Subseasonal characteristics and meteorological causes of surface O3 in different East Asian summer monsoon periods over the North China Plain during 2014-2019

In recent years, surface ozone (O-3) has increased rapidly over the North China Plain (NCP), which is harmful to human health and crop yields. Summertime O-3 over the NCP is remarkably influenced by the East Asian summer monsoon (EASM), while the distinctions of their relations in different monsoon periods are still vague. In this study, we divided the whole monsoon season into the pre-flood period, the Meiyu flood period, and the post-flood period. For each monsoon period, we examined the characteristics and variations of O-3 from 2014 to 2019 and the associated atmospheric circulations individually. Based on a multiple linear regression model, we further quantified the meteorological contributions to the year-by-year variation and increasing trend in O-3 during different monsoon periods. We found that the maximum daily 8 h-average O-3 concentration (O-3_MDA8) exhibits subseasonal variations on the mean level (72.5, 76.7, and 63.8 ppb) and trends (3.2, 4.0, and 1.7 ppb yr(-1)) during the three monsoon periods. Meteorological variations account for 58.9% and 58.6% of the O-3 uptrends in the Meiyu flood and the pre-flood periods but exert adverse effects on the O-3 increase in the postflood period (-27.4%). The positive meteorology-driven trend in the pre-flood period is attributed to increased radiation and temperature, and that in the Meiyu flood period is contributed by the rise in radiation and decrease in relative humidity. While in the post-flood period, the decreasing trend mainly results from the increased precipitation. The subseasonal discrepancy is related to the distinct dominance of wave activities in the mid-and-high latitudes and the western Pacific subtropical high (WPSH) in different monsoon periods. The anomalously high pressure conduces to the high-level O-3 over the NCP in the pre-flood period. Frequent O-3 pollution in the Meiyu flood period is caused by the strong wave amplitudes in the mid-and-high latitudes and a westward stretched and intensified WPSH. While in the post-flood period, the east-west shift of WPSH modulates the precipitation and O-3 variations over the NCP region. Among the three periods, we highlighted the Meiyu flood period for the highest O-3 level and the most drastic increasing trend, during which more strict emission control should be implemented.