Contribution of brown carbon to the light absorption and radiative effect of carbonaceous aerosols from biomass burning emissions in Chiang Mai, Thailand

Chiang Mai suffers from adverse haze associated with heavy biomass burning (BB) during almost every dry season (February to April). As an important source of light-absorbing carbonaceous aerosols (black carbon and brown carbon), BB can have strong radiative effects on local and regional climate. However, studies on characterizing the impacts of BB aerosols on climate in Chiang Mai are quite limited. In this study, we use a global chemical transport model (GEOS-Chem) coupled with the rapid radiative transfer model for GCMs (RRTMG) to estimate the radiative forcing (RF) of BB aerosols in Chiang Mai. Brown carbon (BrC) is included as an absorber and treated as an individual tracer in the model. To our best knowledge, this is the first study to estimate the BrC RF in Chiang Mai. As evaluated, our simulations that were assigned with medium- and high-absorbing kBrC (BrC imaginary refractive index) well reproduces the absorption coefficient of ambient BrC in Chiang Mai. Based on our estimations, 33-40% of total carbonaceous aerosol absorption at 440 nm is attributed to BrC and 60-67% to BC during dry season. As estimated, BrC contributes 14 +/- 3% to the instantaneous RF of total carbonaceous aerosol (IRFCAs) at the top of atmosphere (TOA) and 16 +/- 3% to IRFCAs at surface. Moreover, including BrC in model strengthens (reduces) the surface (TOA) cooling effect of total organic carbon by 9 +/- 5% (9 +/- 3%), indicating the warming effect of BrC in the atmosphere in Chiang Mai.