Atmospheric Mercury -- Global Biogeochemical

Cycles, Sources, and Sinks

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height="4.569444444444445in"}

#### 

#### Richard D. Doctor, John A. Taylor, and Jack D. Shannon

####### 

####### Argonne National Laboratory

######## 9700 S. Cass Avenue

**Argonne, Illinois 60439**

#### January 2000

> > The submitted manuscript has been created by the University of
> > Chicago as Operator of Argonne National Laboratory ("Argonne") under
> > Contract No. W-31-109-Eng-38 with the U.S. Department of Energy. The
> > U.S. Government retains for itself, and others acting on behalf, a
> > paid-up, nonexclusive, irrevocable worldwide license and said
> > article to reproduce, prepare derivative works, distribute copies to
> > the public, and perform publicly and display publicly, by or on
> > behalf of the Government.

########   {#section-2 .unnumbered}

######## Contents

1\. Overview [1](#overview)

2\. Terrestrial Sequestration of Mercury by the Biosphere
[1](#__RefHeading___Toc459010252)

3\. Preindustrial Mercury Flux Estimates 2

4\. Sources of Anthropogenic Mercury Emissions
[2](#__RefHeading___Toc459010256)

> 4.1 Coal and Increasing Mercury Emissions from Market Shifts to
> Western Coals 3
>
> 4.2 Studies of Mercury Emissions from Coal-fired Power Plants 3
>
> 4.3 Studies of Mercury Emissions from Smelters 3
>
> 4.4 Studies of Mercury Emissions from Chloralkali Plants, MSW Sludge,
> and
>
> Gold Refining 4
>
> 4.5 Evidence for High Foreign Mercury Emissions 4
>
> 4.6 Open Questions on Anthropogenic Mercury Emissions
> [4](#__RefHeading___Toc459010258)

5\. Atmospheric Modeling [4](#__RefHeading___Toc459010263)

> 5.1 Eulerian Mercury Transport Models
> [5](#eulerian-mercury-transport-models)
>
> 5.2 Lagrangian Mercury Transport Models
> [5](#__RefHeading___Toc459010265)

6\. Proposed Global Modeling Studies 6

7\. Conclusions [6](#__RefHeading___Toc459010267)

Acknowledgments 7

References 7

## 1. Overview

Measurements of atmospheric mercury (Hg) at remote locations, such as
the North Atlantic, confirm that the levels of mercury in the global
atmosphere are increasing. In spite of considerable effort, estimating
the global movement of mercury through the environment is difficult.
However, anthropogenic emissions of mercury are at least double those
from natural sources and are of such magnitude that they cannot be
attributed to natural sources alone \[Fitzgerald et al. 1998\].
Emissions are considered to be increasing in Asia and decreasing in
North America and Europe.

On the basis of current estimates of the atmospheric burden and the flux
of mercury to the atmosphere, we can estimate the lifetime of
atmospheric elemental mercury (Hgº) to be around one year, while the
lifetime of atmospheric divalent mercury (Hg-II) to be one to three
days. A lifetime of one year implies that Hgº is a global pollutant,
while the much shorter lifetime of Hg-II implies that it is a local to
regional pollutant. Therefore, to understand the link among emissions,
emissions speciation, and final deposition, we need to take a regional
to global approach. Accordingly, in this paper, we propose that the
global biogeochemical cycle of mercury be modeled and studied in detail.
We suggest that an approach combining state‑of-the-art techniques for
inventory estimation with high-resolution global tracer transport
modeling is required to reduce the fundamental scientific uncertainties
associated with the release, transport, transformation, and ultimate
removal through deposition of mercury to the land and ocean.

In the following sections, we review the natural cycle of atmospheric
mercury, industrial and natural emission sources, and atmospheric
modeling studies. In Section 6, we propose a global modeling study of
the biogeochemical cycles of atmospheric mercury.

[]{#__RefHeading___Toc459010252 .anchor}***2. Terrestrial Sequestration
of Mercury by the Biosphere***

Considering the global biogeochemical mercury cycle, Mason et al.
\[1994\] has shown that the atmosphere and ocean are in rapid
equilibrium with respect to atmospheric mercury. The evasion of Hg^0^
from the ocean matches the deposition of mercury, mostly Hg-II, from the
land. Mason pointed to strong evidence that biological processes produce
Hg^0^ from the biological reduction of Hg-II, although the precise
nature of these processes is poorly known. Numerous laboratory and field
studies strongly support the conclusion that the action of phytoplankton
in sunlight converts HG-II to Hg^0^.

While the oceanic inventory of mercury is more than twice that of the
atmospheric inventory (54 Mmol vs. 25 Mmol where Mmol is 10^3^
kg-moles), mercury is deposited into ocean sediments at a rate of less
than 1 Mmol/yr. In contrast, mercury is deposited onto land at a rate of
25 Mmol/yr with a net re-emission to the atmosphere from soils holding
mercury of 5 Mmol/yr. Hence, soils accumulate mercury. The current
inventory on land is estimated to be 5,000  Mmol.

Anthropogenic emissions of the Hg-II form of mercury have large local
impacts, and Mason suggests that 50% of the mercury deposits "locally."
Hence, the deposition of mercury to terrestrial environments is
disproportionately high; 30% of the earth's surface receives 60% of the
atmospheric deposition. The conclusion from a biogeochemical perspective
is that ocean life works very hard to move Hg^0^ from the ocean for
sequestration in the terrestrial biosphere.

***3. Preindustrial Mercury Flux Estimates***

Mason et al. \[1994\] estimated pre-industrial terrestrial fluxes to the
atmosphere from natural sources to be 5 Mmol/yr, with a matching
deposition rate of 5 Mmol/yr. At the same time, preindustrial pelagic
fluxes to the atmosphere were estimated to be 3 Mmol/yr, with a matching
deposition rate of 3 Mmol/yr. Mason et al. believe that both the
atmospheric and oceanic inventories of mercury have increased 300% from
preindustrial levels. The current atmospheric mercury inventory is 25
Mmol; in contrast, the preindustrial level is estimated to have been 8
Mmol. The current oceanic mercury inventory is 54 Mmol; in contrast, the
pre-industrial level is estimated to have been 18 Mmol. The rate of
mercury discharge to the oceans attributable to the flow of surface
waters from the land surface is also assumed to be 300% higher than the
preindustrial rate (\~0.3 Mmol/yr), and ocean sedimentation rates are
assumed to be 300% higher than preindustrial rates (\~0.3 Mmol/yr).

[]{#__RefHeading___Toc459010256 .anchor}***4. Sources of Anthropogenic
Mercury Emissions***

For the United States, a recent U.S. Environmental Protection Agency
(EPA) inventory produced a total for all the domestic mercury emissions
of 144 Mg/yr (or 0.7 Mmol/yr) \[EPA, 1997\]. Anthropogenic emissions of
Hgº and Hg-II are believed to be of similar order, but proper speciation
of mercury emissions remains a vexing issue. These values seem easier to
reconcile with the estimates by Pirrone et al. \[1996\]. The EPA reports
that point sources account for 98% of emissions. However, other U.S.
inventories have somewhat reduced the contribution from point sources
and given more weight to nonpoint sources \[Pai et al. 1998\]. Pai et
al. concluded that point sources accounted for about 89% of the total
48‑state mercury emissions of 146.4 Mg/yr. These point sources are
allocated as follows: 46% are from coal burning, 19% are from municipal
waste combustion, 10% are from medical waste combustion, 10% are from
manufacturing (with the production of chlor-alkali compounds as the
leading cause), and 7% are from fuel oil combustion.

Mason et al. (1994) estimated that 50% of mercury emissions would
deposit locally. The link between mercury levels and local sources of
mercury in both Europe and North America frequently appears in the
literature \[Landers et al. 1998\]. The current best estimate of an
average worldwide background deposition rate is \~16 gm/m^2^yr, on the
basis of four reports for North America and Europe. Recent studies
measuring mercury deposition rates of 19 gm/m^2^yr \[Benoit et al.
1998\] are consistent with this value.

Mercury sampling at Alaskan sites provides an integrated sample of
mercury pollution in the Northern Hemisphere; the results of sampling
from these sites show that global mercury emissions have not abated
\[Engstrom and Swain, 1997\]. It is likely that reduced mercury
emissions from regional sources reduce inputs to Midwestern lakes.
Decreased emissions in the United States are probably the result of the
reduced industrial use of mercury, the use of pollution-control
technologies that incidentally capture mercury, a shift from coal to
natural gas for commercial and residential heating, and a decrease in
uncontrolled waste incineration. Increased stack height and other
factors that favor long-distance transport are possible explanations for
this trend.

**4.1 Coal and Increasing Mercury Emissions from Market Shifts to
Western Coals**

Since the largest single source of mercury is the combustion of coal,
what can be said about mercury levels from this resource?

When neutron activation analysis was used to detect mercury in standard
coals in the Argonne Premium coal bank \[Vorres 1990\], there was a
pattern of grade vs. mercury level that requires considerable knowledge
of the coal basin to interpret consistently \[Richaud et al. 1998\].
Hence, while the mercury values obtained for the given samples are
accurate, many more channel samples need to be analyzed to be certain
that the specific premium coal samples were typical of the larger
population.

Among the eastern coals, the Lewiston-Stockton (WV) high-volatile
bituminous mercury level is 0.178 -gm/gm coal, which essentially matches
Illinois #6 high-volatile bituminous at 0.181 ‑gm/gm coal. However, the
Upper-Freeport (PA) medium-volatile bituminous coals show the highest
measured mercury levels: 1.748 -gm/gm. Pocahontas #3 (VA), a
low-volatile bituminous coal, has a mercury level of 0.112 -gm/gm. The
western low-sulfur coals were represented by Wyodak-Anderson (WY), a
subbituminous coal with a mercury content of 0.292 -gm/gm. If mercury in
the premium coal samples accurately reflects their respective basins,
the recent large market movement toward the use of Western coals to
displace Illinois basin coals suggests that mercury emissions have
increased. For every ton of Illinois coal displaced by moist,
low-heating value Wyoming coal, mercury emissions from that point source
may have increased by 210%.

### **4.2 Studies of Mercury Emissions from Coal-fired Power Plants**

Mercury concentrations were surveyed in fish from twenty-three ponds in
the vicinity of a 543 MW coal-fired power plant in Dickerson, Maryland
\[Pinkney et al. 1997\]. A stratified random sampling design was used to
select ponds within zones delineated by a concentric area mapped at 3,
7, 10, and 15 km from the plant. For each pond, mercury concentrations
were measured by atomic absorption spectrometry in sunfish (bluegill or
green sunfish) in all ponds and largemouth bass. Mean mercury
concentrations in the ponds ranged from 0.01 to 0.38 ppm for sunfish and
0.04 to 0.43 ppm for bass. However, the observed pattern of mercury
bioaccumulation did not match the pattern predicted by a wet deposition
model.

### **4.3 Studies of Mercury Emissions from Smelters**

One group \[Ames et al. 1998; Olmez et al. 1998\] identified atmospheric
particulates by origin and found that approximately 55% of the fine
particulate mercury originated from smelters in Ontario and Quebec, and
25% originated from a mixture of regional sources in the Midwestern
United States. The source identification results for the fine aerosols
were confirmed by examining back‑projected, mixed-layer wind
trajectories. Significant declines in fine-particulate mercury
concentrations were attributed to the installation of particulate
controls at one or more of the copper smelters.

**4.4 Studies of Mercury Emissions from Chloralkali plants, MSW Sludge,
and Gold Refining**

Reports \[Lodenius, 1998\] on the dry and wet deposition near a Finnish
chlor-alkali plant using the moss-bag technique yielded an estimate of
net deposition of 480 gm/m^2^yr. Two‑thirds of the deposition was dry
and one-third wet. This deposition is thirty times the background level.
Even higher impacts have been reported around chloralkali plants in
Portugal \[Inacio et al. 1998\].

According to Carpi et al. \[1997\], using municipal sewage waste as a
soil amendment results in an emission rate of 0.0001 gm/m^2^hr mercury.
If this rate could be maintained day and night throughout the year,
which is not possible because there is only a fixed amount of mercury in
the soil, it would result in an annual impact of 876 gm/m^2^yr \[Malm et
al. 1998\]. Reports of bioaccumulation in plants near an urban site
working on gold‑refining and gold shops found mercury to be three
hundred times in excess of background after forty-five days.

### **4.5 Evidence for High Foreign Mercury Emissions**

Reports of anthropogenic mercury emissions nearly double those from the
United States have been attributed to Siberia. A recent investigation of
natural and anthropogenic mercury emissions from Siberia found
quantitative estimates of mercury emissions of 0.32 Mmol/yr for natural
sources and 1.54 Mmol/yr for anthropogenic sources \[Vasiliev et al.
1998\].

[]{#__RefHeading___Toc459010258 .anchor}

**4.6 Open Questions on Anthropogenic Mercury Emissions**

The 1997 EPA mercury balance is not completely accurate and will likely
shift to show a greater influence from manufacturing as the inventory is
refined. Primary mercury production and processing, coke production, and
oil refining were noted, but they were not included in the 1997 report
because insufficient information was available. These are certain to be
significant sources that need to be investigated and quantified further.

[]{#__RefHeading___Toc459010263 .anchor}***5. Atmospheric Modeling***

Atmospheric modeling studies have been undertaken at the local/regional
scale, primarily in the United States. In the following sections, we
briefly review both Eulerian and Lagrangian approaches to modeling
atmospheric mercury and its deposition.

### **5.1 Eulerian Mercury Transport Models**

Pai et al. \[1998\] developed geographically resolved estimates of
annual average mercury emission rates from current anthropogenic sources
in the 48 contiguous United States. These estimates were made by
applying emission factors to individual facility operating data and to
county-wide source activity levels. Emissions were apportioned to an
Eulerian modeling grid system by using point source coordinates and the
fractions of county areas in each grid cell. Most of the emissions in
the inventory are from the combustion of mercury‑containing fossil fuels
and municipal waste, which are located primarily in the mid-Atlantic,
Great Lakes, and Southeast states. Major uncertainties in the emission
factors used to develop the estimates need to be resolved.

Peterson et al. \[1998\] are developing a comprehensive mercury model
system by using the Eulerian reference frame of the Acid Deposition and
Oxidant Model (ADOM) to study the regional transport of atmospheric
mercury species.

[]{#__RefHeading___Toc459010265 .anchor}**5.2 Lagrangian Mercury
Transport Models**

The Advanced Statistical Trajectory Regional Air Pollution (ASTRAP)
model, developed at Argonne, has been used to model the deposition of
mercury to the Great Lakes Basin \[Shannon and Voldner, 1995\]. The
Regional Lagrangian Model of Air Pollution (RELMAP) is being used to
simulate the emission, transport and diffusion, chemical transformation,
and wet and dry deposition of elemental mercury gas, divalent mercury
gas, and particulate mercury from various point and area‑source types
for the seven point-source categories \[Bullock et al. 1998\].

Schroeder and Munthe \[1998\] advocates intercompartmental
(air-water/soil/vegetation) transfer and biogeochemical cycling of
mercury in an effort to improve current models.

Recently the results of an EPRI-funded study looking at the global
transport of atmospheric mercury using the GISS 8x10 degree chemical
transport model were reported \[Shia et al. 1999\]. Their model
considered mercury chemistry, deposition, and emissions. They concluded
that the global atmospheric transport of mercury is important to the
regional deposition in North America. Mercury is a transboundary issue.
They also observed that aqueous chemistry in water droplets may play an
important role in determining mercury's atmospheric lifetime and
deposition. Finally, they noted the need to further reduce the
uncertainties in model parameters in order to be able to reliably
estimate source and receptor relationships. This effort is encouraging
because the call for reducing uncertainties in the model parameters in
fact focuses on explicitly linking the highly significant biological
contributions to atmospheric mercury emissions.

***6. Proposed Global Modeling Studies***

Global modeling studies provide critical information for determining the
sources and sinks of trace gases \[Mulquiney et al. 1993\]. Global
models have been used extensively to study the biogeochemical cycles of
greenhouse gases, such as CO~2~ \[Taylor 1989, 1990, 1995, 1996; Taylor
and Lloyd 1992; Farquhar et al. 1993; Klinger et al. 1995; Taylor et al.
1996\], CH~4~ \[Taylor et al. 1991\], and N~2~O \[Taylor 1992; Bouwman
et al. 1995, 1996\], as well as other trace compounds, such as CO
\[Erickson and Taylor 1992\], with relatively long atmospheric
lifetimes.

We propose to undertake a study that will incorporate our understanding
of both the natural biogeochemical cycles of mercury with estimates of
the global industrial sources and sinks of mercury into a
high-resolution global Lagrangian atmospheric tracer transport model
\[Taylor 1989\]. Previously, this modeling system was applied
extensively to the study of biogeochemical cycles of greenhouse gases
(as noted above). We seek to understand, at the global scale, the
natural biogeochemical cycle and quantity of mercury, the anthropogenic
release of mercury, and its influence on the natural mercury cycle.
Finally, we propose to investigate the mechanisms producing the observed
increase in atmospheric mercury so that we might be able to predict
future atmospheric mercury levels.

We briefly outline the key research activities:

-   Develop a detailed global emissions inventory for mercury from
    anthropogenic sources for inclusion in the global tracer transport
    model.

-   Develop parameterizations for the exchange of mercury between the
    oceans, land surface, and biosphere for inclusion in the global
    tracer transport model.

-   Develop mercury atmospheric chemistry parameterization for inclusion
    in the global tracer transport model.

-   Perform multiyear global tracer transport model integrations for
    comparison with observations. Study the atmospheric transport,
    chemistry, sources, and sinks of mercury. Quantify sources and sinks
    and atmospheric lifetime.

-   Investigate the release of atmospheric mercury from individual point
    sources and regions and its transport and ultimate removal from the
    atmosphere (i.e., the relationship between sources and sinks of
    atmospheric mercury from a global perspective).

[]{#__RefHeading___Toc459010267 .anchor}***7. Conclusions***

Levels of atmospheric mercury are increasing in the remote troposphere,
far from known sources. Given that global mercury emissions are likely
to be increasing rapidly in the future, improving our scientific
understanding of the global biogeochemical cycle of mercury is urgently
needed. To improve our understanding of the biogeochemical cycle of
atmospheric mercury, we propose to develop an integrated global model
that will incorporate the natural mercury cycle and the influence of the
anthropogenic release of mercury. This will allow us to reduce the
scientific uncertainties associated with the sources and sinks of
atmospheric mercury and test assumptions about the mechanisms producing
the observed increase in atmospheric mercury. Mercury is a global
problem that needs to be addressed from a global perspective.

## Acknowledgments

The work reported here was supported by the U.S. Department of Energy,
Assistant Secretary for Science, under Contract W‑31‑109‑Eng‑38.

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