Increasing Urban OH Levels in China over the Past Decade.

Hydroxyl radical (OH) is the main atmospheric oxidant; therefore, changes in OH concentration are critical for the evolution of atmospheric oxidizing capacity during this century. By constructing a machine learning-based prediction model integrated with multispecies satellite and ground observations, this study investigates the trends and drivers of OH concentrations in 100 cities in China over the past decade. We find a significant increase in urban OH levels (2.5% year-1), particularly in southern and eastern coastal regions. A sharp decline in carbon monoxide emissions is identified as the major associated factor for increased OH concentrations, with the rising trend further amplified by chemical feedbacks from nitrogen oxides emission reductions over more polluted areas. The strong inverse relationship between OH concentration and NO2 lifetime in Shanghai across both decadal-scale and short-term lockdown periods underscores the key role of OH in pollutant removal. These findings reveal an increasing atmospheric oxidation capacity in China's main urban areas, with implications for air quality and climate change.