| Summary: | Increases in ground-level ozone (O<sub>3</sub>) have been observed during the COVID-19 lockdown in many places around the world, primarily due to the uncoordinated emission reductions of O<sub>3</sub> precursors. In Guangzhou, the capital of Guangdong province in South China, O<sub>3</sub> distinctively decreased during the lockdown. Such a phenomenon was attributed to meteorological variations and weakening of local O<sub>3</sub> formation, as indicated by chemical transport models. However, the emission-based modellings were not fully validated by observations, especially for volatile organic compounds (VOCs). In this study, we analyzed the changes of O<sub>3</sub> and its precursors, including VOCs, from the pre-lockdown (Pre-LD) to lockdown period (LD) spanning 1 week in Guangzhou. An observation-based box model was applied to understand the evolution of in-situ photochemistry. Indeed, the ambient concentrations of O<sub>3</sub> precursors decreased significantly in the LD. A reduction of 20.7% was identified for the total mixing ratios of VOCs, and the transportation-related species experienced the biggest declines. However, the reduction of O<sub>3</sub> precursors would not lead to a decrease of in-situ O<sub>3</sub> production if the meteorology did not change between the Pre-LD and LD periods. Sensitivity tests indicated that O<sub>3</sub> formation was limited by VOCs in both periods. The lower temperature and photolysis frequencies in the LD reversed the increase of O<sub>3</sub> that would be caused by the emission reductions otherwise. This study reiterates the fact that O<sub>3</sub> abatement requires coordinated control strategies, even if the emissions of O<sub>3</sub> precursors can be significantly reduced in the short term.
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