Chemical Composition and Hygroscopic Properties of Atmospheric Fine Particles in the Tropical Forest Region of Southwest China

Abstract Biomass burning (BB) significantly influences regional air quality, yet its effects on PM 2.5 characteristics in tropical rainforest regions, particularly across the South Asian background area, remain poorly understood. This study investigated the seasonal variations in the chemical composition and hygroscopicity of water‐soluble PM 2.5 at a tropical rainforest site in Xishuangbanna, Southwest China. Results revealed that dry season aerosols exhibited 3–4 times higher PM 2.5 and water‐soluble matter (WSM) concentrations than wet season samples, with significantly lower hygroscopic growth factors of WSM at 90% RH ( g (90%) WSM = 1.22 vs. 1.34) and hygroscopicity parameters ( κ = 0.11 vs. 0.18), suggesting the dominant influence of biomass burning emissions in the dry season. Chemical analysis showed biomass burning contributed less hygroscopic compounds that suppressed water uptake, while wet season aerosols contained more hygroscopic marine‐influenced components. The hygroscopic behaviors of water‐soluble inorganic and organic matter (WSIM and WSOM) were also revealed using thermodynamic modeling. The hygroscopic growth factors of WSIM and WSOM were both lower in the dry season than in the wet season, which further indicated that BB‐derived aerosols suppressed hygroscopic growth in the dry season, while wet‐season aerosols, influenced by marine and oxidized components, enhanced water uptake. Multiple linear regression analysis further quantitatively confirmed the seasonally distinct influences of biomass‐burning and marine air masses on aerosol hygroscopicity. These findings provide new insights into the seasonal dynamics of PM 2.5 hygroscopicity and chemical composition in a tropical rainforest region and highlight the influence of emission sources and air mass origin on aerosol–water interactions.