Worsening urban ozone pollution in China from 2013 to 2017: The roles of meteorology and anthropogenic emission

China has suffered from increasing levels of ozone pollution in urban areas despite the implementation of various stringent emission reduction measures since 2013. In this study, we conducted numerical experiments with an up-to-date regional chemical transport model to assess the roles of changes in meteorology and anthropogenic emission in summer ozone variations from 2013 to 2017 over China. The model can faithfully reproduce the observed meteorological parameters and air pollutant concentrations and capture the increasing trend in the surface maximum daily 8-hour average (MDA8) ozone (O₃) from 2013 to 2017. An increase of 0.46 ppbv a^-1 (p=0.001) and a slight decrease of 0.17 ppbv a^-1 (p=0.005) in MDA8 O₃ levels were simulated from 2013 to 2017 in urban and rural areas, respectively. The meteorological influence on the ozone trend varied by region and by year and could be comparable with or even larger than the impact of changes in anthropogenic emissions. The variation in biogenic emissions during summer varied across regions and was mainly affected by temperature. China’s midlatitude areas (25°N to 40°N) experienced a significant decrease in MDA8 O₃ due to a decline in biogenic emissions, while higher temperatures in northern (north of 40°N) and southern (south of 25°N) China after 2013 led to an increase in MDA8 O₃ concentrations via an increase in biogenic emissions. We assessed the effects of changes in individual meteorological factors on ozone levels from 2013 to 2017. The results show that the wind field change made a significant contribution to the increase in surface ozone over China by transporting the ozone downward from the upper troposphere and the lower stratosphere. The long-range transport of ozone and its precursors outside the modeling domain also contributed to the increase in MDA8 O₃ on the Tibetan Plateau. The effects of changes in individual pollutant emissions on ozone were simulated. The reduction of NO_x emission increased ozone in urban areas due to non-linear NO_x-VOCs chemistry and decreased aerosol effects; the slight increase in VOCs emission enhanced ozone levels; the reduction of particulate matter(PM) emission increased ozone concentrations by enhancing the photolysis rates and reducing the loss of reactive gases on aerosol surfaces; the reduction of SO₂ emission resulted in a drastic decrease in sulfate concentrations which increase ozone levels through the aerosol effects. In contrast, the reduction of CO emissions helped decrease ozone levels in the past years. On the effects of decreasing levels of aerosol, the drop in heterogeneous uptake of reactive gasses, mainly HO₂ and O₃, was found to be more important than the increase in photolysis rates. The adverse effect on ozone of the reductions of NO_x, SO₂ and PM emissions would have been avoided with ~20% reduction of VOCs emission from 2013 to 2017. Our analysis revealed that the NO_x reduction in the past years has helped to contain the total ozone production in China. However, in order to decrease ozone concentrations in major urban and industrial areas, VOCs emission control should be added to the current NO_x-SO₂-PM policy.