Up to now, a nation-wide scale study of surface ozone (O3) concentrations in China was limited due to scarce observation. Thanks to the establishment of national air quality monitoring network in 2013, surface O3 data from 1402 stations during 2014-2017 were collected to investigate O3 seasonality. Our analysis reveals that the variations of monthly O3 averaged from daily mean concentration during a year show different temporal profiles depending on latitude. A unimodal structure (UMS) is generally found for latitudes over 35°N, whereas a bimodal structure (BMS) is in most of the cases identified south of 35°N. The peak of UMS is found in the period of May to July, whereas the first and second peaks of BMS are found from April to June, and from July to October, respectively. In addition, the seasonality of O3 presents a strong dependence on pseudo-equivalent potential temperature and monsoonal clouds. The onset and retreat of warm and wet air are correlated to the summer minimum in BMS cases and to the sharp decrease of UMS in July. As far as the relationships between O3 and carbon monoxide are concerned, the effects of clean maritime air masses on the summer trough of O3 are not significant for inland sites. Overall, summer monsoon bringing warm and moist air and subsequent clouds leads to the suppression of photochemical production, thereby contributing directly to the geographical distribution of O3 seasonality.
Keywords: Carbon monoxide; Clouds; Ozone; Pseudo-equivalent potential temperature; Summer trough.
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