Impact of Aerosol Direct Effect on Wintertime PM2.5 Simulated by an Online Coupled Meteorology-Air Quality Model over East Asia

This study aims to evaluate the impacts of the aerosol direct effect on simulated concentrations of fine particulate matter (PM 2.5 ) over East Asia, which is controlled by heavy local air pollution and long-range transport. The online coupled Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) modeling system was applied to one-way and two-way simulations (without and with the aerosol direct effect) of a period from January till March of 2014. The differences between the two simulations showed that there were particularly large impacts of the aerosol direct effect on the eastern Asian continent with high aerosol loading. The temporal mean contributions of the direct effect averaged over the regions from Northeast to Central China were a 15% decrease in the surface shortwave radiation, a 9.0% decrease in the planetary boundary layer (PBL) height, and an 8.6% increase in the ground-level PM 2.5 concentration. In addition, there were negative contributions of the direct effect to the PM 2.5 concentration over the ocean from the Sea of Japan to the East China Sea (a 1.0% decrease on average throughout the period). The PM 2.5 decrease over the ocean was likely attributable to a reduction in the secondary PM 2.5 outflow from the continent to the downwind region, which was caused by the increased dry deposition of PM 2.5 precursors from the increased ground-level concentrations within a more stable PBL over the continent. Overall, the substantial decrease in the surface shortwave radiation due to the aerosol direct effect led to enhanced atmospheric stability and therefore increased the ground-level PM 2.5 in the heavily polluted region.