A parameterized description of the ambient aerosol is the basis of a model that treats both gas-particle partitioning and aqueous phase chemical transformations of semivolatile contaminants. Dividing the aerosol population into source, size, hygroscopic, and compositional classes, it is possible to assess the importance of contaminant-aerosol interactions under varying meteorological conditions. Using mercury as a test case, the model provides not only the quantity and speciation of mercury associated with particulate matter for use in dry deposition models and in conjunction with dispersion/meteorological models, but shows conclusively that deliquesced aerosol particles are not simply transporters of adsorbed mercury, but play an active and significant role in the transformation of elemental to oxidized mercury. The sensitivity analysis carried out using a version of the Direct Decoupled Method has shown the transfer of Hg(II) to the gas phase from the aqueous phase to be highly dependent on the chloride ion concentration in the initial parameterization array which describes the ambient aerosol. The chloride ion concentration has a notable effect on the oxidized Hg that is associated with the particle when the chemistry model reaches steady state. The reason for this is clarified by the dependencies of the neutral Hg containing species concentrations on the rates of mass transfer and the initial concentrations. The presence of soot in the aerosol particles is shown to be particularly important in the partitioning of Hg(II) between the gas, aqueous and particulate phases. The implications, given the higher solubility of most oxidized mercury species compared to elemental mercury, are fundamental for the understanding of the cycling and fate of mercury in the environment. ¿ 2000 American Geophysical Union