Environmental Engineering Reference
In-Depth Information
with in situ photochemical production at background sites.
At more urban sites in Baltimore, Maryland, they reported
that local sources contributed to RGM concentrations.
While these measurements of RGM suggest the infl uence of
photochemical production, the major oxidation reactions
producing RGM remain uncertain. The details and uncer-
tainties surrounding mercury oxidation and reduction
reactions are discussed further in the next section.
Some aircraft measurements of Hg(0) show relatively
constant levels as altitude increases (Banic et al., 2003),
while others show depletion of Hg(0) with increasing alti-
tude (Friedli et al., 2004; Talbot et al., 2007). A number
of measurements of RGM at altitudes above the surface
(Landis et al., 2005; Sillman et al., 2007; Swartzendruber
et al., 2006) have shown that RGM is higher there than at
sea level. Swartzendruber et al. (2006) measured RGM at Mt.
Bachelor, Oregon (2.7 km), and observed RGM enhance-
ments up to 600 pg m
-3
at night, associated with downslope
fl ows of free tropospheric air. Sillman et al. (2007) reported
aircraft measurements in the free troposphere (up to 4 km)
between 10 and 250 pg m
-3
, with concentrations increas-
ing with higher altitudes. As total mercury is expected to
be conserved, RGM increases with higher altitudes are con-
sistent with aircraft measurements that show depletion of
Hg(0) with increasing altitude, such as the measurements
of Talbot et al. (2007) of near-total depletion of Hg(0) in
the stratosphere. Single-particle measurements have mea-
sured mercury attached to particles around the tropopause
(Murphy et al., 2006). Mercury is thought to adsorb to
elemental carbon (soot) particles (Seigneur et al., 1998),
but the dynamics of gas-particle exchange for mercury are
not well understood. The infl uence of high-altitude mer-
cury on the surface is uncertain, but this question has been
explored with models as discussed below.
a very rapid decline, accompanied by concurrent increases
in RGM. Hg(0) is depleted and recovers in a series of such
events, which have been measured throughout the Arctic,
sub-Arctic, and Antarctic coasts (Steffen et al., 2008).
These depletion events are highly correlated with deple-
tion events of tropospheric ozone in the Arctic (Simpson
et al., 2007), which are caused by reactions involving halo-
gen chemistry. It is thus thought that halogens, specifi cally
Br, are responsible for AMDEs. At present, it is unknown
how much of the depleted mercury remains in the eco-
system, and how much is revolatilized to the atmosphere,
during AMDEs. Some measurements have indicated that
much of the deposited mercury is revolatilized, but this
remains a topic of active scientifi c interest. In particular,
this is because mercury is of concern in Arctic ecosystems
because of its accumulation in sensitive food chains (Arctic
Monitoring and Assessment Programme [AMAP], 2002).
Mercury deposition during the springtime period of pro-
ductivity could thus contribute to these levels.
Reduction of Hg(II) to Hg(0) is an uncertain process in
the atmosphere. Hg(II) is known to be reduced to Hg(0)
in natural waters, and this process has been observed to
occur in rainwater. It has been hypothesized that an aque-
ous reaction could reduce Hg(II) in the atmosphere, though
its exact mechanism remains unknown. Hg(II) can also
be reduced in power plant plumes (Vijayaraghavan et al.,
2008). The extent to which reduction of Hg(II) occurs in
the atmosphere is important both for the global budget
(Lin et al., 2007) as well as for regional chemistry. As reduc-
tion produces the longer-lived Hg(0), it can lengthen the
lifetime of mercury, and/or reduce regional deposition of
anthropogenically emitted Hg(II). Thus, better constraints
on the oxidation and reduction reactions of mercury are
critical for policy.
Oxidation and Reduction Processes
Deposition Processes
Hg(0) is converted to Hg(II) by oxidation in the atmo-
sphere, which is thought to be a photochemically driven
process. Based on laboratory data, it was previously
thought that O
3
(Hall, 1995) and OH radicals (Pal and
Ariya, 2004; Sommar et al., 2001) were the primary oxi-
dants of mercury in the global atmosphere. However,
more recent theoretical research has demonstrated that
the reactions with O
3
and OH are unlikely to occur under
atmospheric conditions (Calvert and Lindberg, 2005).
At present, it is thought that Br could be the dominant
global oxidant of mercury (Holmes et al., 2006; Seigneur
and Lohman, 2008) and measurements have established
kinetic parameters for its reaction with Hg(0) (Donohoue
et al., 2006).
In polar regions, observations of Hg(0) and RGM have
shown that these species exhibit unusual behavior in
springtime. Shortly after Arctic sunrise, a series of so-
called Atmospheric Mercury Depletion Events (AMDEs)
have been observed to occur, in which Hg(0) levels show
Processes of wet and dry deposition bring mercury from
the atmosphere to the surface. Measurements are available
for wet deposition of mercury through the U.S. Mercury
Deposition Network (MDN) (National Atmospheric Depo-
sition Program, 2009), which was established in 1996. The
MDN measures wet deposition of mercury in weekly pre-
cipitation samples at over 100 sites in the United States,
Canada, and Mexico. This is the most extensive network
of wet deposition monitoring data for mercury that is
available, although some stations in Europe also measure
mercury wet deposition as part of the Co-Operative Pro-
gramme for Monitoring and Evaluation of the Long-Range
Transmissions of Air Pollutants in Europe (EMEP, 2009). In
the United States, wet deposition of mercury varies both
regionally and seasonally. The highest measurements of
wet deposition are in the southeastern United States, and
an additional area of elevated deposition has been mea-
sured near Hg(II) sources in the Midwest (e.g., the Ohio
River Valley region, which has a high concentration of coal
