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Mercury Modeling

EPA

has been working on the development of atmospheric mercury models since the early 1990's when the Regional Lagrangian 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, EPA’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 EPA.

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. 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 mercury simulation capabilities in CMAQ due to the increasing number of pollutants with which mercury is known to react.

EPA 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 and results were published in Ryaboshapko et al. (2002). The second phase of the 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.  Results from this phase of the study 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 this phase of the study were reported in Ryaboshapko et al. (2007b).

As the EMEP study was nearing completion, EPA 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 initial condition and boundary conditions developed from a different global model.  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 from the MDN and Underhill observations. 

Results from the NAMMIS statistical model evaluation are shown in the table below.  For most of the evaluation metrics, CMAQ was found to have superior agreement to the observations.  Results from all three of the regional models tested (CMAQ, REMSAD and TEAM) varied depending on the global model used to define lateral boundary concentrations for mercury (CTM, GEOS-Chem or GRAHM).  All of the regional models used meteorological data provided by the MM5 model. Statistics for the precipitation data obtained from MM5 are also shown in the table.  Obviously, the level of accuracy one can expect from the regional air quality models is limited by the accuracy of the input precipitation data.  It does appear that the best performing air-quality simulations have about the same level of accuracy as the precipitation data provided to those simulations.  Thus, it can be reasoned that significant improvements in the simulation of mercury wet deposition are contingent upon improvements in the modeling of physical meteorology.  Complete descriptions of the NAMMIS study design, participating models, and modeling results are available in two articles published in the Journal of Geophysical Research (Bullock et al., 2008; Bullock et al., 2009).

CMAQ mercury modeling capabilities have been applied to support various EPA regulatory actions for mercury. 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. EPA is currently working with the United Nations Environment Program towards the development of international treaties to reduce mercury emissions to the environment. EPA is working to expand the CMAQ modeling domain to cover the Northern Hemisphere. This will allow CMAQ to provide modeling assessments of the intercontinental transport of mercury and its importance as a global pollutant.


Table 1. Evaluation statistics from the North American Mercury Model Intercomparison Study


 

Contacts: Russell Bullock

Relevant Publications & Presentations:

 

Atmospheric Modeling

Research & Development | National Exposure Research Laboratory

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