Distribution and Formation Causes of PM2.5 and O3 Double High Pollution Events in China during 2013-20

Fine particulate matter (PM2.5) and ozone (O-3) double high pollution (DHP) events have occurred frequently over China in recent years, but their causes are not completely clear. In this study, the spatiotemporal distribution of DHP events in China during 2013-20 is analyzed. The synoptic types affecting DHP events are identified with the Lamb-Jenkinson circulation classification method. The meteorological and chemical causes of DHP events controlled by the main synoptic types are further investigated. Results show that DHP events (1655 in total for China during 2013-20) mainly occur over the North China Plain, Yangtze River Delta, Pearl River Delta, Sichuan Basin, and Central China. The occurrence frequency increases by 5.1% during 2013-15, and then decreases by 56.1% during 2015-20. The main circulation types of DHP events are "cyclone" and "anticyclone", accounting for over 40% of all DHP events over five main polluted regions in China, followed by southerly or easterly flat airflow types, like "southeast", "southwest", and "east". Compared with non-DHP events, DHP events are characterized by static or weak wind, high temperature (20.9 degrees C versus 23.1 degrees C) and low humidity (70.0% versus 64.9%). The diurnal cycles of meteorological conditions cause PM2.5 (0300-1200 LST, Local Standard Time= UTC+ 8 hours) and O-3 (1500-2100 LST) to exceed the national standards at different periods of the DHP day. Three pollutant conversion indices further indicate the rapid secondary conversions during DHP events, and thus the concentrations of NO2, SO2 and volatile organic compounds decrease by 13.1%, 4.7% and 4.4%, respectively. The results of this study can be informative for future decisions on the management of DHP events.