Meteorology-normalized ozone enhancement during the 2022 late-spring COVID-19 lockdown in Beijing

The re-emergence of COVID-19 in late spring (April 29 to June 5) of 2022 compelled the Beijing government to implement a stringent lockdown policy to curb the spread of the virus. In comparison to the first lockdown in the winter of 2020, the late spring lockdown provided a more suitable opportunity to examine how ozone (O3) responds to substantial emission reductions during a photochemically active season. This study investigates the

meteorological and chemical mechanisms underlying the surface O3 enhancement during the 2022 late spring lockdown in Beijing, using a combination of ground-based and satellite observations, along with three meteorology normalization models (Random Forest, Long Short Term Memory, and eXtreme Gradient Boosting). The results indicate that the surface O3 concentration in Beijing increased by 4.9 ppbv during the 2022 lockdown

(compared to the same period in 2021 and 2023). The multiple meteorology normalization models reveal that on average 14.3 % (0.7 ppbv) of surface ozone enhancement was attributed to adverse meteorological conditions, and the remaining 85.7 % (4.2 ppbv) attributed to unfavorable emission factors, including a substantial reduction in nitrogen oxides (NOx) and a slight increase in volatile organic compounds (VOCs). Despite substantial NOx reductions during the lockdown, the O3 formation sensitivity remained VOC-limited, rather than shifting to NOx-limited as expected, highlighting the priority of VOC-targeted management for controlling O3 pollution at the current stage.