Impacts of aerosol direct effects on tropospheric ozone through changes in atmospheric dynamics and photolysis rates.

Aerosol direct effects (ADEs), i.e., scattering and absorption of incoming solar radiation, reduce radiation reaching the ground and the resultant photolysis attenuation can decrease ozone (O) formation in polluted areas. One the other hand, evidence also suggests that ADE-associated cooling suppresses atmospheric ventilation, thereby enhancing surface-level O. Assessment of ADE impacts is thus important for understanding emission reduction strategies that seek co-benefits associated with reductions in both particuate matter and O levels. This study quantifies the impacts of ADEs on tropospheric ozone by using a two-way online coupled meteorology and atmospheric chemistry model, WRF- CMAQ, using a process analysis methodology. Two mani-festations of ADE impacts on O3 including changes in atmospheric dynamics (ᐃDynamics) and changes in photolysis rates (∆Photolysis) were assessed separately through multiple scenario simulations for January and July of 2013 over China. Results suggest that ADEs reduced surface daily maxima 1 h O (DM1O) in China by up to 39μgm through the combination of ∆Dynamics and ∆Photolysis in January but enhanced surface DM1O by up to 4μgm in July. Increased O in July is largely attributed to ∆Dynamics, which causes a weaker O sink of dry deposition and a stronger O source of photochemistry due to the stabilization of the at-mosphere. Meanwhile, surface OH is also enhanced at noon in July, though its daytime average values are reduced in January. An increased OH chain length and a shift towards more volatile organic compound (VOC)-limited conditions are found due to ADEs in both January and July. This study suggests that reducing ADEs may have the potential risk of increasing O in winter, but it will benefit the reduction in maxima O in summer.