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Mercury Modeling

AMAD Research Programs

Model Development

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 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 “multi-pollutant” approach of CMAQ where all pollutants are simulated together just as they exist in the real atmosphere is applied in atmospheric mercury modeling at AMD.

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). Because new information about chemical and physical processes affecting atmospheric mercury is continually being published, refinement of the model code is an ongoing process. The FORTRAN subroutine for the CMAQ aqueous chemistry mechanism is periodically optimized to efficiently calculate mercury chemistry in concert with the standard CMAQ cloud chemistry 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 and studied. The latest public release of CMAQ provides the ability to simulate atmospheric mercury in the “multi-pollutant” version of the model. We found this to be the most efficient way to maintain and disseminate the mercury version of CMAQ due to the increasing number of pollutants with which mercury is known to react.

AMD has participated in two major model intercomparison studies for atmospheric mercury.  The first was the Intercomparison 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. The first phase of this EMEP study involved the simulation of mercury chemistry in a closed cloud volume given a variety of initial conditions. Results obtained from the CMAQ mercury model and the other participating models from Russia, Germany, Sweden, and the United States were compared to identify key scientific and modeling uncertainties. These comparisons were published in a peer-reviewed journal article (Ryaboshapko et al., 2002) and led to some significant changes in some of the participating models, including CMAQ. The second phase of the EMEP study 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. Model simulations were compared to field measurements of elemental mercury gas, reactive gaseous mercury and particulate mercury in air.  The “phase 2” results were reported in Ryaboshapko et al. (2007a). The third and final phase of the EMEP intercomparison involved model simulations for longer periods of time (up to one year) and comparisons to observations of the wet deposition of mercury. Results from “phase 3” of the EMEP study are reported in Ryaboshapko et al. (2007b).

Impact of aerosol feedback on meteorology.

Figure 1: As the EMEP study was nearing completion, AMD organized a second mercury model intercomparison study, this time with a focus on North America. The North American Mercury Model Intercomparison Study (NAMMIS) took advantage of standardized weekly wet deposition samples taken by the Mercury Deposition Network (MDN) as described in Vermette et al. (1995) and separate event-based precipitation samples taken at Underhill, Vermont (Keeler et al., 2005). In addition to CMAQ, two other regional models were tested in the NAMMIS; the Regional Modeling System for Aerosols and Deposition (REMSAD) and the Trace Element Analysis Model (TEAM). All three models were each applied to simulate the entire year of 2001 three times, each time using a different initial condition and boundary condition (IC/BC) data set developed from one of three global models. The NAMMIS provided not only a comparison between regional atmospheric mercury models, but also a measure of the sensitivity of each regional model to uncertainties regarding intercontinental transport. The NAMMIS evaluated each regional model for its agreement to observations of wet deposition of mercury at 63 locations in the U.S. and Canada (Figure 1). Analysis of each model’s average annual wet deposition (Figure 2) found CMAQ in best agreement with observations. Various other statistical comparisons were performed against annual, seasonal and weekly observations. In nearly every case, CMAQ showed superior performance. NAMMIS results regarding model-to-model comparisons are reported in Bullock et al. (2008). Statistical model evaluations against observed wet deposition of mercury are reported in a second paper submitted to the Journal of Geophysical Research in October 2008.

Figure 2: CMAQ mercury modeling capabilities have been applied to support the development of the U.S. EPA’s Clean Air Mercury Rule. They have also been used to provide information regarding mercury deposition from global background concentrations to tribal, state and regional environmental authorities in the development of their water quality protection strategies. AMD plans to maintain and develop atmospheric mercury simulation capabilities in CMAQ to support ongoing environmental assessment and future regulatory action.

Contacts: Russell Bullock

Relevant Publications & Presentations:

 

Atmospheric Modeling

Research & Development | National Exposure Research Laboratory

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