Rapid reappearance of air pollution after cold air outbreaks in North China.

The cold air outbreak (CAO) is the most important way to reduce air pollution during the winter over North 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 airmass after CAO. Based on the analyses in recent winters (2014-2019), we found that the rapid reappearance of air pollution in the CAO decay phase has an occurrence frequency of 74%, and the air quality in more than 50% of CAOs worsens than that before CAO. The reappearance of air pollution tends to occur in the residual cold airmass with weak horizontal flux during the first two days after CAO. By categorizing the CAOs into groups of rapid and slow air pollution reappearance, we found that the residual cold airmass with moderate depth of 150-180 hPa, large negative heat content and small slopes of isentropes is favorable for the rapid reappearance of air pollution. Among these factors, the cold airmass depth is highly consistent with the mixing layer height, below which contains most air pollutants; the negative heat content and slope of isentropes in cold airmass jointly determine the intensity of low-level vertical stability. The rapid reappearance of air pollution is also attributed to the maintenance of residual cold airmass and the above conditions, which is 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. [ABSTRACT FROM AUTHOR]