The atmospheric oxidizing capacity in China - Part 2: Sensitivity to emissions of primary pollutants

Despite substantial reductions in anthropogenic emissions, ozone (O3) pollution remains a severe environmental problem in urban China. These reductions affect ozone formation by altering levels of O3 precursors, intermediates, and the oxidation capacity of the atmosphere. However, the underlying mechanisms driving O3 changes are still not fully understood. Here, we employ a regional chemical transport model to quantify ozone changes due to a specified emission reduction (50 %) for winter and summer conditions in 2018. Our results indicate that reductions in nitrogen oxide (NOx) emissions increase surface O3 concentrations by 15 %–33 % on average across China in winter and by up to 17 % in volatile organic compound (VOC)-limited areas during summer. These ozone increases are associated with a reduced NOx titration effect and higher levels of OH radicals. Reducing NOx emissions significantly decreases the concentration of particulate nitrate, which enhances ozone formation through increased HO2 radical levels due to reduced aerosol uptake and diminished aerosol extinction. Additionally, an enhanced atmospheric oxidative capacity, driven by larger contributions from the photolysis of oxidized VOCs (OVOCs) and OH-related reactions, also favors urban ozone formation. With additional reductions in anthropogenic VOC emissions, increases in summertime ozone (VOC-limited areas) can be offset by reduced production of radicals from VOC oxidations. To effectively mitigate ozone pollution, a simultaneous reduction in the emission of NOx and specific VOC species should be applied, especially regarding alkenes, aromatics, and unsaturated OVOCs, including methanol and ethanol.