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

Air pollution in upper Southeast Asia (SEA) 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. Particulate matter in three size ranges (2.1–1.1 μm, 1.1–0.65 μm, and 0.65–0.43 μm) was 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 FSPM was approximately 1.3 times higher in rural CD (88 ± 34 μg/m³) than in urban CM (66 ± 26 μg/m³). Both areas showed significantly higher concentrations of finer particles, with rural CD detecting 41 ± 17 μg/m³ (46% of FSPM) and urban CM presenting 27 ± 11 μg/m³ (41% of FSPM). Total carbon (TC) concentrations, including organic carbon (OC) and elemental carbon (EC), showed the same pattern as PMs. The most abundant fractions, including OC3 (32–45% of total OC) and EC1 (91–100% of total EC) in FSPM samples, particularly in fine particles (PM1), suggest that biomass burning was the primary source. Source classification based on diagnostic ratios of polycyclic aromatic hydrocarbons (PAHs), OC/EC ratios, combined with primary and secondary formations, revealed that PM_2.1-1.1 was influenced by biomass burning and secondary organic carbon (SOC), while PM_0.65-0.43 originated from both SOC and vehicular emissions. PM_1.1-0.65 in the rural area was probably caused by SOC and other mixed sources, whereas SOC and vehicular emissions were the main source in urban CM. With respect to the effective carbon ratio (ECR), lower values were observed in finer particles (0.37 ± 0.15 in rural CD and 0.36 ± 0.38 in urban CM), suggesting enhanced light-absorbing properties of fine particulate matter during the haze period in the upper SEA region.