Impacts of aerosols and clouds on photolysis frequencies and photochemistry during TRACE-P: 2. Three-dimensional study using a regional chemical transport model

A three-dimensional regional chemical transport model, STEM 2K1, coupled with a detailed radiation model is used to study the influences of aerosols and clouds on photolysis rates and photochemical processes over East Asia-Western Pacific during the TRACE-P period. Measured J-values are compared with those calculated using three-dimensional modeled fields of clouds and aerosols. The model is shown to accurately represent observed J-values over a broad range of conditions. Model studies with and without aerosols and clouds are performed and compared with clear-sky conditions to isolate the various influences. Clouds are shown to have a large impact on photolysis rates during the observation periods of TRACE-P, with J[NO₂] decreased by 20% below clouds and enhanced by ∼30% from 1 km to 8 km. Clouds also exert a dominant influence on short-lived radicals, like OH and HO₂. For March, clouds reduce OH by 23% at altitudes below 1 km and increase OH by ∼25% above 1 km. Asian aerosols contain large amounts of carbonaceous material, inorganic components such as sulfates, and mineral oxides. These aerosols significantly influence J-values and photochemical processess. When averaged over all TRACE-P DC-8 and P-3 flights, the aerosol influence via affecting J-values reduces OH by ∼40% below 1 km, and by ∼24% above 1 km. Aerosols have a stronger impact on longer-lived chemical species than clouds do because aerosols tend to be coemitted with precursors and have a longer contact time with the polluted air masses. The accumulated aerosol impact generally is to reduce O₃ concentrations by about 6 ppbv in the biomass burning plumes emitted from Southeast Asia. In megacity plumes, aerosols can increase NO_x concentration by 40% via reducing its photolytic loss and reduce NO_z concentration by a similar amount. A detailed case study of the DC-8 and P-3 flights on 27 March is used to make comparisons for cloud and aerosol influences. During these flights, the cloud impact on J-values is stronger than the aerosol impact, but aerosols are shown to exert a much stronger accumulated influence on O₃ production.