Enhanced PM2.5 pollution in China due to aerosol-cloud interactions

Aerosol-cloud interactions (aerosol indirect effects) play an important role in regional 19 meteorological variations, which could further induce feedback on regional air quality. While the 20 impact of aerosol-cloud interactions on meteorology and climate has been extensively studied, 21 their feedback on air quality remains unclear. Using a fully coupled meteorology-chemistry 22 model, we find that increased aerosol loading due to anthropogenic activities in China 23 substantially increases column cloud droplet number concentration and liquid water path (LWP), 24 which further leads to a reduction in the downward shortwave radiation at surface, surface air 25 temperature and planetary boundary layer (PBL) height. The shallower PBL and accelerated 26 cloud chemistry due to larger LWP in turn enhance the concentrations of particulate matter with diameter less than 2.5 m (PM2.5) by up to 33.2 g m−3 (25.1%) and 11.0 g m−3 27 (12.5%) in 28 January and July, respectively. Such a positive feedback amplifies the changes in PM2.5 29 concentrations, indicating an additional air quality benefit under effective pollution control 30 policies but a penalty for a region with a deterioration in PM2.5 pollution. Additionally, we show 31 that the cloud processing of aerosols, including wet scavenging and cloud chemistry, could also 32 have substantial effects on PM2.5 concentrations.