Opposite variations of peak and low ozone concentrations in eastern China: Positive effects of NO_x control on ozone pollution

Abstract. Due to the unbalanced emission reductions in ozone precursors in recent years, ozone trends and the causes of their variations in eastern China remain insufficiently understood. To explore the factors controlling ozone trends in eastern China, the long–term ozone precursors observation experiment was conducted. Combined with the satellite and surface measurements, the trend of low, typical and peak ozone concentrations in eastern China was evaluated in detail. Observation results show that the surface peak ozone concentrations significantly decreased (-0.5 % per year) while low ozone concentrations increased (0.3 % per year) in eastern China during May–September 2017–2022. The underlying cause of surface ozone trends in eastern China is anthropogenic emissions (~85 %), and the contribution of meteorological factors is not significant (~15 %). Ozone formation sensitivity is in VOC–limited regime or transition regime during periods (8:00–11:00) of sharp increases in ozone concentrations, and it is usually in NO_x–limited regime when the ozone concentration reaches its peak (~14:00). Substantial reductions in nitrogen oxides (NO_x) emissions have diametrically opposed effects on peak (decreasing) and low (increasing) ozone concentrations, and reducing volatile organic compounds (VOCs) concentrations is the key to reversing the current high ozone level situation in eastern China. In addition, there are obvious interannual variations of surface O₃ formation sensitivity on spatial scales through long–term satellite observations, in which area proportion of VOC–limited regime is decreasing while the area proportion of NO_x–limited regime is increasing. Our results highlight the positive impact of NO_x reduction in controlling peak O₃ levels, and regular changes in the ozone formation sensitivity throughout the day should be taken into account when formulating ozone control policies.