The impact of China's Clean Air Action and future strategies: Actions targeting regionally dominant sources.

Chinese government has implemented a series of measures to reduce air pollution and protect human health, namely Clean Air Action. This initiative has significantly reduced concentrations of common air pollutants, with inhalable particulate matter (PM10), fine particulate matter (PM2.5), nitrogen dioxide (NO2) and sulfur dioxide (SO2) levels decreasing by 40.7, 48.5, 32.3 and 76.2 %, respectively, except for ozone (O3) increasing by 11.1 %. Furthermore, the composition of PM2.5 has changed, and the health risks of polycyclic aromatic hydrocarbons (PAHs) and heavy metals have reduced, highlighting the effectiveness of Clean Air Action. However, the impact of Clean Air Action is complex and multifaceted. The abnormal increase in O3 is primarily due to the complex interactions between nitrogen oxides (NOx), volatile organic compounds (VOCs), and PM2.5, which together affected the photochemical balance. The relative versus absolute shifts in pollutant sources further complicate this picture. Despite overall emission reductions, the relative contributions from biomass burning have increased in some areas, with its share in black carbon in Shanghai rising from 17.9 % in 2013 to 45.0 % in 2019, and its proportion of PAHs in Beijing increasing from 10.2 % in 2014 to 22.5 % in 2021. This inconsistency highlights that the contribution of pollutant sources is relative, not absolute. Systematic research on the molecular-level impacts and the relative shifts in pollution sources provides valuable insights for effective air quality control strategies. Future efforts in China focus on reducing pollutants emissions from some significant contributors such as biomass burning, customizing VOCs/NOx control strategies locally to reduce O3, and continuously monitoring PM2.5 composition to evaluate policy effects.