Direct radiative effects of black carbon and brown carbon from Southeast Asia biomass burning with the WRF-CMAQ two-way coupled model.

Black carbon (BC) and brown carbon (BrC) are significant light-absorbing components of particulate matter that impact weather and climate. Biomass burning (BB) and biofuel (BF) emissions in Southeast Asia are key global sources of BC and BrC. This study utilizes the Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model, integrating a BrC module for the first time, alongside the Global Fire Emissions Database Version 4, to assess the direct radiative effect (DRE) of BC and BrC in March 2015 over Southeast Asia. The novel BrC module parameterizes light absorption based on the BB BC to organic aerosol (OA) ratio in each model grid and time step, aligning better with smog chamber experiments than default coefficients. Results show that BC DRE affects clear sky net radiation at the top of the atmosphere, reaching 14.6 W/m in Indochina and 5.1 W/m in southern China, while BrC DRE reaches 1.9 W/m. Additionally, BC and BrC DRE induce a cooling effect at the Earth's surface, with the maximum reduction of the clear sky downward shortwave radiation by -36.9 W/m and -5.2 W/m, respectively. BC and BrC lead to a tiny decrease (<0.1 °C) in surface temperature and an increase in upper air temperature. Surface O concentration tends to slightly decrease (<1.0 ppbv) due to the DRE of BC and BrC. In comparison to studies using the default coefficient, this result contributes to a more nuanced understanding of the interactions between BC, BrC, and radiation.