Investigating Southeast Asian biomass burning by the WRF-CMAQ two-way coupled model: Emission and direct aerosol radiative effects.

Southeast Asia (SEA) is one of the world's major sources of biomass burning (BB). In this study, the recently released Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) two-way coupled model was used with the Global Fire Emissions Database Version 4, to investigate the effect of BB emissions on the meteorology and air quality over SEA. The results show that the model performance was improved by considering BB emissions. When BB pollutants reach the free troposphere, they can be transported by westerly and southwesterly winds to downstream regions. The contributions of BB were up to 44.3 and 34.5 ppbv to surface O concentrations and 304.9 and 128.1 μg/m to surface PM concentrations in March and April, respectively. The BB emission caused aerosol direct radiative effect (DRE) on the monthly mean clear-sky downward shortwave flux decreased by -39.8 and -14.7 W/m in March and April, respectively. The surface temperature decreased over the land (by a maximum of -0.27 °C in March) and increased over the sea, while the temperature at higher levels tended to increase (to a maximum of 0.14 °C in March). The BB aerosol DRE caused the planetary boundary layer height (PBLH) to decrease by a maximum of -133.0 m in March. Surface O concentrations decreased generally owing to variations in the shortwave flux and temperature. Moreover, the decreased PBLH worsened the diffusion condition within the PBL but also caused larger amounts of BB emissions penetrating into the free atmosphere. This led to a decrease in surface PM concentrations in southern Indochina and Hainan Island and an increase in the rest of the regions. These findings highlight the key effects of BB emissions on local and downwind meteorology and air quality over SEA and demonstrate the practical applications of the WRF-CMAQ coupled model.