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

Abstract:

Chiang Mai, the largest city in Northern Thailand, suffers from severe air pollution associated with strong biomass burning (BB) during dry season (February to April) in recent years. As an important source of black carbon (BC) and light-absorbing organic carbon (brown carbon, BrC), the heavy BB has significant impacts on local and regional climate. However, studies on characterizing the impact of BB-associated light-absorbing aerosols on climate in Chiang Mai are limited. In this study, we used the global chemical transport (CTM) model GEOS-Chem coupled with the rapid radiative transfer model for GCMs (RRTMG) to estimate the radiative forcing (RF) of BC and BrC in Chiang Mai, respectively. BrC was treated as an individual tracer isolating from organic carbon (OC) in the model. The emissions of primary BrC were estimated using a relationship between modified combustion efficiency (MCE) and BrC-to-BC absorption ratio, as described in Jo et al. (2016). Considering that the observed absorptive ability of BrC is highly variable, we conducted three sensitivity runs by assigning different values of imaginary part (k_BrC) of BrC refractive index (RI) to each run. The simulated aerosol mass concentrations and BrC absorption were evaluated by multiple in-situ measurements. We also present the estimates of RFs of BB-associated BC and BrC in Chiang Mai, and analyze the influence of different k_BrC on the simulated absorption and RF of BrC.