Air pollutant concentration trends in China: correlations between solar radiation, PM2.5, and O3.

Atmospheric pollution by fine particulate matter (PM_2.5) and ozone (O₃) is a serious concern in China for its danger to human health and to the environment. As such, it has become, since 2013, the target of governmental emission reduction policies. Subsequently, PM_2.5 concentrations in China have decreased rapidly, but surface O₃ concentration is still measurably increasing in most regions of China. Indeed, although emission reduction policies influence O₃ chemical production and loss processes by their impact on O₃ precursor concentrations, O₃ pollution is also affected by meteorological factors. In this study, we analyzed the spatial distribution and temporal variations of surface solar radiation and aerosol extinction to explain the recent increase in surface O₃ concentration. Our results confirmed a marked PM_2.5 concentration decrease between 2015 and 2019, especially in northern China, and a simultaneous O₃ concentration increase. Surface solar radiation showed geographically consistent increases, likely caused by the decreasing PM_2.5 concentrations and the resulting lower aerosol optical thickness. The surface solar radiation increasing enhanced photochemical O₃ production. We also investigated cloud cover distribution and trends. It demonstrated that the surface solar radiation intensity increase in northern China was caused by decreasing aerosol concentrations, not by cloud cover differences. Moreover, most emission reduction policies focus on sulfur and nitrogen oxides, less on other important O₃ precursors, such as the non-methane volatile organic compounds (NMVOCs). The emission of O₃ precursors has not reached the level of suppressing O₃ pollution. Stricter emission reduction policies are, therefore, required to mitigate O₃ pollution. [ABSTRACT FROM AUTHOR]