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

The significance of Aerosol-photolysis interaction (API) in photochemistry has been emphasized by studies utilizing box models and chemical transport models. Some of them noted that API is closely related to aerosol vertical distributions. However, few studies have considered the actual aerosol vertical distribution when evaluating API due to the lack of observations and the substantial uncertainties in simulation. Herein, we used lidar and radiosonde observations with the GEOS-Chem model 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 were revised. Vertically, photolysis rates changed following AOD, showing 33.4%–73.8% increases at the 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. Besides, the underestimation of ozone levels in winter was improved by 8.5%. PM_2.5 increased by 0.8 µg m⁻³ in winter and 0.2 µg m⁻³ in summer due to the promotion of photochemistry and increased more during pollution, with a maximum daily change of 16.5 µg m⁻³ at Beijing station in winter. The weakened API enhanced nitric acid (HNO₃) formation by increasing the atmospheric oxidizing capacity (13.7% for OH radical) in high NO_x emission areas and this helps explain the strong response of PM_2.5in winter.