Contrasting roles of clouds as a sink and source of aerosols: A quantitative assessment using WRF-Chem over East Asia

Clouds play two contrasting roles in the fate of aerosols as a sink through wet scavenging and a source as a medium for aqueous-phase secondary aerosol formation. The contrasting contributions of clouds to near-surface particulate matter with a diameter less than 2.5 mu m (PM2.5) are quantitatively examined with a particular focus on boundary-layer aerosols and clouds using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem). Overall, the net contribution of wet scavenging to daily-mean PM2.5 is much larger (similar to 5 mu g m(-3) to -22 mu gm(- 3)) than that of cloud chemistry (similar to 0.9 mu g m(- 3)). The effects of wet scavenging are found over a large spatial extent even over no-rainy regions and last for a long time (similar to 2 days). The amount of aerosols scavenged by clouds and rainfall varies greatly, but it increases as the liquid water path (LWP) increases in a general sense. So, aerosols are mostly removed when clouds have large LWPs. For thin clouds with LWPs of 30-80 g m(- 2), the net reduction in PM2.5 due to wet scavenging is barely sensitive to LWP and the role of cloud chemistry becomes non-negligible. A relatively large increase in sulfate mass is found when cloud base height (CBH) is lower than similar to 1.2 km for thin clouds, and the occurrence fraction in which cloud chemistry plays a dominant role over wet scavenging increases up to similar to 30% as CBH becomes lower. These results highlight that fog and/or non-precipitating stratus clouds likely play a substantial role in the formation of aqueous-phase secondary aerosols. A case study reveals that the presence of fog can contribute to increasing sulfate formation at a maximum rate of 1.5 mu g m(- 3) h(-1).