Environmental Engineering Reference
In-Depth Information
Asian anthropogenic contribution
North American anthropogenic contribution
90°N
90°N
60°N
60°N
30°N
30°N
0°
0°
30°S
30°S
60°S
60°S
90°
180°
90°
180°
120°W
60°W
0°
60°E
120°E
180°
120°W
60°W
0°
60°E
120°E
180°
0
20
40
60
80%
0
20
40
60
80%
European anthropogenic contribution
Global anthropogenic contribution
90°N
90°N
60°N
60°N
30°N
30°N
0°
0°
30°S
30°S
60°S
60°S
90°
180°
90°
180°
120°W
60°W
0°
60°E
120°E
180°
120°W
60°W
0°
60°E
120°E
180°
0
20
40
60
80%
0
20
40
60
80%
FIGURE 1.4
GEOS-Chem global chemical transport model estimates of annual average fraction of deposition that is due to anthropogenic
emissions from Asia, North America, Europe, and all anthropogenic sources for 2004. Note that this is simply the direct deposition that does not
include mercury that has been previously deposited to, and reemitted from the ocean and land. (Figures and modeling work provided by S. Strode.)
generally be dispersed in the midlatitude “pollution belt” in
1-2 weeks (Jacob, 1999). During transport, the airmasses tends
to be stretched into long fi laments and begin mixing into to
the global background. Although the pollution and plumes
can be swiftly transported over long distances at higher alti-
tudes, their impact on the surface obviously depends upon
them descending to the surface, which can be a slow process.
There is also transport and exchange of Hg between the
hemispheres, although this is considerably slower, and
requires on the order of 1 year for exchange to occur. The
slower interhemispheric exchange along with the greater
anthropogenic emissions in the northern hemisphere
creates a very useful property, an interhemispheric gradient,
which is a powerful constraint on the sources and global
lifetime of Hg. The interhemispheric gradient provides
clear evidence for increasing anthropogenic emissions,
especially in the northern hemisphere (e.g., Slemr and
Langer, 1992; Lamborg et al., 2002, Strode et al., 2007).
Industrial emissions of RGM and PHg are relatively short-
lived and deposit to surfaces within a day or two of being
emitted. In contrast, most GEM will be exported from the
source region and continue to mix into the hemispheric
background. Thus, the chemical form of Hg emissions are
key to understanding the impacts. In their modeling study,
Seigneur et al. (2004) show that North American industrial
sources contribute between 25 and 32% to total deposition
within the contiguous United States, but that there is
considerable spatial variability in this value (9-81%). This
is due to the signifi cant emissions of RGM and PHg in the
Eastern United States, which makes a signifi cant contribu-
tion to deposition regionally.
On the other hand, GEM will be intercontinentally
transported in 7-10 days, and within 1-2 months it will be
distributed throughout the hemisphere. Slowly, GEM is oxi-
dized to RGM, which is more readily removed by wet and
dry deposits. While the GEM oxidation mechanism remains
uncertain (Lindberg et al., 2007), its impact is felt in a vari-
ety of environments (Swartzendruber et al., 2006; Steffen
et al., 2008; Weiss-Penzias et al., 2009). This is a key process
that makes Hg a global pollutant. Figure 1.4 shows the per-
cent deposition that can be attributed to emissions from
the major industrial regions: Asia, United States, Europe,
and the sum of all anthropogenic sources. Note that this
includes only the deposition due to recent anthropogenic
sources and does not include anthropogenic mercury that
has been previously deposited and reemitted. Each source
region contributes to global Hg deposition in proportion to
its total emissions (see Table 1.1).
What is clear from Figure 1.4 is that Hg is a global pol-
lutant that knows no boundaries. To substantially reduce
deposition and bioaccumulation of Hg in any part of the
world will require reductions in the global emissions.
