Influence of the synoptic systems and inversion characteristics on winter heavy air pollution in the lower atmosphere over Sichuan Basin, China

The Sichuan Basin is located in the eastern part of the Tibetan Plateau, and there is a rapidly developing urban agglomeration over there. Therefore, heavy air pollution events have frequently occurred over many cities in recent decades. In this study, We explored the effects of weather systems on winter heavy air pollution from more than 10 cases of heavy air pollution events occurred during 2006-2018. Most of them were affected by a dry low-pressure system at 700 hPa, at the same time , they were also influenced by the inversion with several layers in the lower atmosphere. It is interesting that the synergistic effects of synoptic patterns and inversions caused by topography on heavy air pollution events. When the urban agglomeration was in front of the low-pressure system and the weather conditions were controlled by a warm southerly air flow, a strong temperature inversion appeared above the atmospheric boundary layer acting as a lid. The local secondary circulation was forced by both of them and was confined within the atmospheric boundary layer. At that time, the horizontal wind speed in the lower atmosphere was low.  As a result, vertical mixing and horizontal dispersion in the atmosphere were poor, a heavy air pollution event was gradually caused. After the low-pressure system had passed over the urban agglomeration, the weather conditions over there were controlled by a dry and cold air flow from the northwest at 700 hPa, foregoing strong inversion layer gradually dissipated, the secondary circulation enhanced and uplifted, and the horizontal wind speed in the lower atmosphere also increased, resulting in a sharp decrease in the concentration of air pollutants. In sum, the strong inversion layers above the lower atmosphere were induced by the low-pressure systems at 700 hPa, both happened a synergistic effect and played a key role in the formation of heavy air pollution during the winter months in this urban agglomeration of The Sichuan Basin. Finally, it was presented that a new conceptual model is about meteorological causes of winter heavy air pollution  in the Sichuan Basin.