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ESTIMATING TRANSPORT AND DEPOSITION OF A SEMI-VOLATILE COMPOUND WITH A REGIONAL PHOTOCHEMICAL MODEL
Citation:
Hutzell, W T. ESTIMATING TRANSPORT AND DEPOSITION OF A SEMI-VOLATILE COMPOUND WITH A REGIONAL PHOTOCHEMICAL MODEL. Presented at 1999 Fall Meeting of the American Geophysical Union, San Francisco, CA, December 13-17, 1999.
Impact/Purpose:
The goal of this research is to develop and test appropriate chemical and physical mechanisms for use in EPA's Models-3 chemical/transport models. These models will be addressing issues of tropospheric photochemistry, fine particles, toxic and semi-volatile substances, and acid deposition. As such, scientifically credible mechanisms for atmospheric gas- and aqueous-phase chemistry as well as heterogeneous chemistry, applicable to the particular pollutant regimes must be included in Models-3.
Description:
To simulate the fate of compounds that are considered semi-volatile and toxic, we have modified a model for regional particulate matter. Our changes introduce a semi-volatile compound into the atmosphere as gaseous emissions from an area source. Once emitted, the gas can transform via OH addition or partition onto ambient particulate matter as a trace species. The partitioning assumes equilibrium between the gas and particulate phases. Concentrations in each phase then depend on the total ambient concentration and partitioning ratios. The latter values include adsorption and absorption processes based on empirical and theoretical work. Besides these chemical and physical processes, the compound undergoes advection, diffusion, and deposition.
The revised model, the Community Multi-scale Air Quality (CMAQ) model, better assesses the atmospheric fate of semi-volatile and toxic compounds because of the way that it estimates particulate matter in the lower troposphere. The estimate uses a tri-modal distribution and internal mixture of inorganic and organic species to describe particulate matter. Air emissions, thermodynamic relations, and gas phase chemistry control production for each species while equations describing aerosol microphysics govern their concentrations over the particulate modes. Particulate concentrations are also controlled by independent transport and deposition algorithms. The CMAQ model then enables assessing how variations in meteorology and particulate matter combine to transform semi-volatile compounds over regional and local scales.
Using the modified model on a herbicide called Atrazine; the application uses emissions of Atrazine predicted by data on annual usage, and a soil model under energy balance conditions. The model domain covers the Eastern US and the simulation spans several days in early July of 1995. Results include atmospheric concentrations and deposition rates. These results may have limited ability to reproduce actual values because the simulation period does not cover a majority of Atrazine emissions. Results illustrate how partitioning can affect the fate of a semi-volatile compound and what sorption processes control partitioning. Results also demonstrate that the CMAQ model can support further research on how regional particulate matter influence atmospheric chemistry and physics.