Climate Impacts of the Biomass Burning in Indochina on Atmospheric Conditions over Southern China

Substantial biomass burning (BB) activities in Indochina during March and April of each year generate aerosols that are transported via westerly winds to southern China. These BB aerosols have both radiative (direct and semi-direct) and indirect effects on the climate. This study evaluates impacts of BB in Indochina during April 2013 on atmospheric conditions in southern China using WRF-Chem sensitivity simulations. We show that the atmosphere becomes drier and hotter under the aerosol radiative effect in southern China, while the changes linked to the indirect effect are opposite. The former (the latter) rises (reduces) surface temperature 0.13°C (0.19°C) and decrease (increase) water vapor mixing ratios 0.23 g kg^(-1) (0.40 g kg^(-1)) at 700 hPa. Atmospheric responses to aerosols in turn affect aerosol dissipation. Specifically, BB aerosols absorb solar radiation and heat the local atmosphere, which inhibits the formation of clouds (reducing low-level cloud about 7%) related to the aerosol semi-direct effect. Less cloud enhances surface solar radiation flux and temperature. Otherwise, northeasterly winds linked to radiative effect suppress water vapor transport. In this case, precipitation reduces 1.09 mm day^(-1), diminishing wet removal and westward transport of aerosols. Under the indirect effect, greater cloud coverage is formed, which reduces surface solar radiation flux and increases local latent heat release. This extra heating promotes air convection and diffusion of pollution. Regional mean precipitation increases 0.49 mm d^(-1), facilitating wet pollution removal. Under indirect effect, aerosol extinction coefficient reduces 0.011 km^(-1) at 2-km height over southern China. However, it increases around 0.002 km^(-1) at 3-km height over southernmost China related to radiative effect. Therefore, atmospheric changes linked to indirect effect play a greater role in removing pollutants from the atmosphere than radiative effect over southern China.