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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 to understand the impact of PM2.5 transport from peatland fires in Indonesia on air quality during the southwest monsoon season. 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 (PCA). 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. During the haze period, both OC and EC were 3.5 times as high as in the normal period. The average total PAHs and BaP-TEQ of PM2.5 during the haze period were ~ 1.3–1.7 and ~ 1.2–1.9 times higher than those in the normal period. The K concentrations significantly increased during haze periods. SO42− dominated throughout the year. The effects of external sources, especially the transboundary haze from peatland fires, were significantly enhanced, because the background air in the study locations was generally clean. 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. This was consistent with the OC/EC and PAH diagnostic ratios. Backward trajectories confirmed that the sources of PM during the haze period were predominantly peatland fires in Sumatra, Indonesia, due to southwest wind. [ABSTRACT FROM AUTHOR]

Abstract Northern Thailand has long been severely affected by haze from biomass burning containing fine and ultrafine aerosols in the dry period. The carbonaceous PM0.1 comprising elemental carbon (EC) and organic carbon (OC) collected during the haze and non-haze periods in Chiang Mai, Thailand was investigated. The PM0.1 levels during the haze periods were about 3 times higher than the non-haze periods, a significant increase. PM0.1 concentration was strongly correlated with atmospheric relative humidity and the number of forest fire hotspots. Carbonaceous aerosol characteristics in PM0.1 were analyzed with the thermal/optical transmittance (TOT) method following the IMPROVE protocol. The concentrations of OC and EC, distribution of OC and EC and OC/EC ratios in PM0.1 were evaluated. Average OC and EC mass concentrations in PM0.1 were 6.8 ± 2.7 and 1.4 ± 0.5 µg m−3 during the haze periods, significantly higher than those during the non-haze periods; 1.9 ± 0.9 and 0.5 ± 0.2 µg m−3. The OC/EC ratio increased linearly with the number of hotspots. This indicated significant contribution from biomass burning to the PM0.1. This was strongly supported by the 48-hr backward trajectory simulation, that indicated both domestic and transboundary aerosol transports. Because both organic and elemental carbon are the light-absorbing carbonaceous aerosols, the increase during the haze periods contributed to regional air quality and climate. This study enhances the understanding of PM0.1 behavior in Chiang Mai, Thailand, during the haze periods in upper southeast Asia.

This study characterizes impacts on PM 2.5 of transboundary haze from peatland fires in Indonesia and local emission sources during 2019–2020 to a large and densely populated city, Hat Yai in southern Thailand. Organic carbon (OC), elemental carbon (EC), water soluble organic carbon (WSOC), water soluble ions (WSI: Cl−, NO 3 −, SO 4 2− and NH 4 +), element tracers (K, Na, Mg and Ca), heavy metals (Cr, Co, Pb, Cd, Ni, Mn) and As, and 16 polycyclic aromatic hydrocarbon (PAHs) components from PM 2.5 samples (n = 18) were measured to identify local and regional emission sources using a chemical mass balance (CMB) model. We used a combination of air mass backward trajectories and CMB source apportionment to identify PM 2.5 sources. An increase of PM 2.5 and chemical component concentrations, during the transboundary haze period, were clearly influenced by aerosols from open biomass burning in Indonesia: PAH concentrations were 2 times higher and OC concentrations, 5 times higher. Secondary organic carbons were predominant during transboundary haze periods, accounting for 52–58% of total OC, indicating higher secondary organic aerosol formation. High K levels demonstrated that the dominant source during this period was biomass burning. Whereas a high level of Ca in the background air came from urban road dust, as well as local biomass burning. Moreover, the increased concentration of SO 4 2−, NH 4 + and NO 3 − during the wet season, as well as transboundary haze periods, was mainly derived the secondary inorganic aerosol formation. Effect on PM 2.5 concentration from a volcanic eruption near the air mass trajectory during the sampling period was minimal. However, it may have contributed slightly to an increase of SO 4 2− concentration in PM 2.5. Sources of PM 2.5 in Hat Yai were clearly influenced by local emission sources, e.g. diesel combustion and biomass burning (rubber wood and rice straw), transboundary haze and also secondary organic and inorganic aerosols. [Display omitted] • Influence of peatland fires in Indonesia was as far as southern Thailand. • High OC/EC in transboundary haze period indicated main source from biomass burning. • Secondary aerosol was important during transboundary haze period. • Transboundary haze caused an increase of PAHs and BaP-TEQ concentrations. • Background PM 2.5 was mainly from diesel combustion and local biomass burning. [ABSTRACT FROM AUTHOR]