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 (O3) and PM2.5 (particulate matter with an aerodynamic equivalent diameter of 2.5 μm 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 O3 and PM2.5. Averaged over the YRD, the observed annual mean concentration of maximum daily 8 h average ozone (MDA8 O3) increased by 36.8 μg m−3 (49.5%) whereas that of PM2.5 decreased by 13.3 μg m−3 (22.1%) over 2014–2019. During warm months of April-October of 2013–2019, the observed regional mean daily concentrations of MDA8 O3 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 O3 and PM2.5 (MDA8 O3 > 160 μg m−3 and PM2.5 > 75 μg m−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 °C, −0.17 m s−1 for days with O3 pollution alone while −6.2%, 1.84 °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.