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

The atmospheric oxidation capacity plays a crucial role in the concentration of photochemical pollutants and primary emissions. Despite dramatic reductions in anthropogenic emissions of key chemical compounds in China, a comprehensive understanding of the corresponding changes in the Atmospheric Oxidative Capacity (AOC) is still not sufficiently understood. Here, a regional chemical model is employed to quantify the sensitivity of air pollutants and photochemical parameters to specified emission reductions in China, considering conditions representative of January and July 2018. The model simulations show that, in winter, a 50% decrease in NOx emissions leads to a 6-12 ppbv increase in surface ozone concentrations across China. In summer, the ozone concentration decreases by 4-8 ppbv in NOx-limited areas, while ozone increases by 10-12 ppbv in VOC-limited areas. This ozone increase is associated with reduced NOx-titration effect and enhanced OH and HO2 radicals. With an additional 50% reduction in anthropogenic VOCs emission, the calculated ozone concentration decreases by 5-10 ppbv in the entire geographic area of China. For AOC parameters, with NOx emission reduction, an increase is also found 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 ozonolysis of alkenes. A large reduction of AOC in summer is led by the reduction in AVOCs emission, with a dominant contribution from the reaction of OH radical with reduced OVOCs, followed by the reactions with fewer alkenes and aromatics. This study highlights the enhanced positive effect of photolysis of OVOCs and ozonolysis of alkenes on atmospheric oxidative capacity in urban areas when NOx emission is reduced. To mitigate ozone rises in urban areas, it is crucial to coordinate the control of NOx emissions with specific VOCs species, including alkenes and aromatics.