Influence of extratropical cyclones on air quality in Beijing

Extratropical cyclones (ETCs) are one of the primary weather systems affecting China. However, atmospheric environmental effects generated under the control of ETCs are rarely reported. In this study, we investigate air quality in Beijing from the perspective of ETCs. The ETCs affecting Beijing from 2014 to 2021 are tracked using the 850 hPa relative vorticity cyclone tracking algorithm. We classify the identified ETCs by cyclogenesis region, which includes 228 northern cyclones (NCs) and 41 southern cyclones (SCs). Generally, NCs are more likely to cause air pollution. For 27.4% and 16.3% of the days under the control of NCs, PM2.5 and PM10 pollution occur, respectively. These are mainly associated with high northwest wind and enhanced sinking motion. SCs are conducive to good or moderate air quality. The PM2.5/PM10 pollution days account for 20.0%/5.0% of total days under the control of SCs, mainly due to enhanced rising motion and increased precipitation. Nevertheless, the influence of NCs/SCs on air quality shows significant seasonal differences. NCs negatively impact air quality in most seasons, except in summer. SCs tend to reduce the probability of air pollution in summer and autumn. Higher chances of air pollution associated with SCs occur in spring and winter. For a typical NC case that causes a dust storm, dust backflow occurs after the NC moves away, resulting in secondary pollution. A typical SC case generates different atmospheric environmental effects owing to the changes in its location. These results highlight the importance of ETCs for air quality in Beijing.