We have developed a three-dimensional Eulerian regional acid deposition model to calculate episodic chemical concentrations and dry and wet deposition of acids in North America. This transport, transformation, and deposition modeling system subdivides the troposphere over the eastern United States, southeastern Canada, and the western Atlantic Ocean into a six-level, 30 by 30 horizontal grid with a horizontal grid size of 80¿80 km2. Transport and vertical diffusion of 24 trace gases and particles are calculated using temporally and spatially varying meteorology, provided by a mesoscale meteorological model. A gas phase chemical reaction mechanism is used to simulate concentrations and chemical conversion rates for 36 species, including 14 stable organics and 11 short-lived radicals. Altitude-, latitude-, and season-dependent photolysis rates for nine reactions in the chemical mechanism are specified using a delta-Eddington radiative transfer model which includes O2 and O3 absorption, scattering and absorption by clouds and aerosols, Rayleigh scattering, and ground relections.

Subgrid scale vertical transport, aqueous chemical conversions, and trace gas and particle scavenging by clouds are parameterized using a one-dimensional dynamical and microphysical cloud model and a box aqueous chemistry and scavenging submodel. The aqueous phase chemistry model includes sulfur oxidation by H2O2, O3, trace metals, and two organic peroxides, with numerous equilibria between all soluble trace species. Dry deposition rates for 13 compounds are computed using species-specific deposition velocities that depend on the local meteorology, season, land type, insolation, and surface wetness conditions. Emissions of SO2, SO4 =, NO, NO2, CO, NH3, and 10 classes of volatile organic compounds are included in the model. Trace gases are emitted into different vertical levels of the model according to a plume rise submodel. Hourly emissions are adjusted according to season and weekday or weekend activities. This model provides a framework to examine the relative importance and sensitivity of numerous physical and chemical processes responsible for the formation and deposition of tropospheric acidity. ¿ American Geophysical Union 1987