Chemically Stratified Urban Boundary Layer: Seasonally Reversing Vertical Changes in the Underlying Chemistry of Atmospheric Oxidation Capacity

The vertical structure of atmospheric oxidation capacity (AOC) and its underlying radical chemistry within the urban boundary layer remains poorly characterized, yet it governs secondary pollutant formation. Here, we combined tower-based vertical observations with process-resolved model simulations in Beijing across winter and summer. We find that AOC generally declined with height in both seasons, but radical concentrations exhibit opposite vertical gradients. Notably, enhanced biogenic volatile organic compounds (VOCs) emissions in summer suppress near-surface OH levels, creating a mid-altitude maximum. However, sharp vertical NO gradients drove an increase in ROx with height in winter. The underlying chemistry of vertical AOC variation shifts with the seasons. In winter, the photolysis of HONO dominates ROx production, and NOx-driven ROx loss prevails. In summer, the photolysis of oxygenated VOCs takes over, with radical self-reactions as the primary ROx sink. These findings demonstrate that surface observations alone may provide an incomplete representation of the whole boundary-layer chemistry.