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Effectiveness of Emission Controls to Reduce the Atmospheric Concentrations of Mercury Mark Sam Castro, and John Sherwell Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03576 • Publication Date (Web): 13 Nov 2015 Downloaded from http://pubs.acs.org on November 24, 2015
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Effectiveness of Emission Controls to Reduce the Atmospheric Concentrations of Mercury
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Mark S. Castro1* and John Sherwell2
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301 Braddock Road, Frostburg, Maryland 21532
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Appalachian Laboratory, University of Maryland Center for Environmental Science,
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Power Plant Research Program, Maryland Department of Natural Resources, Annapolis, Maryland 21401
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*Corresponding author phone: 301 689-7163, e-mail
[email protected] 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
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ABSTRACT: Coal fired power plants in the United States (US) are required to reduce their
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emissions of mercury (Hg) into the atmosphere in order to lower the exposure of Hg to humans.
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The effectiveness of power plant emission controls on the atmospheric concentrations of Hg in
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the US is largely unknown because there are few long-term high-quality atmospheric Hg data
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sets. Here we present the atmospheric concentrations of Hg and sulfur dioxide (SO2) measured
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from 2006 to 2014 at a relatively pristine location in western Maryland that is several (>50 km)
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kilometers downwind of power plants in Ohio (OH), Pennsylvania (PA), and West Virginia
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(WV). Annual average atmospheric concentrations of gaseous oxidized mercury (GOM), SO2,
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fine particulate mercury (PBM2.5), and gaseous elemental mercury (GEM) declined by 75%,
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75%, 43%, and 13%, respectively, and were strongly correlated with power plant Hg emissions
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from the upwind states.
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emissions from power plants in the US had their intended impact to reduce regional Hg
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pollution.
These results provide compelling evidence that reductions in Hg
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1. INTRODUCTION
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Mercury pollution is a serious threat to human health throughout the world. Many people,
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particularly pregnant women and their fetuses, and young children are highly susceptible to the
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neurological effects of methyl Hg (1,2). Human exposure to methyl Hg results primarily from the
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consumption of contaminated fish. In the US, all fifty states had fish consumption advisories in
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2011 because of the high concentrations of methyl Hg in many species of fish (3). The methyl
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Hg in these fish is formed from inorganic Hg that originates from both natural and anthropogenic
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sources.
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Important anthropogenic sources of Hg in the US have included power plants and waste
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incinerators. In 1990, coal fired power plants (46.4 Mg yr-1), medical waste incinerators (44.8
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Mg yr-1), and municipal waste incinerators (51.4Mg yr-1) accounted for 71.5% of the total
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anthropogenic Hg emissions of 199.5 Mg yr-1 (4). To reduce these anthropogenic emissions, the
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US Environmental Protection Agency (EPA) implemented the 1990 Solid Waste Combustion
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Rule, which targeted emissions from incineration sources. By 1999, total annual Hg emissions
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from medical (1.5 Mg yr-1) and municipal (4.4 Mg yr-1) waste incinerators were reduced 97%
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and 91%, respectively, and power plant Hg emissions (43.5 Mg yr-1) became the dominant
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anthropogenic source of Hg, accounting for 43% of the total (102 Mg yr-1) anthropogenic Hg
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emissions (4). In 2005, the EPA issued the Clean Air Mercury Rule (CAMR) to reduce Hg
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emissions from US power plants. CAMR was patterned after the Clean Air Interstate Rule
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(CAIR), a cap and trade program designed to reduce SO2 emissions from US power plants (5).
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The same emissions controls used by power plants to reduce SO2 emissions also remove some of
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the oxidized Hg in the flue gases, providing an important co-benefit of CAIR. In 2008, the court
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vacated CAMR because power plant Hg emissions must be regulated under a different section of
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the Clean Air Act. Subsequently, in 2011, the EPA issued the Mercury and Air Toxic Standards
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(MATS), which were designed to reduce Hg emissions from power plants by 90% (40 Mg yr-1 to
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4 Mg yr-1) upon full compliance in April 2016 (6). Recently, however, the US Supreme Court
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ruled that MATS needs to be reexamined by the D.C. Circuit Court. Two potential outcomes of
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this review include MATS being amended to account for the monetary costs of compliance or
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MATS can be vacated, which would force the EPA to develop new regulations to reduce Hg
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emissions from US power plants.
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The effectiveness of power plant emission controls on the atmospheric concentrations of
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GEM, PBM2.5 and GOM are largely unknown in the US. However, global-scale models predict
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that domestic sources of Hg in North America account for only 20% to 32% of the atmospheric
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Hg deposited in the contiguous US (7,8). This implies that Hg emission reductions would have
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little to no impact of atmospheric Hg deposition in many regions of the US. However, these
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models also predicted strong regional variations in the contributions made by domestic
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anthropogenic sources to the atmospheric deposition of Hg (7,8). For example, some regions in
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the eastern US, particularly those downwind of large sources of Hg, are predicted to receive
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between 60% and 80% of their atmospheric Hg deposition from domestic anthropogenic sources.
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These regions of the US may benefit from domestic reductions in Hg emissions from power
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plants. Thus, accurate measurements of the atmospheric concentrations of Hg at appropriate
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locations are needed to assess these model predictions and, more importantly, to evaluate the
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effectiveness of air pollution control policies in the US.
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The rural and relatively pristine ecosystems in the Appalachian region of western
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Maryland are downwind of several large power plant sources of Hg in the Ohio River Valley (9).
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Several of the impacts of these power plant Hg emissions on ecosystems of western Maryland
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have previously been reported from watershed, atmospheric, and modelling studies. Highlights
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from these studies include elevated concentrations of Hg in fish in streams and reservoirs
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(10,11), some of the highest Hg wet deposition rates (15 to 23 ug m-2 yr-1) in the US (12),
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elevated inputs (15 ug m-2 yr-2) of Hg to the forest floor from litter fall (13), and very high rates
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(3 to 8 ug m-2 yr-1) of GOM and PBM2.5 dry deposition (14,15). More specifically, GOM and
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PBM2.5, two Hg species with relative short atmospheric lifetimes, accounted for 54% of the total
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dry deposition of Hg in western Maryland, which was 2 to 11 times greater than their
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contribution to dry deposition at other sites in the eastern US (15). In addition, a regional scale
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Lagrangian model predicted that power plant Hg emissions from the states of OH, PA, and WV
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contributed 50 % of the Hg that was deposited in Maryland from these states (16). Collectively,
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these studies imply that western Maryland may be an ideal location to evaluate the effectiveness
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of US power plant emission controls on the atmospheric concentrations of Hg in regions
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downwind of major sources. Note, that the prevailing westerly winds transport air pollutants
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from western Maryland to the highly populated regions along the east coast of the US. Thus, the
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purpose of this study is to determine if power plant emission reductions have affected the
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atmospheric concentrations of GEM, GOM, and PBM2.5 entering western Maryland before being
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transported to population centers further east.
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2. EXPERIMENTAL SECTION
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2.1 Study Site. Our measurements were made at the Piney Reservoir Ambient Air Monitoring
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(PRAMS, N 39o 42’ 19.16” W 079o 00’45.1”) station in Garrett County, Maryland (Figure 1).
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PRAMS is surrounded by deciduous forests and agricultural lands. There are relatively few
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sources (9.1 x 10-6 Mg yr-1) of Hg within 50 km of PRAMS, but there are several sources of Hg
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(0.6 Mg yr-1) within 150 km (17). Winds arriving at PRAMS are commonly from the west (240
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to 320o), more than twice as frequent as winds from other directions. Winds from the west travel
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over several large power plants in PA and OH before arriving at PRAMS. Winds from the south,
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most common in summer, also travel over large coal-fired power plants in WV, before arriving
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at PRAMS. For perspective, the upwind states of PA, OH, and WV are often ranked in the top
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ten (OH 2nd, PA 3rd and WV 7th) power plant Hg emitting states in the US, accounting for 16%
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(4.6 Mg yr-1) of the total US power plant Hg emissions (28.6 Mg yr-1) in 2010 (9).
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2.2 Atmospheric Measurements. Ambient air concentrations of GOM, GEM and PBM2.5 were
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measured using a Tekran 2537A, and Tekran 1130 and 1135 speciation units. The inlet of this
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system was about 5 m off the ground. This system operated on a three-hour cycle. During the
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first two hours of this cycle, GEM concentrations were measured every five minutes for twenty-
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four consecutive five-minute periods. Simultaneously, GOM was adsorbed onto a KCl coated
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annular denuder while PBM2.5 was collected on a regenerable particulate filter. After every two
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hour sampling period, the denuder and particulate filter were purged with Hg free air and the
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GOM and PBM2.5 were pyrolyzed to GEM and analyzed using the Tekran 2537A. This system
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was calibrated every three days using a Hg permeation source in the Tekran 2537A. This
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permeation source and the accuracy and precision of the Tekran 2537A was checked every six
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months using a Tekran 2505 calibration system.
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The Tekran mercury speciation system was operated and maintained using standard
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protocols endorsed by atmospheric mercury community (18,19). Recently, however, it has been
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reported that the Tekran 1130 speciation unit may underestimate the atmospheric concentrations
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of GOM (20). At this time, however, it is not clear if the results from this instrument inter-
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comparison can be generalized to all field measurements. For example, there may be many
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different forms of GOM in the atmosphere in different regions of the US that may or may not
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behave the same as the forms of GOM of used in this inter-comparison (19). In addition, there
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were other problems with this inter-comparison experiment that still need to be resolved before
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we modify our sampling protocols (19). Note, however, if this bias exists then it would affect our
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entire GOM data set, not just the GOM concentrations after the reductions in Hg emissions from
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power plants. Thus, the reductions in the GOM concentrations reported here would still occur
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with or without this bias.
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Sulfur dioxide concentrations were measured using an Ecotech SO2 analyzer. This
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instrument uses pulsed UV fluorescence for the detection of SO2 in ambient air. This system is
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calibrated, operated and maintained using standard protocols approved by the USEPA. This
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instrument measures SO2 at 50 Hz. We integrate these measurements to compute 1-minute
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averages. For this study, we used hourly averaged SO2 concentrations.
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2.3 Statistical Analyses. The statistical package in Sigma Plot 12.3 was used to analyze our
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data. The non-parametric Spearman Rank Order Correlation was used to test for significant
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correlations (p
