Long-term trend of O in a mega City (Shanghai), China: Characteristics, causes, and interactions with precursors.

In recent years, ozone (O) is often the major pollutant during summertime in China. In order to better understand this problem, a long-term measurement of ozone (from 2006 to 2015) and its precursors (NO and VOCs) as well as the photochemical parameter (UV radiation) in a mega city of China (Shanghai) is analyzed. The focus of this study is to investigate the trend of O and the causes of the O trend in large cities in China. In order to understand the relationship between the O precursors and O formation, two distinguished different sites of measurements are selected in the study, including an urbanization site (XJH-Xujiahui) and a remote site (DT-Dongtan). At the XJH site, there are high local emissions of ozone precursors (such as VOCs and NO), which is suitable to study the effect of O precursors on O formation. In contrast, at the DT site, where there are low local emissions, the measured result can be used to analyze the background conditions nearby the city of Shanghai. The analysis shows that there were long-term trends of O and NO concentrations at the urban site (XJH) from 2006 to 2015 (O increasing 67% and NO decreasing 38%), while there were very small trends of O and NO concentrations at the background site (DT). The analysis for causing the O trend suggests that (1) the large O increase at the urban area (XJH) was not due to the regional transport of O; (2) the measurement of solar radiation had not significant trend during the period, and was not the major cause for the long-term O trend; (3) the measurement of VOCs had small change during the same period, suggesting that the trend in NO concentrations at the urban site (XJH) was a major factor for causing the long-term change of O at the urban area of Shanghai. As a result, the O and NO concentrations from 2006 to 2015 at the urban area of Shanghai were strongly anti-correlated, suggesting that the extremely high NO concentration in the urban area depressed the O concentrations. It is interesting to note that the anti-correlation between O and NO was in an un-linearly relationship. Under high O concentration condition, the ratio of Δ[O]/Δ[NO] was as large as -1.5. In contrast, under low O concentrations, the ratio of Δ[O]/Δ[NO] was only -0.2. This result suggested that when O concentration was high, it was more sensitive to NO concentration, while when O concentration was low, it was less sensitive to NO concentration. This study provides useful insights for better understanding the causes of the long-term-trend of regional O pollution nearby Shanghai, and has important implication for air pollution control in large cities in China. Due to the fact that NO and VOCs are not only precursors for O, but also are important precursors for particular matter (PM). If reduction of NO leads to decrease in PM, but increase in O, the NO emission control become a very complicated issue and need to carefully design a comprehensive control method.