Role of the ambient aerosol in the atmospheric processing of semivolatile contaminants: A parameterized numerical model (Gas-Particle Partitioning (GASPAR))

A parameterized description of the ambient aerosol is the basis of a model that treats both gas-particle partitioning and aqueous phase chemical trans formations of semivolatile contaminants. Dividing the aerosol population in to source, size, hygroscopic, and compositional classes, it is possible to assess the importance of contaminant-aerosol interactions under varying met eorological conditions. Using mercury as a test case, the model provides no t only the quantity and speciation of mercury associated with particulate m atter for use in dry deposition models and in conjunction with dispersion/m eteorological models, but shows conclusively that deliquesced aerosol parti cles are not simply transporters of adsorbed mercury, but play an active an d significant role in the transformation of elemental to oxidized mercury. The sensitivity analysis carried out using a version of the Direct Decouple d 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 init ial parameterization array which describes the ambient aerosol. The chlorid e ion concentration has a notable effect on the oxidized Hg that is;associa ted with the particle when the chemistry model reaches steady state. The re ason for this is clarified by the dependencies of the neutral Hg containing species concentrations on the rates of mass transfer and the initial conce ntrations. The presence of soot in the aerosol particles is shown to be par ticularly important in the partitioning of Hg(II) between the gas, aqueous and particulate phases. The implications, given the higher solubility of mo st oxidized mercury species compared to elemental mercury, are fundamental for the understanding of the cycling and fate of mercury in the environment .