A state-of-the-art gas phase chemical mechanism for modeling atmospheric chemistry on a regional scale is presented. The second generation Regional Acid Deposition Model (RADM2) gas phase chemical mechanism, like its predecessor RADM1, is highly nonlinear, since predicted ozone, surfate, nitric acid and hydrogen peroxide concentrations are complicated functions of NOx and nonmethane hydrocarbon concentrations. The RADM2 chemical mechanism is an upgrade of RADM1 in that (1) three classes of higher alkanes are used instead of one, (2) a more detailed treatment of aromatic chemistry is used, (3) the two higher alkene classes now represent intemal and terminal alkenes, (4) ketones and dicarbonyl species are treated as classes distinct from aldehydes, (5) isoprene is now included as an explicit species, and (6) there is a more detailed treatment of peroxy radical-peroxy radical reactions. As a result of these improvements the RADM2 mechanism simulates the concentrations of peroxyacetyl nitrate, HNO3, and H202 under a wide variety of environmental conditions. Comparisons of RADM2 mechanism with the RADM1 mechanism predictions and selected environmental chamber experimental results indicate that for typical atmospheric conditions, both mechanisms reliably predict 03, surfate and nitric acid concentrations. The RADM2 mechanism gives lower and presumably more realistic predictions of H202 because of its more detailed treatment of peroxy radical-peroxy radical reactions. INTRODUCYIONOne of the most important components of any regional air quality model is its gas phase chemical mechanism. Gas phase chemical transformation rates, as well as emissions, transport and deposition, determine the distribution of gas phase species. The relationship between emissions of reactive organic species, sulfur and nitrogen oxides and regional air pollution effects associated with ozone and acid deposition is highly dependent on the gas phase chemistry of the polluted continental troposphere. Transformation rates also affect transport and deposition rates of trace species, which are highly dependent on the chemical form of the species.Aqueous phase reactions in clouds are major contributors to atmospheric acidification, thus it is very important for the gas phase mechanism to predict correct concentrations of species that are needed for aqueous-phase chemistry. Midday summertime gas phase sulfur dioxide and nitrogen dioxide oxidation rates are only about 5 and 40% per hour, respectively [Calvert and Stockwell, 1983; Stockwell, 1986; Calvert et al., 1985]. However, gas phase chemistry supplies reactants such as ozone and hydrogen peroxide to cloud water, where the conversion rates of sulfur dioxide to sulfate can be several hundred percent per hour or more [Calvert et al., 1985]. These rapid aqueous phase oxidation rates depend on the gas phase concentrations, solubility, and rate of mass transfer of oxidizing agents such as hydrogen peroxide, ozone, methyl hydrogen peroxide, peroxy acetic acid, and HO and HO 2 radicals [Calvert et al., 1985...

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 reflections. 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.

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