MSA\u003csub\u003eg\u003c/sub\u003e concentrations for 60% to 100% RH at 273 K and aerosol pH range estimated for CAPRICORN-2. Aerosol composition used in the model was from 30 Jan filter sample and different pH concentrations were achieved by varying NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e concentrations. (d) pH dependence of particulate fraction of MSA (MSA\u003csub\u003ep\u003c/sub\u003e/MSA\u003csub\u003eg\u003c/sub\u003e+MSA\u003csub\u003ep\u003c/sub\u003e) for CAPRICORN-2 observations (circles) and E-AIM IV modelled MSA (crosses), coloured by logarithm of ALWC calculated for CAPRICORN-2. Black lines represent theoretical curves of particulate MSA fraction for several different ALWCs at 273 K.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-6905825/v1/296da580f755ac160a090f7c.png"},{"id":85693477,"identity":"bc9ddfbb-ed13-4625-b4c1-2de69f822945","added_by":"auto","created_at":"2025-06-30 17:35:07","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":343799,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic linking large-scale circulation in the Antarctic polar cell and MSA production, transport, and partitioning.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-6905825/v1/2278511e88eeac392a304d66.png"},{"id":99211853,"identity":"de281685-24b9-4ec2-abf5-a81a8027e679","added_by":"auto","created_at":"2025-12-30 08:11:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2725394,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6905825/v1/ee8b6667-782c-4afa-912c-9ea7afd33b39.pdf"},{"id":85693474,"identity":"6908644b-689b-4c02-bcfd-e46b439f4879","added_by":"auto","created_at":"2025-06-30 17:35:07","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2138324,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"CAPRICORN2papersupplementMiljevicetal.docx","url":"https://assets-eu.researchsquare.com/files/rs-6905825/v1/a5a09507a160be2f4f72d9c7.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Aerosol Acidity Controls Methanesulfonic Acid Evaporation From Aerosols During Antarctic Katabatic Outflow","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe connection between ocean biogeochemistry and the atmosphere is critical for understanding Southern Ocean and Antarctic aerosols, their interaction with clouds and how they might be impacted by a changing climate [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Recent work has shown that biogenic aerosols increase cloud condensation nuclei and modulate cloud properties over the coastal East Antarctica [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. These biogenic aerosols are produced primarily from dimethyl sulfide (DMS; CH\u003csub\u003e3\u003c/sub\u003eSCH\u003csub\u003e3\u003c/sub\u003e ) [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] and potentially other volatile sulfur-based compounds [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] emitted by ocean microbiota. Once emitted in the atmosphere, DMS is chemically transformed through complex oxidation pathways, ultimately leading to the production of sulfuric acid (SA; H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e) and methanesulfonic acid (MSA; CH\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003eH). These are compounds that, due to their low volatilities, can nucleate to form new particles and/or condense onto existing aerosol particles, thereby promoting growth into sizes capable of acting as Cloud Condensation Nuclei (CCN).\u003c/p\u003e \u003cp\u003eGaseous DMS is oxidised in the atmosphere via two main pathways. The first involves hydrogen atom abstraction, primarily by hydroxyl radical (.OH), but also by halogen radicals, such as chlorine (Cl.), and bromine oxide (BrO), and the nitrate radical (NO\u003csub\u003e3\u003c/sub\u003e.) [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. This pathway is favoured at higher temperatures (e.g. lower latitudes) and predominantly produces sulfur dioxide (SO\u003csub\u003e2\u003c/sub\u003e), which is efficiently converted to H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. The second pathway involves hydroxyl radical addition, which is favoured at lower temperatures (e.g. polar environments and/or the free troposphere) and the dominant product is MSA [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. This temperature dependence has been demonstrated in the field through an increase in the ratio of particulate MSA to non-sea-salt sulfate (nss-SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e) with latitude, in regions where nss-SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e is minimally influenced by anthropogenic activities in the Southern Hemisphere [\u003cspan additionalcitationids=\"CR9 CR10\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. In addition, recent chamber experiments have shown an order of magnitude increase in the gas phase MSA/H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e ratio when temperature is decreased from 25 to -10 \u003csup\u003eo\u003c/sup\u003eC [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMSA is also the dominant product of DMS aqueous phase oxidation by O\u003csub\u003e3\u003c/sub\u003e and OH radicals, which occurs in deliquesced particles or cloud droplets [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Modelling studies have proposed this to be an important MSA formation mechanism [\u003cspan additionalcitationids=\"CR14 CR15\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] and Kecorius et al have suggested that upon droplet evaporation, particulate MSA can partition into the gas phase [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Consistent with this, previous field studies have observed a negative correlation between gaseous MSA and relative humidity (RH) and, based on that, hypothesized that MSA may evaporate from particles as RH and aerosol liquid water content (ALWC) decreases [\u003cspan additionalcitationids=\"CR18 CR19\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. This process will be further enhanced by aerosol acidity, with more acidic aerosol promoting MSA partitioning into the gas phase [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Overall, the prevalent DMS oxidation mechanism \u0026ndash; and the resulting oxidation products - will depend on environmental conditions, such as temperature and humidity, availability of different oxidants, and cloud presence. Despite this, large-scale chemical transport and climate models use fixed yields of SO\u003csub\u003e2\u003c/sub\u003e and MSA to simulate sulfate aerosol formation and most consider only gas phase reactions. Although DMS chemistry has been studied for decades, there are still large uncertainties in the amount and spatial distribution of aerosols resulting from its oxidation [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDuring austral summer the polar sector of the Southern Ocean has the highest concentrations of aqueous DMS on the planet [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. However, studies investigating DMS fluxes into the Southern Ocean atmosphere, as well as the yields of its terminal oxidation products (MSA and SA), are scarce. In this study we focus on simultaneous measurements of gaseous MSA and SA in the Antarctic West Pacific Ocean sector (90\u0026ndash;160\u0026deg;E), highlighting a unique eight-day period during the CAPRICORN-2 (2018) Southern Ocean voyage characterised by elevated gaseous MSA concentrations. We show that this period coincides with katabatic outflow from the Antarctic continent and, by combining observations with thermodynamic modelling of MSA gas \u0026ndash; particle partitioning, we provide evidence for MSA evaporation from aerosols being the source of enhanced gas-phase MSA. We demonstrate that katabatic outflow results in highly acidic and biogenically dominated aerosols and identify aerosol pH to be the primary driver of MSA evaporation.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eGaseous MSA and SA during CAPRICORN-2\u003c/h2\u003e \u003cp\u003eThe CAPRICORN-2 (Clouds, \u003cem\u003eAerosols\u003c/em\u003e, Precipitation, Radiation, and Atmospheric Composition over the Southern Ocean) voyage took place between 10 Jan and 22 Feb 2018 on board the Australian Research Vessel (RV) Investigator. The RV Investigator travelled southward from Hobart, Tasmania and reached\u0026thinsp;~\u0026thinsp;65\u003csup\u003eo\u003c/sup\u003e S where it transected the longitudinal area between 130\u003csup\u003eo\u003c/sup\u003e and 150\u003csup\u003eo\u003c/sup\u003e E. A prominent feature of the CAPRICORN-2 voyage was an eight-day long period (29 Jan \u0026ndash; 6 Feb) of persistently high levels of gaseous MSA (MSA\u003csub\u003eg\u003c/sub\u003e 2 (\u0026plusmn;\u0026thinsp;1) \u0026times; 10\u003csup\u003e7\u003c/sup\u003e molecules cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e), roughly an order of magnitude higher than the average of the rest of the data (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). Gaseous sulfuric acid (SA\u003csub\u003eg\u003c/sub\u003e), on the other hand, did not exhibit any substantial increase during the \u0026ldquo;high MSA\u0026rdquo; period. SA\u003csub\u003eg\u003c/sub\u003e concentrations were on average an order of magnitude lower than those of MSA\u003csub\u003eg\u003c/sub\u003e during the campaign and two orders of magnitude lower than MSA\u003csub\u003eg\u003c/sub\u003e during the \u0026ldquo;high MSA\u0026rdquo; period (Supplementary Fig.\u0026nbsp;1). Our observed gaseous MSA\u003csub\u003eg\u003c/sub\u003e/SA\u003csub\u003eg\u003c/sub\u003e ratio during the \u0026ldquo;high MSA\u0026rdquo; period is remarkably high (~\u0026thinsp;100 during the \u0026ldquo;high MSA\u0026rdquo; period) and substantially higher than previously reported for the Southern Ocean and Antarctica [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan additionalcitationids=\"CR27 CR28\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. As this is the first study focusing on the atmospheric MSA\u003csub\u003eg\u003c/sub\u003e and SA\u003csub\u003eg\u003c/sub\u003e in the Antarctic West Pacific Ocean sector (90\u0026ndash;160\u0026deg;E), these unique observations might reflect large-scale compositional differences between oceanic sectors of coastal Antarctica.\u003c/p\u003e \u003cp\u003eThroughout the whole voyage SA\u003csub\u003eg\u003c/sub\u003e exhibits a typical diurnal pattern (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) dictated by solar radiation intensity, reflecting the diurnal pattern of OH radicals needed for the oxidation of SO\u003csub\u003e2\u003c/sub\u003e into H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. On average, intensities of SA\u003csub\u003eg\u003c/sub\u003e increase by a factor of 2 around midday. This pattern has been previously observed in different marine environments, including polar regions [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan additionalcitationids=\"CR31\" citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. This indicates that local photooxidation of DMS and possibly other volatile sulfur compounds is the dominant pathway of the SA\u003csub\u003eg\u003c/sub\u003e production in the sampled region. MSA\u003csub\u003eg\u003c/sub\u003e diurnal behaviour (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec) is presented separately for the \u0026ldquo;high MSA\u0026rdquo; period and the rest of the voyage due to an order of magnitude difference in MSA\u003csub\u003eg\u003c/sub\u003e concentrations between those two periods. While the 8-day \u0026ldquo;high MSA\u0026rdquo; period shows up to 70% increase in median MSA\u003csub\u003eg\u003c/sub\u003e intensity around midday, attributing this to photochemical production might be unwarranted, as MSA\u003csub\u003eg\u003c/sub\u003e day-to-day variability during this short period does not consistently align with the solar radiation cycle (Supplementary Fig.\u0026nbsp;2). Moreover, data from the remainder of the sampling period show that MSA\u003csub\u003eg\u003c/sub\u003e shows no diurnal pattern and, on average, has no significant difference in concentrations between day and night. (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). Such absence of any particular diurnal trend for MSA\u003csub\u003eg\u003c/sub\u003e has been previously observed in studies focusing on the Southern Ocean and Antarctic region [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e], while marine atmosphere studies outside of polar regions reported diurnal behaviour of MSA\u003csub\u003eg\u003c/sub\u003e similar to that of SA\u003csub\u003eg\u003c/sub\u003e [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Modelling studies have proposed that a large portion of MSA is formed via aqueous phase oxidation of DMS and its intermediate products [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. In addition, it has been previously suggested that particulate MSA could partition into the gas phase depending on temperature, humidity and particle acidity [\u003cspan additionalcitationids=\"CR18 CR19 CR20 CR21\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. This supports our observed MSA\u003csub\u003eg\u003c/sub\u003e trends. A more detailed analysis of MSA\u003csub\u003eg\u003c/sub\u003e behaviour in relation to RH and particle acidity will be presented in subsequent sections.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eAir masses origin during the “high MSA” period\u003c/h3\u003e\n\u003cp\u003eThe period of high gaseous MSA occurred at high latitudes (\u0026gt;\u0026thinsp;63\u003csup\u003eo\u003c/sup\u003e S), coinciding with the ship\u0026rsquo;s southward progression into latitudes typically influenced by the polar cell (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). Elevated MSA\u003csub\u003eg\u003c/sub\u003e concentrations persisted during the ship\u0026rsquo;s eastward travel (140\u003csup\u003eo\u003c/sup\u003e to 150\u003csup\u003eo\u003c/sup\u003e E at ~\u0026thinsp;64\u003csup\u003eo\u003c/sup\u003eS), and declined after the ship moved northward (\u0026lt;\u0026thinsp;63\u003csup\u003eo\u003c/sup\u003e S). This compositional change is consistent with previous observations [\u003cspan additionalcitationids=\"CR35 CR36\" citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e] and Humphries et al [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e] coined a term \u0026ldquo;Atmospheric Compositional Front of Antarctica\u0026rdquo; to describe a distinct change in aerosol composition at a boundary that varies spatially and temporally between 60 and 65\u003csup\u003eo\u003c/sup\u003e S in the East Antarctic region. However, a later southward transect through similarly high latitudes near ~\u0026thinsp;132\u003csup\u003eo\u003c/sup\u003e E cell did not lead to a comparable increase in gaseous MSA, suggesting compositional variability not wholly dependent on latitude. The majority of air masses during the \u0026ldquo;high MSA\u0026rdquo; period have recent influence from the Antarctic continent (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed). While a few Antarctic and Southern Ocean studies have reported gaseous MSA concentrations [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan additionalcitationids=\"CR27 CR28\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e], no other campaigns have documented such a pronounced increase in gaseous MSA associated with continental air mass influence.\u003c/p\u003e \u003cp\u003eIn contrast, during a subsequent period (9\u0026ndash;10 February), despite air masses also originating from Antarctica, there was no significant increase in MSA\u003csub\u003eg\u003c/sub\u003e. This demonstrates that Antarctic air masses do not uniformly result in atmospheric compositional changes. This variability led us to explore additional factors that may have influenced our observations.\u003c/p\u003e \u003cp\u003eWe found that the tenfold increase in MSA\u003csub\u003eg\u003c/sub\u003e that we observed on 29 Jan coincided with a sharp and sustained drop in both absolute humidity and temperature throughout the entire \u0026ldquo;high MSA\u0026rdquo; period (Supplementary Fig.\u0026nbsp;3a). Such dry, cold air from the Antarctic continent is often linked to katabatic outflow [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. Katabatic winds are an important climatological feature of Antarctica, occurring when cold, dense air descends from the Antarctic plateau along the slope of the continent under the influence of gravity. Previous studies have shown that there is a spatial variability in katabatic drainage patterns, driven primarily by topography, and have identified the zones of drainage convergence or \u0026ldquo;confluence zones\u0026rdquo;, where katabatic winds are intensified and more persistent [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. One such zone is the coastal region of Adelie land around 142\u003csup\u003eo\u003c/sup\u003e E, a region of the strongest observed surface winds on Earth [\u003cspan additionalcitationids=\"CR41\" citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. The 8-day-long \u0026ldquo;high MSA\u0026rdquo; event occurred while the ship was in that region (140\u003csup\u003eo\u003c/sup\u003e \u0026ndash; 150\u003csup\u003eo\u003c/sup\u003e E). Katabatic winds are known to promote vertical mixing and disrupt shallow low-level inversions, facilitating the entrainment of air from the free troposphere [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. These winds are also associated with snowfall sublimation and in Adelie land have been shown to decrease the amount of precipitation reaching the ground by up to 35% [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. This suggests that katabatic winds played a key role in modulating MSA levels during CAPRICORN-2.\u003c/p\u003e\n\u003ch3\u003eAerosol properties during the “high MSA” period\u003c/h3\u003e\n\u003cp\u003eTo explore aerosol properties during the \u0026ldquo;high MSA\u0026rdquo; period, we investigated the aerosol number size distribution and PM1 chemical composition (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). A notable feature in the number size distribution is a prominent and persistent accumulation mode throughout the entire \u0026ldquo;high MSA\u0026rdquo; period. This period was also marked by persistently increased CCN concentrations (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), averaging at 238\u0026thinsp;\u0026plusmn;\u0026thinsp;45 cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e compared to 121\u0026thinsp;\u0026plusmn;\u0026thinsp;68 cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e for the rest of the CAPRICORN-2 voyage (values given for 0.35% supersaturation as a good proxy for marine cloud-effective supersaturations [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]). This is consistent with the latitudinal distribution of aerosol and CCN concentrations across the Southern Ocean reported in Humphries ae al [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e] that shows an increase in these aerosol parameters at the southernmost latitude bin (65-70\u003csup\u003eo\u003c/sup\u003e S).\u003c/p\u003e \u003cp\u003ePM1 composition data reveal that particulate SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e and MSA increase during the \u0026ldquo;high MSA\u0026rdquo; period in the same manner as gaseous MSA (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). There is a strong correlation between MSA\u003csub\u003eg\u003c/sub\u003e and particulate SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e (R\u0026thinsp;=\u0026thinsp;0.71), and MSA\u003csub\u003eg\u003c/sub\u003e and particulate MSA (R\u0026thinsp;=\u0026thinsp;0.62) during the \u0026ldquo;high MSA\u0026rdquo; period (Supplementary Fig.\u0026nbsp;4a and b). A very similar relationship was found between MSA\u003csub\u003eg\u003c/sub\u003e and CCN number concentration (R\u0026thinsp;=\u0026thinsp;0.68; Supplementary Fig.\u0026nbsp;4c). Previous studies have shown the same trends for particulate MSA and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, and CCN [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e], but this is the first time that such a strong alignment between gaseous MSA and CCN has been observed.\u003c/p\u003e \u003cp\u003eDuring the \u0026ldquo;high MSA\u0026rdquo; period, particulate SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e increases approximately 4 times compared to the rest of the data, and particulate MSA increases approximately 6 times compared to the rest of the data. This results in an increase of the MSA/SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e ratio from 0.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08 to 0.41\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06 (Supplementary Fig.\u0026nbsp;5). NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e also increases almost 3 times during the \u0026ldquo;high MSA\u0026rdquo; period. At the same time, Na\u003csup\u003e+\u003c/sup\u003e and Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e ions, as markers for sea spray aerosols, drop down substantially (approximately 2.5 and 20 times, respectively) (Supplementary Fig.\u0026nbsp;6a and b). During the \u0026ldquo;high MSA\u0026rdquo; period, SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e and MSA represented on average 88% of all measured species, whereas during the rest of the voyage, these species made up on average 21% of all measured species (Supplementary Fig.\u0026nbsp;6c and d). Supplementary Fig.\u0026nbsp;6 illustrates a significant difference between average PM1 ionic composition during the \u0026ldquo;high MSA\u0026rdquo; period compared to the rest of the measuring period. Overall, the observed increase in gaseous MSA was clearly reflected in the particle phase and demonstrates that during the \u0026ldquo;high MSA\u0026rdquo; period PM1 was strongly dominated by species of biogenic origin.\u003c/p\u003e \u003cp\u003eFurther investigation of the PM1 composition points to two important observations - summarised in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and d, and in the text below - that further underscores the distinct characteristics of the \u0026ldquo;high MSA\u0026rdquo; period:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003e \u003cem\u003eCl\u003c/em\u003e \u003csup\u003e \u003cem\u003e-\u003c/em\u003e \u003c/sup\u003e \u003cem\u003e/Na\u003c/em\u003e\u003csup\u003e\u003cem\u003e+\u003c/em\u003e\u003c/sup\u003e \u003cem\u003eratio\u003c/em\u003e: Another notable feature of the \u0026ldquo;high MSA\u0026rdquo; period is a strong, almost complete chloride depletion (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). Cl⁻/Na⁺ molar ratio of 1.18 is typical of seawater and expected to be the same or similar for freshly produced sea salt particles. Values lower than this suggest Cl⁻ displacement as HCl by less volatile acids (H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, MSA, HNO\u003csub\u003e3\u003c/sub\u003e) or heterogeneous oxidation by O\u003csub\u003e3\u003c/sub\u003e and OH radicals. However, given the low reaction rate constants for sea spray aerosol oxidation by O₃ and OH radicals, displacement by acidic species is considered to be the dominant pathway [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. Cl\u003csup\u003e-\u003c/sup\u003e depletion in marine aerosols has been previously observed all around the world, including polar regions [\u003cspan additionalcitationids=\"CR48 CR49\" citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e] Here, we observe up to 98% Cl⁻ loss during the \"high MSA\" period (assuming all Na⁺ originates from sea spray aerosol). This near complete Cl\u003csup\u003e-\u003c/sup\u003e depletion suggests significant chemical ageing of sea spray aerosols, with Cl\u003csup\u003e-\u003c/sup\u003e being replaced by H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and MSA. This is supported by 4\u0026ndash;6 times increase in PM1 SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e and MSA during the \u0026ldquo;high MSA\u0026rdquo; period (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea).\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003e \u003cem\u003eIon balance\u003c/em\u003e: Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed shows the concentration of H⁺ in PM1, calculated from ionic charge balance, where a value of 0 indicates that all anions are fully neutralised by cations and values\u0026thinsp;\u0026gt;\u0026thinsp;0 (excess anions) suggest the presence of an unmeasured cation, assumed to be H\u003csup\u003e+\u003c/sup\u003e. Conversely, values below 0 (deficit of anions) imply a missing anion, likely OH\u003csup\u003e-\u003c/sup\u003e. For most days during CAPRICORN-2, the charge balance was \u0026gt;\u0026thinsp;0 indicating an excess of anions and hence an acidic nature of aerosols. During the \u0026ldquo;high MSA\u0026rdquo; period, the excess of anions increased approximately fivefold, suggesting a substantial rise in aerosol acidity. To further explore aerosol acidity, we used the Extended Aerosol Inorganic Model IV (E-AIM IV) that predicts aerosol pH based on bulk ionic composition and environmental conditions (temperature and RH). Details on input parameters are given in the Supplementary Information. The modelled pH values (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed) closely mirror the trends in ionic charge balance, supporting the evidence of more acidic aerosol conditions during the \u0026ldquo;high MSA\u0026rdquo; period. Specifically, the aerosol was 1.2 pH units more acidic during this period (-0.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4) compared to the average across the rest of the campaign (0.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.7). Supplementary Fig.\u0026nbsp;8 shows that the pH output for two different E-AIM model settings (II and IV) has the same trend for the majority of the measuring period and an excellent agreement for the \u0026ldquo;high MSA\u0026rdquo; period (on average, not more than 0.2 pH units difference). These are the first observationally constrained estimates of aerosol acidity over the high latitude Southern Ocean, specifically sea ice region of East Antarctica. Our results indicate consistently acidic aerosols throughout the sampling period, with pH values ranging from \u0026minus;\u0026thinsp;1.4 to 2. This aligns with recent global estimates based on different aircraft campaigns around the globe showing that aerosol pH decreases with remoteness\u0026ndash; from pH values around 3 near polluted continental regions to -1 in remote marine environments [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e].\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eGas–Particle Partitioning of MSA in Antarctic Outflow: Influence of RH and Aerosol Acidity\u003c/h3\u003e\n\u003cp\u003eGiven the strong positive correlation between MSA\u003csub\u003eg\u003c/sub\u003e and particulate SO₄\u0026sup2;⁻ and MSA, as well as CCN concentrations during the \u0026ldquo;high MSA\u0026rdquo; period, along with the highly acidic aerosols and dry air masses, we investigated the potential evaporation of MSA from the surface of particles. We used the E-AIM to simulate MSA gas \u0026ndash; particle partitioning (see Methods for details), where Model II represents marine secondary aerosols and Model IV represents an internally mixed aerosol of primary sea spray and secondary species. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea both E-AIM configurations successfully simulate the observed MSA\u003csub\u003eg\u003c/sub\u003e pattern during the \u0026ldquo;high MSA\u0026rdquo; period, with estimated concentrations within the same order of magnitude as observations. Our observations combined with E-AIM model results provide direct evidence that MSA undergoes evaporation from the surface of aerosols under ambient marine boundary layer conditions.\u003c/p\u003e \u003cp\u003eSeveral studies have suggested that MSA may evaporate from particles as RH decreases [\u003cspan additionalcitationids=\"CR18 CR19\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. However, gas \u0026ndash; particle partitioning of semivolatile acids, such as MSA, can be affected by both RH, which governs aerosol liquid water content (ALWC), and aerosol acidity [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. A reduction in ALWC promotes evaporation by decreasing the particle's capacity to retain dissolved species, while simultaneously contributing to a drop in aerosol pH. Very low aerosol pH (\u0026lt;\u0026thinsp;0) reflects a substantial shortage of neutralizing bases (e.g., NH₃), promoting the formation of the protonated form of strong acids like MSA, which increases their tendency to partition into the gas phase, particularly at low RH. The \u0026ldquo;high MSA\u0026rdquo; period coincided with low RH (Supplementary Fig.\u0026nbsp;3b) and highly acidic aerosols (pH\u0026thinsp;\u0026lt;\u0026thinsp;0) and we show there is a strong inverse linear relationship between logMSA\u003csub\u003eg\u003c/sub\u003e and aerosol pH (R = -0.73), with the highest MSA\u003csub\u003eg\u003c/sub\u003e concentrations also coinciding with the lowest RHs (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). To investigate which of these two factors MSA partitioning is more sensitive to, we performed an E-AIM IV simulation at a constant temperature of 273 K, varying RH between 60% and 100%, using PM1 composition from the \u0026ldquo;high MSA\u0026rdquo; period and adjusting only NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e concentrations to obtain different pH values. The results indicate that under the pH and RH conditions characteristic for CAPRICORN-2, MSA partitioning is more sensitive to changes in pH (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). For example, when RH changes from 60 to 90% MSA\u003csub\u003eg\u003c/sub\u003e stays within the same order of magnitude and for a change in p Therefore, aerosol acidity and pH changes to values\u0026thinsp;\u0026lt;\u0026thinsp;0 seem to be the main driver of MSA evaporation. It is important to note is that despite the ten-fold increase in MSA\u003csub\u003eg\u003c/sub\u003e during the \u0026ldquo;high MSA\u0026rdquo; period, these gas phase concentrations represent a small fraction of the total MSA (\u0026lt;\u0026thinsp;10%). Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed shows the theoretical pH dependence of particulate fraction of MSA (MSA\u003csub\u003ep\u003c/sub\u003e/MSA\u003csub\u003ep\u003c/sub\u003e+MSA\u003csub\u003eg\u003c/sub\u003e) calculated for several different ALWCs [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e], along with observational and E-AIM IV modelled data for CAPRICORN-2 and demonstrates excellent agreement of our observed data with theoretical predictions of MSA partitioning. This provides further evidence of pH-driven partitioning of MSA.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Discussion and atmospheric implications","content":"\u003cp\u003eAntarctic coastal waters are known to be one of the most biologically productive regions of the world during austral summer, resulting in high DMS and other volatile sulfur compounds production [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Antarctica is also surrounded by a band of low pressure between 60\u003csup\u003eo\u003c/sup\u003e and 65\u003csup\u003eo\u003c/sup\u003e S known as the circumpolar trough [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e], which is further enhanced by frequent cyclones and mesocyclones around its coast [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. These low pressure systems drive a net upward transport of Antarctic coastal air masses, which can have two important consequences for the atmospheric processing of DMS: (1) aqueous phase oxidation of DMS in clouds formed due to atmospheric uplift, with the subsequent free-tropospheric evaporation of cloud droplets leading to biogenically enriched particles; (2) free-tropospheric oxidation of DMS, predominantly via OH addition pathway, which is preferred at low temperature conditions [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], with the free troposphere providing favourable environmental conditions (low temperature, low condensation sink) for H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and/or MSA-driven new particle formation. Both scenarios will result in increased MSA production and overall increased particulate MSA/SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e ratio. This agrees with our CAPRICORN-2 observations (Supplementary Fig.\u0026nbsp;5). Subsidence of free-tropospheric air over Antarctica and its entrainment into the boundary layer is promoted by the continental outflow and katabatic drainage of air masses over the slopes of the continent. Antarctic surface winds have a strong directional constancy of around 90% [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e] and are frequently attributed to katabatic outflow, downslope winds driven by gravity due to radiative cooling and densification of air over the sloping surface. Northward air flow (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed) in combination with a decrease in temperature and absolute humidity (Supplementary Fig.\u0026nbsp;3a) provides strong evidence for katabatic outflow during the \u0026ldquo;high MSA\u0026rdquo; period. We show that these air masses carry highly acidic particles (reaching pH of up to -1.5) dominated by DMS oxidation products (~\u0026thinsp;90% of anions). In addition, these particles are almost completely depleted in Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e, which is driven by the accumulation of low vapour pressure acids (i.e. H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and MSA) and insufficient availability of ammonia to neutralise them. This is particularly important as most chemical transport models tend to underestimate aerosol acidity in remote marine regions, which leads to underestimation of water uptake and underestimation of aerosol direct radiative effect [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. In summary, we suggest that these highly acidic, biogenically dominated aerosol particles are the result of long-range transport that is governed by the dominant large-scale circulation pattern within the Antarctic polar cell.\u003c/p\u003e \u003cp\u003eFurthermore, we demonstrate that such compositional make-up, in combination with low RH characteristic of katabatic winds, leads to pH-driven evaporation of MSA, revealing a previously unconfirmed process in sulfur cycling. Many aerosol transformation processes are driven by acidity, including gas \u0026ndash; particle partitioning of acids and bases [\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e]. Previous studies have reported a positive correlation between the decrease in RH and an increase in gaseous MSA, suggesting MSA evaporation at lower RH. While RH and aerosol pH are closely linked through aerosol liquid water content, we show that this process is more sensitive to pH. Processes outlined here are schematically illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe partitioning of MSA into the gas phase can extend its atmospheric lifetime by reducing its removal through wet deposition. In addition, the evaporation of MSA could potentially have implications for CCN number concentrations. Evaporation of MSA from the surface of highly acidic particles will be more pronounced for smaller particles due to the Kelvin effect. As the observed MSA partitioning is pH-driven, particulate MSA in externally mixed aerosols could potentially partition from more acidic particles to less acidic particles. The partitioning of MSA and both its dependence and influence on the aerosol size distribution and potential implication for CCN concentrations needs further exploration.\u003c/p\u003e \u003cp\u003eDMS oxidation represents a key process influencing CCN and cloud droplet number in the Southern Ocean atmosphere. It is expected that DMS emissions will increase in a warming climate due to increase in near-surface wind speeds and sea surface temperatures [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]. MSA is a non-negligible DMS oxidation product and yet it is not well represented in models [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. It is still an open question of whether the partitioning of MSA into the gas phase from the aerosol surface is a significant source of boundary layer gaseous MSA compared to direct gas-phase production from boundary layer DMS oxidation. If it is, as our study suggests, then models must account for this process when simulating gas phase MSA and comparing it with observations. Capturing DMS chemistry along the entire transport pathways, including the free troposphere and over the continent, is essential for improving the accuracy of chemical transport models and global climate models.\u003c/p\u003e \u003c/div\u003e"},{"header":"Methodology","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003eGaseous MSA and SA measurements\u003c/h2\u003e \u003cp\u003eMSA and SA were measured using a high-resolution time-of-flight Chemical Ionisation Mass Spectrometer with nitrate reagent ions (NO\u003csub\u003e3\u003c/sub\u003e-CIMS) [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e, \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]. Nitrate ions ((HNO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003ex\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e; x\u0026thinsp;=\u0026thinsp;0\u0026ndash;2) were generated by introducing HNO\u003csub\u003e3\u003c/sub\u003e vapour into 20 L min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e HEPA-filtered sheath air flow and exposing it to X-ray radiation within the chemical ionisation inlet. The HNO\u003csub\u003e3\u003c/sub\u003e vapour was generated by passing 3 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of N\u003csub\u003e2\u003c/sub\u003e over a concentrated (70% wt) HNO\u003csub\u003e3\u003c/sub\u003e reservoir. Outdoor air was sampled through a 2 m, \u0026frac12;\u0026rdquo; stainless-steel, non-heated tube at 30 L min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e,with 10 L min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e directed to the NO\u003csub\u003e3\u003c/sub\u003e-CIMS and 20 L min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e as auxiliary flow to reduce residence time in the tubing. Diffusional losses in the sampling line were calculated based on the diffusion coefficient of sulfuric acid [\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e] and applied to both SA and MSA.\u003c/p\u003e \u003cp\u003eThe intensity of SA and MSA was calculated by summing ions originating from each of these two species, normalising them to the sum of the reagent ions and multiplying by a calibration factor. While the nitrate-CIMS was not directly calibrated for sulfuric acid in this study, literature-reported calibration factors are broadly consistent across studies and a median calibration factor derived from 21 literature values was used (details in SI). The same calibration factor was used for MSA based on the assumption that ionisation of these species is equal and proceeds at the kinetic limit due to having a lower proton affinity than nitric acid, and transmission losses in the instrument are comparable.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eAerosol measurements\u003c/h3\u003e\n\u003cp\u003eAerosols were sampled using a common sampling inlet located at the bow of the ship, 18 m above sea level, with the flowrate of the common sampling line set to 180 L min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The details of the sampling inlet and standard atmospheric measurement capabilities onboard RV Investigator are given in [\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e]. For all aerosol data, as well as gaseous sulfuric acid, periods influenced by ship exhaust were removed based on the method developed by Humphries et al [\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e]. This resulted in removing 14% of the data.\u003c/p\u003e \u003cp\u003eThe aerosol number size distribution was measured over the mobility diameter range 4\u0026ndash;661 nm with two scanning mobility particle sizers (SMPS) - GRIMM Nano-SMPS (4-409 nm) and TSI long-column SMPS (15\u0026ndash;661 nm). A merged size distribution was calculated at 5 min time resolution, combining the GRIMM Nano-SMPS size bins up to 35.4 nm and the TSI long-column SMPS size bins up to 661 nm.\u003c/p\u003e \u003cp\u003eCCN number concentrations were measured with a DMT CCN-100 counter, cycling between 1.05%, 0.65%, 0.55%, 0.45%, 0.35% and 0.25% supersaturation (SS). This sequence was repeated hourly, giving 10 min of data at each supersaturation setting [\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e]. In this study, only 0.35% SS data is used.\u003c/p\u003e \u003cp\u003eThe chemical composition of non-refractory submicron aerosol was measured using an Aerodyne time-of-flight aerosol chemical speciation monitor (ToF-ACSM) equipped with a PM1 aerodynamic lens and the standard vaporiser. 10-minute averages of particulate sulfate and MSA are presented here. MSA concentrations were estimated from the m/z 79 peak (CH\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e fragment) as described in [\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e], but were largely below the detection limit outside of the \u0026ldquo;high MSA\u0026rdquo; period. In remote marine environments chloride originates primarily from sea salt aerosol; however, sea salt particles are refractory and as such are not efficiently vaporised in the ACSM. NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e measured by the ACSM was below the detection limit during the whole measurement campaign.\u003c/p\u003e \u003cp\u003eAlongside the ToF-ACSM measurements, PM\u003csub\u003e1\u003c/sub\u003e aerosol was collected on 47 mm quartz filters, sampling between 20 and 48 hr. To avoid contamination by exhaust aerosol from the Research Vessel itself, a switching controller was employed, which paused sampling when relative wind direction was between 90 and 270◦ and the concentrations of particles exceeded a threshold value 1000 Anion and cation concentrations are determined with a Dionex ICS-3000 reagent-free ion chromatograph as described in [\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e]. Measured inorganic ions included Na\u003csup\u003e+\u003c/sup\u003e, NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e, Ca \u003csup\u003e2+\u003c/sup\u003e, Mg\u003csup\u003e2+\u003c/sup\u003e, SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, Br\u003csup\u003e\u0026minus;\u003c/sup\u003e and Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e, and the organic ions included methane sulphonate (MS\u003csup\u003e\u0026minus;\u003c/sup\u003e), oxalate (C\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e), acetate (CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e\u0026minus;\u003c/sup\u003e) and formate (HCOO\u003csup\u003e\u0026minus;\u003c/sup\u003e). All values reported in this manuscript are blank corrected.\u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eRV Investigator underway data\u003c/h2\u003e \u003cp\u003eStandard meteorological parameters (temperature, relative humidity, pressure, wind speed and direction) were measured from two locations on the ship (port and starboard) at ~\u0026thinsp;25 m above sea level and they are a part of continuously collected \u0026ldquo;underway data\u0026rdquo;.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eBack trajectory analyses\u003c/h2\u003e \u003cp\u003eTo estimate the origin of sampled airmasses, we performed backward simulation of airmass trajectories using NOAA\u0026rsquo;s Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model [\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e]. The meteorological data from the Global Data Assimilation System (GDAS) at 0.5 degrees resolution was used for the HYSPLIT trajectory calculations. 5-day-long backtrajectories were initiated at ship\u0026rsquo;s location for each hour of the voyage and an arrival height of 100 m above sea level.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eExtended Aerosol Inorganics Model (E-AIM)\u003c/h2\u003e \u003cp\u003eE-AIM (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://www.aim.env.uea.ac.uk/aim/aim.php\u003c/span\u003e\u003cspan address=\"http://www.aim.env.uea.ac.uk/aim/aim.php\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e, last access: 23.12.2024) is a widely used thermodynamic model utilised here to model gas-particle partitioning of MSA and to estimate aerosol acidity based on aerosol bulk chemical composition and environmental conditions (T, RH) [\u003cspan additionalcitationids=\"CR63\" citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e]. E-AIM Models II and IV were used. Model II is a thermodynamic equilibrium model for the H\u003csup\u003e+\u003c/sup\u003e - NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e - nssSO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e - NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e - H\u003csub\u003e2\u003c/sub\u003eO mixture system, here used as a representative of secondary aerosol. Model IV includes H\u003csup\u003e+\u003c/sup\u003e-NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-Na\u003csup\u003e+\u003c/sup\u003e-SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e-Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e-H\u003csub\u003e2\u003c/sub\u003eO, here used to represent an internal mixture of primary sea spray aerosol and secondary inorganic species. In addition to concentrations of ionic species (further details in SI), including H\u003csup\u003e+\u003c/sup\u003e estimated from charge balance, both models had total MSA (MSA\u003csub\u003ep\u003c/sub\u003e and MSA\u003csub\u003eg\u003c/sub\u003e) included. The MSA thermodynamic properties used in the model are the same as the ones reported in Baccarini et al [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. As E-AIM can use only Na\u003csup\u003e+\u003c/sup\u003e, other metal ions (Ca\u003csup\u003e2+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e, Mg\u003csup\u003e2+\u003c/sup\u003e) were accounted for by converting them to charge equivalent amounts of Na\u003csup\u003e+\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eE-AIM provides molar fraction based H\u003csup\u003e+\u003c/sup\u003e activity coefficients, which were converted to molality-based coefficients following IUPAC recommendations for pH calculations (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://goldbook.iupac.org/terms/view/P04524\u003c/span\u003e\u003cspan address=\"https://goldbook.iupac.org/terms/view/P04524\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis research was supported by a grant of sea time on RV Investigator from the CSIRO Marine National Facility (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://ror.org/01mae9353\u003c/span\u003e\u003cspan address=\"https://ror.org/01mae9353\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e). The Authors wish to thank the CSIRO Marine National Facility (MNF) for its support in the form of sea time on RV Investigator, support personnel, scientific equipment and data management. In particular, we thank the technical and IT support personnel on board the voyage, with particular thanks to the Seagoing Instrumentation Team ashore.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eMallet M et al (2023) \u003cem\u003eUntangling the influence of Antarctic and Southern Ocean life on clouds.\u003c/em\u003e Elementa: Science of the Anthropocene., 11(1)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMallet MD et al (2025) Biological enhancement of cloud droplet concentrations observed off East Antarctica. npj Clim Atmospheric Sci 8(1):113\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCharlson RJ et al (1987) Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature 326(6114):655\u0026ndash;661\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWohl C et al (2024) Marine emissions of methanethiol increase aerosol cooling in the Southern Ocean. Sci Adv 10(48):eadq2465\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBarnes I et al (2006) Dimethyl Sulfide and Dimethyl Sulfoxide and Their Oxidation in the Atmosphere. Chem Rev 106(3):940\u0026ndash;975\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFaloona I (2009) Sulfur processing in the marine atmospheric boundary layer: A review and critical assessment of modeling uncertainties. Atmos Environ 43(18):2841\u0026ndash;2854\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChen J et al (2023) Atmospheric Gas-Phase Formation of Methanesulfonic Acid, vol 57. Environmental Science \u0026amp; Technology, pp 21168\u0026ndash;21177. 50\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBates TS et al (1992) Variations in the methanesulfonate to sulfate molar ratio in submicrometer marine aerosol particles over the south Pacific Ocean. J Geophys Research: Atmos 97(D9):9859\u0026ndash;9865\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChen L et al (2012) Latitudinal distributions of atmospheric MSA and MSA/nss-SO42\u0026thinsp;\u0026ndash;\u0026thinsp;ratios in summer over the high latitude regions of the Southern and Northern Hemispheres. J Geophys Research: Atmos, 117(D10).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYan J et al (2019) Significant Underestimation of Gaseous Methanesulfonic Acid (MSA) over Southern Ocean, vol 53. Environmental Science \u0026amp; Technology, pp 13064\u0026ndash;13070. 22\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDavison B et al (1996) Dimethyl sulfide and its oxidation products in the atmosphere of the Atlantic and Southern Oceans. Atmos Environ 30(10):1895\u0026ndash;1906\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eShen J et al (2022) High Gas-Phase Methanesulfonic Acid Production in the OH-Initiated Oxidation of Dimethyl Sulfide at Low Temperatures, vol 56. Environmental Science \u0026amp; Technology, pp 13931\u0026ndash;13944. 19\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHoffmann EH et al (2016) \u003cem\u003eAn advanced modeling study on the impacts and atmospheric implications of multiphase dimethyl sulfide chemistry.\u003c/em\u003e Proceedings of the National Academy of Sciences, 113(42): pp. 11776\u0026ndash;11781\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChen Q et al (2018) DMS oxidation and sulfur aerosol formation in the marine troposphere: a focus on reactive halogen and multiphase chemistry. Atmos Chem Phys 18(18):13617\u0026ndash;13637\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKecorius S et al (2023) Rapid growth of Aitken-mode particles during Arctic summer by fog chemical processing and its implication. PNAS Nexus, 2(5)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003evon Glasow R et al (2004) Model study of multiphase DMS oxidation with a focus on halogens. Atmos Chem Phys 4(3):589\u0026ndash;608\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBaccarini A et al (2021) Low-Volatility Vapors and New Particle Formation Over the Southern Ocean During the Antarctic Circumnavigation Expedition. J Geophys Research: Atmos, 126(22): p. e2021JD035126.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMauldin III (1999) Observations of H2SO4 and MSA during PEM-Tropics-A. J Geophys Research: Atmos 104(D5):5801\u0026ndash;5816\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eScholz W et al (2023) \u003cem\u003eMeasurement report: Long-range transport and the fate of dimethyl sulfide oxidation products in the free troposphere derived from observations at the high-altitude research station Chacaltaya (5240 m a.s.l.) in the Bolivian Andes.\u003c/em\u003e Atmos. Chem. Phys., 23(2): pp. 895\u0026ndash;920\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang Y et al (2014) Surface and free tropospheric sources of methanesulfonic acid over the tropical Pacific Ocean. Geophys Res Lett 41(14):5239\u0026ndash;5245\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDada L et al (2022) A central arctic extreme aerosol event triggered by a warm air-mass intrusion. Nat Commun 13(1):5290\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHodshire AL et al (2019) The potential role of methanesulfonic acid (MSA) in aerosol formation and growth and the associated radiative forcings. Atmos Chem Phys 19(5):3137\u0026ndash;3160\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDavis DD et al (1998) DMS oxidation in the Antarctic marine boundary layer: Comparison of model simulations and held observations of DMS, DMSO, DMSO2, H2SO4(g), MSA(g), and MSA(p). J Phys Res 103:1657\u0026ndash;1678\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFung KM et al (2022) Exploring dimethyl sulfide (DMS) oxidation and implications for global aerosol radiative forcing. Atmos Chem Phys 22(2):1549\u0026ndash;1573\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHulswar S et al (2022) Third revision of the global surface seawater dimethyl sulfide climatology (DMS-Rev3). Earth Syst Sci Data 14(7):2963\u0026ndash;2987\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBrean J et al (2021) Open ocean and coastal new particle formation from sulfuric acid and amines around the Antarctic Peninsula. Nat Geosci 14(6):383\u0026ndash;388\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJokinen T et al (2018) Ion-induced sulfuric acid\u0026ndash;ammonia nucleation drives particle formation in coastal Antarctica. Sci Adv 4(11):eaat9744\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMauldin RL et al (2004) Measurements of OH, HO2\u0026thinsp;+\u0026thinsp;RO2, H2SO4, and MSA at the South Pole during ISCAT 2000. Atmos Environ 38(32):5423\u0026ndash;5437\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eQu\u0026eacute;l\u0026eacute;ver LLJ et al (2022) Investigation of new particle formation mechanisms and aerosol processes at Marambio Station, Antarctic Peninsula. Atmos Chem Phys 22(12):8417\u0026ndash;8437\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBardouki H et al (2003) \u003cem\u003eGaseous (DMS, MSA, SO\u0026thinsp;\u0026lt;\u0026thinsp;sub\u0026thinsp;\u0026gt;\u0026thinsp;2, H\u0026thinsp;\u0026lt;\u003c/em\u003e\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e\u0026thinsp;sub\u0026thinsp;\u0026gt;\u0026thinsp;2\u0026thinsp;SO\u0026thinsp;\u0026lt;\u0026thinsp;sub\u0026thinsp;\u0026gt;\u0026thinsp;4\u0026lt;!--\u0026lt;/i--\u0026gt;\u003c/span\u003e\u003cspan address=\"http://\u0026thinsp;sub\u0026thinsp;%3E\u0026thinsp;2\u0026thinsp;SO\u0026thinsp;%3C\u0026thinsp;sub\u0026thinsp;%3E\u0026thinsp;4%3C!--%3C/i--%3E\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003cem\u003esub\u0026thinsp;\u0026gt;\u0026thinsp;and DMSO) and particulate (sulfate and methanesulfonate) sulfur species over the northeastern coast of Crete.\u003c/em\u003e Atmos. Chem. Phys., 3(5): pp. 1871\u0026ndash;1886\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBerresheim H et al (2002) \u003cem\u003eGas-aerosol relationships of H2SO4, MSA, and OH: Observations in the coastal marine boundary layer at Mace Head, Ireland.\u003c/em\u003e Journal of Geophysical Research: Atmospheres, 107(D19): p. PAR 5-1-PAR 5\u0026ndash;12\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePeltola M et al (2023) Chemical precursors of new particle formation in coastal New Zealand. Atmos Chem Phys 23(7):3955\u0026ndash;3983\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang Y et al (2024) Measurements of particulate methanesulfonic acid above the remote Arctic Ocean using a high resolution aerosol mass spectrometer. Atmos Environ 331:120538\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAlroe J et al (2020) Marine productivity and synoptic meteorology drive summer-time variability in Southern Ocean aerosols. Atmos Chem Phys 20(13):8047\u0026ndash;8062\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHumphries RS et al (2021) Southern Ocean latitudinal gradients of cloud condensation nuclei. Atmos Chem Phys 21(16):12757\u0026ndash;12782\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHumphries RS et al (2016) Unexpectedly high ultrafine aerosol concentrations above East Antarctic sea ice. Atmos Chem Phys 16(4):2185\u0026ndash;2206\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSimmons JB et al (2021) Summer aerosol measurements over the East Antarctic seasonal ice zone. Atmos Chem Phys 21(12):9497\u0026ndash;9513\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJefferson AJ et al (1998) OH photochemistry and methane sulfonic acid formation in the coastal Antarctic boundary layer. Oceanogr Literature Rev 6:922\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChambers SD et al (2018) Characterizing Atmospheric Transport Pathways to Antarctica and the Remote Southern Ocean Using Radon-222. Front Earth Sci, Volume 6\u0026ndash;2018\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eParish TR et al (1987) The surface windfield over the Antarctic ice sheets. Nature 328(6125):51\u0026ndash;54\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eParish TR et al (2007) Reexamination of the Near-Surface Airflow over the Antarctic Continent and Implications on Atmospheric Circulations at High Southern Latitudes. Mon Weather Rev 135(5):1961\u0026ndash;1973\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWendler G et al (1997) On the extraordinary katabatic winds of Ad\u0026eacute;lie Land. J Geophys Research: Atmos 102(D4):4463\u0026ndash;4474\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eParish TR et al (2003) The Role of Katabatic Winds on the Antarctic Surface Wind Regime. Mon Weather Rev 131(2):317\u0026ndash;333\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGrazioli J et al (2017) \u003cem\u003eKatabatic winds diminish precipitation contribution to the Antarctic ice mass balance.\u003c/em\u003e Proceedings of the National Academy of Sciences, 114(41): pp. 10858\u0026ndash;10863\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSanchez KJ et al (2021) Measurement report: Cloud processes and the transport of biological emissions affect southern ocean particle and cloud condensation nuclei concentrations. Atmos Chem Phys 21(5):3427\u0026ndash;3446\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAyers GP et al (1991) Seasonal relationship between cloud condensation nuclei and aerosol methanesulphonate in marine air. Nature 353(6347):834\u0026ndash;835\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSu B et al (2022) A review of atmospheric aging of sea spray aerosols: Potential factors affecting chloride depletion. Atmos Environ 290:119365\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGon\u0026ccedil;alves SJ et al (2021) Photochemical reactions on aerosols at West Antarctica: A molecular case-study of nitrate formation among sea salt aerosols. Sci Total Environ 758:143586\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLegrand M et al (2017) Year-round records of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) \u0026ndash; Part 1: Fractionation of sea-salt particles. Atmos Chem Phys 17(22):14039\u0026ndash;14054\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKerminen V-M et al (2000) Chemistry of sea-salt particles in the summer Antarctic atmosphere. Atmos Environ 34(17):2817\u0026ndash;2825\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNault BA et al (2021) Chemical transport models often underestimate inorganic aerosol acidity in remote regions of the atmosphere. Commun Earth Environ 2(1):93\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNenes A et al (2020) Aerosol pH and liquid water content determine when particulate matter is sensitive to ammonia and nitrate availability. Atmos Chem Phys 20(5):3249\u0026ndash;3258\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSimmonds I et al (2003) Synoptic Activity in the Seas around Antarctica. Mon Weather Rev 131(2):272\u0026ndash;288\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJoge SD et al (2025) \u003cem\u003eClimate warming increases global oceanic dimethyl sulfide emissions.\u003c/em\u003e Proceedings of the National Academy of Sciences, 122(23): p. e2502077122\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJokinen T et al (2012) Atmospheric sulphuric acid and neutral cluster measurements using CI-APi-TOF. Atmos Chem Phys 12(9):4117\u0026ndash;4125\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKurt\u0026eacute;n, T., et al., \u003cem\u003eThe effect of H\u0026thinsp;\u0026lt;\u0026thinsp;sub\u0026thinsp;\u0026gt;\u0026thinsp;2\u0026thinsp;SO\u0026thinsp;\u0026lt;\u0026thinsp;sub\u0026thinsp;\u0026gt;\u0026thinsp;4 \u0026ndash; amine clustering on chemical ionization mass spectrometry (CIMS) measurements of gas-phase sulfuric acid.\u003c/em\u003e Atmos. Chem. Phys., 2011. 11(6): pp. 3007\u0026ndash;3019\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHanson DR et al (2000) Diffusion of H2SO4 in Humidified Nitrogen: Hydrated H2SO4. J Phys Chem A 104(8):1715\u0026ndash;1719\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHumphries RS et al (2019) Identification of platform exhaust on the RV Investigator. Atmos Meas Tech 12(6):3019\u0026ndash;3038\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHumphries R et al (2021) CAPRICORN2 - Atmospheric aerosol measurements from the RV Investigator voyage IN2018_V01. Commonwealth Scientific and Industrial Research Organisation\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZorn SR et al (2008) Characterization of the South Atlantic marine boundary layer aerosol using an aerodyne aerosol mass spectrometer. Atmos Chem Phys 8(16):4711\u0026ndash;4728\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDraxler R et al (1998) An overview of the HYSPLIT_4 modeling system for trajectories, dispersion, and deposition. Aust Meteorol Mag 47:295\u0026ndash;308\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eClegg SL et al (2006) Thermodynamic Models of Aqueous Solutions Containing Inorganic Electrolytes and Dicarboxylic Acids at 298.15 K. 2. Systems Including Dissociation Equilibria. J Phys Chem A 110(17):5718\u0026ndash;5734\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eClegg SL et al (2006) Thermodynamic Models of Aqueous Solutions Containing Inorganic Electrolytes and Dicarboxylic Acids at 298.15 K. 1. The Acids as Nondissociating Components. J Phys Chem A 110(17):5692\u0026ndash;5717\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWexler AS et al (2002) \u003cem\u003eAtmospheric aerosol models for systems including the ions H+, NH4+, Na+, SO42\u0026ndash;, NO3\u0026ndash;, Cl\u0026ndash;, Br\u0026ndash;, and H2O.\u003c/em\u003e Journal of Geophysical Research: Atmospheres, 107(D14): p. ACH 14-1-ACH 14\u0026ndash;14\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6905825/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6905825/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMethanesulfonic acid (MSA), a key oxidation product of dimethyl sulfide (DMS), plays a crucial role in the atmospheric sulfur cycle and in the formation of cloud condensation nuclei (CCN). MSA contributes significantly to aerosol growth and, potentially, the modulation of cloud microphysical properties, particularly in remote marine and polar regions where CCN concentrations are relatively low. Here we focus on an eight-day period of elevated gaseous MSA observed along the coastal region of East Antarctica that coincided with persistent katabatic outflow. We show that this outflow brings biogenically dominated, highly acidic aerosols with elevated gaseous MSA resulting from evaporation off the surface of these aerosol particles. While MSA evaporation is promoted by a decrease in relative humidity, we show that aerosol acidity is the primary driver of this process. These results provide new insights into processes involved in the marine sulfur cycle, which should be included when using observations of DMS oxidation products to guide model evaluation and development. Furthermore, they reveal the highly acidic nature of Southern Ocean aerosols and highlight the importance of aerosol acidity on atmospheric processes.\u003c/p\u003e","manuscriptTitle":"Aerosol Acidity Controls Methanesulfonic Acid Evaporation From Aerosols During Antarctic Katabatic Outflow","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-30 17:27:02","doi":"10.21203/rs.3.rs-6905825/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"communications-earth-and-environment","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"commsenv","sideBox":"Learn more about [Communications Earth and Environment](https://www.nature.com/commsenv/)","snPcode":"","submissionUrl":"","title":"Communications Earth \u0026 Environment","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Communications Series","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"f882cf2d-a77a-41a8-b1d7-56ca742ae7ea","owner":[],"postedDate":"June 30th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":50300578,"name":"Physical sciences/Chemistry/Environmental chemistry/Atmospheric chemistry"},{"id":50300579,"name":"Earth and environmental sciences/Climate sciences/Atmospheric science/Atmospheric chemistry"},{"id":50300580,"name":"Earth and environmental sciences/Climate sciences/Atmospheric science/Atmospheric dynamics"}],"tags":[],"updatedAt":"2025-12-30T08:11:07+00:00","versionOfRecord":{"articleIdentity":"rs-6905825","link":"https://doi.org/10.1038/s43247-025-03041-2","journal":{"identity":"communications-earth-and-environment","isVorOnly":false,"title":"Communications Earth \u0026 Environment"},"publishedOn":"2025-11-28 05:00:00","publishedOnDateReadable":"November 28th, 2025"},"versionCreatedAt":"2025-06-30 17:27:02","video":"","vorDoi":"10.1038/s43247-025-03041-2","vorDoiUrl":"https://doi.org/10.1038/s43247-025-03041-2","workflowStages":[]},"version":"v1","identity":"rs-6905825","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6905825","identity":"rs-6905825","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}