Water-soluble ions composition of ambient PM2.5 in relation to traffic and biomass burning sources

Abstract

Upper Northern Thailand (UNT) has suffered from annual smoke-haze episode during dry season. In this study, seasonal PM2s concentrations and chemical characteristics in urban and rural areas in 2019 were investigated. The objectives were to monitor ambient PM2.s and their water-soluble ions from traffic and biomass burning sources and to find the seasonal variation of water-soluble ion characteristics in urban and rural area for source identification. PM2s samples were collected in 24-hour basis from urban (Chiang Mai and Lampang) and rural (Chiang Dao and Mae Sariang) areas in 3provinces across UNT. The Average PM2s concentrations for hot dry season or haze season in descending order were Chiang Dao (115+50 ug m3), Lampang (110+29 1g m-3), Mae Sariang (80t33 ug m3 and Chiang Mai (77:41 ug m ). The average PM.5 concentrations in wet season ranged between 20-53 jg m 3, while those of cool dry season were 36-60 ug m-3. Source identification by Principal Component Analysis in Chiang Mai, Lampang and Mae Sariang in hot dry season were found to be mixed sources,characterized by dominating Secondary Inorganic Aerosols (SIA), including SO4, NH4+ and NO3 as well as Kt in forms of KNO3 and (NH4)2SO4. This referred to open burning, traffic emission and transboundary pollution. Conversely, sources of PM.s in Chiang Dao rural site were identified as mainly from biomass burning, indicated by primary and secondary potassium salts (KCl and KNO3). Despite high active fire counts in rural Mae Sariang, PM2.s had similar source identity to urban area rather than Chiang Dao may resulted from the area being more of receptor site, with mainly transboundary pollution from western direction. Active fire counts in Mae Sariang were also relatively low compared to those of Chiang Dao, where fire activities were constantly high throughout the season. Other minor sources included soil, both from surface soil combustion and soil resuspension, and sea salt from western and southwestern wind. PM2s source identification in non-haze season were more distinguishable between urban and rural areas, depending on the local emission sources. Urban areas had (NH4)2SO4 as main SIA species while rural areas had a trace of primary aerosols from biomass burning (KCl).