Long-term trend of ozone in southern China reveals future mitigation strategy for air pollution

The 14-year trends of surface ozone were examined using observations from 12 urban sites and 4 regional sites over the Pearl River Delta (PRD) region in China over the 2006-2019 period. Surface ozone exhibits upward trends in the range of 0.28-1.02 ppb yr(-1) at urban sites and mixed trends in the range of-0.29 - 0.21 ppb yr(-1) at regional sites. Meanwhile, surface O-x (O-x = O-3+NO2) mixing ratios exhibit downward trends at 9 urban sites and 3 regional sites in the range of-0.01 to-0.54 ppb yr(-1), revealing that the ozone rise at urban sites is predominantly attributed to the reduced titration effect by nitric oxide (NO). The further decline of surface NO2 mixing ratios could drive the decreases in O(x )mixing ratios and the increases in ozone mixing ratios at urban sites. In addition, surface O-x concentrations show a good correlation with aerosol concentrations, implying the dominant contributions of secondary production to aerosol and the consistent control strategies of ozone and aerosol pollution in PRD. For the abatement of ozone pollution, our results highlight the synergistic control of key precursors including nitrogen oxides and volatile organic compounds to counteract the reduced NO titration effect by inhibiting the photochemical production of ozone. At last, we also highlight the importance of establishing more regional sites in comprehensively assessing spatiotemporal changes in surface air pollutants in China.