The second generation regional acid deposition model chemical mechanism for regional air quality modeling

Abstract

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...

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