Insights on chemical characteristics and potential sources of PM2.5 in the inner city of Bangkok during the COVID-19 lockdown

The COVID-19 lockdown provided a unique opportunity to investigate PM 2.5 chemical characteristics under reduced anthropogenic emissions in urban Bangkok. This study comprehensively characterized PM 2.5 and its chemical components (carbonaceous species, water-soluble ions, and heavy metals) at an inner-city park during the wet season (September-October 2021, n = 30) and dry season (January-February 2022, n = 30). The source of PM 2.5 were determined using Principal Component Analysis (PCA). The potential source regions were determined by using Potential Source Contribution Function (PSCF) model and back trajectory analysis. Mean PM 2.5 concentrations were 13.6 ± 6.6 μg/m 3 in the wet season and 19.9 ± 9.0 μg/m 3 in the dry season. Carbonaceous species constituted the dominant PM 2.5 fraction, accounting for 29-48% of total mass, while secondary organic carbon (SOC) contributed up to 84% of total organic carbon (OC) during the dry season. SO 4 2– was the most abundant water-soluble ion in both seasons, and enrichment factor analysis indicated that most trace metals were primarily derived from anthropogenic sources. PCA identified four major PM 2.5 sources: primary combustion, vehicle emissions, and industrial emissions, sea salt, and dust resuspension. PSCF analysis revealed that local emissions were the primary contributors to PM 2.5 pollution, with additional influence from northeastern Thailand during the dry season due to biomass-burning activities. Overall, the results demonstrate that despite reduced anthropogenic activities during the COVID-19 lockdown, local vehicular and industrial emissions remained the dominant PM 2.5 sources in Bangkok, highlighting the importance of targeted emission-control strategies and regional cooperation for effective urban air quality management.