Environmental Engineering Reference
In-Depth Information
power). Deposition can vary seasonally, and is generally
highest in the summer months in the eastern United States.
Dry deposition could be more important than wet depo-
sition to many ecosystems, though few measurements are
available. Methods for mercury dry deposition are not well
developed (Lyman et al., 2009), and thus the total global
magnitude of dry deposition is unknown. Measurements
of the deposition velocity of Hg(II) to forest canopies and
wetlands are very high (Lindberg et al., 1998; Poissant et al.,
2004), as expected for a species of high solubility. The impor-
tance of dry deposition of Hg(0) is unknown. Uptake of Hg(0)
by vegetation is thought to occur at the leaf interior, con-
trolled by gas exchange at the stomata (Lindberg et al., 1992).
While measured deposition velocities for Hg(0) are much
slower than those for Hg(II) (Lindberg et al., 1995; Poissant
et al., 2004), the signifi cantly higher concentrations of Hg(0)
in the atmosphere mean that this could be an important
atmospheric sink. However, as the Hg(0) land-atmosphere
and ocean-atmosphere fl ux is bidirectional, measurements
and models must take this into account in estimating the
net fl ux. Because the total amount of deposition of mercury
is roughly equal to its source to the atmosphere, the total
amount of dry deposition in particular is a key constraint in
the global biogeochemical budget of mercury.
and Selin et al. (2007, 2008) showed that GEOS-Chem agreed
with mean concentrations at land-based sites as well as spa-
tial variations. A large number of models have also been com-
pared with constraints from MDN measurements (Bullock et
al., 2009; Seigneur et al., 2004; Selin and Jacob, 2008).
Lin et al. (2006, 2007) have used the CMAQ mercury
model in an extensive evaluation of the sensitivity of
the atmospheric behavior of mercury to different model
assumptions about chemistry and deposition processes.
They suggested that chemical speciation and kinetics intro-
duce the greatest uncertainties in atmospheric mercury
modeling. Bullock et al. (2008) conducted a model inter-
comparison of the regional mercury models CMAQ (Bullock
and Brehme, 2002), Regional Modeling System for Aerosols
and Deposition (REMSAD) (ICF, 2005), and the Trace
Element Analysis Model (TEAM) (Pai et al., 1997). They
found signifi cant differences among the models, driven
both by initial and boundary conditions and by model
processes. In their study, initial and boundary conditions
were supplied by three different global models, CTM-Hg
(Shia et al., 1999; Seigneur et al., 2001), GEOS-Chem (Selin
et al., 2007), and the Global/Regional Atmospheric Heavy
Metals (GRAHM) Model (Dastoor and Larocque, 2004). For
some mercury species, monthly average boundary condi-
tions varied by over an order of magnitude, especially at
higher altitudes. Bullock et al. (2009) compared wet deposi-
tion measurements to output from these models, and found
that adjusting for errors in precipitation data improved the
agreements between models and observations.
One application of mercury modeling that is of particu-
lar interest to policy makers involves diagnosing and attrib-
uting the sources of mercury in deposition. Seigneur et al.
(2004) used CTM-Hg to calculate that on average 25-32%
of deposition to the United States is from North American
sources, but at some locations their contribution was as high
as 81%. They also estimated that Asian sources contributed
5-36%. Cohen et al. (2004) used the Hybrid Single-Particle
Lagrangian Integrated Trajectory (HYSPLIT) model to inves-
tigate the sources of mercury to the Great Lakes, and found
that coal combustion was the largest contributor. Selin and
Jacob (2008) estimated, using the GEOS-Chem model, that
North American sources contributed 20% on average to
U.S. deposition, exceeding 50% in the industrial Midwest
and Northeast. They also estimated that high-altitude RGM
contributed over 50% to U.S. deposition, in particular con-
tributing to high levels of deposition in the U.S. Southeast in
summertime from convective scavenging.
Atmospheric Models and Applications
Atmospheric models can help to constrain uncertainties in
the global mercury cycle, evaluate the importance of vari-
ous chemical reactions, and assist in policy-making appli-
cations. A variety of atmospheric models have been applied
to mercury at scales from regional to global (Bullock et al.,
2008). In addition, modeling applications have been used
to estimate the global biogeochemical budget of mercury.
Lamborg et al. (2002) used a simple, multibox model of
mercury to estimate the present-day and preindustrial global
biogeochemical budgets of mercury, constrained by the inter-
hemispheric gradient and the enhancement of deposition
since the preindustrial era. Mason and Sheu (2002), in con-
trast, scaled up from individual measurements to estimate
preindustrial and present-day cycles. Sunderland and Mason
(2007) used an ocean cycling model to assess preindustrial
and present ocean fl uxes, and Selin et al. (2008) constructed
preindustrial and present-day cycles using a coupled three-
dimensional land-ocean-atmosphere model. Estimates of
global mercury fl uxes vary, with the largest uncertainties in
fl uxes to and from the ocean and land (Selin, 2009).
Despite the uncertainties in modeling mercury, substantial
insights can nevertheless be gained from their application in
combination with measurements. Most mercury models show
reasonable agreement with data on atmospheric Hg(0) and
wet deposition (although, as suggested by Lin et al. [2006],
it should be recognized that model uncertainties could be
compensating for each other). For example, Shia et al. (1999)
reported agreement with spatial and seasonal trends for the
Chemical Transport Model for Mercury (CTM-Hg) model,
Future Challenges
Though concentrations of mercury in the atmosphere
are low, it is atmospheric transport that makes mercury
a global pollution concern. Understanding the pathways
by which mercury travels long distances in the environ-
ment thus requires a better understanding of the reactions
and processes that mercury undergoes in the atmosphere.
Search WWH ::




Custom Search