both the amounts of each emitted from each source (i.e. a
speciated emissions inventory) and the rates of the
interconversion processes, which vary from point to point
and over time in the atmosphere.  These interconversion
processes are dependent on a number of factors, including,
for example, the presence and concentrations of other
compounds in the atmosphere that oxidize or reduce
mercury.
1.6
It is also important that ambient monitoring programs
attempt to make measurements of each of these different
forms; however, there are very few such measurements
made.  This is another of several limitations precluding a
comprehensive understanding of atmospheric mercury
phenomena.
dissolved Hg(II) to and from soot surfaces within a droplet
(Seigneur et al., 1998), but, the relationship of adsorbed
Conceptual Model of Atmospheric Mercury Deposition
Hg(II) species and Hg(p) species has not been well
characterized.  In this NOAA-HYSPLIT modeling, it was
The conceptual model emerging for atmospheric mercury
assumed that Hg(p) is insoluble, but Hg(II) formed
deposition to the Great Lakes (and other water bodies) can
reversible complexes with soot in aqueous droplets.
perhaps be summarized as follows (see for example, Vette
et al., 2002).
Emissions:  Speciation and Subsequent
Transformations
The dry and wet deposition of mercury to the lakes is
generally dominated by the Hg(II) and Hg(p) forms.
Most natural emissions or re-emissions of previously
deposited mercury are believed to be primarily elemental
In the lake, there are a number of conversion pro-
mercury (e.g. Schroeder and Munthe, 1998; Scholtz et al.,
cesses, and some of the deposited mercury is eventu-
2003; Gustin 2003).  However, emissions from many
ally transformed to methylmercury, the most environ-
significant current anthropogenic sources, such as coal-
mentally significant species, and is then available for
fired electrical utilities or municipal or medical waste
bioaccumulation in the ecosystem.
incinerators, are often mixtures of the three forms.  An
estimation of the proportions of the various forms from
0
In the lake, some mercury - believed to be largely Hg
different source types will be presented later in the
(Vette et al., 2002) - resides in the water column in
discussion of specific emission data.
dissolved form or associated with suspended sedi-
ments.
As mentioned earlier, each of the above forms of mercury
can be transformed into the other in the atmosphere.
Some mercury in the lake is incorporated into the
Because these reactions are relatively prolonged, and given
sediments; it may reside there or be resuspended, or
that wet and dry deposition of elemental mercury is a
be incorporated, after conversion to methylmercury by
relatively inefficient process, the atmospheric lifetime of
bacteria in the sediments, into the food chain.
Hg0 is believed to be on the order of 0.5-1 year (e.g. Tokos
et al., 1998) allowing for the global distribution of this
Some elemental mercury - and potentially a small
mercury species.  Background concentrations of approxi-
amount of the methylmercury (Rolfhus et al., 2003) -
mately 1.5-2.5 ng/m3 arising from this global circulation are
in the lake may volatilize from the lake surface for
found worldwide, even in the absence of local sources.
transport via the atmosphere to other locales, repre-
senting a loss of mercury from that particular
The other forms of mercury are more readily deposited by
waterbody.
wet and dry processes, and likely have typical atmospheric
lifetimes of a few days to a few weeks.  Thus, these forms,
0
This volatilization or surface exchange process of Hg
when emitted, typically have more local and regional
across the lake’s surface is believed to be comparable
impacts.
to that of PCB and other similar semi-volatile pollut-
ants; the direction and magnitude of the net flux will
Because of the distinct atmospheric deposition behavior of
depend on the water and air temperatures, wind
the different species, it is crucial to have good estimates of
speed, and other meteorological and aquatic variables,
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