Characterizations of ozone, NOx, and VOCs measured in Shanghai, China

Shanghai Meteorological Bureau (SMB) has established a VOC laboratory and an observational station network for VOC, NOx and O3 measurements in the Shanghai region. The measurements became operational since 2005. In this study, we investigate spatial and temporal variability of O3 and its precursors (NOx and VOCs), and impacts of the precursors on O3 formation. A chemical mechanism model (NCAR-MM) is applied to assess the sensitivity of the O3 formation to NOx and VOC concentrations. The results show that O3 concentrations are higher in rural area than in center of the city, suggesting that there are O3 depression processes in center of city and air pollutant emissions are not favorable for the O3 chemical formation. The analysis suggests that the NO and NO2 concentrations in center of the city are lower during weekend than weekday due to less traffic and industrial activities. By contrast, the O3 concentrations are higher during weekend than weekday. During weekend, the reduction of NO2 leads to enhancement in OH radicals, resulting in the higher O3 production. This enhancement of weekend O3 is off-set by decrease in VOC concentrations. The chemical model study suggests that the decrease of NOx alone could result in a larger weekend O3 enhancement than the measured result. The model sensitivity study also suggests that O3 concentration decreases when NOx concentration is enhanced. Both model and measurement results indicate that the O3 formation is clearly under VOC-sensitive regime in the city of Shanghai. As a result, in the urban area, the O3 production is strongly depressed by high NOx concentrations, resulting in lower O3 concentration in center of the city than in rural area of the city. This study further shows that the highly reactive VOCs (aromatics) play important roles in determining the O3 formation. The ratio of toluene/benzene suggests that automobiles play important roles in the O3 formation in the city of Shanghai. Further increase in automobiles in Shanghai could lead to potential high O3 concentration in the future.