The Atmospheric Oxidizing Capacity in China: Part 2. Sensitivity to emissions of primary pollutants

Abstract. The Atmospheric Oxidation Capacity (AOC), often referred to as the self-cleansing ability of the atmosphere, considerably affects the concentrations of photochemical air pollutants. Despite substantial reductions in anthropogenic emissions of key chemical compounds in China, the mechanisms that determine the changes in the atmospheric oxidation capacity are still not sufficiently understood. Here, a regional chemical transport model is employed to quantify the sensitivity of air pollutants and photochemical parameters to specified emission reductions in China for conditions of January and July 2018 as representative. The model simulations show that, in winter, a 50 % decrease in nitrogen oxides (NO_x) emissions leads to an 8–10 ppbv (15–20 %) increase in surface ozone concentrations across China. In summer, the ozone concentration decreases by 2–8 ppbv (3–12 %) in NO_x-limited areas, while ozone increases by up to 12 ppbv (15 %) in volatile organic compounds (VOCs)-limited areas. This ozone increase is associated with a reduced NO_x-titration effect and higher levels of hydroperoxyl (HO₂) radical due to decreased aerosol uptake. With an additional 50 % reduction in anthropogenic VOCs emission, the predicted ozone concentration decreases by 5–12 ppbv (6–15 %) in the entire geographic area of China, with an exception in the areas, where the role of BVOCs is crucial to ozone formation. Further, the adopted reduction in NO_x emission leads to an increase of AOC by 18 % in VOC-limited areas. This specific increase is associated with the combined effect of enhanced radical cycles associated with the photolysis of oxidized VOCs (OVOCs) and the oxidation of alkenes by hydroxyl (OH) radical and O₃. A large reduction of daytime AOC in summer results from the reduction in anthropogenic VOCs emission, with a dominant contribution from the reaction of OH radical with reduced alkenes, followed by the reactions with depleted aromatics and OVOCs. This study highlights that photolysis of OVOCs and oxidation of alkenes in urban areas when NO_x emission is reduced leads to an increase in O₃. To mitigate ozone rises in urban areas, a joint reduction in the emission of NO_x and specific VOCs species, including alkenes and aromatics and photodegradable OVOCs, should be implemented.