Long‐Term Evolution of Atmospheric Carbonyls in Megacity Beijing: Source, Trend, and Impact on the Atmospheric Oxidation Capacity

Abstract This study investigates the evolution of atmospheric carbonyl compounds in Beijing over the past two decades by combining observations during the COVID‐19 pandemic and post‐pandemic period (2022–2023) with literature data from 2004 to 2023. Formaldehyde (FA), acetaldehyde (AA), and acetone (AC) were consistently the dominant carbonyl species. During the summer of 2022, under pandemic‐related restrictions, short‐lived carbonyls such as FA and AA dominated, primarily driven by localized traffic emissions. In contrast, in 2023, both the concentration and fractional contribution of the longer‐lived AC increased, likely reflecting enhanced regional transport associated with the recovery of socio‐economic activities. Despite these changes, total carbonyl concentrations exhibited a sustained decline from 2013 to 2023, indicating the effectiveness of China's clean air policies, with additional reductions during pandemic lockdowns. Observation‐based model (OBM) further showed that carbonyl photolysis, particularly which of FA, was an important source of HOx (OH + HO 2 ) radicals and substantially enhanced atmospheric oxidation capacity, contributing 40%–58% of HOx production during periods of carbonyl accumulation in the cold season of 2022. However, the relative contribution of carbonyl photolysis to total HOx production decreased from approximately 43% before 2013 to about 21% during 2022–2023, attributable to stringent emission controls under the Air Pollution Prevention and Control Action Plan and reduced anthropogenic activities during the pandemic. Overall, these results demonstrate the success of long‐term emission‐reduction strategies while highlighting the increasing influence of regional transport on urban carbonyl pollution in Beijing.