Model analysis of meteorology and emission impacts on springtime surface ozone in Shandong.

Air quality observations showed that surface ozone (O) concentrations over Shandong increased significantly in springtime in recent years, especially 2017. The observed 90th percentile of hourly O concentrations (O-h_90) in May increased from 148.4 μg/m in 2016 to 176.2 μg/m in 2017. To investigate the reasons of significant increase of O in spring of 2017, seven sensitivity cases were performed with the RAMS-CMAQ modeling system to identify the impacts of meteorological conditions (M) and emissions (E) on O concentrations in May of Shandong during the time period 2016-2018. The regional O-h_90 in May of Shandong were 103.0, 120.3 and 86.3 μg/m in 2016, 2017 and 2018, respectively. It was found that the positive effects from favorable meteorology were the dominant reasons that resulted in the high O concentration in May 2017. When compared to 2017 standard simulation (17E17M), the differences of meteorological conditions led to the decrease of 17.5 and 33.8 μg/m in regional O-h_90 of May in 2016 (17E16M) and 2018(17E18M), while small changes (0.6 and - 0.3 μg/m) appeared in that of May 2016 (16E17M) and 2018(18E17M) due to emission differences. Since there were few differences in the wind speeds of May between three years, the higher temperature and lower relative humidity significantly contributed to O formation in May 2017 compared to May of 2016 and 2018. Besides, the amount of cloud fraction (CF), which has an indirect influence on the surface temperature and photochemical production of ozone by its impacts on the insolation, was the least in May 2017 compared to that in May 2016 and 2018 over Shandong. The distributions of changes in CF had obvious negative correlations with that of O vertical column concentrations and temperature. Thus, the fewer cloud fraction may play a key role in O formation of May 2017.