Characteristics of PM2.5 concentrations in a tropical dry deciduous forest of Thailand: Forest fires and urban air pollution seriously affect air quality in forest ecosystems

Particulate matter with an aerodynamic diameter equal to or less than 2.5 μm (PM 2.5 ) remains a serious issue in Asia. To address the lack of observational data and investigate the status of PM 2.5 in the forests, we collected PM 2.5 above a tropical dry deciduous forest in a remote area of northeastern Thailand during the dry season. Notably, a large-scale surface fire occurred at the forest site during the mid-dry season. We also analyzed the PM 2.5 data collected by the Pollution Control Department of Thailand and the satellite-based aerosol optical depth (AOD) over Southeast Asia. Three episodes of elevated PM 2.5 concentration were observed at the site. Based on the ground-based measurements and AOD, two episodes were attributable to the long-range transport from urban areas in Vietnam and Thailand. The proportion of SO 4 2− increased in the two episodes, indicating SO 4 2− was a key indicator for identifying the sources of air pollution. While NH 4 + , NO 3 − , and SO 4 2− dominated approximately 83% of the total concentration of water-soluble inorganic ions (WSII) during the non-fire periods, the forest fire substantially altered the chemical composition, with K + and Cl − accounting for approximately 42%. The maximum concentration of total WSII in PM 2.5 during the forest fire period reached 88.0 μg m −3 , which was approximately three times higher than that during the non-fire periods (26.4 μg m −3 ). The forest site served not only as a receptor of urban air pollution but also as a large source of PM 2.5 . Enhancing regional cooperation and forest management is necessary in this region. • We measured PM 2.5 above a dry deciduous forest in a remote area of Thailand. • Forest fire altered typical PM 2.5 composition by generating biomass burning aerosols. • Forest served as a receptor of long-range transported urban/industrial air pollution. • Forest fire considerably raised PM 2.5 concentration and served as a large source.