Sources and Transport - Mercury in the Environment - page 5

per kilogram of body weight ingestion basis (due to
more effi cient absorption in the gut). It is believed
to be present in the atmosphere in only negligible
concentrations, but it is thought to be ubiquitous in the
deeper ocean (Mason et al., 1998).

on larger particles would be excluded from the reported
concentration.
The amount of mercury extracted from the particles
can be dependent on the technique used. Waterborne
particulate mercury is generally determined by
fi ltration, addition of BrCl, reduction with SnCl 2 , and
purging with a clean, inert gas. Measuring airborne
PHg also requires capturing particles on a fi lter. The
fi lters can be analyzed in the aqueous phase using
the previously described technique. Or, the mercury
on particles can be thermally reduced/desorbed, and
quantifi ed as GEM (Landis et al., 2002).

MMHg is monomethylmercury, CH 3 Hg+. MMHg is
signifi cantly more toxic than Hg 0 on a milligrams
per kilogram of body weight ingestion basis (due to
more effi cient absorption in the gut) and readily bio-
accumulates up the food chain (National Research
Council, 2000). MMHg has not been reliably detected in
the open oceans apart from the Equatorial Pacifi c Ocean
(Fitzgerald et al., 2007).

The Changing Global Cycle of Mercury

OPERATIONALLY DEFINED FORMS OF MERCURY
DGHg is dissolved gaseous Hg. It is a fraction of
mercury measured in water that is defi ned by its ability
to be volatilized only by purging with a clean, inert gas.
It includes DMHg (Fitzgerald et al., 2007).

Sediments and Ice Cores as Archives
of Geochemical Cycles

Lake sediment cores and glacial ice cores have been used
as historical records of preindustrial and anthropogenic
deposition. Trace metal and hydrocarbon concentrations
in cores have been shown to accurately refl ect the impact
of industrialization on air concentrations and increased
deposition of pollutants to the earth (e.g., Wong et al.,
1984) and oceans (Véron et al., 1987). Mercury has also
been studied in ice cores and lake sediments, and similar
increases in deposition are seen across a wide range of
geologic and hydrologic environments (e.g., Swain et al.,
1992; Schuster et al., 2002). These records are powerful
evidence of the recent anthropogenic infl uence on the
global Hg cycle.

HgR is reactive Hg dissolved in water. It is defi ned
based on its ability to be volatilized after reduction with
SnCl 2 , and purging with a clean, inert gas (Fitzgerald
et al., 2007).

Hg(II) has also been used as an operationally defi ned
fraction of dissolved Hg. It is determined by subtracting
mercury that is readily volatilized (DGHg) from
reactive Hg (HgR) (Mason et al., 1998). It has been
used as a measure of bio-available mercury, but is
known to not be universally appropriate (Fitzgerald
et al., 2007).

Hg-Col is colloidal mercury. It is mercury associated
with colloidal matter that can be trapped on an
ultrafi ne membrane after fi ltration of larger particulate
matter. Colloidal mercury is generally considered to be
larger than 1000 Da (molecular weight) but smaller than
0.1-0.5 µm (Guentzel et al., 1996).

The Preindustrial Cycle

A diagram of the simplifi ed global mercury cycle is shown
in Figure 1.2 (after Mason and Sheu, 2002). Preindustrial
values are shown in parentheses below the modern val-
ues. The glacial and sediment records have shown that
in the millennium before industrialization, mercury and
other metals had a relatively steady deposition fl ux, with
an occasional perturbation due to volcanic activity (e.g.,
Schuster et al., 2002). This implies that the net fl ux of mer-
cury coming into the atmosphere approximately equaled
the net fl ux deposited to the land and oceans. There is sub-
stantial evasion from the ocean and land, but it is nearly
balanced by deposition. There is also local recycling of
mercury over the ocean surface (not shown) that makes
no contribution to the net evasion or deposition (Strode
et al., 2007). Rivers also make a small contribution to the
open ocean (~1% of ocean reservoir), but this is omitted
from the fi gure for the sake of simplicity. In the preindus-
trial cycle, the annual fl ux into and out of the atmosphere
(~2-4000 tons/yr) is similar to the total airborne bur-
den, suggesting a lifetime for atmospheric Hg of approxi-
mately 1 year (Mason and Sheu, 2002; Selin et al., 2008).
In the oceans, the total burden is more than a factor of

RGM or reactive gaseous mercury, refers to Hg that
can be captured on a KCl surface (Landis et al., 2002).
RGM is believed to consist primarily of gaseous Hg(II)
compounds. It is regarded as the fraction of airborne
Hg that is readily deposited to the surface via wet or
dry deposition. The exact chemical form of RGM is not
known, but likely candidates include HgO (Hall, 1995),
HgCl 2 (Landis et al., 2002), and HgBr 2 (Holmes et al.,
2009). In many reports, RGM and gaseous Hg(II) are
used interchangeably, but it is important to recognize
that RGM is an operation defi nition whereas Hg(II) is a
chemical defi nition.

PHg, or particulate-bound mercury, refers to mercury
that is extracted from particles, either airborne or
waterborne. The observed PHg concentration can be
dependent on the size of particles that are collected; for
example, most airborne PHg measurements include only
particles 2.5 µm (aerodynamic diameter), so mercury