Spatial distribution of wintertime air pollution in major cities over eastern China: Relationship with the evolution of trough, ridge and synoptic system over East Asia

Hourly air quality index (AQI) data from the Ministry of Environment Protection of China are used to investigate spatial distribution of air pollution in major cities over eastern China (ECN) in winter 2015. The wintertime averaging identifies a pollution center in east Shanxi (IV, the identifier number in Fig. 1), south Hebei (III), west Shandong (V) and Henan (VI) provinces with mean AQI > 100. Four typical types of air pollution spatial distribution are identified. The low level southerly winds are found to be associated with the occurrence of regional air pollution episodes over ECN. The spatial distribution of air pollution is mainly related to the low level meridional wind, which is determined by the specific evolution process of the trough, ridge and synoptic system. The longitudinal position, the deepening (or weakening) and the southward extension (or northward retreat) of the trough over East Asia are the crucial factors determining where the regional air pollution could occur. The influence of precipitation on air pollution is complicated; the initial stage of precipitation with limited precipitation amount promotes the secondary formation and hygroscopic growth of aerosol due to increased supply of moisture, thus favors an aggravation of air pollution; in contrast, only the persisting and intensifying of precipitation are accompanied with more effective wet scavenging and an improvement of air quality. Fireworks emission is found to exacerbate air pollution during the Spring Festival, resulting in a unique distribution of air pollution over a large area of China, suggesting that prohibiting fireworks is necessary under adverse synoptic situation.