Co-occurrence of ozone and PM2.5 pollution in the Yangtze River Delta over 2013-2019: Spatiotemporal distribution and meteorological conditions

We examined the spatial-temporal variations of surface-layer ozone (O-3) and PM2.5 (particulate matter with an aerodynamic equivalent diameter of 2.5 mu m or less) observed from April 2013 to December 2019 in the Yangtze River Delta (YRD) region to identify the O3-PM2.5 relationship and to focus on the co-polluted days by O-3 and PM2.5. Averaged over the YRD, the observed annual mean concentration of maximum daily 8 h average ozone (MDA8 O-3) increased by 36.8 mu gm(3) (49.5%) whereas that of PM2.5 decreased by 13.3 mu gm(3) (22.1%) over 2014-2019. During warm months of April-October of 2013-2019, the observed regional mean daily concentrations of MDA8 O-3 and PM2.5 had a small positive correlation of 0.23, and this correlation coefficient became 0.44 when the long term trends were removed from the concentrations. The days with co-pollution of MDA8 O-3 and PM2.5 (MDA8 O-3 > 160 mu gm(3) and PM2.5 > 75 mu gm(3)) were observed frequently, which reached 54 days in Shanghai and 71 days in Jiangsu province during 2013-2019. Such co-polluted days in the YRD were found to occur mainly in the months of April, May, June, and October. The occurrence of co-pollution in the YRD is found to be mainly dependent on relative humidity, surface air temperature, and wind speed. The mean anomalous values of these three variables were, respectively,-7.3%, 0.46 degrees C,-0.17 m s(-1) for days with O-3 pollution alone while-6.2%, 1.84 degrees C, and-0.40 m s(-1) for days with co-pollution. Four typical weather patterns were identified to be associated with the co-polluted days. Our results provide better understanding of the complex air pollution and have implications for the control of such co-polluted events.