An Unprecedented Ozone Reduction at a Coastal Background Site in South China: Drivers and Implications

Ozone (O3) concentration in the regional atmosphere of South China was shown to increase. Here, we identified an unprecedented O3 reduction at a high rate of 2.79 ± 0.08 ppbv yr–1 during 2018–2023, contrasting with the long-term rise in the level of O3 at the same site. We found that this change was unlikely to be caused by COVID-19 lockdowns. Carbon monoxide and some volatile organic compounds (e.g., formaldehyde and toluene) also decreased. Significant O3 reductions occurred in the summer and autumn, with summer showing a higher percentage decline (9.51% yr–1). An ensemble machine learning model was developed to eliminate meteorological impacts on the O3 trends, which was shown to explain 24.5% of the year-round O3 reduction rate. The Weather Research and Forecasting model, coupled with Community Multiscale Air Quality, quantified the meteorological contribution of 54.0% to significant O3 reduction in August 2018 to 2023. While the model well reproduced O3 in August 2018, it overestimated the level of O3 in August 2023 when using emissions derived from simple extrapolation. Nevertheless, the remaining half of the O3 reduction rate in August was likely attributed to changes in anthropogenic emissions on land. This study also highlights the need for timely updates to emission inventories for effective air quality policy evaluations.