Rapid reappearance of air pollution after cold air outbreaks in northern and eastern China

The cold air outbreak (CAO) is the most important way to reduce air pollution during the winter over northern and eastern China. However, a rapid reappearance of air pollution is usually observed during its decay phase. Is there any relationship between the reappearance of air pollution and the properties of CAO? To address this issue, we investigated the possible connection between air pollution reappearance and CAO by quantifying the properties of the residual cold air mass after CAO. Based on the analyses of recent winters (2014-2022), we found that the rapid reappearance of air pollution in the CAO decay phase has an occurrence frequency of 63 %, and the air quality in more than 50 % of CAOs worsens compared to that before CAO. The reappearance of air pollution tends to occur in the residual cold air mass with a weak horizontal flux during the first 2 d after CAO. By categorizing the CAOs into groups of rapid and slow air pollution reappearance, we found that the residual cold air mass with a moderate depth of 150-180 hPa, a large negative heat content, and small slopes of isentropes is favorable for the rapid reappearance of air pollution. Among these factors, the cold air mass depth is highly consistent with the mixing layer height, below which most air pollutants are found; the negative heat content and slope of isentropes in the cold air mass jointly determine the intensity of low-level vertical stability. The rapid reappearance of air pollution is also attributed to the maintenance of the residual cold air mass and the above conditions, which are mainly regulated by the dynamic transport process rather than diabatic cooling or heating. Furthermore, analysis of the large-scale circulation of CAOs in their initial stage shows that the anticyclonic (cyclonic) pattern in northern Siberia (northeastern Asia) can be recognized as a precursor for the rapid (slow) reappearance of air pollution after the CAO.

Automated summary

The study finds that the cold air outbreak is effective in reducing air pollution during winter in northern and eastern China, but air pollution tends to rapidly reappear during the decay phase. The properties of the residual cold air mass, such as depth, heat content, and slope of isentropes, are related to the reappearance of air pollution. The large-scale circulation patterns in the initial stage of the cold air outbreak can also serve as a precursor for the rapid reappearance of air pollution.