United States Environmental Protection Agency	Office of Research and Development

National Exposure Research Laboratory
Research Abstract

Government Performance Results Act Goal: Clean Air

Significant Research Findings:

Atmospheric Chemistry of Mercury Compounds

Scientific Problem A full assessment of the atmospheric release of compounds listed in the EPA

and Policy Issues	Urban Air Toxics Strategy requires high quality chemical mechanisms that can

be used to determine atmospheric lifetimes and fates. This information is
needed to predict (1) ambient concentrations that affect human exposure levels
and (2) dry and wet deposition rates that control uptakes of compounds to
ecologically sensitive land surfaces and aquatic bodies and potentially lead to
indirect human exposures through ingestion. To address this issue the U.S.
Environmental Protection Agency (EPA) has designed a research program to
develop the necessary information to construct chemical mechanisms for air
toxic compounds. The first stage of the program that began in FY01 consists
of conducting a critical literature review of the atmospheric chemistry of air
toxic compounds. The results will be used (1) to identify research required to
fill in gaps and reduce uncertainties in the chemical mechanisms and (2) to
produce, where possible, literature-based chemical mechanisms. The first set of
compounds to be evaluated are inorganic mercury (Hg) compounds. Volatile
inorganic Hg compounds are emitted into the atmosphere mainly as elemental
mercury (Hg") and to a lesser extent as oxidized compounds (Hg11). Once
emitted Hg" may react with atmospheric free radicals to form Hg11 compounds
that readily deposit by dry and wet processes to land and aquatic surfaces.
Deposited Hg compounds can be transformed by natural biological compounds
into toxic methyl mercury that is bio-concentrated in the aquatic food chain
more than a million fold. Human consumption of Hg-contaminated marine and
fresh water fish can lead to significant adverse health effects from (1) transport
of methyl mercury to the brain through the blood-brain barrier and (2)
penetration into the placenta. The chemical mechanistic data gathered under
this research program will be used to develop chemical mechanisms for air
quality models that will be used by EPA and States to help characterize risk
from and evaluate control strategies for Hg compounds.

Research Approach A review of the peer reviewed literature will be undertaken to assess the state

of science of atmospheric chemistry of inorganic Hg compounds. The review
includes (1) evaluating rate constants for gas phase reactions of Hg" with
reactive gas phase constituents including 03, OH, CI and N03; (2) determining
the extent to which oxidation products of such reactions have been identified;

(3)	assessing similar rate constant and product data for reactions of dissolved
Hg and Hg11 compounds in aqueous media including fog water and cloud water;

(4)	evaluating the status of equilibrium constants for aqueous reactions between
the Hg2+ and atmospherically relevant anions including CI", OH", and S032"; (5)

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assessing the extent to which Hg compounds partition between the gas and
aerosol phases. Based on the literature review, recommendations for what
should be included in a chemical mechanism for inorganic Hg compounds are
presented along with a list of research recommendations for filling gaps and
reducing uncertainties in the mechanisms.

Results and	Results of the literature review reveal that significant uncertainties and gaps

Implications	remain in our understanding of the atmospheric chemistry of inorganic Hg

compounds. The atmospheric chemistry of Hg compounds is highly complex,
far more complicated than that of hydrocarbons involved in smog formation, a
major focus of the EPA's model development efforts over the past 30 years.
Both Hg° and Hg11 compounds are reactive, semivolatile, water-soluble
compounds that can partition between the gas and aerosol phases and between
the gas phase and atmospheric aqueous phases, including cloud water and fog
water. In addition to undergoing gas phase reactions, Hg compounds, upon
dissolution into aqueous media, are subject to a complex series of reactions
with a variety of oxidizing and reducing agents. To date only a limited number
of laboratory investigations have been carried out to investigate these chemical
processes. Based on these data chemical mechanisms have been developed for
predicting ambient Hg concentrations. The complexities of the proposed
chemical mechanisms range from simple mechanisms up to multi-phase
mechanisms that include gas phase reactions, gas-aerosol phase partitioning,
partitioning between the gas phase and ambient aqueous media including cloud
water and fog water, and aqueous reactions involving dissolved Hg
compounds. While existing models are important first generation tools for
evaluating Hg control strategies and characterizing the potential risks from Hg
exposure, the proposed chemical mechanisms remain largely unevaluated and
gaps and uncertainties remain in the parameterizations of key chemical
processes.

The literature review revealed a number of significant uncertainties and gaps in
the gas phase chemistry. To date the gas phase reactions of 03 and OH with
Hg° are the most studied Hg gas phase reactions, although even for these
compounds there are considerable uncertainties in the rate constant
measurements and there are no direct measurements of the oxidation products.
There are no rate constant measurements or product studies for the reactions
of Hg° with other oxidants including H02 and CI. For this reason the gas phase
atmospheric lifetime of Hg°, an important parameter in air quality models,
remains uncertain. The aqueous chemistry, while highly complex, is better
understood than gas phase chemistry. Hg compounds are taken up into
aqueous media, including cloud water and fog water, where they can be
oxidized to Hg11 compounds, some of which may be volatile and can be
released into the atmosphere. On the other hand, dissolved Hg11 compounds
are subject to reactions with reductants, among them H02 radicals, forming
Hg° that can be released into the atmosphere. While many aqueous Hg
reactions have been studied individually, there have been no investigations of
Hg reactions in aqueous media of the chemical diversity of ambient cloud
water, fog water, and precipitation samples that include a wide range of
dissolved reactive molecules as well as many ionic complexes, some of which

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are solids. Finally, the interaction of Hg compounds with ambient aerosols has
only very recently been addressed, and significant uncertainties in the
parameterization of the partitioning mechanism between the gas and aerosol
phases remain.

Based on this review of the literature, a set of guidelines are provided that
describe the chemical processes that should be included in a chemical
mechanism for inorganic Hg compounds. A number of published chemical
mechanisms include many of these processes and may be sufficient to estimate
ambient gas phase Hg° concentrations. However, additional laboratory studies
are required to develop high quality chemical mechanisms for accurately
predicting ambient concentrations of Hg° and Hg11 in both the gas and aerosol
phases as well as in cloud water, fog water, and precipitation. To address this
issue, a set of research recommendations are presented.

This research project directly supports ORD's research to improve the
scientific foundation of human health risk assessment under the Government
Performance and Results Act (GPRA) Goal 1: Clean Air; Objective 2:
Eliminate Risks from Air Toxics; Sub-Objective 1: Conduct Air Toxics
Research. The results of this project address GPRA annual performance goal
(APG) 06: In Fiscal Year 2000, develop an air quality model incorporating (1)
air toxics as their chemistry and emissions become known and (2) source
emissions and control information for both mobile and stationary sources to
guide cost-effective risk management.

Research

Collaboration and

Publications

The review of the atmospheric chemistry of inorganic Hg compounds was
conducted by a National Exposure Research Laboratory staff scientist. This
research findings will be presented at several conferences and are contained in
the following manuscript:

Edney, EO. "Atmospheric Chemistry of Inorganic Mercury Compounds." Journal
of Air and Waste Management Association. (Submitted in
2001.)

Future Research	Future research will include the development of a research program for

carrying out laboratory chemistry studies to fill in gaps and reduce uncertainties
in the atmospheric chemistry of Hg compounds.

Contacts for	Questions and inquiries can be directed to:

Additional	Edward O. Edney, Ph.D.

Information	(JS EPA, Office of Research and Development

National Exposure Research Laboratory
Research Triangle Park, NC 27711

Phone: 919/541-3905
E-mail: edney.edward@epa.gov

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