Long-Term Trends of Persistent Synoptic Circulation Events in Planetary Boundary Layer and Their Relationships With Haze Pollution in Winter Half Year Over Eastern China

Air quality in China has been gradually improving because of recent emission control policies, but synoptic circulations in the planetary boundary layer have become increasingly conducive to haze formation. The effect of persistent synoptic circulation (PSC) on long-term variation in haze episodes remains to be fully elucidated. This study identified and analyzed PSC similarity and its influence on haze during the winter half year from 1961 to 2013. We analyzed sea level pressure and geopotential heights at the 850-hPa level by using correlation coefficients and mean Hamming distance to quantify PSC similarity. A total of 1,754 PSC events were identified, of which 236 and 167 PSC events of 868 and 625 days, respectively, were associated with six polluted types and three clean types, respectively. The variations in occurrence frequencies of polluted PSC events exhibited different trends in the past decades over eastern China: an increase in 1961-1979, no obvious change in 1980-1999, and a rapid increase since 2000. The durations of polluted and clean PSC events in the planetary boundary layer over eastern China have tended to become longer and shorter, respectively, over the past decades. Our analyses suggest that the reduction in Arctic sea ice in autumn may be favorable for less cold activities and more polluted PSC events with longer durations in eastern China, positively contributing to the formation and maintenance of persistent haze pollution. This work provides convincing evidence that the occurrence frequencies and durations of haze episodes increased because of the increase in pollution-related PSC events.