A THEORETICAL ESTIMATE OF O-3 AND H2O2 DRY DEPOSITION OVER THE NORTHEAST UNITED-STATES

Estimates of short-term, regional-scale spatial distributions of ozone (O3) and hydrogen peroxide (H2O2) dry deposition over the northeast U.S. are presented. Dry deposition fluxes to surfaces are computed using a regional tropospheric chemistry model with deposition velocities which vary with local meteorology, land type, insolation, seasonal factors and surface wetness. A compilation of O3 surface resistances is presented based on a survey of O3 dry deposition measurements. The surface resistance for H2O2 is assumed to be small under most conditions, causing H2O2 to dry deposit at a rate which is frequently limited by surface-layer turbulence. Regional patterns of dry deposition velocities for these oxidants over the northeast U.S. are computed using landuse data and meteorological information predicted using a mesoscale meteorology model. Domain-averaged O3 deposition velocities during a spring period reach a mid-day peak of 0.7-0.8 cm s-1, and drop to 0.1-0.2 cm s-1 at night. Domain-averaged H2O2 deposition velocities at a height of approximately 80 m are predicted to reach a mid-day peak of 1.6-2.0 cm s-1, and fall to 0.6-0.9 cm s-1 at night. Time-averaged surface-layer H2O2 concentrations show a latitude dependence, with higher concentrations in the south. H2O2 concentrations are significantly reduced due to efficient wet removal and chemical destruction during the passage of a cyclonic frontal system. In contrast, O3 concentrations are predicted to rise during the passage of a frontal system due to efficient vertical exchange of midtropospheric air into the boundary layer during convective conditions, followed by synoptic-scale subsidence occurring in the high pressure airmass following a cyclone. Maximum O3 deposition during this 3-day springtime period occurs in polluted agricultural areas. In contrast, H2O2 dry deposition exhibits a latitude dependence with maximum 3-day accumulations occurring in the south. Domain-averaged mid-day deposition rates for O3 and H2O2 were 45-50 .mu.mol m-2 h-1 and 4-5 .mu.mol m-2 h-1. At night, deposition rates were approximately 5-10 .mu.mol m-2 h-1 and 1.5-2.5 .mu.mol m-2 h-1 for O3 and H2O2. These model results show that regional patterns of oxidant dry deposition are strongly influenced by oxidant concentrations, atmospheric stability, surface roughness, and numerous other surface and meteorological factors. Each of these factors must be well-characterized before regional patterns of biological damage associated with oxidant dry deposition can be quantified.