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[NO 2 ] 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 HO 2 . 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 processes. 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 O 3 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 O 3 production.