Decisive role of ozone formation control in winter PM_2.5 mitigation in Shenzhen, China

During the COVID-19 lockdown, atmospheric PM_2.5 in the Pearl River Delta (PRD) showed the highest reduction in China, but the reasons, being a critical question for future air quality policy design, are not yet clear. In this study, we analyzed the relationships among gaseous precursors, secondary aerosols and atmospheric oxidation capacity in Shenzhen, a megacity in the PRD, during the lockdown period in 2020 and the same period in 2021. The comprehensive observational datasets showed large lockdown declines in all primary and secondary pollutants (including O₃). We found that, however, the daytime concentrations of secondary aerosols during the lockdown period and normal period were rather similar when the corresponding odd oxygen (O_x≡O₃+NO₂, an indicator of photochemical processing avoiding the titration effect of O₃ by freshly emitted NO) were at similar levels. Therefore, reduced O_x, rather than the large reduction in precursors, was a direct driver to achieve the decline in secondary aerosols. Moreover, O_x was also found to determine the spatial distribution of intercity PM_2.5 levels in winter PRD. Thus, an effective strategy for winter PM_2.5 mitigation should emphasize on control of winter O₃ formation in the PRD and other regions with similar conditions.