Seasonal variations in brown carbon properties in tropical forest region of southwest China: impacts from biomass burning and secondary formation

Brown carbon (BrC) is an important type of organic aerosol that can significantly affect the climate and air quality because of its important role in radiation balance and visibility impairment. Despite its importance, the optical properties and sources of BrC remain poorly characterized, especially in tropical rainforest regions. Here, we collected yearlong PM_2.5 samples from a typical tropical forest site in southwest China. We combined optical measurements and chemical analyses to investigate the optical properties and sources of water-soluble BrC. We show that the average absorbance coefficient of BrC (Abs₃₆₅) was higher in the dry season than wet season, coinciding with increased water-soluble organic carbon concentrations during the dry season. Correlation analyses revealed a strong association between Abs₃₆₅ and biomass-burning (BB) organic markers (R² > 0.6), along with a moderate correlation with secondary organic aerosol (SOA) markers (R² > 0.3) in the dry season, indicating that enhanced biomass-burning activities substantially increased BrC levels. However, such a clear association was not observed in the wet season. These results suggest that biomass burning was the major source of BrC in the dry season, followed by secondary formation, while no single dominant source of BrC was apparent in the wet season. The estimation of the contribution of solar absorption by BrC relative to elemental carbon shows that BrC contributed over 29 % of light absorption in the near-UV range in both seasons, indicating an important role of BrC in solar absorption. Overall, our work gains insights into the optical properties and possible sources of BrC in tropical forest regions.