Environmental Engineering Reference
In-Depth Information
The CAMR was fi nalized in March 2005 to reduce
emissions of mercury from coal power plants. This rule
(along with the Clean Air Interstate Rule), would reduce mer-
cury emissions from coal power plants in the United States
from 48 tons/yr to 15 tons/yr by the year 2018. However,
the rule proposes to use a “cap and trade” method, whereby
not all power plants need to reduce their emissions uni-
formly. Under a cap-and-trade system, one plant can reduce
their emissions more than is required and sell the result-
ing “credits” to a plant that did not reduce their emissions
as much, or at all. This could result in “hotspots,” where
mercury deposition remains high and unaffected by the
national emission reductions (e.g., Evers et al., 2007). The
situation is further complicated by several lawsuits regard-
ing the way the United States Environmental Protection
Agency (EPA) regulated mercury via the CAMR. Prior to
2005, mercury was listed as a Hazardous Air Pollutant
(HAP) in section 112 of the U.S. Clean Air Act (see http://
www.epa.gov/oar/caa/caa112.txt). This would require the
“maximum achievable” control of a listed pollutant on a
plant-by-plant basis and would be inconsistent with a cap-
and-trade approach. As part of its March 2005 decision,
mercury was de-listed as a hazardous air pollutant and
the CAMR regulations were put into place by the EPA. As
a result of this action by the EPA, the CAMR rules were
challenged in court by a broad coalition of states, Native
American groups, and an array of health and environmen-
tal organizations. On February 8, 2008, the U.S. Court of
Appeals for the District of Columbia overturned the EPA's
CAMR. Thus, at the time of this writing (mid-2009), the
fi nal form for any rules on mercury emissions from coal
power plants in the United States are in question.
Mercury emissions that result from low-temperature vol-
atilization (e.g., evaporation of mercury from concentrated
mining waste) are emitted nearly 100% as GEM. Sources
that involve high-temperature combustion (e.g., coal com-
bustion or metal smelting) are more likely to contain some
mercury in other forms. Depending on the coal type and
combustion conditions, RGM and PHg could be as much
as 46% of the mercury emissions (Seigneur et al., 2001).
Globally, anthropogenic mercury emissions are believed
to be 53% GEM, 37% RGM, and 10% PHg (Pacyna et al.,
2006). For the United States, emissions are reported as 50%
GEM, 46% RGM, and 4% PHg. For China, the Hg emissions
are reported to be 57% GEM, 33% RGM, and 10% PHg. As
mentioned previously, RGM and PHg will primarily deposit
locally, so the large emissions in China have a signifi cant
contribution to deposition within Asia (Jaffe and Strode,
2008). Unfortunately, the values given above have signifi -
cant uncertainty, and thus limit our ability to model the
relative importance of global versus regional sources at any
particular location.
Reemission of Previously Deposited Mercury
Evidence for reemission of previously deposited mercury
has been shown in a number of studies. For example,
Landis and Keeler (2002) estimated the evasion of mercury
from Lake Michigan to be 38% of the annual wet and
dry deposition fl ux. A study using mercury isotopes in a
Canadian lake showed conclusively that recently deposited
mercury could be reemitted to the atmosphere (Southworth,
et al., 2007). Nearly all reemissions of mercury are in the
form of Hg
0
, regardless of how the mercury entered the
system.
With respect to reemission, the key question is how
much of the emissions from any one source is natural and
how much is due to anthropogenic activities that may have
taken place months to years ago? This question is impor-
tant in that it directs us to identify the natural component
of the global mercury cycle against which human-caused
changes can be understood (e.g., the anthropogenic contri-
bution to Hg in fi sh). However, quantifying the total frac-
tion of current emissions that is natural versus anthropo-
genic is a challenging task. Probably the ice-core records
and the lake-sediment cores, which document historic
deposition trends, are the best evidence for large-scale
changes in global mercury cycling (see the section on “The
Changing Global Cycle of Mercury,” above).
Chemical Speciation
While most natural mercury sources emit Hg
0
(GEM),
this is not the case for anthropogenic emissions, which
consist of a mix of particle-bound mercury (PHg), Hg(II)
compounds (or RGM), and Hg
0
. The relative proportions
of these is specifi c to each facility and source type. The
chemical speciation is important for the simple reason that
the different forms have vastly different lifetimes and thus
vastly different impacts on the local environment. Hg
0
has
a long enough lifetime in the atmosphere (about 1 year)
that it mixes throughout the globe before reentering the
terrestrial cycle, whereas PHg and RGM are removed from
the atmosphere in a matter of hours to days and are thus
much more important for local and regional bioaccumula-
tion. In short, GEM is largely a global problem, whereas
PHg and RGM are of regional concern. A further com-
plication is that in some industries, stack tests are often
only required to measure total mercury, without regard to
the chemical form. For understanding and modeling the
deposition and environmental infl uence, the chemical
form can easily be more important than the total amount
being emitted.
Mercury Emissions: Summary, Uncertainty,
and Validation
Current estimates are that emissions of mercury are
6000-11,000 tons/yr, with the sources divided approxi-
mately equally between natural, direct anthropogenic, and
reemissions from past activities. Table 1.1 gives a sum-
mary of the direct anthropogenic emissions from several
