Measurement and modeling of O3 variability in Shanghai, China: Application of the WRF-Chem model

Since 2005, Shanghai Meteorological Bureau (SMB) has established an observational network for measuring VOC, NOx, O3 and aerosols in the Shanghai region. In this study, a rapid O3 changes from Aug/02/2007 to Aug/11/2007 was observed in the region. During this 10 day period, the noontime O3 maximum decreased from 100 to 130 ppbv to about 20–30 ppbv. In order to analyze the processes in controlling this rapid change of O3 during this short period, a newly developed regional chemical/dynamical model (WRF-Chem) is applied to study O3 variability in the Shanghai region. The model performances are evaluated by comparing the model calculation to the measurement. The result shows that the calculated magnitudes and diurnal variations of O3 are close to the measured results in city sites, but are underestimated at a rural petroleum industrial site, suggesting that the emissions from petroleum factories around this rural site are significantly underestimated and need to be improved. The calculated rapid changes of O3 concentrations, O3 precursors, and aerosols are consistent with the measured results, suggesting that the model is suitable to study the causes of this rapid O3 change. The model analysis indicates that weather conditions play important roles in controlling the surface O3 in the Shanghai region. During summer, there is a persistent sub-tropical high pressure system (SUBH) in southeast of Shanghai over Pacific Ocean. During the earlier time of the period (Aug/02–Aug/05), the SUBH system was weak, resulting in weak surface winds. With the calm winds, a noticeable noontime sea-breeze produced an inflow from ocean to land, generating a cycling pattern of wind directions. As a result, the high O3 concentrations were trapped in the Shanghai region, with a maximum concentration of 100–130 ppbv. By contrast, during the later time of the period (Aug/06–Aug/11), the SUBH was enhanced, resulting in strong surface winds. The high O3 concentrations formed in the city were rapidly transported to the downwind region of the city, resulting in low O3 concentrations in the Shanghai region. This study illustrates that the WRF-Chem model is a useful tool for studying the high variability of O3 concentrations in Shanghai, which has important implication for the prediction of high O3 concentration events in the city.