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 3) formation in polluted areas. One the other hand, evidence also suggests that ADE-associated cooling suppresses atmospheric ventilation, thereby enhancing surface-level O 3. Assessment of ADE impacts is thus important for understanding emission reduction strategies that seek co-benefits associated with reductions in both particulate matter and O 3 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 manifestations of ADE impacts on O 3 including changes in atmospheric dynamics (ΔDynamics) and changes in photolysis rates (ΔPhotolysis) 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 3 (DM1O 3) in China by up to 39 µg m −3 through the combination of ΔDynamics and ΔPhotolysis in January but enhanced surface DM1O 3 by up to 4 µg m −3 in July. Increased O 3 in July is largely attributed to ΔDynamics, which causes a weaker O 3 sink of dry deposition and a stronger O 3 source of photochemistry due to the stabilization of the atmosphere. 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 3 in winter, but it will benefit the reduction in maxima O 3 in summer.