STUDYING THE EFFECTS OF AEROSOLS ON VERTICAL PHOTOLYSIS RATE COEFFICIENT AND TEMPERATURE PROFILES OVER AN URBAN AIRSHED

This paper discusses the effects of size-and composition-resolved aero sols on photolysis and temperatures within and above an urban airshed. With respect to photolysis, three-dimensional simulations indicated t hat (1) in regions of the boundary layer where absorption of ultraviol et (UV) radiation was strong, aerosols reduced photolysis coefficients of W-absorbing gases; (2) in regions of the boundary layer where UV s cattering dominated UV absorption by aerosols, aerosols enhanced photo lysis coefficients of UV-absorbing gases; (3) aerosols increased photo lysis coefficients for visible-absorbing gases since visible scatterin g always exceeded visible absorption by aerosols; (4) scattering and w eakly absorbing aerosols above the boundary layer increased photolysis coefficients above the boundary layer for all absorbing gases; and (5 ) increases in aerosol absorption extinction within the boundary layer reduced photolysis coefficients above the boundary layer for all abso rbing gases. Photolysis coefficients changes due to aerosols decreased near-surface ozone mixing ratios in Los Angeles by 5-8%. With respect to temperatures, simulations indicated that aerosols increased radiat ive heating rates at all altitudes but decreased surface solar irradia nces during the day. Surface irradiance reductions cooled the ground, reducing mechanical and thermal turbulent heat fluxes back to the boun dary layer, cooling near-surface air, and stabilizing the boundary lay er. During the night, aerosols decreased boundary-layer heating rates but increased downward infrared irradiances to the ground. Warmer grou nd temperatures increased mechanical turbulent heat fluxes to the boun dary layer, increasing nighttime near-surface temperatures. Thus, aero sols affected temperatures primarily through ground-atmosphere turbule nt heat transfer.