TABLE 3
Typical ambient air concentrations and atmospheric duration of mercury species
Concentration
Henry’s Law
Temporal and
3
Constant
Spatial Scale
ng/m
Elemental Mercury: – Hg 0
1 – 3
0.3
Global Lifetime:
Months to a year
Divalent Mercury (e.g. HgCl2)
0.01 – 0.1
0.00004
Local/Regional Lifetime:
~ 1 – 10 days
Particulate Mercury – Hg(p)
0.01 – 0.1
Local/Regional Lifetime:
~ 1 – 10 days
Oxidized mercury Hg(II)
Lin and Pehkonen, 1999).  Each of the above three forms of
mercury has distinctly different wet and dry deposition
When in the gas phase, this form of mercury is frequently
behavior, and the interconversion of one form to another
referred to as reactive gaseous mercury.  Little is known
in the atmosphere must be considered in any realistic
about the actual species that comprise the pool of oxidized
atmospheric model.
mercury in the atmosphere, although one species of
probable significance is mercuric chloride (HgCl2).  This
Elemental mercury in the gas phase can be oxidized to
form of mercury is defined to some extent through its
Hg(II) by a number of agents, including ozone (O3) (Hall
behavior in measurement devices and by its water solubility
1995), hydrogen peroxide (H202) (Tokos et al., 1998),
and/or its lower volatility or relative adherence to some
chlorine (Cl2), (Calhoun and Prestbo, 2001) and hydroxyl
surfaces.  It predominantly exists in the gas phase and in
radical (OH) (Sommar et al., 2001), and, in the aqueous
atmospheric water droplets (where they are present).  It
phase (e.g. cloud droplets), by several different oxidizing
also has an affinity for and can be adsorbed to soot in the
species, including O3 (Munthe 1992), OH-1 (Lin and
atmosphere, and can be associated with this substance
Pehkonen, 1997), hydrochlorous acid (HOCl), and hy-
particularly within the aqueous droplets.  At most locations,
pochlorite ion (OCl-1 )(Lin and Pehkonen, 1998).
oxidized mercury makes up only a small percent of the
total mercury in the atmosphere.
There also appear to be important heterogeneous halogen-
mediated oxidation reactions that play a very significant
Particulate mercury Hg(p)
role in the “Arctic sunrise” mercury depletion events
(Lindberg et al., 2002).  Measurements of relatively
Perhaps even less is known about the precise species that
elevated concentrations of Hg(II) at high altitudes (Landis
make up the pool of particulate mercury in the atmo-
2001) suggest that these or similar reactions may also be of
sphere.  This form of mercury is also defined operationally,
significance in the upper atmosphere, but this possibility
as the amount of mercury associated with particulate
requires further examination.
matter in the atmosphere.  This is not a reference to
discrete particles of mercury, but rather to mercury species
In addition, divalent mercury Hg(II) can be reduced to
within and on the surface of atmospheric particles.
elemental mercury (Hg0) by sulphurous acid (HSO3-1),
Mercuric oxide (HgO) may be an important Hg(p) species,
formed in pH-dependent amounts from dissolved sulphur
but this is not known with any certainty.  It is likely that, on
dioxide (SO2).  Until recently, it was thought that the
average, particulate mercury may be less bioavailable after
hydroperoxyl radical (HO2) also reduced Hg(II), but recent
being deposited to ecosystems.  Like oxidized mercury, this
determinations by Gardfeldt and Jonnson (2003) have
form of mercury also typically accounts for only a small
indicated that this reaction may be of much less importance
percent of the total concentration of atmospheric mercury
than it originally appeared to be.
at most locations.
The chemical behavior of Hg(p) is not well understood.
Interaction of Mercury Species and
For example, the extent to which it is dissolved whenever
Other Constituents in the Atmosphere
the particle it is associated with becomes a droplet is
uncertain, and the assumed solubility varies among
The atmospheric chemistry of mercury is complex
different atmospheric chemistry models.  Moreover, it is
(Ryaboshapko et al., 2002, 2003; Schroeder et al., 1991;
known that there is a reversible adsorption/desorption of
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