The temporal and spatial distribution of the correlation between PM2.5 and O-3 contractions in the urban atmosphere of China

The mass concentrations of fine particles (PM2.5) have decreased significantly in China in recent years, while surface ozone pollution shows an opposite trend. To better understand the combined PM2.5 and O-3 pollution in China's urban atmosphere, the Spearman correlation between PM2.5 mass concentration and daily maximum 8-hour average ozone concentration (MDA8 O-3) was analyzed in this study, as well as the correlation between the concentrations of PM2.5 and O-x (O-x=O-3 +NO2). Since 2015, the number of days when both O-3 and PM2.5 concentrations exceed the national ambient air quality standards has decreased significantly with the decrease in PM2.5 concentrations. The pollution combined O-3 and PM2.5 usually occurs in April and May in the Beijing-Tianjin-Hebei (BTH) area. It is worth noting that the correlations between PM2.5 and MDA8 O-3 concentrations depend on regions and seasons on south of 40 degrees N in China. A stronger positive correlation between the concentrations of PM2.5 and MDA8 O-3 in the Pearl River Delta (PRD) area was obtained throughout the year (R>0.6). In the BTH area, this type of relationship occurs only in summer (R similar to 0.5) whereas in winter, a weak negative correlation between PM2.5 and MDA8 O-3 concentrations was observed (R<-0.2). This may be due to a higher primary contribution of PM2.5 and a low concentration of O-3 due to low photochemical production compared to summer in wintertime. For the case of O-3 concentration exceeds national ambient air quality standards in Beijing and Xuzhou, MDA8 O-3 and PM2.5 concentrations correlate positively when PM2.5 = 50 mu g/m(3). In contrast, when PM2.5 > 50 mu g/m(3), a weak negative correlation was observed (R similar to-0.1), suggesting that high concentrations of particulate matter may inhibit O-3 formation on days when is polluted. However, the mechanism of such phenomenon remains confusing due to the complex relationship between the production of O-3 and PM2.5. Meanwhile, PM2.5 and MDA8 O-3 concentrations show a stronger positive correlation during daytime when O-3 concentration exceeds the national ambient air quality standards, which is due to the strong photochemical formation of O-3 as well as secondary aerosols. Furthermore, the positive correlation coefficients between Ox and PM2.5 are significantly higher than those between PM2.5 and MDA8 O-3 in most cities. This result is consistent with the predominant contribution of secondary aerosol to PM2.5 mass concentrations, both in wintertime and in summertime, after the stringent control of primary source emissions. In addition, the chemical concentration of water-soluble inorganic composition in PM2.5 samples collected in urban Beijing was analyzed in this work using ion chromatography in 2020. Comparing the correlation coefficients between PM2.5 and MDA8 O-3 concentrations, there are stronger positive correlations in the concentrations between MDA8 O-3 and the ratio between PM2.5 mass concentration and secondary inorganic aerosols (SNA= sulfate + nitrate + ammonium). This result may be due to the fact that a high concentration of O-3 can promote the formation of secondary aerosols.