NERL Research Abstract

EPA's National Exposure Research Laboratory
GPRA Goal 1 - Clean Air
	APMs # 233 and # 528	

Significant Research Findings

Extension of the EPA Models-3/

Community Multiscale Air Quality Modeling System
to Atmospheric Mercury

Purpose	Many areas of the United States currently have public advisories regarding the

consumption of mercury-contaminated fish. Atmospheric deposition is the
primary source of this contamination in nearly every case. To help identify the
air emission sources of mercury, new CMAQ software has been developed to
simulate the emission, transport, chemical and physical transformation, and wet
and dry deposition of atmospheric mercury. The new pollutant species added to
CMAQ for atmospheric mercury are: elemental mercury (Hg°), mercuric
chloride (HgCl2), mercuric oxide gas (HgO(g)), mercuric oxide aerosol
(HgO(a)), and a general mercuric aerosol (HgA) resulting from the evaporation
of cloud water containing various dissolved mercury compounds.

Research The CMAQ model with the new mercury science modules (CMAQ-Hg) has
Approach been developed from the June 2000 CMAQ public release version. Current
understanding of atmospheric mercury indicates that emissions of sulfur
dioxide, volatile organic compounds, and carbon-rich particulate aerosols have
important effects on chemical and physical transformations of mercury in air
and in cloud water. Thus, the CMAQ's "one-atmosphere" approach involving
comprehensive multi-pollutant simulation is well suited to the study of
atmospheric mercury cycling and the eventual assessment of source-receptor
relationships. In addition to the pre-existing CMAQ model chemistry, the
CMAQ-Hg model simulates two gas-phase reactions, eight aqueous-phase
reactions, and six dissociation equilibria for mercury and mercury compounds.
These reactions were all identified in the scientific literature along with their
associated rate and equilibrium coefficients. In addition to chemical reactions,
a physical adsorption of all divalent mercury compounds to carbon aerosol
suspended in cloud water is also simulated. Anthropogenic emissions of
mercury for the CMAQ-Hg model are based on an emissions inventory
developed at EPA's Office of Air Quality Planning and Standards and
chemical/physical emissions speciation assumptions developed at NERL.

National Exposure Research Laboratory - September 2000

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Major	A 24-hour simulation of emission, transport, transformation, and wet and dry

Findings	deposition of mercury, along with all the standard pollutant species of the

CMAQ model, was performed for a northeast United States test domain for
July 13, 1995. While the spatial coverage of precipitation for this period was
rather small, the cloud chemistry of the model was active over a large fraction
of the model domain, especially during the afternoon and evening. Most of the
simulated dry deposition of mercury for this modeled period is attributable to
dry deposition of emissions of HgCl2 gas before they are involved in cloud
chemistry and wet deposition. Both the wet and dry deposition patterns show
maximum values for the 24-hour simulation period that are reasonable based on
general observations in the United States over the past few years. Despite
significant measurement uncertainties that remain in the atmospheric chemistry
and emissions of mercury, this research version of CMAQ-Hg can be used now
along with supporting observational data to help identify the most important
uncertainties in the context of current research regarding atmospheric mercury
cycling. As the understanding of atmospheric mercury improves, advances can
be efficiently incorporated into the modular CMAQ modeling framework.

Research

Collaboration

and

Publications

The research leading to this new version of the CMAQ model benefitted from
collaborative work for atmospheric chemistry of mercury with scientists at the
University of Michigan under EPA Cooperative Agreement CR827079, and by
NERL's participation in a mercury model intercomparison involving modeling
researchers from the United States, Germany, Sweden, and Russia.

Bullock, O.R., Jr. Current methods and research strategies for modeling atmospheric mercury.

Fuel Processing Technology 65/66: 459-471, 2000.

Future	Model simulations will be conducted to study the CMAQ model's ability to

Research replicate mercury wet deposition patterns over the United States and also to
perform diagnostic studies using intensive surface and aircraft data collected
over Florida.

Questions about CMAQ-Hg modeling system may be directed to:

O. Russell Bullock, Jr.

U.S. Environmental Protection Agency

National Exposure Research Laboratory (MD-80)

Research Triangle Park, NC 27711

Phone: (919)541-1349

E-mail: bullock.russell@epa.gov

National Exposure Research Laboratory - September 2000

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