What drove the rapid ozone reduction at a background site in South China during 2018-2023?

Ozone (O3) concentration in the regional atmosphere of South China were shown to increase, but the recent variations in the context of dramatic emissions fluctuations remain unknow. In contrast to the overall increase determined elsewhere, O3 at a regional background site in South China decreased at a surprisingly high rate of -2.79 ppbv yr-1 during 2018-2023, which has not been seen before and was unlikely to be explained by the COVID-19 lockdowns. Significant reductions in O3 were only observed in summer and autumn. Three statistical methods were used to eliminate the impacts of meteorological variations on O3 trends, which were shown not to be the main cause of the O3 reduction. With the 2017 emission inventories (EIs), the Weather Research and Forecasting model coupled with Community Multiscale Air Quality (WRF-CMAQ) well reproduced the O3 in August 2018. Sensitivity tests indicated that meteorological variations explained at most half of the O3 reduction rate. Despite the significant drop in cargo throughput at Hong Kong (HK) terminals, ship emissions made a higher contribution to O3 in August 2023 than in the same period of 2018. The unexplained O3 decrease was likely due to reduced anthropogenic emissions in mainland China and HK. Simulations with a simple extrapolation based on the 2017 EIs did show that emission changes in the neighboring Pearl River Delta led to a decrease in O3. However, updated emissions are needed to clearly understand the effects of local and regional emission changes.