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Mercury Modeling
Research Programs
ASMD has been working on the development of atmospheric mercury models since the early 1990's when the Regional Lagranian Model of Air Pollution (RELMAP) was adapted to simulate mercury in support of EPA’s Mercury Study Report to Congress. As the scientific understanding of atmospheric mercury processes continued to develop in the late 1990's, it became apparent that Lagrangian-type models, also known as “puff” models, would have difficulties simulating the complex chemical and physical interactions of mercury with other pollutants that were being discovered. Thus, AMD’s focus for atmospheric mercury model development was moved to the Community Multi-scale Air Quality model (CMAQ). The CMAQ simulates atmospheric processes within a 3-dimensional array of pre-defined finite volume elements and can model complex interactions between all of the pollutants that might exist within each volume element. The CMAQ was previously developed to simulate photochemical oxidants, acidic and nutrient pollutants, and aerosol particulate matter, all of which have been shown to interact with mercury in air and in cloud water and influence its deposition to sensitive aquatic ecosystems. The “one atmosphere” approach of CMAQ where all pollutants are simulated together just as they exist in the real atmosphere has been extended to atmospheric mercury modeling at AMD.
Model Development
Figure 1
A number of modification were made to the standard CMAQ model to allow it to simulate atmospheric mercury which are described in detail in Bullock and Brehme (2002). New information about chemical and physical processes affecting mercury continues to be published, and refinement of the mercury version of CMAQ (CMAQ-Hg) is an ongoing process. The model’s treatment for the sorption of Hg2+ compounds to elemental carbon aerosol in aqueous suspension in cloud water is currently under examination. The Fortran subroutine for the CMAQ-Hg aqueous chemistry mechanism was recently optimized to more efficiently calculate the mercury chemistry in concert with the standard CMAQ mechanism. Further modification of the CMAQ-Hg chemical mechanisms for mercury in both the gaseous and aqueous phases is expected as additional chemical reactions are identified.
AMD is participating in an intercomparison study of numerical models for long-range atmospheric transport of mercury sponsored by the European Monitoring and Evaluation Programme (EMEP) and organized by EMEP’s Meteorological Synthesizing Center - East in Moscow, Russia. This first phase of the model intercomparison involved the simulation of mercury chemistry in a closed cloud volume given a variety of initial conditions. Results obtained from the CMAQ mercury model (CMAQ-Hg) and the other participating models from Russia, Germany, Sweden, and the United States were compared to identify key scientific and modeling uncertainties. The results were published in a report to the EMEP governing body (Ryaboshapko et al., 2001) and in a peer-reviewed article in the scientific literature (Ryaboshapko et al., 2002). The second phase of the model intercomparison involved full-scale model simulations of the emission, transport, transformation, and deposition of mercury over Europe for two short periods of 10 to 14 days each and comparison of the modeling results to field measurements of elemental mercury gas, reactive gaseous mercury, and particulate mercury in air. These results were also reported to the EMEP governing body (Ryaboshapko et al., 2003) and are in preparation for publication in a scientific journal. A third phase of intercomparison is now nearing completion which involves model simulations for longer periods of time (up to one year) and comparisons to observations of the wet deposition of mercury.
An evaluation of the CMAQ-Hg model against observations of wet deposition of mercury in North America has also been conducted. Measurements of the wet deposition of mercury obtained from the Mercury Deposition Network (MDN) at 11 locations in the central and eastern parts of the United States (see figure 1) were compared to CMAQ-Hg model simulations during four weeks in the spring of 1995. The results showed relatively good agreement between modeled and observed wet deposition of mercury for the period from April 4 to May 2 (see figure 2). The results shown here are a slight improvement from those described in Bullock and Brehme (2002) due to recent model code optimizations described above. Further CMAQ-Hg model evaluation is being performed against MDN data collected throughout the year of 2001 all across the United States and southern Canada. Results from these studies are expected to be published in various EPA reports and in the scientific literature.
Figure 2
- Bullock, O.R., Jr. and Brehme, K.A. (2002) Atmospheric mercury simulation using the CMAQ model: formulation description and analysis of wet deposition results. Atmospheric Environment 36, 2135-2146.
- Ryaboshapko, A., Ilyin, I., Bullock, R., Ebinghaus, R., Lohman, K., Munthe, J., Petersen, G., Seigneur, C., and Wängberg, I. (2001) Intercomparison study of numerical models for long-range atmospheric transport of mercury - Stage I.. Comparison of chemical modules for mercury transformations in a cloud/fog environment. MSC-East Technical Report 2/2001 (available at http://www.msceast.org/publications.html)
- Ryaboshapko, A., Bullock, R., Ebinghaus, R., Ilyin, I., Lohman, K., Munthe, J., Petersen, G., Seigneur, C., and Wängberg, I.. (2002) Comparison of mercury chemistry models. Atmospheric Environment 36, 3881-3898.
- Ryaboshapko, A., Artz, R., Bullock, R., Christensen, J., Cohen, M., Dastoor, A., Davignon, D., Draxler, R., Ebinghaus, R., Ilyin, I., Munthe, J., and Syrakov, D. (2003) Intercomparison study of numerical models for long-range atmospheric transport of mercury - Stage II. Comparison of modeling results with observations obtained during short-term measuring campaigns. MSC-East Technical Report 1/2003 (available at http://www.msceast.org/publications.html)