Impacts of atmospheric circulation patterns and cloud inhibition on aerosol radiative effect and boundary layer structure during winter air pollution in Sichuan Basin, China

Persistent winter aerosol pollution frequently occurs in the Sichuan Basin (SCB) due to its unfavourable weather conditions, such as low wind, wetness, and cloudiness. Based on long-term observational data analyses from 2015-2021, it has been found that the four representative stations in the SCB often simultaneously experience PM2.5 pollution accompanied by variations in meteorological conditions above 850 hPa, which indicates a connection between regional winter air pollution in the SCB and large-scale synoptic patterns. The dominant 850 hPa synoptic patterns of winter in the SCB were classified into six patterns using T-model principal component analysis: (1) strong high pressure in the north, (2) east high-west low (EHWL) pressure, (3) weak high pressure in the north, (4) a weak ridge of high pressure after the trough, (5) a low trough (LT), and (6) strong high pressure. Pattern 2 characterized by the EHWL pressure system, and Pattern 5, featured with LT, was identified as having key synoptic patterns for the beginning and accumulation of pollution processes. Pattern 1, characterized by a strong high pressure in the north, was the cleanest pattern associated with reduced PM2.5 concentrations. The EHWL and LT patterns were associated with a remarkably high cloud liquid content attributed to upper southerly winds introducing humid air. Clouds reduce solar radiation through reflection and scattering, resulting in more stable stratification and aerosol accumulation. This cloud radiation interaction (CRI) was more pronounced in the LT pattern due to denser isobaric lines and stronger southerly winds than in the EHWL pattern. Numerical simulation experiments utilizing WRF-Chem indicated that there is an upper-level heating during afternoon and surface cooling in the morning forced by the aerosol radiation interaction (ARI) under the EHWL and LT patterns. Additionally, strong surface cooling in the evening influenced by valley winds could be found. With wet and cloudy synoptic forcing, ARI directly affects the stability of the boundary layer and is modulated through CRI inhibition. For example, Chongqing exhibited lower PM2.5 concentrations and stronger ARI compared to the western and southern SCB due to lower cloud liquid content and weaker CRI inhibition on the ARI. The CRI inhibition caused a 50 % reduction in solar radiation and boundary layer height during the daytime under the LT pattern, which was larger than that under the EHWL pattern. This study comprehensively analysed the spatial disparities in cloud inhibition on the ARIs, their impacts on the boundary layer structure, and the discrepancies of these interactions under different synoptic patterns during pollution processes. The findings have important implications for the effective management of pollution processes in cloudy and foggy weather.