Multilayer inversion formation and evolution during persistent heavy air pollution events in the Sichuan Basin, China

Abstract

The Sichuan Basin (SB) is one of the most air-polluted areas in China. The deep mountain-basin orography and the unique geographic location on the leeward slope of the Tibetan Plateau cause multilayer inversions to occur frequently in winter, which greatly increases the possibility of local heavy pollution. However, the genesis and development mechanisms of the multilayer inversions during the heavy pollution event are unclear. In this study, radiosonde measurements, air quality data, and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis data were employed to investigate the formation and evolution of the multilayer inversions during 10 persistent heavy pollution events from 2015 to 2019. The results revealed that >90% of the persistent heavy pollution events in the SB included multilayer inversions. The inversions occurred near the ground (i.e., boundary layer inversion) and at heights of approximately 2000–3800 m above the ground (i.e., lower troposphere inversion, LTI). The large-scale pressure patterns in the middle of the troposphere forced the formation and evolution of the multilayer inversions during the persistent heavy pollution events. Strong mid-level warm air advection induced by a large-scale warm high-pressure ridge was the dominant factor in the formation and maintenance of the multilayer inversion. Furthermore, strong cold advection associated with an approaching large-scale cold low-pressure trough played a prominent role in the weakening and disappearance of the multilayer inversion, leading to the end of the heavy pollution event. The warming over the SB peaked at two heights successively: 650–600 hPa at the start of an event and 700 hPa in the middle of an event. Accordingly, the LTI was enhanced from the start to the middle of an event, and its height decreased. The downward extension of the LTI compressed the diffusion space of pollutants within the basin, leading to further deterioration of the air quality. The southwesterly warm advection at 750–700 hPa triggered by the orographic forcing of the Tibetan Plateau also modulated the generation and development of the LTI.