Large-scale synoptic drivers of co-occurring summertime ozone and PM2.5 pollution in eastern China

Surface ozone (O-3) pollution during summer (June-August) over eastern China has become more severe in recent years, resulting in a co-occurrence of surface O-3 and PM2.5 (particulate matter with aerodynamic diameters <= 2.5 mu m in the air) pollution. However, the mechanisms regarding how the synoptic weather pattern (SWP) might influence this compound pollution remain unclear. In this study, we applied the T-mode principal component analysis (T-PCA) method to objectively classify the occurrence of four SWPs over eastern China, based on the geopotential heights at 500 hPa during summer (2015-2018). These four SWPs over eastern China were closely related to the western Pacific subtropical high (WPSH), exhibiting significant intraseasonal and interannual variations. Based on ground-level air quality observations, remarkable spatial and temporal disparities of surface O-3 and PM2.5 pollution were also found under the four SWPs. In particular, there were two SWPs that were sensitive to compound pollution (Type 1 and Type 2). Type 1 was characterized by a stable WPSH ridge with its axis at about 22 degrees N and the rain belt located south of the Yangtze River Delta (YRD); Type 2 also exhibited WPSH dominance (ridge axis at similar to 25 degrees N) but with the rain belt (over the YRD) at a higher latitude compared to Type 1. In general, SWPs have played an important role as driving factors of surface O-3-PM2.5 compound pollution in a regional context. Our findings demonstrate the important role played by SWPs in driving regional surface O-3-PM2.5 compound pollution, in addition to the large quantities of emissions, and may also provide insights into the regional co-occurring high levels of both PM2.5 and O-3 via the effects of certain meteorological factors.

Automated summary

Surface ozone (O-3) pollution during summer over eastern China has worsened in recent years, with a co-occurrence of surface O-3 and PM2.5 pollution under four different synoptic weather patterns (SWPs), two of which (Type 1 and Type 2) are particularly sensitive to compound pollution.