PM2.5 and O3 pollution during 2015-2019 over 367 Chinese cities: Spatiotemporal variations, meteorological and topographical impacts.

The strict Clean Air Action Plan has been in place by central and local government in China since 2013 to alleviate haze pollution. In response to implementation of the Plan, daytime PM2.5 (particulate matter with aerodynamic diameter less than 2.5 μm) showed significant downward trends from 2015 to 2019, with the largest reduction during spring and winter in the North China Plain. Unlike PM2.5, O3 (ozone) showed a general increasing trend, reaching 29.7 μg m-3 on summer afternoons. Increased O3 and reduced PM2.5 simultaneously occurred in more than half of Chinese cities, increasing to approximately three-fourths in summer. Declining trends in both PM2.5 and O3 occurred in only a few cities, varying from 19.1% of cities in summer to 33.7% in fall. Meteorological variables helped to decrease PM2.5 and O3 in some cities and increase PM2.5 and O3 in others, which is closely related to terrain. High wind speed and 24 h changing pressure favored PM2.5 dispersion and dilution, especially in winter in southern China. However, O3 was mainly affected by 24 h maximum temperature over most cities. Soil temperature was found to be a key factor modulating air pollution. Its impact on PM2.5 concentrations depended largely on soil depth and seasons; spring and fall soil temperature at 80 cm below the surface had largely negative impacts. Compared with PM2.5, O3 was more significantly affected by soil temperature, with the largest impact at 20 cm below the surface and with less seasonal variation.