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. The cities of Chengdu, Deyang, andMianyang in the northwest Sichuan Basin are part of a rapidly developingurban agglomeration adjoining the eastern slopes of the Tibetan Plateau.Heavy air pollution events have frequently occurred over these cities in recentdecades, but the effects of meteorological conditions on these pollutionevents are unclear. We explored the effects of weather systems on winterheavy air pollution from 1 January 2006 to 31 December 2012 and from1 January 2014 to 28 February 2017. A total of 10 heavy air pollution events occurredduring the research period and 8 of these took place while the region wasaffected by a dry low-pressure system at 700 hPa. When the urbanagglomeration was in front of the low-pressure system and the weatherconditions were controlled by a warm southerly air flow, a strongtemperature inversion appeared above the atmospheric boundary layer acting asa lid. Forced by this strong inversion layer, the local secondary circulationwas confined to the atmospheric boundary layer, and the horizontal windspeed in the lower troposphere was low. As a result, vertical mixing andhorizontal dispersion in the atmosphere were poor, favoring the formation ofheavy air pollution events. After the low-pressure system had transited overthe region, the weather conditions in the urban agglomeration were controlledby a dry and cold air flow from the northwest at 700 hPa. The stronginversion layer gradually dissipated, the secondary circulation enhanced anduplifted, and the horizontal wind speed in the lower troposphere alsoincreased, resulting in a sharp decrease in the concentration of airpollutants. The strong inversion layer above the atmospheric boundary layerinduced by the low-pressure system at 700 hPa thus played a key role in theformation of heavy air pollution during the winter months in this urbanagglomeration. This study provides scientific insights for forecasting heavyair pollution in this region of China.

Impact of low-pressure systems on winter heavy air pollution in the northwest Sichuan Basin, China