Simulated photochemical response to observational constraints on aerosol vertical distribution over North China

Abstract. The significance of aerosol-photolysis interaction (API) in atmospheric photochemistry has been emphasized by studies utilizing box models and chemical transport models. However, few studies have considered the actual aerosol vertical distribution when evaluating API effects due to the lack of observations and the uncertainties in model simulation. Herein, we integrated lidar and radiosonde observations with the chemical transport model (GEOS-Chem) to quantify the response of photochemistry to observational constraints on aerosol vertical distribution across different seasons in North China. The underestimation of aerosol optical depth (AOD) in lower layers and the overestimation in upper layers in GEOSChem model were revised. In response, photolysis rates changed following AOD, showing 33.4 %–73.8 % increases at the surface. Surface ozone increased by an average of 0.9 ppb and 0.5 ppb in winter and summer and the default API impact on ozone reduced by 36 %–56 %. The weaker response in summer can be related to the compensatory effects of stronger turbulence mixing in the boundary layer. The long-lasting underestimation of ozone levels in winter was also greatly improved. Due to the enhanced photochemistry, PM_2.5 increased by 0.8 µg m⁻³ in winter and 0.2 µg m⁻³ in summer and increased strongly during pollution events with a maximum daily change of 16.5 µg m⁻³ at Beijing station in winter. The weakened API effect in turn enhanced nitric acid formation by increasing atmospheric oxidizing capacity (13.7 % increase for OH radical) in high NO_x emission areas and this helps explain the strong response of PM_2.5 in winter.