remoteness and the presence of adjacent relatively large
point and areal sources.  For Lake Michigan, the contribu-
tion from the Chicago region stands out, due to its signifi-
cant emissions and proximity to the lake; this particular
region appears to be somewhat important for the other
lakes as well.  For Lake Erie and Lake Ontario, contribu-
tions from the Ohio River Valley appear to be very signifi-
cant, again, due to the high emissions in this region and the
comparative proximity to these lakes.  In general, for each
of the lakes, contributions from the United States appear to
be somewhat greater than contributions from Canada.
The geographical distribution of mercury contributions is
illustrated in another manner in Figure 20, where the
emissions and contributions are shown as a function of
distance away from each lake.  For Lake Superior and Lake
Huron, significant deposition occurs from sources 200-
consider the merits of various proposed mercury emission
1500 km away from the lakes.  For the other Great Lakes,
reduction programs for this sector.
there is a more significant contribution from sources closer
to the lakes, but, even for these lakes, the regional and
long-range contribution is significant.
1.6.4
Model Evaluation
Atmospheric Deposition Contributions of Mercury
In any modeling study, it is important to ground-truth the
from Different Source Types
predictions to ensure that the simulations are providing
reasonable results.  Since the science of atmospheric
In light of the limitations outlined in the discussion of
mercury modeling is evolving, and there are still significant
emissions inventories earlier in this report, ascribing por-
uncertainties in the understanding of key processes, such
tions of the deposition to different source categories is an
model evaluation exercises are perhaps even more critical.
imprecise exercise.  However, preliminary estimates of the
impact of different source categories on deposition to the
Previous HYSPLIT Model Evaluations
Great Lakes can be developed and are shown in Figure 21.
As noted earlier, the HYSPLIT model has been extensively
This figure illustrates the per capita contribution from four
evaluated over many years for simulations of a variety of
broad source categories comprising the total inventory -
compounds, and has been shown to be capable of provid-
fuel combustion, incineration, metallurgical operations,
ing reasonable simulations.  For example, as mentioned
and manufacturing.   This last category also includes
earlier, many aspects of this modeling methodology were
emissions from a few other non-manufacturing categories.
used in a recent dioxin modeling analysis that provided
One feature is the apparent relative importance, even on a
results consistent with available ambient measurements.  In
per capita basis, of fuel combustion in the United States to
addition, this mercury modeling methodology has been
mercury deposition in the Great Lakes.  Most of the
applied in an ongoing mercury modeling intercomparison
mercury emissions from this fuel combustion category
study by Meteorological Synthesizing Center-East for the
come from various coal combustion activities (see Figure
European Monitoring and Evaluation Program
10), predominantly the coal fired electrical utility sector.  In
(Ryaboshapko et al., 2003) over a European modeling
Canada, by comparison, on a per capita basis, metallurgical
domain, along with several other atmospheric mercury
operations appear to have been a dominant source sector
models from research groups around the world.  In this
in 1996.
exercise, the HYSPLIT_4-based mercury-modeling method-
ology showed a very encouraging capacity to simulate
As noted earlier, during and since the period under
atmospheric mercury, and was comparable in its capabili-
consideration by the model - the mid point of the previous
ties to the other models.
decade - substantial reductions have been achieved in
emissions from both municipal and medical waste incinera-
Comparison with Results from the Lake Michigan Mass
tion.  The Canadian metallurgical operations have subse-
Balance Study
quently also lowered their mercury emissions dramatically.
However, emissions from the coal fired electrical utility
A ground-truthing exercise has been carried out by
sector have remained relatively stable  while governments
comparing the model predictions deposition to Lake
36