1.6
MODELING OF ATMOSPHERIC MERCURY TRANSPORT AND DEPOSITION
TO THE GREAT LAKES BASIN USING THE NOAA-HYSPLIT MODEL
States to the Great Lakes basin (1997-99 Priorities and
1.6.1
Introduction
1.5
Progress Under the Great Lakes Water Quality Agreement,
1.6
M
1999).
ercury contamination is increasingly recognized as
a critical environmental issue in many parts of the
In addition to the dioxin application, the NOAA-HYSPLIT
world, and the Great Lakes basin is no exception.
model has been extensively evaluated in simulations of the
In addition to significant public health concerns, addressed
transport and deposition of a variety of compounds and
elsewhere in this document, there are substantive eco-
contaminants, and has been shown to be capable of
nomic implications associated with this contamination as
providing outputs and estimates that compare well with
well.  The presence of persistent toxic substances in
ambient measurements and other comparable data (Cohen
freshwater and marine fish has resulted in restrictions on
et al., 1997; Draxler and Hess, 1998; Draxler, 1991, 2000;
consumption of various forms of seafood, and, as shown in
Stein et al., 2000; Rolph et al., 1992, 1993; McQueen and
Figure 3, mercury levels are driving the increased place-
Draxler, 1994, Draxler et al., 1994).  In the dioxin modeling
ment of advisories on individual water bodies.  As limits on
analysis, it also provided results consistent with ambient
human consumption of many commercially- and
measurements (Cohen et al., 2002).  One of the principal
recreationally-important freshwater and marine species are
advantages of using this methodology is that, if adequate
tightened further, the negative economic impact becomes
supporting data, particularly good quality emissions
more pronounced.
inventories, are available unusually detailed source-receptor
relationships can be derived.
Atmospheric deposition is a significant loading pathway for
mercury, and in many cases is the dominant loading
In considering application of the NOAA-HYSPLIT atmo-
pathway to many ecosystems.  Available evidence suggests
spheric transport model to the determination of mercury
that this is true for the Great Lakes.  Mass balance calcula-
emission sources of significance to deposition in the Great
tions for Lake Michigan (Mason and Sullivan, 1997) and
Lakes basin, several issues integral to the nature of mercury
Lake Superior (Dolan et al., 1993) indicate that atmo-
must be examined and managed.
spheric deposition accounts for approximately 75 percent
of the overall mercury loading to these lakes.
Nature of the Contaminant
Building on its experience in modeling the sources and
There are many mercury species of environmental impor-
input of dioxin to the Great Lakes basin, the IAQAB agreed
tance.  However, relatively little is known about the precise
that its contribution under the 2001-2003 biennial Great
speciation of mercury in the atmosphere.  Notwithstanding
Lakes priorities of the IJC would be an attempt to model
this limited understanding, scientists generally classify
the transport and deposition of mercury from continental
atmospheric mercury into three general categories
sources to the basin.
(Schroeder and Munthe, 1998); some selected information
on the behavior of these three classes of mercury in the
Rationale
atmosphere is given in Table 3.
While there have been several mercury modeling efforts in
0
Elemental mercury Hg
North America (Bullock et al., 1998; Bullock and Brehme,
2002; Dvonch et al., 1998; Lin et al., 2001; Pai et al., 1997;
Seigneur et al., 2001; Shannon and Voldner, 1995; Xu et
Elemental mercury is simply atomic mercury.  Its limited
al., 2000a,b,c), none has developed detailed source-
solubility, shown by the value of its Henry’s Law Constant,
receptor relationships for the Great Lakes, as advocated in
results in only small amounts being dissolved in atmo-
Annex 15 of the Great Lakes Water Quality Agreement.
spheric water droplets and its relative volatility means that
During the IJC’s 1997-1999 biennial priority cycle, the
little will be adsorbed onto the surface of aerosol particles;
IAQAB focused on refinement of the application of the
thus, it exists primarily in the gas phase in the atmosphere.
NOAA-HYSPLIT model to the transport and deposition of
In most locations, particularly those at some distance from
polychlorinated dibenzo-p-dioxins and dibenzofurans
large emission sources, the great majority of the mercury in
(PCDD/F or “dioxin”) from across Canada and the United
the atmosphere (~ 95 percent) would be found in this form.
27