Sources and Formation of Fine Size-Fractionated Particulate Matters During Smoke Haze Episode in Northern Thailand in Relation to Polycyclic Aromatic Hydrocarbons and Carbonaceous Composition

Abstract

Air pollution in upper Southeast Asia has become critical particularly regarding atmospheric particles. This study aims to investigate the sources and formation of fine size-fractionated particulate matters (FSPMs), as well as their potential impact on climate. Three size ranges of PMs including 2.1 - 1.1 μm, 1.1 - 0.65 μm, and 0.65 - 0.43 μm, were collected using cascade impactors in rural Chiang Dao (CD) and urban Chiang Mai (CM) during the smoke haze period (March – April 2019) in Chiang Mai, Thailand. The average concentration of those FSPMs was significantly higher in the rural CD (87.85 ± 34.42 µg/m3) than in the urban CM (65.57 ± 25.79 µg/m3). Moreover, significantly higher concentrations of PMs were observed in the finer particle sizes in both areas. Consequently, concentrations of total carbon (TC), including organic carbon (OC) and elemental carbon (EC), showed the same pattern as PMs. Furthermore, the most abundant fractions of OC3 and EC1 in FSPMs samples, particularly in fine particles (PM1), were observed, indicating biomass burning as its primary source. Classification of sources based on diagnostic ratios of polycyclic aromatic hydrocarbons (PAHs) and OC/EC ratios, combined with primary and secondary formations, revealed that PM2.1-1.1 was influenced by biomass burning and secondary organic aerosol (SOA), while PM0.65-0.43 were generated from vehicular emissions and SOA. PM1.1-0.65 in the rural area was probably caused by SOA and other mixed sources, whereas it was emitted from vehicular combustion in the urban CM. Referring to the effective carbon ratio (ECR), the FSPMs, especially the finest PMs, are associated with the absorption of solar radiation and contribute to atmospheric warming during smoke haze periods in this region.

Keywords: Air pollution, Carbonaceous aerosols, PAHs, Biomass burning, Secondary organic carbon, Climate impact