Contributions of unknown ionizable precursors for perfluoroalkyl acids in the air over a wastewater treatment plant and a landfill

Contributions of unknown ionizable precursors for perfluoroalkyl acids in the air over a wastewater treatment plant and a landfill | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Contributions of unknown ionizable precursors for perfluoroalkyl acids in the air over a wastewater treatment plant and a landfill Bin Wang, Yiming Yao, Hongwen Sun, Hongyan Zhao, Qingxia Yu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7284069/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 02 Dec, 2025 Read the published version in Environmental Geochemistry and Health → Version 1 posted 10 You are reading this latest preprint version Abstract Wastewater treatment plant (WWTP) and landfill are important point sources for emission of per- and polyfluoroalkyl substance (PFAS). However, occurrence and risks of emerging and unknown PFAS in the atmosphere of these sources have not been clarified. An optimized novel cartridge-based active air sampler for both neutral and ionizable PFAS was used to collect air samples from a WWTP and a landfill in Tianjin, China. For target PFAS, the concentrations of neutral PFAS were comparable with those of ionizable PFAS in air samples from both sources. 8:2 Fluorotelomer alcohol (8:2 FTOH) was the predominant neutral PFAS in all the air samples, while ultrashort-chain perfluoroalkyl carboxylic acids (PFCAs) were dominant ionizable PFAS. The optimized total oxidizable precursor (TOP) assay indicated that unknown precursors contributed 46.4–58.4 mol%. This study further revealed the unknown PFAS profiles and their risk in the atmosphere contributed by WWTP and landfill. Emerging PFAS PFAA precursors TOP assay atmosphere WWTP landfill Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction Per- and polyfluoroalkyl substances (PFAS) are a class of synthetic organic fluorinated compounds, which have been widely produced and applied in various industrial and commercial fields since late 1940s for their unique properties. 1 Due to potential high persistency, bioaccumulation, and toxicity of C8-based perfluoroalkyl acids (PFAAs), perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) have been listed under Annexes B and A of the Stockholm Convention in 2009 and 2019, respectively. 2 , 3 Additionally, perfluorohexane sulfonic acid (PFHxS) has been listed under the Stockholm Convention in 2022. 4 It was proposed that long-chain perfluorocarboxylic acids (PFCAs) with perfluorinated carbon chain lengths from 8 to 20 should be listed as persistent organic pollutants (POPs). 5 As long-chain PFAS have been phased out after more than 20 years of control, emerging PFAS substitutes have been manufactured and produced, e.g. alternatives to PFOA and PFOS. In many applications, long-chain PFAS are often replaced by short-chain congeners that are not regulated. For example, 3M has commercialized surface treatment products containing C4 side-chain fluoromers since 2003. 6 In addition, a number of novel ether-containing PFAS have been manufactured as substitutes for long-chain PFAS, e.g. perfluoroether carboxylic acids (PFECAs) and perfluoroether sulfonic acids (PFESAs), which are enhanced for degradability by inserting ether bonds into shorter perfluorocarbon chains. 7 In the production of fluoroperic high-performance materials, some PFECAs, including perfluoro-2-propoxypropanoic acid (HFPO-DA; trade name: GenX), are used as processing aids instead of PFOA. 8 It is estimated that the annual production of GenX in Europe is 10 t-100 t. 9 Since 1970s, F-53B, with major components of 6:2 and 8:2 chlorinated polyfluorinated ether sulfonic acids (Cl-PFESAs), has been widely used as fog inhibitor in chromium plating industry in China in substitution for PFOS, and the annual consumption from 2006 to 2015 was 10 t-14 t. 10 Thus, we should take attention on the occurrence of emerging PFAS in the environment. There are thousands of PFAS with clear chemical structure according to the PFAS Master List by the Environmental Protection Agency, US. However, no more than 30 PFAS were targeted for analysis in most studies, especially related with atmosphere. 11 Rarely screened PFAS may prove to have similar or higher toxicity than conventionally measured PFAS, 12 with potential environmental and human health risks. Initial development of the total oxidized precursor (TOP) assay was applied to urban runoff and find the contribution of unknown PFAA precursors. 13 TOP assay has been further applied to a variety of environmental matrices such as soil, sediment, and leachate. 13 – 23 In our previous study, the application of TOP assay to precipitation for the first time suggested that unknown ionizable PFAS precursors occur in the atmosphere while no direct application on air samples has been conducted yet. However, little is known about the potential unknown PFAS profiles in the air over WWTP and landfill. In order to increase flexibility and reduce matrix effects, an active air sampler using a cartridge-based composite sorbent of WAX/HC-C18 developed in our previous study was used and optimized with an increasing flow rate for outdoor use to collect both neutral and ionizable PFAS in the atmosphere. 24 Accordingly, the distribution characteristics of PFAS in the air over WWTP and landfill sites were analyzed. The contribution of unknown PFAA precursors in the air samples and the main types of PFAA precursors were estimated using the TOP assay. 2. Materials and methods 2.1 Chemicals and Reagents. Twenty-seven ionizable and seven neutral target PFAS were analyzed in this study and detail information of the native and internal standards is given in Table S1 in the Supporting Information (SI). Ammonium hydroxide was purchased from Aladdin Industrial Corporation (Los Angeles, CA, USA). HPLC-grade methanol and formic acid was purchased from Fisher Scientific (Hampton, NH, USA). Milli-Q water was generated using an ultrapure water purification system (Millipore, Billerica, MA, USA). HPLC-grade ammonium acetate was purchased from CNW Technologies (Shanghai, China). 2.2 Sample Collection. From June to August 2020, eight air samples were collected from a municipal WWTP and a municipal landfill from Tianjin, China. Detail information of two source sites and sampling points were listed in Table S2 (SI). Three sampling points were set at the grid before primary sedimentary tank (W-Inlet), aeration tank (W-AT), and secondary sedimentary tank (W-outlet) of the WWTP, respectively. For the landfill, five sampling points were set at the center of the landfill (L-C), above the leachate (L-L), 2 km upwind (L-U2), 2 km downwind (L-D2) and 5 km downwind (L-D5), respectively. The dominant wind direction during the sampling period was south or southeast. For the collection of air samples, the collection time of each sample is about 12 h and the sampling volume of 5 m 3 was guaranteed. Three sets of self-designed active air samplers (Fig. S1) are placed at each sampling point were collected for three consecutive days. Four field blanks were collected at each sampling site to expose the cartridge for 5 min during sampling. Sampling cartridges were conditioned and air-dried with ethyl acetate and methanol before sampling. After sampling, the sampling cartridges were wrapped in aluminum foil and stored in a polypropylene ziplock bag. The samples were transported back to the laboratory and stored at -20°C until analysis. The validation of the air sampling cartridges using WAX/HC-C18 composite sorbents can be referred to our previous study. 24 2.3 Sample Pretreatment . Before elution, the cartridge was spiked with 5 ng of each neutral and ionizable mass-labelled PFAS. Neutral and ionizable PFAS were eluted with 5 mL ethyl acetate (E1) and 4 mL methanol containing 0.1% ammonia (E2), sequentially. The two eluents were purified by Supelclean ENVI-Carb SPE cartridges (500 mg, 6 cc, SUPELCO, USA). The ENVI- Carb SPE column was first activated three times with 1 mL ethyl acetate/methanol. Then sample, and start to collect; The SPE column was then washed three times with 1 mL of ethyl acetate/methanol and merged with the activated collection solution. The eluents were concentrated under a gentle stream of ultrapure nitrogen at 40°C to a constant volume of 0.5 mL, respectively. An aliquot of 200 µL E1 supernatant was transferred to an autosampler for analysis of neutral PFAS. Another 200 µL of E1 supernatant was transferred to another autosampler, evaporated to dryness, and redissolved with an aliquot of 200 µL E2 supernatant for analysis of ionizable PFAS. TOP Assay. Using another sampling cartridge, the eluents of E1 and E2 were combined and evaporated to dryness at 40°C under a gentle stream of ultrapure nitrogen. The residual was redissolved with 15 mL of Milli-Q water and ultrasonically mixed for 2 min. Each sample was dosed with 0.243 g potassium persulfate (60 mM) and 0.225 mL 10 N sodium hydroxide solution (150 mM). The sample was ultrasonically mixed at a constant temperature (20°C) until the salt was completely dissolved. The sample was then placed in a thermostatic water bath at 85°C for 6 h. After reaction, the sample was cooled to room temperature in an ice water bath, and then the pH of the sample was adjusted to 5.0–9.0 with concentrated hydrochloric acid. Before extraction, 5 ng of each mass-labelled ionizable PFAS was added to the sample. The Cleanert IC-Ba/Ag/H cartridge was used to remove excess anions such as SO 4 2− and Cl − from the sample in improved recovery for ultra-short chain PFAS. 25 The Cleanert IC-Ba/Ag/H cartridge was used as follows: Firstly, the cartridge was conditioned with 5 mL methanol and 10 mL Milli-Q water and allowed for a 10-min balance. The sample was loaded onto the cartridge and collected at a rate of < 2 mL/min. For a complete recovery, PFAAs that may be retained by the cartridge were eluted with 6 mL methanol containing 0.1% ammonia. The sample was further enriched with an Oasis WAX cartridge according to our previous method. 14 , 26 The details were as follows: Firstly, the cartridge was conditioned with 4 mL methanol solution containing 0.1% ammonia, 4 mL methanol, and 4 mL Milli-Q water. Then, the pretreated sample was loaded onto the cartridge at a rate of < 2 mL/min. After loading the sample, the cartridge was washed with 5 mL Milli-Q and then conditioned with 5 mL ammonium acetate buffer solution (2.5 mM, pH = 4). After centrifuging at 3,000 rpm for 5 min, the cartridge was eluted with 4 mL methanol and methanol containing 0.1% ammonia that was collected from the Cleanert IC-BA/Ag/H pretreatment. The eluents were combined and concentrated to 0.5 mL under a gentle stream of ultrapure nitrogen at 40 ℃. The concentrate was purified with dispersive ENVI-Carb and centrifuged at 15,000 rpm for 10 min. An aliquot of 200 µL supernatant was transferred to an autosampler for instrumental analysis. 2.4 Instrumental Analysis . An Agilent 7890 gas chromatography interfaced with a 5975C triple-axis mass spectrometry (MS) and a DB-WAX column (30 m×0.25 mm i.d., 0.25 mm film thickness, Agilent J&W, USA) were used for analysis of neutral PFAS. The injection volume was 3 µL and the injection was operated in a splitless mode at 200˚C with an initial pressure of 40 psi, which returned to 10 psi at 1 min and followed by an injector purge. The initial oven temperature was kept at 60 ˚C for 1 min, ramped at 2 ˚C/min to 75 ˚C, followed by 10 ˚C/min to 135 ˚C and 20˚C/min to 235 ˚C, and then held for 4 min. Helium was used as the carrier gas. The MS system was operated in positive chemical ionization mode with methane used as reagent gas. Selected ion monitoring was applied for data acquisition. An Agilent 1260 liquid chromatography (LC) interfaced with an Agilent 6460 triple quadrupole tandem MS/MS (Agilent Technologies, USA) was operated with electrospray negative ionization and in multiple-reaction monitoring mode for analysis of ionizable PFAS. Nitrogen with a purity of 99.9% was used as desolvation gas at a flow rate of 11 L/min and a temperature of 350 ˚C. The nebulizer gas pressure was 50 psi. The capillary voltage was 4 kV. The dwell time was 200 ms. The injection volume was 10 µL. The extracts were analyzed using different separation approaches for specific classes of analytes. An ion-exchange RSpak JJ-50 2D column (2.0 mm i.d.×150 mm length, 5 µm; Shodex, Japan) was used for separation of short-chain PFCAs (C2–C4). An isocratic condition of mobile phase was used with a composition of 20% of 50 mM ammonium acetate in Milli-Q Water (pH = 9) and 80% of methanol (v/v) at a flow rate of 350 µL/min. An X-terra MS C18 column (2.1 mm i.d.×150 mm, 5µm, Waters, Ireland) was used for separation of ionizable PFAS (≥ C4). Milli-Q water and methanol both containing 2.5 mM ammonium acetate were used as mobile phases and the gradient was operated as 10% methanol uniformly increasing to 58% at 0.8 min, to 100% at 12.8 min, reversing to original conditions at 14.3 min, and held to 26 min until equilibrium. The flow rate was kept at 350 µL/min. For both columns, the temperature was kept at 40 ˚C. 2.5 Quality Assurance and Quality Control (QA/QC) . Each batch of air samples was pretreated accompanied with a procedural blank, and an instrument blank was run every 10 samples during instrumental analysis. The spiked recoveries of neutral PFAS in atmospheric samples were 75–95%, and the spiked recoveries of ionizable PFAS in unoxidized and oxidized atmospheric samples were 73–116% and 68–111%, respectively. The target PFAS were all quantified using a calibration curve at levels of 0.2, 0.5, 1, 2, 5, 10, 20, 50, and 100 µg/L with a constant level of mass-labelled standards at 10 µg/L. Before testing each batch of samples, the linearity and reproducibility of each calibration curve shall fit well (R 2 ≥ 0.99). The concentrations of PFAS were all corrected with the mean of procedural blanks if with detection. The instrumental limits of quantification, process gaps, matrix spiked recoveries, and method limits of detection of target compounds are shown in Table S3. Except for trifluoroacetic acid (TFA) and PFOA, other target compounds were not detected in the procedural blanks of unoxidized and oxidized air samples. All values below MDL were denoted as undetected by n.d., and were replaced by MDL/√2 when statistical analyses were performed. For TOP assay, two C8 PFAA precursors, 8:2 fluorotelomer sulfonic acid (8:2 FTSA) and perfluorooctane sulfonamide (FOSA), were selected as model precursors in the extracted air sample matrix to verify the mass balance of the TOP assay before and after oxidation. Oxidation of 8:2 FTSA by TOP assay produced a typical reproducible PFCA mixture pattern, and oxidation of FOSA by TOP assay produced PFOA. The total molar yields of 8:2 FTSA and FOSA to produce PFCA mixture or PFOA from three parallel sets of experiments were 99% and 101%, respectively (Table S4), which are consistent with previously reported molar yields. 3. Results and discussion 3.1 Levels of target PFAS. 3.1.1 Neutral PFAS . N -methyl perfluorooctane sulfonamide/sulfonamidoethanol ( N -MeFOSA/Es) and N -ethyl perfluorooctane sulfonamide/sulfonamidoethanol ( N -EtFOSA/Es) were not detected in all the samples. The concentrations of ∑FTOHs were 2,816 − 56,343 pg/m 3 in air samples, with median concentration of 4,162 pg/m 3 at WWTP and 4,727 pg/m 3 at the landfill (Table S5). The air concentrations of ∑FTOHs at L-C in this study were 1–2 magnitude orders higher than those sampled in 2016, when the highest concentration of 2,100 pg/m 3 was at the L-C. 27 6:2, 8:2, and 10:2 FTOHs were detected in all the site except for 10:2 FTOH over the secondary sedimentary tank. 8:2 FTOH was predominated neutral PFAS with median contribution of 79.7% in atmospheric samples of 8 sampling points (Fig. 1 /Table S5). Different concentrations and compiles of 3 FTOHs were found in different sampling points (Fig. 1 ). Highest level of ∑FTOHs were found in atmosphere above leachate of the landfill (L-L, 56,343 pg/m 3 ), which was higher than other sampling points of 1 magnitude order. Longer-chain 8:2 FTOH and 10:2 FTOH in L-L were both in highest levels of all sampling points, which was also higher than in landfilling center of the landfill (L-C). This may be due to L-L was next to old-age landfilling area, while L-C was around with new waste and neutral PFAS were not fully released. Neutral PFAS in contaminating centers of the WWTP (W-Inlet) and landfill (L-L, L-C) were higher than other peripheral sampling points. Therefore, landfill and WWTPs were important point sources of neutral PFAS to atmosphere. Additionally, longer-chain FTOHs in the landfill were in higher proportion (8:2 FTOH: 80.7% of landfill v.s. 58.9% of WWTP; 10:2 FTOH: 18.8% of landfill v.s. 0.44% of WWTP), while shorter-chain 6:2 FTOH in WWTP were in higher proportion (2.08% of landfill v.s. 40.1% of WWTP). This may due to wastewater in WWTP updated rapidly and neutral PFAS composition in corresponding atmosphere can reflect current release, while the landfill is a place where waste accumulates over time and neutral PFAS composition in corresponding atmosphere can reflect long-time release. Similarly, 8:2 FTOH was dominating neutral PFAS in China and Canada landfill atmosphere, 27 , 28 and 6:2 FTOH was dominating neutral PFAS in Canada WWTP atmosphere. 28 Compared with previous study, 6:2 FTOH concentration has increased in the landfill. 14 Recently, 6:2 FTOH concentration has been found increasing year by year in a WWTP in Beijing, China. 29 Thereout, point source type may influence PFAS compositions in atmosphere, and production and use of PFAS recently transfer from long chain to short chain. 3.1.2 Ionizable PFAS . 33 ionizable PFAS were analyzed in source point atmosphere samples, and perfluorodecane sulfonic acid (PFDS), 4:2 FTSA, 8:2 Cl-PFESA, HFPO-DA, dodecafluoro-3H-4,8-dioxnonanoic acid (ADONA) and10:2 diPAP were not detected in all samples. Total concentrations of ionizable PFAS (∑i-PFAS) were 4,275 − 13,360 pg/m 3 in air samples, with median concentration of 5,670 pg/m 3 in WWTP and 8,075 pg/m 3 in landfill (Table S5). Ultrashort-chain PFCA (C2-C3) were dominating pollutant type of i-PFAS in all sampling points with median contribution of 77.4% (median concentration: 4,162 pg/m 3 ) in WWTP and 85.4% (median concentration: 4,727 pg/m 3 ) in landfill (Fig. 2 ). Especially, contribution of TFA to ∑i-PFAS was 52.7–77.9% (median: 63.9%) in WWTP and 72.8–79.2% (median: 75.4%) in landfill, which was dominating i-PFAS. Concentrations of ∑i-PFAS (C ≥ 4) were 896-1,946 pg/m 3 in atmosphere samples, with median concentrations of 1,282 pg/m 3 in WWTP and 1,260 pg/m 3 in landfill. Concentrations of ∑i-PFAS (C ≥ 4) in this study were higher than that sampling in 2016 Tianjin landfills (highest level in central area: 820 pg/m 3 ), 27 and also higher than that in Chinese city atmospheric particulates (median concentration: 351 pg/m 3 ). 30 PFCAs (C4-C12) were dominating pollutant type with median contribution of 52.1% and 56.3% to ∑i-PFAS (C ≥ 4) in WWTP and landfill, respectively, and PFOA was predominating ∑i-PFAS (C ≥ 4) (median contribution: 29.8% in WWTP and 30.3% in landfill) and perfluorobutanoic acid (PFBA) followed with median contribution of 12.8% and 17.8% in WWTP and landfill, respectively. Compared with atmosphere sampling in 2016 Tianjin landfills, 27 PFBA level (180–590 pg/m 3 ) was similar with that in this study (129–416 pg/m 3 ), while PFOA level (11–100 pg/m 3 ) was obviously lower than that in this study (223-1,937 pg/m 3 ), indicating PFOA and its precursors were consistently used in China. Also, PFOA was dominating i-PFAS in Chinese city outdoor atmospheric particulates. 30 Concentrations of ∑PFSAs were 60.3–565 pg/m 3 (median: 160 pg/m 3 in WWTP and 84.1 pg/m 3 in landfill) in atmosphere samples of 8 sampling points, which significantly higher than that in atmosphere sampling in 2016 Tianjin landfills. The contributions of ∑PFSAs to ∑i-PFSAs (C ≥ 4) were 4.32–38.8% (median: 12.5% in WWTP and 5.43% in landfill), and PFOS was the predominant PFSA with median contribution of 8.96% and 2.45% to ∑i-PFSAs (C ≥ 4) in WWTP and landfill, respectively, and followed by perfluorobutane sulfonic acid (PFBS). High contribution of PFOA and PFOS in Tianjin source point atmosphere indicates that environment and health risks caused by C8 PFAAs and their precursors in China deserve further attentions. Emerging PFAS also been detected with considerable concentrations, especially 6:2 FTSA and OBS. Concentrations of 6:2 FTSA was 17–227 pg/m 3 (median: 18 pg/m 3 in WWTP and 77 pg/m 3 in landfill) in atmosphere samples of 8 sampling points with contributions of 4.32%-38.8% (median: 12.5% in WWTP and 5.43% in landfill) to ∑i-PFAS (C ≥ 4), and 6:2 FTSA was also found in Chinese urban atmospheric particulates. 30 6:2 FTSA is a degradation product of fluoromeric surfactants, 31 and used as a substitute for PFOS in metal plating. 30 OBS was also detected in 1.7–582 pg/m 3 (median: 4.8 pg/m 3 in WWTP and 166 pg/m 3 in landfill) in sampling atmosphere, and the highest level was found in atmosphere above landfill leachate (L-L). Used as surfactants in petroleum production, OBS was first identified in surface water of oil fields in China, 32 and were also detected in atmospheric particulate matter in Chinese cities. 30 Concentrations of 6:2 FTAB were 17–24 pg/m 3 in atmosphere of WWTP, and 6:2 Cl-PFESA were 2-6.2 pg/m 3 and 0.67–1.7 pg/m 3 in atmosphere of WWTP and landfill, respectively. As the main component of the inhibitory agent in chrome-plating industry, 6:2 Cl-PFESA is widely detected in Chinese environment including river water, surface water, sea water, sediments, sludge, biological samples and human serum. 33 , 34 6:2 fluorotelomer sulfonamide betaine (6:2 FTAB) is a fluoromeric surfactant, which is mainly detected in soil, groundwater, surface water and sediments in areas affected by firefighting activities in North America and Europe. 20 , 35 – 37 Chen et al. also extensively detected 6:2 FTAB in surface water, groundwater, seawater and sediments in China. 33 Perfluoroethylcyclohexanesulfonate (PFECHS) used as a PFOS substitute was also detected of 10.3 pg/m 3 in L-C atmosphere. Four intermediates of PFAS (i.e. 6:2 and 8:2 fluorotelomer (unsaturated) carboxylic acids (6:2 and 8:2 FT(U)As)) were also detected in the sampling atmosphere. The median concentrations of 8:2 FTUCA and 6:2 FTCA were 0.69 pg/m 3 and 26.9 pg/m 3 , respectively, with detection rates of 63.6% and 54.5%, respectively. 3.2 Unknown PFAA-precursors. 3.2.1 Increasing PFAAs . To investigate PFAS mass balance in source point atmosphere, this study has improved TOP assay and analyze unknown PFAA precursors in all sampling points. After oxidation of TOP assay, PFAA precursors in source point atmosphere transformed into PFCAs (increased PFCAs, ΔPFCAs) (Table S6). Concentrations of ∑Δ[PFCAs] were 2,188 − 12,718 ng/g (median: 5,292 ng/g in WWTP and 7,399 ng/g in landfill) with the highest in L-C and the lowest in L-L (Fig. 3 ). Ultrashort-chain PFCAs were principle oxidation produce with contributions of 43.8–97.8% (median: 84.1% in WWTP and 90.1% in landfill) to ∑Δ[C2-C12 PFCAs], and TFA contributed 30.5–75.7% (median: 74.3% in WWTP and 59.5% in landfill) and perfluoropropionic acid (PFPrA) contributed 9.71–54.7% (median: 9.87% in WWTP and 31.1% in landfill). Concentrations of ∑Δ[C4-C12 PFCAs] were 198-1,229 ng/g (median: 1153 ng/g in WWTP and 696 ng/g in landfill) with the highest in W-Inlet and the lowest in L-D5 (Fig. 3 ). PFBA predominated the increase in oxidized atmosphere samples with median concentration of 54.4% and 47.8% to ∑Δ[C4-C12 PFCAs] in WWTP and landfill, respectively, and followed by PFOA with median contribution of 21.9%. 3.2.2 Contribution of Unknown PFAA-precursors . PFAA precursors in atmosphere samples were oxidized into PFCA mixtures with different carbon chains by TOP assay, and it offers a reliable method to predict unknown PFAA-precursors in source point atmosphere samples. After the TOP assay, concentrations of ∑PFAS were 69.9–382 pmol/m 3 (median: 105 pmol/m 3 in WWTP and 146 pmol/m 3 in landfill), with the highest in W-Inlet (133 pmol/m 3 ) and L-L (382 pmol/m 3 ), respectively (Table S7). Unknown PFAA-precursor concentrations were 13.0-97.9 pmol/m 3 (median: 40.8 pmol/m 3 in WWTP and 55.1 pmol/m 3 in landfill), with the highest in W-AT (61.1 pmol/m 3 ) and L-U2 (97.9 pmol/m 3 ), respectively. Unknown PFAA-precursors contributed 6.99 mol%-67.1 mol% (median: 58.4 mol% in WWTP and 46.4 mol% in landfill) to ∑PFAS, with the highest contribution in W-Outlet (58.4 mol%) and L-U2 (67.1 mol%), respectively (Fig. 4 ). C2-C3 unknown PFAA-precursors were the predominant unknown precursor type, and their concentrations were 7.63-95.0 pmol/m 3 (median: 36.7 pmol/m 3 in WWTP and 52.6 pmol/m 3 in landfill), with the highest in W-AT (57.0 pmol/m 3 ) and L-U2 (95.0 pmol/m 3 ), respectively. C2-C3 unknown PFAA-precursors contributed 5.74 mol%-65.1 mol% to ∑PFAS, with the highest contribution in W-AT (54.5 mol%) and L-U2 (65.1 mol%), respectively. However, since C2-C3 PFAA precursors have contained non-PFAS sources, their contribution may be overestimated. Without considering ultrashort-chain PFAAs, ∑PFAS (C4-C12) were 3.54–268 pmol/m 3 (median: 14.6 pmol/m 3 in WWTP and 12.1 pmol/m 3 in landfill) in all sampling atmosphere, with the highest in W-Inlet (91.8 pmol/m 3 ) and L-L (268 pmol/m 3 ), respectively. C4-C12 unknown PFAA-precursor concentrations were 0.49–5.40 pmol/m 3 (median: 4.05 pmol/m 3 in WWTP and 2.49 pmol/m 3 in landfill), with the highest in W-Inlet (5.40 pmol/m 3 ) and L-U2 (2.94 pmol/m 3 ), respectively. C4-C12 unknown PFAA-precursors contributed 0.48 mol%-28.3 mol% (median: 27.1 mol% in WWTP and 14.0 mol% in landfill) to ∑PFAS (C4-C12), with the highest contribution in W-AT (27.6 mol%) and L-D2 (28.3 mol%), respectively. 3.2.3 Prediction of Unknown C4, C6 and C8 PFAA-precursors . Table 1 Estimated concentrations of C8-, C6-, and C4-precursors (ng/g as PFOA, PFHxA, and PFBA) and their ratios to ∆[PFOA], ∆[PFHxA], and ∆[PFBA]generated upon oxidation in atmosphere. WWTP Landfill W-Inlet W-AT W-Outlet L-U2 L-L L-C L-D2 L-D5 [C8-precursors] min 1.4 457 147 206 73 146 101 160 [C8-precursors] max 77 582 258 371 214 269 190 193 [C6-precursors] min 48 0 64 0 7 0 0 8.4 [C6-precursors] max 238 85 220 144 194 113 37 29 [C4-precursors] min 987 532 421 302 106 455 525 0 [C4-precursors] max 2,282 2,896 2403 1,752 770 2,514 2,800 1.2 [C8-precursors] min /ΔPFOA 0.05 0.95 0.8 0.91 0.93 0.91 0.87 1 [C8-precursors] max /ΔPFOA 3.05 1.21 1.41 1.65 2.71 1.67 1.65 1.2 [C6-precursors] min /ΔPFHxA 0.76 0 0.75 0 0.15 0 0 0.46 [C6-precursors] max /ΔPFHxA 3.79 5 2.54 4.99 4.14 3.86 3.86 1.58 [C4-precursors] min /ΔPFBA 0.95 0.97 0.92 0.91 0.73 0.95 0.99 0 [C4-precursors] max /ΔPFBA 2.21 5.26 5.26 5.26 5.26 5.26 5.26 5.26 As shown in Table 1 , the concentrations of potential C8-PFAA-precursors in atmospheric samples ranged from 1.4–582 pg/m 3 (PFOA equivalent concentration). The minimum concentrations of potential C8 PFAA-precursors in atmospheric samples were 1.4–457 pg/m 3 (PFOA equivalent concentration) with the lowest point in W-Inlet and the highest in W-Outlet, while the maximum concentrations of potential C8 PFAA-precursors in atmospheric samples were 77–582 pg/m 3 (PFOA equivalent concentration) with the lowest point in W-Inlet and the highest in W-AT. Predicted [C8 precursors] max /∆[PFOA] ranged from 1.20 to 3.05 (median: 1.41 in WWTP and 1.65 in landfill). Therefore, PFOS precursors were the main potential C8-PFAA-precursors in atmospheric samples other than W-Inlet. Similar to estimation of potential C8-PFAA-precursor concentrations and types, potential C6-PFAA-precursor concentrations were estimated based on perfluoropentanoic acid (PFPeA) and perfluoroheptanoic acid (PFHpA) generated in the TOP assay. Concentrations of potential C6-PFAA-precursors in atmospheric samples ranged from 0-238 pg/m 3 (perfluorohexanoic acid (PFHxA) equivalent concentration). The minimum concentrations of potential C6-PFAA-precursors were 0–64 pg/m 3 (PFHxA equivalent concentration) with the highest value in W-Outlet, and all PFHxA generated in W-AT, L-U2, L-C, and L-D2 were derived from C7 and above fluoromeric precursors. The maximum concentrations of potential C6-PFAA-precursors were 29–238 pg/m 3 (PFHxA equivalent concentration), with the lowest value in L-D5 and the highest value in W-Inlet. Predicted [C6 precursors] max /∆[PFHxA] ranged from 1.58 to 5.00 (median: 3.79 in WWTP and 3.86 in landfill). Hence, a greater proportion of potential C6-PFAA-precursors may be PFHxS precursors in W-Outlet and L-D5, while potential C6-PFAA-precursor type on a larger proportion may be fluorine-polymer precursors in W-Inlet, W-AT, L-U2, L-L, L-C and L-D2. With reference to the assumptions in the potential C6-PFAA-precursor concentration and type estimation method, the potential C4-PFAA-precursor concentration was estimated based on PFPrA and PFPeA generated in the TOP assay. The concentrations of potential C4-PFAA-precursors in atmospheric samples ranged from 0–2,896 pg/m 3 (PFBA equivalent concentration). The minimum concentrations of potential C4 PFAA-precursors in atmospheric samples were 0-987 pg/m 3 (PFBA equivalent concentration) with the lowest point in L-D5 and the highest in W-Inlet, while the maximum concentrations of potential C4 PFAA-precursors in atmospheric samples were 1.2-2,896 pg/m 3 (PFBA equivalent concentration) with the lowest point in L-D5 and the highest in W-AT. Predicted [C4 precursors] max /∆[PFBA] ranged from 2.21 to 5.26 (median: 5.26). Therefore, potential C4-PFAA-precursor type on a larger proportion may be fluorine-polymer precursors in atmospheric samples other than W-Inlet. 4. Conclusions Overall, the present study comprehensively illustrated the occurrence, migration and transformation of known and unknown PFAS in atmosphere from two PFAS contaminated source points of WWTP and landfill. Firstly, an optimized atmospheric active sampler was successfully used to collect atmospheric samples minimizing matrix and impurity effects, which provides a new sampling idea for the non-targeted analysis of atmospheric samples. Also, considerable amounts of neutral and ionizable PFAS in atmospheric samples highlights the potential risks of both PFAAs and their precursors in point sources of the WWTP and landfill, and efforts are needed to explore the transformation mechanisms of PFAS in atmosphere. The different occurrence of known PFAS in WWTP and landfill revealed the dominant PFAS types for different types of contaminated sites, which provides data and theoretical support for the precise removal of PFAS. In addition, this study oxidized PFAS in atmospheric samples by the TOP assay, and the contribution of unknown PFAA-precursors revealed the underestimation of PFAS risks in the atmosphere. The TOP assay emphasized the existence and potential risks of unknown PFAS in atmosphere, and predicted C4-, C6-, and C8-PFAA precursor types. Based on this study, migration and transformation mechanisms of emerging and unknown PFAS in atmosphere needs further exploration. Declarations Author Contribution Bin Wang: Writing—original draft, Methodology, Investigation, Formal analysis. Yiming Yao: Writing—review and editing, Supervision, Methodolog. Hongwen Sun: Writing—review and editing, Supervision, Methodology. Hongyan Zhao: Writing—review and editing.Qingxia Yu: Writing—review and editing. All authors reviewed the manuscript. References Dong, J.; Kim, S.; Young, S. D.; Li, C.; Jin, Z.; Lee, D.; Olivares, C. 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Chen, C.; Wang, J.; Li, L.; Xu, W.; Liu, J., Comparison of fluorotelomer alcohol emissions from wastewater treatment plants into atmospheric and aquatic environments. Environment International 2020, 139 , 105718. Yu, N.; Guo, H.; Yang, J.; Jin, L.; Wang, X.; Shi, W.; Zhang, X.; Yu, H.; Wei, S., Non-target and suspect screening of per- and polyfluoroalkyl substances in airborne particulate matter in China. Environmental Science & Technology 2018, 52 , (15), 8205–8214. Phillips, M. M. M.; Dinglasan-Panlilio, M. J. A.; Mabury, S. A.; Solomon, K. R.; Sibley, P. K., Fluorotelomer acids are more toxic than perfluorinated acids. Environmental Science & Technology 2007, 41 , (20), 7159–7163. Xu, L.; Shi, Y.; Li, C.; Song, X.; Qin, Z.; Cao, D.; Cai, Y., Discovery of a novel polyfluoroalkyl benzenesulfonic acid around oilfields in Northern China. Environmental Science & Technology 2017, 51 , (24), 14173–14181. Chen, H.; Munoz, G.; Sung Vo, D.; Zhang, L.; Yao, Y.; Zhao, Z.; Yi, L.; Liu, M.; Sun, H.; Liu, J.; Sauve, S., Occurrence and distribution of per- and polyfluoroalkyl substances in Tianjin, China: The contribution of emerging and unknown analogues. Environmental Science & Technology 2020, 54 , (22), 14254–14264. Munoz, G.; Liu, J.; Duy, S. V.; Sauve, S., Analysis of F-53B, Gen-X, ADONA, and emerging fluoroalkylether substances in environmental and biomonitoring samples: A review. Trends In Environmental Analytical Chemistry 2019, 23 , e00066. Munoz, G.; Desrosiers, M.; Duy, S. V.; Labadie, P.; Budzinsk, H.; Liu, J.; Sauve, S., Environmental occurrence of perfluoroalkyl acids and novel fluorotelomer surfactants in the freshwater fish catostomus commersonii and sediments following firefighting foam deployment at the Lac-Megantic railway accident. Environmental Science & Technology 2017, 51 , (3), 1231–1240. Mejia-Avendano, S.; Munoz, G.; Duy, S. V.; Desrosiers, M.; Benoit, P.; Sauve, S.; Liu, J., Novel fluoroalkylated surfactants in soils following firefighting foam deployment during the Lac-Megantic railway accident. Environmental Science & Technology 2017, 51 , (15), 8313–8323. D'Agostino, L. A.; Mabury, S. A., Certain perfluoroalkyl and polyfluoroalkyl substances associated with aqueous film forming foam are widespread in Canadian surface Waters. Environmental Science & Technology 2017, 51 , (23), 13603–13613. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 02 Dec, 2025 Read the published version in Environmental Geochemistry and Health → Version 1 posted Editorial decision: Revision requested 13 Sep, 2025 Reviews received at journal 07 Sep, 2025 Reviews received at journal 07 Sep, 2025 Reviewers agreed at journal 20 Aug, 2025 Reviewers agreed at journal 19 Aug, 2025 Reviewers agreed at journal 18 Aug, 2025 Reviewers invited by journal 11 Aug, 2025 Editor assigned by journal 06 Aug, 2025 Submission checks completed at journal 05 Aug, 2025 First submitted to journal 03 Aug, 2025 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. 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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-7284069","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":502196139,"identity":"0fab5df0-da05-4806-8f9f-f60cbd2c0157","order_by":0,"name":"Bin Wang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA90lEQVRIiWNgGAWjYJACZgYDBh5+ZvYDBz5USMjJE6tFTrK9J/HgjDMWxoYNRGlhYDA26DlgfJi3rSKR4QAB5QbHzx5+XVBwJ3GDRELCAd55EgmMDcwPH93Ap+VMXpr1DINnidslEg8ckNwmkcfOwGZsnINHi9mBHDNjHoPDiTtnAG0x3CZRzNjAwyaNV8v5NxAtG24kGBxInCOR2HCAkJYbOcaPgVqMDc4cMDhwsIEILfY33pgxzzA4DArkhIMNxySMDZsJ+EWyP8f4c8Gfw6CoPPz5T02dnDx788PH+LQAAZsEKp8Zv3Kwkg+E1YyCUTAKRsGIBgC0KFSe6eEhvgAAAABJRU5ErkJggg==","orcid":"","institution":"Guizhou University","correspondingAuthor":true,"prefix":"","firstName":"Bin","middleName":"","lastName":"Wang","suffix":""},{"id":502196142,"identity":"588ac703-2895-42c2-8261-1b9b0914606f","order_by":1,"name":"Yiming Yao","email":"","orcid":"","institution":"Nankai University","correspondingAuthor":false,"prefix":"","firstName":"Yiming","middleName":"","lastName":"Yao","suffix":""},{"id":502196144,"identity":"d77d0b27-a11d-44e3-b755-43ec0a439a8c","order_by":2,"name":"Hongwen Sun","email":"","orcid":"","institution":"Nankai University","correspondingAuthor":false,"prefix":"","firstName":"Hongwen","middleName":"","lastName":"Sun","suffix":""},{"id":502196145,"identity":"0c9be8bc-1d1d-4e56-a551-b921725870cd","order_by":3,"name":"Hongyan Zhao","email":"","orcid":"","institution":"Guizhou University","correspondingAuthor":false,"prefix":"","firstName":"Hongyan","middleName":"","lastName":"Zhao","suffix":""},{"id":502196148,"identity":"51980c4c-69a3-4481-82c0-dbd4b1e523a9","order_by":4,"name":"Qingxia Yu","email":"","orcid":"","institution":"Guizhou University","correspondingAuthor":false,"prefix":"","firstName":"Qingxia","middleName":"","lastName":"Yu","suffix":""}],"badges":[],"createdAt":"2025-08-03 14:53:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7284069/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7284069/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10653-025-02856-4","type":"published","date":"2025-12-02T15:57:51+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":89421762,"identity":"a5bfe9fa-50df-489c-9c0e-b9ec744380b9","added_by":"auto","created_at":"2025-08-19 19:01:31","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":129547,"visible":true,"origin":"","legend":"\u003cp\u003eConcentrations (a) and contribution (b) of neutral PFAS in atmosphere.\u003c/p\u003e","description":"","filename":"floatimage16.png","url":"https://assets-eu.researchsquare.com/files/rs-7284069/v1/7b8a826df81c93722b9fdb5c.png"},{"id":89421772,"identity":"a0608945-8339-4e40-abdd-76915128a18d","added_by":"auto","created_at":"2025-08-19 19:01:31","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":274971,"visible":true,"origin":"","legend":"\u003cp\u003eConcentrations and contribution of ionizable PFAS in atmosphere. Figures (a) and (b) are for i-PFAS (C2-C12), and figures (c) and (d) are for i-PFAS (C4-C12).\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7284069/v1/1590d8d325dc1d375bcca30c.png"},{"id":89422124,"identity":"bb37ae69-87b0-4d80-8a10-7844f0231587","added_by":"auto","created_at":"2025-08-19 19:09:31","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":121910,"visible":true,"origin":"","legend":"\u003cp\u003eIncreased concentrations (a) and contributions (b) of PFCAs.\u003c/p\u003e","description":"","filename":"floatimage36.png","url":"https://assets-eu.researchsquare.com/files/rs-7284069/v1/4e896f0455d21000aaea8cd0.png"},{"id":89421773,"identity":"d516b27d-604e-4886-8f5f-f2382a4349d2","added_by":"auto","created_at":"2025-08-19 19:01:32","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":218187,"visible":true,"origin":"","legend":"\u003cp\u003eMolar contribution of unknown PFAA-precursors (C2-C12) (a) and unknown PFAA-precursors (C4-C12) (b).\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7284069/v1/bb992731e809c8e69ca9fd06.png"},{"id":97723883,"identity":"5d8701e1-35af-4f94-8ff9-65a5c2ca7570","added_by":"auto","created_at":"2025-12-08 16:09:06","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1335732,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7284069/v1/080d4625-2915-409c-87c1-a3b36f271cae.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Contributions of unknown ionizable precursors for perfluoroalkyl acids in the air over a wastewater treatment plant and a landfill","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003ePer- and polyfluoroalkyl substances (PFAS) are a class of synthetic organic fluorinated compounds, which have been widely produced and applied in various industrial and commercial fields since late 1940s for their unique properties.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e Due to potential high persistency, bioaccumulation, and toxicity of C8-based perfluoroalkyl acids (PFAAs), perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) have been listed under Annexes B and A of the Stockholm Convention in 2009 and 2019, respectively.\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e Additionally, perfluorohexane sulfonic acid (PFHxS) has been listed under the Stockholm Convention in 2022.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e It was proposed that long-chain perfluorocarboxylic acids (PFCAs) with perfluorinated carbon chain lengths from 8 to 20 should be listed as persistent organic pollutants (POPs).\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eAs long-chain PFAS have been phased out after more than 20 years of control, emerging PFAS substitutes have been manufactured and produced, e.g. alternatives to PFOA and PFOS. In many applications, long-chain PFAS are often replaced by short-chain congeners that are not regulated. For example, 3M has commercialized surface treatment products containing C4 side-chain fluoromers since 2003.\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e In addition, a number of novel ether-containing PFAS have been manufactured as substitutes for long-chain PFAS, e.g. perfluoroether carboxylic acids (PFECAs) and perfluoroether sulfonic acids (PFESAs), which are enhanced for degradability by inserting ether bonds into shorter perfluorocarbon chains.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e In the production of fluoroperic high-performance materials, some PFECAs, including perfluoro-2-propoxypropanoic acid (HFPO-DA; trade name: GenX), are used as processing aids instead of PFOA.\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e It is estimated that the annual production of GenX in Europe is 10 t-100 t.\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e Since 1970s, F-53B, with major components of 6:2 and 8:2 chlorinated polyfluorinated ether sulfonic acids (Cl-PFESAs), has been widely used as fog inhibitor in chromium plating industry in China in substitution for PFOS, and the annual consumption from 2006 to 2015 was 10 t-14 t.\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e Thus, we should take attention on the occurrence of emerging PFAS in the environment.\u003c/p\u003e\u003cp\u003eThere are thousands of PFAS with clear chemical structure according to the PFAS Master List by the Environmental Protection Agency, US. However, no more than 30 PFAS were targeted for analysis in most studies, especially related with atmosphere.\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e Rarely screened PFAS may prove to have similar or higher toxicity than conventionally measured PFAS,\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e with potential environmental and human health risks. Initial development of the total oxidized precursor (TOP) assay was applied to urban runoff and find the contribution of unknown PFAA precursors.\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e TOP assay has been further applied to a variety of environmental matrices such as soil, sediment, and leachate.\u003csup\u003e\u003cspan additionalcitationids=\"CR14 CR15 CR16 CR17 CR18 CR19 CR20 CR21 CR22\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e In our previous study, the application of TOP assay to precipitation for the first time suggested that unknown ionizable PFAS precursors occur in the atmosphere while no direct application on air samples has been conducted yet. However, little is known about the potential unknown PFAS profiles in the air over WWTP and landfill.\u003c/p\u003e\u003cp\u003eIn order to increase flexibility and reduce matrix effects, an active air sampler using a cartridge-based composite sorbent of WAX/HC-C18 developed in our previous study was used and optimized with an increasing flow rate for outdoor use to collect both neutral and ionizable PFAS in the atmosphere.\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e Accordingly, the distribution characteristics of PFAS in the air over WWTP and landfill sites were analyzed. The contribution of unknown PFAA precursors in the air samples and the main types of PFAA precursors were estimated using the TOP assay.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Chemicals and Reagents.\u003c/h2\u003e\u003cp\u003eTwenty-seven ionizable and seven neutral target PFAS were analyzed in this study and detail information of the native and internal standards is given in Table S1 in the Supporting Information (SI). Ammonium hydroxide was purchased from Aladdin Industrial Corporation (Los Angeles, CA, USA). HPLC-grade methanol and formic acid was purchased from Fisher Scientific (Hampton, NH, USA). Milli-Q water was generated using an ultrapure water purification system (Millipore, Billerica, MA, USA). HPLC-grade ammonium acetate was purchased from CNW Technologies (Shanghai, China).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Sample Collection.\u003c/h2\u003e\u003cp\u003eFrom June to August 2020, eight air samples were collected from a municipal WWTP and a municipal landfill from Tianjin, China. Detail information of two source sites and sampling points were listed in Table S2 (SI). Three sampling points were set at the grid before primary sedimentary tank (W-Inlet), aeration tank (W-AT), and secondary sedimentary tank (W-outlet) of the WWTP, respectively. For the landfill, five sampling points were set at the center of the landfill (L-C), above the leachate (L-L), 2 km upwind (L-U2), 2 km downwind (L-D2) and 5 km downwind (L-D5), respectively. The dominant wind direction during the sampling period was south or southeast. For the collection of air samples, the collection time of each sample is about 12 h and the sampling volume of 5 m\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e was guaranteed. Three sets of self-designed active air samplers (Fig. S1) are placed at each sampling point were collected for three consecutive days. Four field blanks were collected at each sampling site to expose the cartridge for 5 min during sampling. Sampling cartridges were conditioned and air-dried with ethyl acetate and methanol before sampling. After sampling, the sampling cartridges were wrapped in aluminum foil and stored in a polypropylene ziplock bag. The samples were transported back to the laboratory and stored at -20\u0026deg;C until analysis. The validation of the air sampling cartridges using WAX/HC-C18 composite sorbents can be referred to our previous study.\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 \u003cem\u003eSample Pretreatment\u003c/em\u003e.\u003c/h2\u003e\u003cp\u003eBefore elution, the cartridge was spiked with 5 ng of each neutral and ionizable mass-labelled PFAS. Neutral and ionizable PFAS were eluted with 5 mL ethyl acetate (E1) and 4 mL methanol containing 0.1% ammonia (E2), sequentially. The two eluents were purified by Supelclean ENVI-Carb SPE cartridges (500 mg, 6 cc, SUPELCO, USA). The ENVI- Carb SPE column was first activated three times with 1 mL ethyl acetate/methanol. Then sample, and start to collect; The SPE column was then washed three times with 1 mL of ethyl acetate/methanol and merged with the activated collection solution. The eluents were concentrated under a gentle stream of ultrapure nitrogen at 40\u0026deg;C to a constant volume of 0.5 mL, respectively. An aliquot of 200 \u0026micro;L E1 supernatant was transferred to an autosampler for analysis of neutral PFAS. Another 200 \u0026micro;L of E1 supernatant was transferred to another autosampler, evaporated to dryness, and redissolved with an aliquot of 200 \u0026micro;L E2 supernatant for analysis of ionizable PFAS.\u003c/p\u003e\u003cp\u003e\u003cem\u003eTOP Assay.\u003c/em\u003e Using another sampling cartridge, the eluents of E1 and E2 were combined and evaporated to dryness at 40\u0026deg;C under a gentle stream of ultrapure nitrogen. The residual was redissolved with 15 mL of Milli-Q water and ultrasonically mixed for 2 min. Each sample was dosed with 0.243 g potassium persulfate (60 mM) and 0.225 mL 10 N sodium hydroxide solution (150 mM). The sample was ultrasonically mixed at a constant temperature (20\u0026deg;C) until the salt was completely dissolved. The sample was then placed in a thermostatic water bath at 85\u0026deg;C for 6 h. After reaction, the sample was cooled to room temperature in an ice water bath, and then the pH of the sample was adjusted to 5.0\u0026ndash;9.0 with concentrated hydrochloric acid.\u003c/p\u003e\u003cp\u003eBefore extraction, 5 ng of each mass-labelled ionizable PFAS was added to the sample. The Cleanert IC-Ba/Ag/H cartridge was used to remove excess anions such as SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e and Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e from the sample in improved recovery for ultra-short chain PFAS.\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e The Cleanert IC-Ba/Ag/H cartridge was used as follows: Firstly, the cartridge was conditioned with 5 mL methanol and 10 mL Milli-Q water and allowed for a 10-min balance. The sample was loaded onto the cartridge and collected at a rate of \u0026lt;\u0026thinsp;2 mL/min. For a complete recovery, PFAAs that may be retained by the cartridge were eluted with 6 mL methanol containing 0.1% ammonia. The sample was further enriched with an Oasis WAX cartridge according to our previous method.\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e The details were as follows: Firstly, the cartridge was conditioned with 4 mL methanol solution containing 0.1% ammonia, 4 mL methanol, and 4 mL Milli-Q water. Then, the pretreated sample was loaded onto the cartridge at a rate of \u0026lt;\u0026thinsp;2 mL/min. After loading the sample, the cartridge was washed with 5 mL Milli-Q and then conditioned with 5 mL ammonium acetate buffer solution (2.5 mM, pH\u0026thinsp;=\u0026thinsp;4). After centrifuging at 3,000 rpm for 5 min, the cartridge was eluted with 4 mL methanol and methanol containing 0.1% ammonia that was collected from the Cleanert IC-BA/Ag/H pretreatment. The eluents were combined and concentrated to 0.5 mL under a gentle stream of ultrapure nitrogen at 40 ℃. The concentrate was purified with dispersive ENVI-Carb and centrifuged at 15,000 rpm for 10 min. An aliquot of 200 \u0026micro;L supernatant was transferred to an autosampler for instrumental analysis.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4 \u003cem\u003eInstrumental Analysis\u003c/em\u003e.\u003c/h2\u003e\u003cp\u003eAn Agilent 7890 gas chromatography interfaced with a 5975C triple-axis mass spectrometry (MS) and a DB-WAX column (30 m\u0026times;0.25 mm i.d., 0.25 mm film thickness, Agilent J\u0026amp;W, USA) were used for analysis of neutral PFAS. The injection volume was 3 \u0026micro;L and the injection was operated in a splitless mode at 200˚C with an initial pressure of 40 psi, which returned to 10 psi at 1 min and followed by an injector purge. The initial oven temperature was kept at 60 ˚C for 1 min, ramped at 2 ˚C/min to 75 ˚C, followed by 10 ˚C/min to 135 ˚C and 20˚C/min to 235 ˚C, and then held for 4 min. Helium was used as the carrier gas. The MS system was operated in positive chemical ionization mode with methane used as reagent gas. Selected ion monitoring was applied for data acquisition.\u003c/p\u003e\u003cp\u003eAn Agilent 1260 liquid chromatography (LC) interfaced with an Agilent 6460 triple quadrupole tandem MS/MS (Agilent Technologies, USA) was operated with electrospray negative ionization and in multiple-reaction monitoring mode for analysis of ionizable PFAS. Nitrogen with a purity of 99.9% was used as desolvation gas at a flow rate of 11 L/min and a temperature of 350 ˚C. The nebulizer gas pressure was 50 psi. The capillary voltage was 4 kV. The dwell time was 200 ms. The injection volume was 10 \u0026micro;L. The extracts were analyzed using different separation approaches for specific classes of analytes. An ion-exchange RSpak JJ-50 2D column (2.0 mm i.d.\u0026times;150 mm length, 5 \u0026micro;m; Shodex, Japan) was used for separation of short-chain PFCAs (C2\u0026ndash;C4). An isocratic condition of mobile phase was used with a composition of 20% of 50 mM ammonium acetate in Milli-Q Water (pH\u0026thinsp;=\u0026thinsp;9) and 80% of methanol (v/v) at a flow rate of 350 \u0026micro;L/min. An X-terra MS C18 column (2.1 mm i.d.\u0026times;150 mm, 5\u0026micro;m, Waters, Ireland) was used for separation of ionizable PFAS (\u0026ge;\u0026thinsp;C4). Milli-Q water and methanol both containing 2.5 mM ammonium acetate were used as mobile phases and the gradient was operated as 10% methanol uniformly increasing to 58% at 0.8 min, to 100% at 12.8 min, reversing to original conditions at 14.3 min, and held to 26 min until equilibrium. The flow rate was kept at 350 \u0026micro;L/min. For both columns, the temperature was kept at 40 ˚C.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.5 \u003cem\u003eQuality Assurance and Quality Control (QA/QC)\u003c/em\u003e.\u003c/h2\u003e\u003cp\u003eEach batch of air samples was pretreated accompanied with a procedural blank, and an instrument blank was run every 10 samples during instrumental analysis. The spiked recoveries of neutral PFAS in atmospheric samples were 75\u0026ndash;95%, and the spiked recoveries of ionizable PFAS in unoxidized and oxidized atmospheric samples were 73\u0026ndash;116% and 68\u0026ndash;111%, respectively. The target PFAS were all quantified using a calibration curve at levels of 0.2, 0.5, 1, 2, 5, 10, 20, 50, and 100 \u0026micro;g/L with a constant level of mass-labelled standards at 10 \u0026micro;g/L. Before testing each batch of samples, the linearity and reproducibility of each calibration curve shall fit well (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;\u0026ge;\u0026thinsp;0.99). The concentrations of PFAS were all corrected with the mean of procedural blanks if with detection. The instrumental limits of quantification, process gaps, matrix spiked recoveries, and method limits of detection of target compounds are shown in Table S3. Except for trifluoroacetic acid (TFA) and PFOA, other target compounds were not detected in the procedural blanks of unoxidized and oxidized air samples. All values below MDL were denoted as undetected by n.d., and were replaced by MDL/\u0026radic;2 when statistical analyses were performed.\u003c/p\u003e\u003cp\u003eFor TOP assay, two C8 PFAA precursors, 8:2 fluorotelomer sulfonic acid (8:2 FTSA) and perfluorooctane sulfonamide (FOSA), were selected as model precursors in the extracted air sample matrix to verify the mass balance of the TOP assay before and after oxidation. Oxidation of 8:2 FTSA by TOP assay produced a typical reproducible PFCA mixture pattern, and oxidation of FOSA by TOP assay produced PFOA. The total molar yields of 8:2 FTSA and FOSA to produce PFCA mixture or PFOA from three parallel sets of experiments were 99% and 101%, respectively (Table S4), which are consistent with previously reported molar yields.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Levels of target PFAS.\u003c/h2\u003e\u003cdiv id=\"Sec10\" class=\"Section3\"\u003e\u003ch2\u003e\u003cem\u003e3.1.1 Neutral PFAS\u003c/em\u003e.\u003c/h2\u003e\u003cp\u003e\u003cem\u003eN\u003c/em\u003e-methyl perfluorooctane sulfonamide/sulfonamidoethanol (\u003cem\u003eN\u003c/em\u003e-MeFOSA/Es) and \u003cem\u003eN\u003c/em\u003e-ethyl perfluorooctane sulfonamide/sulfonamidoethanol (\u003cem\u003eN\u003c/em\u003e-EtFOSA/Es) were not detected in all the samples. The concentrations of \u0026sum;FTOHs were 2,816\u0026thinsp;\u0026minus;\u0026thinsp;56,343 pg/m\u003csup\u003e3\u003c/sup\u003e in air samples, with median concentration of 4,162 pg/m\u003csup\u003e3\u003c/sup\u003e at WWTP and 4,727 pg/m\u003csup\u003e3\u003c/sup\u003e at the landfill (Table S5). The air concentrations of \u0026sum;FTOHs at L-C in this study were 1\u0026ndash;2 magnitude orders higher than those sampled in 2016, when the highest concentration of 2,100 pg/m\u003csup\u003e3\u003c/sup\u003e was at the L-C.\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e 6:2, 8:2, and 10:2 FTOHs were detected in all the site except for 10:2 FTOH over the secondary sedimentary tank. 8:2 FTOH was predominated neutral PFAS with median contribution of 79.7% in atmospheric samples of 8 sampling points (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e/Table S5).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eDifferent concentrations and compiles of 3 FTOHs were found in different sampling points (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Highest level of \u0026sum;FTOHs were found in atmosphere above leachate of the landfill (L-L, 56,343 pg/m\u003csup\u003e3\u003c/sup\u003e), which was higher than other sampling points of 1 magnitude order. Longer-chain 8:2 FTOH and 10:2 FTOH in L-L were both in highest levels of all sampling points, which was also higher than in landfilling center of the landfill (L-C). This may be due to L-L was next to old-age landfilling area, while L-C was around with new waste and neutral PFAS were not fully released. Neutral PFAS in contaminating centers of the WWTP (W-Inlet) and landfill (L-L, L-C) were higher than other peripheral sampling points. Therefore, landfill and WWTPs were important point sources of neutral PFAS to atmosphere. Additionally, longer-chain FTOHs in the landfill were in higher proportion (8:2 FTOH: 80.7% of landfill v.s. 58.9% of WWTP; 10:2 FTOH: 18.8% of landfill v.s. 0.44% of WWTP), while shorter-chain 6:2 FTOH in WWTP were in higher proportion (2.08% of landfill v.s. 40.1% of WWTP). This may due to wastewater in WWTP updated rapidly and neutral PFAS composition in corresponding atmosphere can reflect current release, while the landfill is a place where waste accumulates over time and neutral PFAS composition in corresponding atmosphere can reflect long-time release. Similarly, 8:2 FTOH was dominating neutral PFAS in China and Canada landfill atmosphere,\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e and 6:2 FTOH was dominating neutral PFAS in Canada WWTP atmosphere.\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e Compared with previous study, 6:2 FTOH concentration has increased in the landfill.\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e Recently, 6:2 FTOH concentration has been found increasing year by year in a WWTP in Beijing, China.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e Thereout, point source type may influence PFAS compositions in atmosphere, and production and use of PFAS recently transfer from long chain to short chain.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\u003ch2\u003e\u003cem\u003e3.1.2 Ionizable PFAS\u003c/em\u003e.\u003c/h2\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e33 ionizable PFAS were analyzed in source point atmosphere samples, and perfluorodecane sulfonic acid (PFDS), 4:2 FTSA, 8:2 Cl-PFESA, HFPO-DA, dodecafluoro-3H-4,8-dioxnonanoic acid (ADONA) and10:2 diPAP were not detected in all samples. Total concentrations of ionizable PFAS (\u0026sum;i-PFAS) were 4,275\u0026thinsp;\u0026minus;\u0026thinsp;13,360 pg/m\u003csup\u003e3\u003c/sup\u003e in air samples, with median concentration of 5,670 pg/m\u003csup\u003e3\u003c/sup\u003e in WWTP and 8,075 pg/m\u003csup\u003e3\u003c/sup\u003e in landfill (Table S5). Ultrashort-chain PFCA (C2-C3) were dominating pollutant type of i-PFAS in all sampling points with median contribution of 77.4% (median concentration: 4,162 pg/m\u003csup\u003e3\u003c/sup\u003e) in WWTP and 85.4% (median concentration: 4,727 pg/m\u003csup\u003e3\u003c/sup\u003e) in landfill (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Especially, contribution of TFA to \u0026sum;i-PFAS was 52.7\u0026ndash;77.9% (median: 63.9%) in WWTP and 72.8\u0026ndash;79.2% (median: 75.4%) in landfill, which was dominating i-PFAS.\u003c/p\u003e\u003cp\u003eConcentrations of \u0026sum;i-PFAS (C\u0026thinsp;\u0026ge;\u0026thinsp;4) were 896-1,946 pg/m\u003csup\u003e3\u003c/sup\u003e in atmosphere samples, with median concentrations of 1,282 pg/m\u003csup\u003e3\u003c/sup\u003e in WWTP and 1,260 pg/m\u003csup\u003e3\u003c/sup\u003e in landfill. Concentrations of \u0026sum;i-PFAS (C\u0026thinsp;\u0026ge;\u0026thinsp;4) in this study were higher than that sampling in 2016 Tianjin landfills (highest level in central area: 820 pg/m\u003csup\u003e3\u003c/sup\u003e),\u003csup\u003e27\u003c/sup\u003e and also higher than that in Chinese city atmospheric particulates (median concentration: 351 pg/m\u003csup\u003e3\u003c/sup\u003e).\u003csup\u003e30\u003c/sup\u003e PFCAs (C4-C12) were dominating pollutant type with median contribution of 52.1% and 56.3% to \u0026sum;i-PFAS (C\u0026thinsp;\u0026ge;\u0026thinsp;4) in WWTP and landfill, respectively, and PFOA was predominating \u0026sum;i-PFAS (C\u0026thinsp;\u0026ge;\u0026thinsp;4) (median contribution: 29.8% in WWTP and 30.3% in landfill) and perfluorobutanoic acid (PFBA) followed with median contribution of 12.8% and 17.8% in WWTP and landfill, respectively. Compared with atmosphere sampling in 2016 Tianjin landfills,\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e PFBA level (180\u0026ndash;590 pg/m\u003csup\u003e3\u003c/sup\u003e) was similar with that in this study (129\u0026ndash;416 pg/m\u003csup\u003e3\u003c/sup\u003e), while PFOA level (11\u0026ndash;100 pg/m\u003csup\u003e3\u003c/sup\u003e) was obviously lower than that in this study (223-1,937 pg/m\u003csup\u003e3\u003c/sup\u003e), indicating PFOA and its precursors were consistently used in China. Also, PFOA was dominating i-PFAS in Chinese city outdoor atmospheric particulates.\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e Concentrations of \u0026sum;PFSAs were 60.3\u0026ndash;565 pg/m\u003csup\u003e3\u003c/sup\u003e (median: 160 pg/m\u003csup\u003e3\u003c/sup\u003e in WWTP and 84.1 pg/m\u003csup\u003e3\u003c/sup\u003e in landfill) in atmosphere samples of 8 sampling points, which significantly higher than that in atmosphere sampling in 2016 Tianjin landfills. The contributions of \u0026sum;PFSAs to \u0026sum;i-PFSAs (C\u0026thinsp;\u0026ge;\u0026thinsp;4) were 4.32\u0026ndash;38.8% (median: 12.5% in WWTP and 5.43% in landfill), and PFOS was the predominant PFSA with median contribution of 8.96% and 2.45% to \u0026sum;i-PFSAs (C\u0026thinsp;\u0026ge;\u0026thinsp;4) in WWTP and landfill, respectively, and followed by perfluorobutane sulfonic acid (PFBS). High contribution of PFOA and PFOS in Tianjin source point atmosphere indicates that environment and health risks caused by C8 PFAAs and their precursors in China deserve further attentions.\u003c/p\u003e\u003cp\u003eEmerging PFAS also been detected with considerable concentrations, especially 6:2 FTSA and OBS. Concentrations of 6:2 FTSA was 17\u0026ndash;227 pg/m\u003csup\u003e3\u003c/sup\u003e (median: 18 pg/m\u003csup\u003e3\u003c/sup\u003e in WWTP and 77 pg/m\u003csup\u003e3\u003c/sup\u003e in landfill) in atmosphere samples of 8 sampling points with contributions of 4.32%-38.8% (median: 12.5% in WWTP and 5.43% in landfill) to \u0026sum;i-PFAS (C\u0026thinsp;\u0026ge;\u0026thinsp;4), and 6:2 FTSA was also found in Chinese urban atmospheric particulates.\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e 6:2 FTSA is a degradation product of fluoromeric surfactants,\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e and used as a substitute for PFOS in metal plating.\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e OBS was also detected in 1.7\u0026ndash;582 pg/m\u003csup\u003e3\u003c/sup\u003e (median: 4.8 pg/m\u003csup\u003e3\u003c/sup\u003e in WWTP and 166 pg/m\u003csup\u003e3\u003c/sup\u003e in landfill) in sampling atmosphere, and the highest level was found in atmosphere above landfill leachate (L-L). Used as surfactants in petroleum production, OBS was first identified in surface water of oil fields in China,\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e and were also detected in atmospheric particulate matter in Chinese cities.\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e Concentrations of 6:2 FTAB were 17\u0026ndash;24 pg/m\u003csup\u003e3\u003c/sup\u003e in atmosphere of WWTP, and 6:2 Cl-PFESA were 2-6.2 pg/m\u003csup\u003e3\u003c/sup\u003e and 0.67\u0026ndash;1.7 pg/m\u003csup\u003e3\u003c/sup\u003e in atmosphere of WWTP and landfill, respectively. As the main component of the inhibitory agent in chrome-plating industry, 6:2 Cl-PFESA is widely detected in Chinese environment including river water, surface water, sea water, sediments, sludge, biological samples and human serum.\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e 6:2 fluorotelomer sulfonamide betaine (6:2 FTAB) is a fluoromeric surfactant, which is mainly detected in soil, groundwater, surface water and sediments in areas affected by firefighting activities in North America and Europe.\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan additionalcitationids=\"CR36\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e Chen et al. also extensively detected 6:2 FTAB in surface water, groundwater, seawater and sediments in China.\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e Perfluoroethylcyclohexanesulfonate (PFECHS) used as a PFOS substitute was also detected of 10.3 pg/m\u003csup\u003e3\u003c/sup\u003e in L-C atmosphere. Four intermediates of PFAS (i.e. 6:2 and 8:2 fluorotelomer (unsaturated) carboxylic acids (6:2 and 8:2 FT(U)As)) were also detected in the sampling atmosphere. The median concentrations of 8:2 FTUCA and 6:2 FTCA were 0.69 pg/m\u003csup\u003e3\u003c/sup\u003e and 26.9 pg/m\u003csup\u003e3\u003c/sup\u003e, respectively, with detection rates of 63.6% and 54.5%, respectively.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Unknown PFAA-precursors.\u003c/h2\u003e\u003cdiv id=\"Sec13\" class=\"Section3\"\u003e\u003ch2\u003e\u003cem\u003e3.2.1 Increasing PFAAs\u003c/em\u003e.\u003c/h2\u003e\u003cp\u003eTo investigate PFAS mass balance in source point atmosphere, this study has improved TOP assay and analyze unknown PFAA precursors in all sampling points. After oxidation of TOP assay, PFAA precursors in source point atmosphere transformed into PFCAs (increased PFCAs, ΔPFCAs) (Table S6). Concentrations of \u0026sum;Δ[PFCAs] were 2,188\u0026thinsp;\u0026minus;\u0026thinsp;12,718 ng/g (median: 5,292 ng/g in WWTP and 7,399 ng/g in landfill) with the highest in L-C and the lowest in L-L (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Ultrashort-chain PFCAs were principle oxidation produce with contributions of 43.8\u0026ndash;97.8% (median: 84.1% in WWTP and 90.1% in landfill) to \u0026sum;Δ[C2-C12 PFCAs], and TFA contributed 30.5\u0026ndash;75.7% (median: 74.3% in WWTP and 59.5% in landfill) and perfluoropropionic acid (PFPrA) contributed 9.71\u0026ndash;54.7% (median: 9.87% in WWTP and 31.1% in landfill). Concentrations of \u0026sum;Δ[C4-C12 PFCAs] were 198-1,229 ng/g (median: 1153 ng/g in WWTP and 696 ng/g in landfill) with the highest in W-Inlet and the lowest in L-D5 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). PFBA predominated the increase in oxidized atmosphere samples with median concentration of 54.4% and 47.8% to \u0026sum;Δ[C4-C12 PFCAs] in WWTP and landfill, respectively, and followed by PFOA with median contribution of 21.9%.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section3\"\u003e\u003ch2\u003e\u003cem\u003e3.2.2 Contribution of Unknown PFAA-precursors\u003c/em\u003e.\u003c/h2\u003e\u003cp\u003ePFAA precursors in atmosphere samples were oxidized into PFCA mixtures with different carbon chains by TOP assay, and it offers a reliable method to predict unknown PFAA-precursors in source point atmosphere samples. After the TOP assay, concentrations of \u0026sum;PFAS were 69.9\u0026ndash;382 pmol/m\u003csup\u003e3\u003c/sup\u003e (median: 105 pmol/m\u003csup\u003e3\u003c/sup\u003e in WWTP and 146 pmol/m\u003csup\u003e3\u003c/sup\u003e in landfill), with the highest in W-Inlet (133 pmol/m\u003csup\u003e3\u003c/sup\u003e) and L-L (382 pmol/m\u003csup\u003e3\u003c/sup\u003e), respectively (Table S7). Unknown PFAA-precursor concentrations were 13.0-97.9 pmol/m\u003csup\u003e3\u003c/sup\u003e (median: 40.8 pmol/m\u003csup\u003e3\u003c/sup\u003e in WWTP and 55.1 pmol/m\u003csup\u003e3\u003c/sup\u003e in landfill), with the highest in W-AT (61.1 pmol/m\u003csup\u003e3\u003c/sup\u003e) and L-U2 (97.9 pmol/m\u003csup\u003e3\u003c/sup\u003e), respectively. Unknown PFAA-precursors contributed 6.99 mol%-67.1 mol% (median: 58.4 mol% in WWTP and 46.4 mol% in landfill) to \u0026sum;PFAS, with the highest contribution in W-Outlet (58.4 mol%) and L-U2 (67.1 mol%), respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). C2-C3 unknown PFAA-precursors were the predominant unknown precursor type, and their concentrations were 7.63-95.0 pmol/m\u003csup\u003e3\u003c/sup\u003e (median: 36.7 pmol/m\u003csup\u003e3\u003c/sup\u003e in WWTP and 52.6 pmol/m\u003csup\u003e3\u003c/sup\u003e in landfill), with the highest in W-AT (57.0 pmol/m\u003csup\u003e3\u003c/sup\u003e) and L-U2 (95.0 pmol/m\u003csup\u003e3\u003c/sup\u003e), respectively. C2-C3 unknown PFAA-precursors contributed 5.74 mol%-65.1 mol% to \u0026sum;PFAS, with the highest contribution in W-AT (54.5 mol%) and L-U2 (65.1 mol%), respectively. However, since C2-C3 PFAA precursors have contained non-PFAS sources, their contribution may be overestimated.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eWithout considering ultrashort-chain PFAAs, \u0026sum;PFAS (C4-C12) were 3.54\u0026ndash;268 pmol/m\u003csup\u003e3\u003c/sup\u003e (median: 14.6 pmol/m\u003csup\u003e3\u003c/sup\u003e in WWTP and 12.1 pmol/m\u003csup\u003e3\u003c/sup\u003e in landfill) in all sampling atmosphere, with the highest in W-Inlet (91.8 pmol/m\u003csup\u003e3\u003c/sup\u003e) and L-L (268 pmol/m\u003csup\u003e3\u003c/sup\u003e), respectively. C4-C12 unknown PFAA-precursor concentrations were 0.49\u0026ndash;5.40 pmol/m\u003csup\u003e3\u003c/sup\u003e (median: 4.05 pmol/m\u003csup\u003e3\u003c/sup\u003e in WWTP and 2.49 pmol/m\u003csup\u003e3\u003c/sup\u003e in landfill), with the highest in W-Inlet (5.40 pmol/m\u003csup\u003e3\u003c/sup\u003e) and L-U2 (2.94 pmol/m\u003csup\u003e3\u003c/sup\u003e), respectively. C4-C12 unknown PFAA-precursors contributed 0.48 mol%-28.3 mol% (median: 27.1 mol% in WWTP and 14.0 mol% in landfill) to \u0026sum;PFAS (C4-C12), with the highest contribution in W-AT (27.6 mol%) and L-D2 (28.3 mol%), respectively.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section3\"\u003e\u003ch2\u003e\u003cem\u003e3.2.3 Prediction of Unknown C4, C6 and C8 PFAA-precursors\u003c/em\u003e.\u003c/h2\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eEstimated concentrations of C8-, C6-, and C4-precursors (ng/g as PFOA, PFHxA, and PFBA) and their ratios to ∆[PFOA], ∆[PFHxA], and ∆[PFBA]generated upon oxidation in atmosphere.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"10\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u003cp\u003eWWTP\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colspan=\"5\" nameend=\"c10\" namest=\"c6\"\u003e\u003cp\u003eLandfill\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eW-Inlet\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eW-AT\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eW-Outlet\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eL-U2\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eL-L\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003eL-C\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c9\"\u003e\u003cp\u003eL-D2\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c10\"\u003e\u003cp\u003eL-D5\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e[C8-precursors]\u003csub\u003emin\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e457\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e147\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e206\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e73\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e146\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e101\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e160\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e[C8-precursors]\u003csub\u003emax\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e77\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e582\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e258\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e371\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e214\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e269\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e190\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e193\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e[C6-precursors]\u003csub\u003emin\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e48\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e64\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e8.4\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e[C6-precursors]\u003csub\u003emax\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e238\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e85\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e220\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e144\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e194\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e113\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e37\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e29\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e[C4-precursors]\u003csub\u003emin\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e987\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e532\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e421\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e302\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e106\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e455\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e525\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e[C4-precursors]\u003csub\u003emax\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2,282\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2,896\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2403\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1,752\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e770\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e2,514\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e2,800\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e1.2\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e[C8-precursors]\u003csub\u003emin\u003c/sub\u003e/ΔPFOA\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.05\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.95\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.91\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.93\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.91\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0.87\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e[C8-precursors]\u003csub\u003emax\u003c/sub\u003e/ΔPFOA\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3.05\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.41\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1.65\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e2.71\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e1.67\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e1.65\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e1.2\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e[C6-precursors]\u003csub\u003emin\u003c/sub\u003e/ΔPFHxA\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.76\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.75\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0.46\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e[C6-precursors]\u003csub\u003emax\u003c/sub\u003e/ΔPFHxA\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3.79\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2.54\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e4.99\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e4.14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e3.86\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e3.86\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e1.58\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e[C4-precursors]\u003csub\u003emin\u003c/sub\u003e/ΔPFBA\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.95\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.97\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.92\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.91\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.73\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.95\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0.99\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e[C4-precursors]\u003csub\u003emax\u003c/sub\u003e/ΔPFBA\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2.21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5.26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e5.26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e5.26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e5.26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e5.26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e5.26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e5.26\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eAs shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the concentrations of potential C8-PFAA-precursors in atmospheric samples ranged from 1.4\u0026ndash;582 pg/m\u003csup\u003e3\u003c/sup\u003e (PFOA equivalent concentration). The minimum concentrations of potential C8 PFAA-precursors in atmospheric samples were 1.4\u0026ndash;457 pg/m\u003csup\u003e3\u003c/sup\u003e (PFOA equivalent concentration) with the lowest point in W-Inlet and the highest in W-Outlet, while the maximum concentrations of potential C8 PFAA-precursors in atmospheric samples were 77\u0026ndash;582 pg/m\u003csup\u003e3\u003c/sup\u003e (PFOA equivalent concentration) with the lowest point in W-Inlet and the highest in W-AT. Predicted [C8 precursors]\u003csub\u003emax\u003c/sub\u003e/∆[PFOA] ranged from 1.20 to 3.05 (median: 1.41 in WWTP and 1.65 in landfill). Therefore, PFOS precursors were the main potential C8-PFAA-precursors in atmospheric samples other than W-Inlet.\u003c/p\u003e\u003cp\u003eSimilar to estimation of potential C8-PFAA-precursor concentrations and types, potential C6-PFAA-precursor concentrations were estimated based on perfluoropentanoic acid (PFPeA) and perfluoroheptanoic acid (PFHpA) generated in the TOP assay. Concentrations of potential C6-PFAA-precursors in atmospheric samples ranged from 0-238 pg/m\u003csup\u003e3\u003c/sup\u003e (perfluorohexanoic acid (PFHxA) equivalent concentration). The minimum concentrations of potential C6-PFAA-precursors were 0\u0026ndash;64 pg/m\u003csup\u003e3\u003c/sup\u003e (PFHxA equivalent concentration) with the highest value in W-Outlet, and all PFHxA generated in W-AT, L-U2, L-C, and L-D2 were derived from C7 and above fluoromeric precursors. The maximum concentrations of potential C6-PFAA-precursors were 29\u0026ndash;238 pg/m\u003csup\u003e3\u003c/sup\u003e (PFHxA equivalent concentration), with the lowest value in L-D5 and the highest value in W-Inlet. Predicted [C6 precursors]\u003csub\u003emax\u003c/sub\u003e/∆[PFHxA] ranged from 1.58 to 5.00 (median: 3.79 in WWTP and 3.86 in landfill). Hence, a greater proportion of potential C6-PFAA-precursors may be PFHxS precursors in W-Outlet and L-D5, while potential C6-PFAA-precursor type on a larger proportion may be fluorine-polymer precursors in W-Inlet, W-AT, L-U2, L-L, L-C and L-D2.\u003c/p\u003e\u003cp\u003eWith reference to the assumptions in the potential C6-PFAA-precursor concentration and type estimation method, the potential C4-PFAA-precursor concentration was estimated based on PFPrA and PFPeA generated in the TOP assay. The concentrations of potential C4-PFAA-precursors in atmospheric samples ranged from 0\u0026ndash;2,896 pg/m\u003csup\u003e3\u003c/sup\u003e (PFBA equivalent concentration). The minimum concentrations of potential C4 PFAA-precursors in atmospheric samples were 0-987 pg/m\u003csup\u003e3\u003c/sup\u003e (PFBA equivalent concentration) with the lowest point in L-D5 and the highest in W-Inlet, while the maximum concentrations of potential C4 PFAA-precursors in atmospheric samples were 1.2-2,896 pg/m\u003csup\u003e3\u003c/sup\u003e (PFBA equivalent concentration) with the lowest point in L-D5 and the highest in W-AT. Predicted [C4 precursors]\u003csub\u003emax\u003c/sub\u003e/∆[PFBA] ranged from 2.21 to 5.26 (median: 5.26). Therefore, potential C4-PFAA-precursor type on a larger proportion may be fluorine-polymer precursors in atmospheric samples other than W-Inlet.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eOverall, the present study comprehensively illustrated the occurrence, migration and transformation of known and unknown PFAS in atmosphere from two PFAS contaminated source points of WWTP and landfill. Firstly, an optimized atmospheric active sampler was successfully used to collect atmospheric samples minimizing matrix and impurity effects, which provides a new sampling idea for the non-targeted analysis of atmospheric samples. Also, considerable amounts of neutral and ionizable PFAS in atmospheric samples highlights the potential risks of both PFAAs and their precursors in point sources of the WWTP and landfill, and efforts are needed to explore the transformation mechanisms of PFAS in atmosphere. The different occurrence of known PFAS in WWTP and landfill revealed the dominant PFAS types for different types of contaminated sites, which provides data and theoretical support for the precise removal of PFAS. In addition, this study oxidized PFAS in atmospheric samples by the TOP assay, and the contribution of unknown PFAA-precursors revealed the underestimation of PFAS risks in the atmosphere. The TOP assay emphasized the existence and potential risks of unknown PFAS in atmosphere, and predicted C4-, C6-, and C8-PFAA precursor types. Based on this study, migration and transformation mechanisms of emerging and unknown PFAS in atmosphere needs further exploration.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eBin Wang: Writing\u0026mdash;original draft, Methodology, Investigation, Formal analysis. Yiming Yao: Writing\u0026mdash;review and editing, Supervision, Methodolog. Hongwen Sun: Writing\u0026mdash;review and editing, Supervision, Methodology. Hongyan Zhao: Writing\u0026mdash;review and editing.Qingxia Yu: Writing\u0026mdash;review and editing. All authors reviewed the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eDong, J.; Kim, S.; Young, S. D.; Li, C.; Jin, Z.; Lee, D.; Olivares, C. 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V.; Desrosiers, M.; Benoit, P.; Sauve, S.; Liu, J., Novel fluoroalkylated surfactants in soils following firefighting foam deployment during the Lac-Megantic railway accident. \u003cem\u003eEnvironmental Science \u0026amp; Technology\u003c/em\u003e 2017, \u003cem\u003e51\u003c/em\u003e, (15), 8313\u0026ndash;8323.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eD'Agostino, L. A.; Mabury, S. A., Certain perfluoroalkyl and polyfluoroalkyl substances associated with aqueous film forming foam are widespread in Canadian surface Waters. \u003cem\u003eEnvironmental Science \u0026amp; Technology\u003c/em\u003e 2017, \u003cem\u003e51\u003c/em\u003e, (23), 13603\u0026ndash;13613.\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":"environmental-geochemistry-and-health","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"egah","sideBox":"Learn more about [Environmental Geochemistry and Health](https://www.springer.com/journal/10653)","snPcode":"10653","submissionUrl":"https://submission.nature.com/new-submission/10653/3","title":"Environmental Geochemistry and Health","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Emerging PFAS, PFAA precursors, TOP assay, atmosphere, WWTP, landfill","lastPublishedDoi":"10.21203/rs.3.rs-7284069/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7284069/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWastewater treatment plant (WWTP) and landfill are important point sources for emission of per- and polyfluoroalkyl substance (PFAS). However, occurrence and risks of emerging and unknown PFAS in the atmosphere of these sources have not been clarified. An optimized novel cartridge-based active air sampler for both neutral and ionizable PFAS was used to collect air samples from a WWTP and a landfill in Tianjin, China. For target PFAS, the concentrations of neutral PFAS were comparable with those of ionizable PFAS in air samples from both sources. 8:2 Fluorotelomer alcohol (8:2 FTOH) was the predominant neutral PFAS in all the air samples, while ultrashort-chain perfluoroalkyl carboxylic acids (PFCAs) were dominant ionizable PFAS. The optimized total oxidizable precursor (TOP) assay indicated that unknown precursors contributed 46.4\u0026ndash;58.4 mol%. This study further revealed the unknown PFAS profiles and their risk in the atmosphere contributed by WWTP and landfill.\u003c/p\u003e","manuscriptTitle":"Contributions of unknown ionizable precursors for perfluoroalkyl acids in the air over a wastewater treatment plant and a landfill","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-19 19:01:27","doi":"10.21203/rs.3.rs-7284069/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-09-13T18:39:28+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-08T03:55:09+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-07T20:58:45+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"161993926140953662512863347735755967838","date":"2025-08-20T12:56:01+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"157159157945012395323575020380728986354","date":"2025-08-19T08:44:06+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"271039669006980559364626957853023660000","date":"2025-08-18T12:59:45+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-08-11T22:19:40+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-06T16:47:44+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-08-05T18:29:24+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Geochemistry and Health","date":"2025-08-03T14:40:24+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-geochemistry-and-health","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"egah","sideBox":"Learn more about [Environmental Geochemistry and Health](https://www.springer.com/journal/10653)","snPcode":"10653","submissionUrl":"https://submission.nature.com/new-submission/10653/3","title":"Environmental Geochemistry and Health","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"f3d16561-b55b-4da2-b6a2-24bc11665c69","owner":[],"postedDate":"August 19th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-12-08T16:01:58+00:00","versionOfRecord":{"articleIdentity":"rs-7284069","link":"https://doi.org/10.1007/s10653-025-02856-4","journal":{"identity":"environmental-geochemistry-and-health","isVorOnly":false,"title":"Environmental Geochemistry and Health"},"publishedOn":"2025-12-02 15:57:51","publishedOnDateReadable":"December 2nd, 2025"},"versionCreatedAt":"2025-08-19 19:01:27","video":"","vorDoi":"10.1007/s10653-025-02856-4","vorDoiUrl":"https://doi.org/10.1007/s10653-025-02856-4","workflowStages":[]},"version":"v1","identity":"rs-7284069","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7284069","identity":"rs-7284069","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}