The temporal and spatial distribution of the correlation between PM<sub>2.5</sub> and O<sub>3</sub> contractions in the urban atmosphere of China

Abstract

<p indent="0mm">The mass concentrations of fine particles (PM_2.5) have decreased significantly in China in recent years, while surface ozone pollution shows an opposite trend. To better understand the combined PM_2.5 and O₃ pollution in China’s urban atmosphere, the Spearman correlation between PM_2.5 mass concentration and daily maximum 8-hour average ozone concentration (MDA8 O₃) was analyzed in this study, as well as the correlation between the concentrations of PM_2.5 and O_{<italic>x</italic>} (O_{<italic>x</italic>}=O₃+NO₂). Since 2015, the number of days when both O₃ and PM_2.5 concentrations exceed the national ambient air quality standards has decreased significantly with the decrease in PM_2.5 concentrations. The pollution combined O₃ and PM_2.5 usually occurs in April and May in the Beijing-Tianjin-Hebei (BTH) area. It is worth noting that the correlations between PM_2.5 and MDA8 O₃ concentrations depend on regions and seasons on south of 40°N in China. A stronger positive correlation between the concentrations of PM_2.5 and MDA8 O₃ in the Pearl River Delta (PRD) area was obtained throughout the year (<italic>R</italic>&gt;0.6). In the BTH area, this type of relationship occurs only in summer (<italic>R</italic>~0.5) whereas in winter, a weak negative correlation between PM_2.5 and MDA8 O₃ concentrations was observed (<italic>R</italic>&lt;–0.2). This may be due to a higher primary contribution of PM_2.5 and a low concentration of O₃ due to low photochemical production compared to summer in wintertime. For the case of O₃ concentration exceeds national ambient air quality standards in Beijing and Xuzhou, MDA8 O₃ and PM_2.5 concentrations correlate positively when PM_2.5 ≤ <sc>50 μg/m³.</sc> In contrast, when PM_2.5&gt;<sc>50 μg/m³,</sc> a weak negative correlation was observed (<italic>R</italic>~–0.1), suggesting that high concentrations of particulate matter may inhibit O₃ formation on days when is polluted. However, the mechanism of such phenomenon remains confusing due to the complex relationship between the production of O₃ and PM_2.5. Meanwhile, PM_2.5 and MDA8 O₃ concentrations show a stronger positive correlation during daytime when O₃ concentration exceeds the national ambient air quality standards, which is due to the strong photochemical formation of O₃ as well as secondary aerosols. Furthermore, the positive correlation coefficients between O_{<italic>x</italic>} and PM_2.5 are significantly higher than those between PM_2.5 and MDA8 O₃ in most cities. This result is consistent with the predominant contribution of secondary aerosol to PM_2.5 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 PM_2.5 samples collected in urban Beijing was analyzed in this work using ion chromatography in 2020. Comparing the correlation coefficients between PM_2.5 and MDA8 O₃ concentrations, there are stronger positive correlations in the concentrations between MDA8 O₃ and the ratio between PM_2.5 mass concentration and secondary inorganic aerosols (SNA= sulfate + nitrate + ammonium). This result may be due to the fact that a high concentration of O₃ can promote the formation of secondary aerosols.

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