Air Pollutant Correlations in China: Secondary Air Pollutant Responses to NOx and SO2 Control

摘要

PM_(2.5) concentrations have decreased remarkably in China in recent years, coinciding with a more rapid decrease in SO_(2) concentrations and a slower decrease in NO_(2) concentrations, while O_(3) concentrations increased. Correlations between PM_(2.5) and key gaseous pollutants were studied to identify linked trends as a means of understanding the impacts of air pollution control in China. In most cities, the PM_(2.5)–NO_(2) correlation coefficients were higher than the PM_(2.5)–SO_(2) correlation coefficients, and the gap tended to expand as air quality improved. Multiple linear regression also indicated that PM_(2.5) concentrations were more sensitive to changes in NO_(2) than in SO_(2). The rate of decrease in the PM_(2.5) concentration with a decreasing NO_(2) concentration is nearly 3 times higher than that with SO_(2). These results support the priority of controlling NO_(x ) to further reduce PM_(2.5) pollution in China. The chemistry behind this was twofold: (1) NO_(x ) can be converted into nitrate, and (2) NO_(x ) contributes to atmospheric oxidation capacity. The decrease in PM_(2.5) concentration always coincided with an increase in O_(3) concentration when the PM_(2.5) concentration was higher than 50 μg m~(–3). However, the correlation between PM_(2.5) and O_(3) tended to change from negative to positive as air quality improved, indicating O_(3) and PM_(2.5) control could both benefit from reducing the concentrations of gas precursors.