PM2.5 and chemical compositions in a naturally clean background air of Thailand's deep south, impact of transboundary haze from peatland fires and source apportionment by Principal Component Analysis

Abstract Atmospheric particulate matter smaller than 2.5 micron (PM2.5) was evaluated at four sites in the lower southern part of Thailand during 2019-2020. Mass concentration and chemical bound-PM, including carbon composition e.g. organic carbon (OC) and elemental carbon (EC), polycyclic aromatic hydrocarbons (PAHs), and inorganic elements, were analyzed. The PM2.5 emission sources were identified by Principal Components Analysis. The average mass concentrations of PM2.5 in the normal period, which represents clean background air, from four sites was 3.5-5.1 µg/m3, whereas during the haze period, it rose to 5.4-13.5 µg/m3. The average OC/EC ratio in PM2.5 was 2.7-3.8 during the normal period, suggesting mixed sources of both vehicle combustion and biomass burning, whereas during strong haze period, the ratio rose to 3.3-5.2 indicating that biomass burning was a dominant source. This was consistent with the PAH diagnostic ratios. The average total PAHs and BaP-TEQ of PM2.5 during strong haze period were ~1.3-1.7 and ~1.2-1.9 times higher than those the normal period. The effects of external sources, especially the transboundary haze from peatland fires, are quantitatively pronounced because the background air in the study locations was generally clean. The PCA indicated that vehicle emission, local biomass burning, and secondary particles played a key role during normal period, whereas open biomass burning dominated during the haze phenomena. Backward trajectory simulations confirmed that the sources of PM during haze period was dominated by peatland fire in Sumatra, Indonesia due to south-west wind.