A two-pollutant strategy for improving ozone and particulate air quality in China

Fine particulate matter (PM_2.5) decreased by 30–40% across China during 2013–2017 in response to the governmental Clean Air Action. However, surface ozone pollution worsened over the same period. Model simulations have suggested that the increase in ozone could be driven by the decrease in PM_2.5, because PM_2.5 scavenges hydroperoxy (HO₂) and NO_x radicals that would otherwise produce ozone. Here we show observational evidence for this effect with 2013–2018 summer data of hourly ozone and PM_2.5 concentrations from 106 sites in the North China Plain. The observations show suppression of ozone pollution at high PM_2.5 concentrations, consistent with a model simulation in which PM_2.5 scavenging of HO₂ and NO_x depresses ozone concentrations by 25 ppb relative to PM_2.5-free conditions. PM_2.5 chemistry makes ozone pollution less sensitive to NO_x emission controls, emphasizing the need for controlling emissions of volatile organic compounds (VOCs), which so far have not decreased in China. The new 2018–2020 Clean Air Action plan calls for a 10% decrease in VOC emissions that should begin to reverse the long-term ozone increase even as PM_2.5 continues to decrease. Aggressive reduction of NO_x and aromatic VOC emissions should be particularly effective for decreasing both PM_2.5 and ozone.