Chemical signatures, source characteristics, and transboundary transport of PM0.49 in Thailand–Myanmar border region

Air pollution, particularly biomass burning (BB)–derived particulate matter (PM), remains a major challenge in Southeast Asia, affecting atmospheric processes, climate, human health, and socio-economic systems. This study presents the chemical characterization and source contributions of PM 0.49 at a rural site along the Thailand–Myanmar border in 2023. PM 0.49 concentrations showed pronounced temporal variability, with smoke-haze (SH) concentrations (67.9 ± 46.0 µg/m 3 ) nearly six times higher than those during non-haze periods (non-SH; 10.9 ± 4.4 µg/m 3 ). Organic carbon (OC) dominated during SH, accounting for 46% of PM 0.49 mass, followed by secondary inorganic aerosols (SO 4 2− , NO 3 − , NH 4 + ; 13%) and BB tracers such as K + (1.4%) and levoglucosan (2.7%), indicating strong BB influence. K⁺ exhibited strong correlations with levoglucosan ( r = 0.95) and major anions ( r = 0.52–0.94), highlighting its dual role as both a neutralizing cation and a BB tracer in both fresh and aged BB aerosols. Elevated OC to elemental carbon ratios (OC/EC; 13.4 ± 4.6) and char-EC/soot-EC ratios (9.68 ± 4.82) indicate smoldering BB as the principal source. Positive matrix factorization attributed 57% of PM 0.49 to BB during SH (up to 69% during peak phase), whereas soil dust dominated during non-SH conditions (70%). Backward trajectories and fire hotspot analyses suggest substantial cross-border transport from Myanmar. Furthermore, the seasonal shift in the effective carbon ratio (ECR), from 0.63 (SH) to 1.13 (non-SH), indicates a transition from light-absorbing to light-scattering aerosol dominance, underscoring the role of PM 0.49 in aerosol transformation under BB influence. These findings suggest air quality and potential climate implications, supporting coordinated mitigation.