Long-term trend of O3 in a mega City (Shanghai), China: Characteristics, causes, and interactions with precursors.

In recent years, ozone (O3) 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 (NOx 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 O3 and the causes of the O3 trend in large cities in China. In order to understand the relationship between the O3 precursors and O3 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 NOx), which is suitable to study the effect of O3 precursors on O3 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 O3 and NOx concentrations at the urban site (XJH) from 2006 to 2015 (O3 increasing 67% and NOx decreasing 38%), while there were very small trends of O3 and NOx concentrations at the background site (DT). The analysis for causing the O3 trend suggests that (1) the large O3 increase at the urban area (XJH) was not due to the regional transport of O3; (2) the measurement of solar radiation had not significant trend during the period, and was not the major cause for the long-term O3 trend; (3) the measurement of VOCs had small change during the same period, suggesting that the trend in NOx concentrations at the urban site (XJH) was a major factor for causing the long-term change of O3 at the urban area of Shanghai. As a result, the O3 and NOx concentrations from 2006 to 2015 at the urban area of Shanghai were strongly anti-correlated, suggesting that the extremely high NOx concentration in the urban area depressed the O3 concentrations. It is interesting to note that the anti-correlation between O3 and NOx was in an un-linearly relationship. Under high O3 concentration condition, the ratio of Δ[O3]/Δ[NOx] was as large as -1.5. In contrast, under low O3 concentrations, the ratio of Δ[O3]/Δ[NOx] was only -0.2. This result suggested that when O3 concentration was high, it was more sensitive to NOx concentration, while when O3 concentration was low, it was less sensitive to NOx concentration. This study provides useful insights for better understanding the causes of the long-term-trend of regional O3 pollution nearby Shanghai, and has important implication for air pollution control in large cities in China. Due to the fact that NOx and VOCs are not only precursors for O3, but also are important precursors for particular matter (PM). If reduction of NOx leads to decrease in PM, but increase in O3, the NOx emission control become a very complicated issue and need to carefully design a comprehensive control method.