Memorandum: Emissions Overview: Hazardous 

Air Pollutants in Support of the Final Mercury 

and Air Toxics Standard

EPA-454/R-11-014

November 2011

Memorandum: Emissions Overview: Hazardous Air Pollutants in 

Support of the Final Mercury and Air Toxics Standard

Marc Houyoux

and

Madeleine Strum

Emission Inventory and Analysis Group

Air Quality Assessment Division

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Air Quality Assessment Division

Emissions Inventory and Analysis Group

Research Triangle Park, North Carolina
1

MEMORANDUM

To: Toxics Rule docket, number EPA-HQ-OAR-2009-0234

From: Marc Houyoux, group leader Emission Inventory and Analysis Group

Madeleine Strum, Emission Inventory and Analysis Group

Subject: Emissions Overview: Hazardous Air Pollutants in Support of the Final Mercury and Air Toxics 

Standard

Date: December 1, 2011

1 Introduction

The purpose of this document is to summarize the mercury (Hg) and non-Hg Hazardous Air 

Pollutants (HAP) emissions from U.S. electric generating utilities (EGUs) associated with the final Mercury 

and Air Toxics Standard (MATS). This information includes both background information about these 

pollutants as well as emissions totals from several emissions cases developed for the final MATS. In this 

memo, we summarize current HAP emissions estimates from U.S. EGUs, the estimates used in the nationalscale Hg risk assessment (Hg Risk TSD) for years 2005 and 2016, and the estimates for the future impact of 

the rule when fully implemented (in 2016).

2 Hg Emissions

2.1 Hg Emissions Overview

Mercury is an element. There is a fixed amount of it in the world. As long as it is bound up, for 

example in coal, it cannot affect people or the environment. Once it is released, for example via the 

combustion process, it enters the environment and becomes available for chemical conversion. Once 

emitted, Hg remains in the environment and can bioaccumulate in organisms or be remitted through natural 

processes. Mercury is emitted through natural and anthropogenic processes, and previously deposited Hg

from either process may be re-emitted. The majority of natural Hg emissions arise from volcanoes, 

geothermal activity, Hg-enriched topsoil, oceans/lakes, and vegetation. The category of natural Hg

emissions may or may not include biomass burning, due to the difficulty in determining whether biomass 

burning events are started by anthropogenic or natural causes. Generally, natural Hg emissions are 

dominated by the elemental form of mercury, and as a result have more impact on the global pool of Hg than 

on deposition in the region of the emissions.

Mercury is known to exist in the atmosphere in three forms: (1) elemental mercury, Hg0, (2) gaseous 

oxidized mercury, Hg+2, and (3) particulate bound mercury, Hgp. Hg0dominates total mercury composition 

in the atmosphere (greater than 95%) and has a much greater residence time than Hg+2 or Hgp

1

. Hg0ambient 

concentrations are spatially uniform due to the long atmospheric residence time and a more uniform 

distribution of sources due to re-emission of previously deposited mercury. Hg+2 and HgP concentrations are 

locally elevated near sources due to their shorter atmospheric lifetime. Total mercury deposition is likely to 

be dominated by all sources of Hg0including global sources, while deposition of Hg+2 and Hgp is primarily 

from local and regional sources.

2

 

1

Schroeder, W. H. and J. Munthe (1998). "Atmospheric mercury - An overview." Atmospheric Environment 32(5): 809-822.

2 Marsik, F. J., G. J. Keeler, et al. (2007). "The dry-deposition of speciated mercury to the Florida Everglades: Measurements and 

modeling." Ibid. 41(1): 136-149.
2

The categories for anthropogenic Hg emissions include the combustion of fossil-fuels, cement 

production, waste incineration, metals production, and other industrial processes. Anthropogenic Hg 

emissions consist of all three forms of Hg.

Mercury re-emissions include previously deposited Hg originating from both natural and 

anthropogenic sources. At this time, it is not possible to determine the original source of previously 

deposited Hg, i.e., whether its source is natural emissions or re-emissions from previously deposited 

anthropogenic Hg.3,4,5 Current publications on this topic estimate that half of re-emitted Hg originates from 

anthropogenic sources.6,7

Current estimates of total global Hg emissions based on a 2005 inventory range from 6,600 to 7,500 

metric tons per year (mt/yr). The United Nations Environment Programme (UNEP) estimates of global Hg 

emissions for 2005 are somewhat lower, at 5,600 mt/yr8,9. Global anthropogenic Hg emissions, excluding 

biomass burning, have been estimated by many researchers. UNEP’s 2005 estimate is approximately 1,900 

mt/yr (with a range of 1,200 to 3,000 mt/yr)

10 and the 2005 estimate by Pirrone, et al. is approximately 2,400 

mt/yr.

11

 Global fossil-fuel fired power plants total approximately 500 to 800 mt/yr, a large fraction (25 to 35 

percent) of the total global anthropogenic emissions.12

There are large uncertainties regarding projected mercury global inventories. To model Hg deposition 

for the national-scale Hg risk assessment, initial and boundary conditions were based on a GEOS-CHEM 

simulation using a 2000-based global inventory that includes approximately 7,000 mt/yr of mercury 

emissions13. Recent research shows that Hg emissions from China were consistent between 2000 and 2006 

(approximately 1,300 mt/yr)14, so no adjustments were made to the 2000-based global Hg emissions for the 

2005 simulation. Recent research has shown that ambient mercury concentrations have been decreasing in 

 

3

 Lindberg, S., Bullock, R., Ebinghaus, R., Engstrom, D., Feng, X., Fitzgerald, W., et al. (2007). A Synthesis of Progress and 

Uncertainties in Attributing the Sources of Mercury in Deposition. Ambio, 36(1), 19-33.

4

 Lohman, K., Seigneur, C., Gustin, M., & Lindberg, S. (2008). Sensitivity of the global atmospheric cycle of mercury to 

emissions. Applied Geochemistry, 23(3), 454-466.

5

 Seigneur, C., Vijayaraghavan, K., Lohman, K., Karamchandani, P., & Scott, C. (2004). Global Source Attribution for Mercury 

Speciation in the United States. Environmental Science and Technology(38), 555-569.

6

 Mason, R., Pirrone, N., & Mason, R. P. (2009). Mercury emissions from natural processes and their importance in the global 

mercury cycle. In Mercury Fate and Transport in the Global Atmosphere (pp. 173-191): Springer U.S.

7

 Selin, N. E., Jacob, D. J., Park, R. J., Yantosca, R. M., Strode, S., Jaeglé, L., et al. (2007). Chemical cycling and deposition of 

atmospheric mercury: Global constraints from observations. J. Geophys. Res, 112, 1071-1077.

8

 UNEP (United Nations Environment Programme), Chemicals Branch, 2008. The Global Atmospheric Mercury 

Assessment:Sources, Emissions and Transport, UNEP Chemicals, Geneva. 

9 The 5,600 metric ton estimate is from the Figure on page 37 of the report. The report also provides ranges in the executive 

summary.

10 Study on Mercury Sources and Emissions and Analysis of the Cost and Effectiveness of Control Measures “UNEP Paragraph 29 

study”, UNEP (DTIE)/Hg/INC.2/4. November, 2010.

11 Pirrone, N., Cinnirella, S., Feng, X., Finkelman, R. B., Friedli, H. R., Leaner, J., et al. (2010). Global mercury emissions to the 

atmosphere from anthropogenic and natural sources. Atmospheric Chemistry and Physics Discussions, 10(2), 4719-4752.

12 Study on Mercury Sources and Emissions and Analysis of the Cost and Effectiveness of Control Measures “UNEP Paragraph 29 

study”, UNEP (DTIE)/Hg/INC.2/4. November, 2010.

13 Selin, N.E., Jacob, D.J., Park, R.J., Yantosca, R.M., Strode, S., Jaegle, L., Jaffe, D., 2007. Chemical cycling and deposition of 

atmospheric mercury: Global constraints from observations. Journal of Geophysical Research-Atmospheres 112.

14 Streets, D.G., Zhang, Q., Wu, Y., 2009. Projections of Global Mercury Emissions in 2050. Environmental Science & Technology 43, 2983-

2988.
3

the northern hemisphere since 200015, so no adjustments were made to the boundary condition inflow for the 

2016 simulations.

2.2 Estimating Current Hg Emissions

In 1990, Hg emissions for coal and oil EGUs in the U.S. were 59 tons16 out of a total of 246 tons17, 

and by 1999 Hg emissions for U.S. EGUs were 4918 out of 115 tons. In 2005, they were 5319 tons out of a 

total of 105 tons. As part of the development of the MATS, EPA completed in 2010 an Information 

Collection Request (ICR) that gathered the latest available emissions data from coal- and oil-fueled EGUs. 

The ICR data have been used in several ways, including to calculate EPA’s best estimate of current 

emissions, called the “current base” from U.S. EGUs. At proposal, the current base Hg emissions estimate 

(labeled “2010” at proposal) was 29 tons, and after the revisions made and described here for the final 

MATS, the current base estimate remains 29 tons of Hg from U.S. EGUs.

While the current base Hg emissions estimate is not a basis for the MATS rule, the risk assessment, 

or the Regulatory Impact Analysis (RIA), EPA received numerous comments on its value as part of the 

proposed rule. These comments compared the current base estimate to EPA’s future year Hg emissions 

estimate from U.S. EGUs and called into question the future-year estimate since it was the same as the 

current base estimate. Since the current base estimates use a 2007-2009 average heat input, it did not seem 

likely that the current base estimate would be the same as future years that include additional controls from 

other rules. As a result, EPA has reviewed and updated its methods for the current base emissions, and these 

updates are described in this section. These changes resulted in a decrease of 0.3 tons/year, which rounds to 

the same 29 ton/yr value included with the proposed MATS.

The calculations underlying the current base inventories are available in the MATS docket in an 

Excel® workbook entitled “MATS_Final_Current_Base_HAP_inven.xlsx”.

EPA updated the current base national inventories for the final rule in the following three ways:

Updated list of units assumed to be subject to the rule for the purposes of calculating the national 

inventory values. Within a given facility, some units are subject to the rule and some are not 

depending on each unit’s capacity and fuels.

Revised EFs for untested units. These revisions included outlier statistical tests of the underlying 

stack test data for each pollutant to improve the quality of the estimates. This approach was also 

consistent with the EF method used to estimate emissions for untested units in the non-Hg risk case 

studies.

Revised the heat input to use the 2007-2009 average CEM-based heat input for units reporting to the 

CEM program, rather than ICR-based heat input. For other units, EPA used the same approach as at 

proposal, which was based on the ICR data and also represents a 2007-2009 average.

 

15 Slemr, F., Brunke, E.G., Ebinghaus, R., Kuss, J., 2011. Worldwide trend of atmospheric mercury since 1995. Atmospheric Chemistry and 

Physics 11, 4779-4787.

16 2008 EPA Report on the Environment available at http://www.epa.gov/ncea/roe/index.htm

17 This value was transposed as 264 tons in the version of this memorandum included with the MATS proposal.

18 ftp://ftp.epa.gov/pub/EmisInventory/finalnei99ver3/haps/summaries

19 Emission Inventory Technical Support Document (TSD) for the proposed MATS rule
4

To update the list of assumed affected units in the current base inventory, EPA compared the primary 

fuels listed for each unit in the NEEDS v4.0 and NEEDS v4.1020 databases as well as 2005 data available 

from the Energy Information Administration. The updated list of affected units is provided in the current 

base workbook, including the reason a unit was added to, or excluded from, the original list of units 

developed for the MATS ICR.

Mercury EFs were developed based on reported data from 323 units (boilers), with data from 6 of 

these units excluded as outliers. Using these data, EPA computed unit-specific EFs and industry-wide 

average EFs associated with boiler configuration, which considers combinations of fuel type, boiler type, and 

control devices. The average EFs were assigned to untested boilers based on their configuration. The boilerspecific EFs from the test data and the average EFs were applied to the units reporting data. The EFs were 

combined with average heat input for 2007-2009 for all units subject to the rule. The approaches and data 

are available in “Emission Factors Supporting Documentation for the Final Mercury and Air Toxics 

Standards,” which is available in the MATS docket.

For the final rule, EPA revised the approach for the operating data (heat input). For proposal, the 

operating data gathered as part of the ICR was used, which included calculating an average throughput using 

the 3-year average capacity factor for 2007 to 2009 and the boiler maximum heat input. For the final rule, 

heat input data were compiled from the CEM program for 2007, 2008, and 2009 when available for units 

included in that program. For units not reporting to the CEM program, the same approach as at proposal was 

used: the operating data collected as part of the rule were used by calculating the 3-year average capacity

factor for 2007 to 2009 and applying to the boiler maximum heat input.

One goal of using the average 2007-2009 approach to estimate the current base emissions was to

limit the impact of the economic downturn on Hg emissions estimates, since lower throughput of EGUs 

caused by decreased electricity demand is not a long-term or enforceable decrease in emissions. To evaluate 

this impact, EPA computed demand-adjusted U.S. EGU emissions for 2002 through 2010 based on the 

current, ICR-based boiler configurations (fuels, technologies, controls). The heat input data were based on 

the year-specific CEM data where available. For units that did not report to EPA using CEMs for a given 

year, the heat input was held constant at the 2007-2009 average. Table 1 shows the resulting demandadjusted emissions. The emissions estimate for 2009 represents a low point in Hg demand-adjusted 

emissions assuming current boiler configurations for all years, whereas 2007 and 2008 are relatively 

consistent with historical values. Demand-adjusted increases start to occur in 2010, which is about 1 

ton/year higher than 2009 based on demand impacts alone. Thus, a 2007-2009 average will include some 

impact of temporary decreased demand since it includes 2009. Since CEM data were used only where 

available and a 2007-2009 average heat input is used for other units, this approach will tend to somewhat 

dampen the resulting trend; however, this is a reasonable guide to help understand the impact of demand 

decreases on emissions levels.

 

20

 http://www.epa.gov/airmarkets/progsregs/epa-ipm/BaseCasev410.html#needs
5

Table 1: Demand-adjusted ICR-based U.S. EGU Hg emissions

Year Total Mercury using ICR-based boiler 

configurations (tons/year)

2002 29.5

2003 30.1

2004 29.8

2005 30.3

2006 29.9

2007 30.5

2008 29.6

2009 25.5

2010 26.8

To further evaluate EPA’s current base Hg emissions estimate for MATS, EPA reviewed the data 

available from industry as reported into the Toxics Release Inventory (TRI). Table 2 provides the TRI-based 

emissions and illustrates that while industry-reported emissions in TRI were lower than EPA estimates in 

2005, the 2010 TRI compilation of 33.5 tons/year is higher than the EPA current base estimate for MATS of 

29 tons/year. Since facilities estimate and report their emissions to the TRI Program, facilities are 

responsible for estimating emissions using a variety of method reporting codes21. These methods include 

continuous monitoring data, periodic or random monitoring data or measurements, mass balance 

calculations, using published general emission factors or site-specific emission factors, and other approaches. 

Since the methods used by industry will be a complex composite of these different methods, and EPA’s 

approach for MATS uses a single method based on site-specific and general emission factors using the latest 

source test data, it is not surprising that the estimates are different.

Table 2: Industry-reported emissions from the Toxics Release Inventory

Year Total mercury (tpy)

2005 48.2

2006 46.7

2007 46.5

2008 44.8

2009 35.6

2010 33.5

2.3 Hg Emissions Trend and Projections

Table 3 shows past U.S. emissions of Hg, the revised current emissions estimates, and future 

emission projections for the year 2016. In 2005, 53 out of 105 tons of Hg came from combustion of coal and 

oil at U.S. EGUs, or approximately 50%. The current estimate shows a decrease in these emissions to 29

tons, which includes reductions resulting from the installation of Hg controls to comply with state Hg-

 

21 See TRI 2010 reporting instructions for form R, http://www.epa.gov/tri/report/rfi/ry2010rfi_061511.pdf, pg 46-47.

0.00

10.00

20.00

30.00

40.00

50.00

60.00

2005 2006 2007 2008 2009 2010

Hg (tons/yr)
6

specific rules, voluntary reductions, as well as from the co-benefits of Hg reductions from control devices 

installed for the reduction of SO2 and PM as a result of state and federal actions, such as New Source Review 

(NSR) enforcement actions. The 2016 Hg estimate of 27 tons for U.S. EGUs was projected using the 

Integrated Planning Model (IPM), which uses a different methodology to project emissions.

22

 The 2016 

base-case scenario has been updated since the proposal to reflect known federal measures for all sectors, 

including the recently finalized Cross-State Air Pollution Rule (CSAPR), but without state Hg rules.

23. It 

also reflects projected economic changes and fuel usage for the EGU and mobile sectors. Based on this 

modeling, the estimated U.S. EGU proportion of Hg emissions in 2016 is 42%. More information on IPM is 

available in the “Documentation Supplement for EPA Base Case v.4.10_MATS – Updates for Final Mercury 

and Air Toxics Standards (MATS) Rule,” which is available in the MATS docket. 

Table 3: U.S. anthropogenic Hg emissions trends for U.S. EGUs.

19901

(tons)

19992

(tons)

20053

(tons)

Current Base4

(tons)

Final 2016 Base Case5

(tons)

U.S. EGUs 59 49 53 29 27

Non-EGU Anthropogenic

U.S. Hg 205 66 52 Not Available6

35

Total U.S. Anthropogenic 

Hg 264 115 105 Not Available6

64

Percent of U.S. Anthropogenic Hg Emissions from U.S. EGUs

1990 1999 2005 Current Base 2016

22% 43% 50% Not Available642%

1. 2008 EPA Report on the Environment available at http://www.epa.gov/ncea/roe/index.htm, which also has 2002 

emissions that used a consistent approach as 1990.

2. ftp://ftp.epa.gov/pub/EmisInventory/finalnei99ver3/haps/summaries/. The 1999 estimates used different methods 

than the 1990 and 2005 methods, and so trends shown here also reflect changes in methods.

3. Emission Inventory Technical Support Document (TSD) for the proposed MATS rule

4. The estimate of the current base emissions of Hg may underestimate Hg emissions from U.S. EGUs due to targeting 

of the 2010 ICR on the best performing EGUs.

5. The 2016 base case scenario represents predicted emissions including known federal measures for all sectors, 

and excludes state Hg rules for EGUs. It reflects projected economic changes and fuel usage for the EGU 

and mobile sectors. The term “Final” is used to indicate that the EGUs were projected utilizing the final 

version of the IPM version 4.10_MATS developed for the final MATS rule as opposed to the Interim 

version. “Base” indicates that these are the projected emissions in absence of the Final MATS policy.

6. Information on recent U.S. EGU emissions was obtained using an ICR for EGUs only. This same 

information is not available for other sources, which were not covered by the ICR.

Table 4 below provides the projections for non-EGU emission sources that include expected future 

reductions from existing and proposed regulations for HAPs that are expected to be implemented prior to 

 

22

 IPM is a multi-regional, dynamic, deterministic linear program model of the U.S. power sector that determines the least cost 

solution to meeting a set of environmental constraints while still meeting specified electric demand. For more detail on IPM see: 

http://www.epa.gov/airmarkets/progsregs/epa-ipm/index.html

23 2016 base Hg emissions from U.S. EGUs were estimated to be 29 tons/year at proposal. The future-year emissions are 

described further in Section 4.
7

2016. These regulations include the NESHAPs that affect the following industries: Portland Cement; 

Industrial, Commercial and Institutional Boilers and Process Heaters; Gold Ore Mining and Production, 

Electric Arc Furnaces, Hazardous Waste Incineration, and Mercury Cell Chlor-alkali facilities. The 

projections also include the replacement of a smelter and a pulp and paper plant, which will result in the 

elimination of Hg emissions from those sources, along with other known plant closures. This projection 

shows that U.S. EGU emissions will continue to comprise a significant, dominant portion of the total U.S. 

anthropogenic inventory in 2016. In 2016, Hg emissions from U.S. EGUs are projected to comprise 27 tons 

out of a total of 62 tons. More information on the non-EGU projections of Hg is available in the Emission 

Inventory Technical Support Document (TSD) for the proposed MATS rule. This document has not been 

updated for the final rule.

Table 4: Anthropogenic Hg emissions and projections in the Continental U.S.

Category

2005 

Mercury 

(tons)

2016 Base 

Mercury 

(tons)

Electric Generating Units 53 27

Portland Cement Manufacturing 7.5 1.1

Stainless and Non-stainless Steel Manufacturing: Electric Arc Furnaces 7.0 4.6

Industrial, Commercial, Institutional Boilers & Process Heaters 6.4 4.6

Chemical Manufacturing 3.3 3.3

Hazardous Waste Incineration 3.2 2.1

Mercury Cell Chlor-Alkali Plants 3.1 0.3

Gold Mining 2.5 0.7

Municipal Waste Combustors 2.3 2.3

Sum of other source categories (each of which emits less than 2 tons) 17 16

Total 105 62

3 Non-Hg HAP Emissions

Fossil-fuel fired boilers emit a variety of metal HAP, organic HAP and HAP that are acid gases. 

Acid gas and metal HAP emissions are discussed below.

The same three updates made for the Hg national inventory were made for the acid gases and non-Hg 

metal emissions: a slightly updated list of units are assumed to be subject to the rule, improved heat input 

data based on the CEM program is used where available, and improved EF estimation methods are used to 

exclude statistical outliers. The emissions factors (EFs) used to estimate current non-Hg HAP emissions are 

consistent with the EFs used to calculate chromium, nickel, and arsenic emissions for EGUs without 

available facility-specific stack test data in the revised Non-Hg Case Study Chronic Inhalation Risk 

Assessment.
8

3.1 Acid Gases

Acid gas emissions from U.S. EGUs include hydrogen chloride (HCl), hydrogen fluoride (HF), 

chlorine (Cl2) and hydrogen cyanide (HCN). Table 5 shows emissions of certain acid gases from EGUs, 

based on the 2010 ICR data by fuel type. Although Cl2 was not included in the data collected from the 2010 

ICR, Cl2 gas is expected to be emitted at significantly lower amounts than HCl (ratios of HCl-to- Cl2 can be 

as high as 200:124).

Similar to the Hg estimates, the current base emissions estimates for the acid gases are based on

emission factors generated from boiler-specific emission tests. These emission factors are based on test data 

from 330 boilers for hydrogen chloride, 324 boilers for hydrogen fluoride, and 314 boilers for hydrogen 

cyanide. Unit-specific emission factors were developed and applied from unit test averages where available; 

otherwise, average emission factors were applied. Current-year estimates are not available for other U.S. 

anthropogenic emission sources, so the latest available complete set of data, 2005, were used to approximate 

the percentage contribution of U.S. EGUs to total U.S. anthropogenic emissions. As illustrated by Table 5, 

U.S. EGUs are estimated to emit the majority of HCl and HF nationally. Additional documentation on the 

inventory calculations and emission factors used for each unit is available in the inventory workbook 

identified in Section 2.2. Additional information on the approach to calculate binned emission factors is 

available in “Emission Factors Supporting Documentation for the Final Mercury and Air Toxics Standards”, 

available in the docket.

 Table 5: Summary of acid gas emissions from U.S. EGU sources

Selected Nationwide Acid HAP current base

emissions (tons/yr) from U.S. EGUs

2005 Acid HAP 

Emissions (tons/yr)

from the inventory 

used for the National 

Air Toxics 

Assessment (NATA)1

Percent of U.S. 

Anthropogenic

Emissions 

from EGUs, 

based on the 

inventory used 

for the 2005 

Coal NATA IGCC2

PC3 Oil

Total 

Current 

Base EGU4

Total 

2005

EGU

2005 

NonEGU 

Hydrogen 

Cyanide5

5,459 0 84 84 5,627 1,200 14,000 8%

Hydrogen 

Chloride

99,567 1 81 330 99,980 350,000 78,000 82%

Hydrogen 

Fluoride

25,658 1 7 67 25,733 47,000 28,000 62%

Total 130,680 3 172 480 131,334 398,200 120,000 77%

1 2005 NATA: http://www.epa.gov/ttn/atw/nata2005/ EGU emissions were extracted from the total using the MACT 

code field (1808)

2

IGCC = integrated gasification combined cycle

3

PC = petroleum coke

4

The estimate of the current base emissions for acid gases may underestimate total EGU emissions due to targeting of 

the 2010 ICR on the best performing EGUs.

5 Used cyanide emissions for hydrogen cyanide

For the final rule, average emission factors for the untested units were recomputed using improved 

 

24

 200:1 is based on testing done at the EPA’s pilot facility in Research Triangle Park, NC. Personal communication, Nick 

Hudson.
9

methods. The approaches and underlying data are available in “Emission Factors Supporting Documentation 

for the Final Mercury and Air Toxics Standards,” which is available in the docket. For acid gases, the data 

and calculations used for the average emission factors are available in the spreadsheet 

“MATS_Final_Utility_Boiler_Mercury_and_Acid_Gases.xlsx”.

3.2 Metal HAP

Metals are emitted primarily because they are present in fuels. Table 6 shows selected metals emitted 

by EGUs and emission estimates based on data from the 2010 ICR. Similar to the Hg and acid gas estimates, 

the current base emissions for the metal HAPs are based on emission factors generated from boiler-specific 

emission tests. The number of tests used for each metal was different depending on the available data and 

the impact of outlier tests applied to the analysis. Unit-specific emission factors were developed and applied 

from unit test averages where available; otherwise, average emission factors were applied. Current-year

estimates are not available for other U.S. anthropogenic emission sources, so the latest available complete set 

of data, 2005, was used to approximate the percentage contribution of U.S. EGUs to total U.S. anthropogenic 

emissions.

Table 6: Summary of metal emissions from U.S. EGU sources.

Selected Nationwide Metal HAP currentbase emissions(tons/yr) from EGUs

2005 Metal HAP 

Emissions from the 

inventory used for the 

National Air Toxics 

Assessment (NATA)1

Percent of U.S. 

Anthropogenic 

Emissions from 

EGUs, based on the 

inventory used for the 

Coal IGCC2 PC3 Oil 2005 NATA

Total 

Current 

Base

EGU4

Total 2005

EGU

2005 NonEGU

Antimony 8 0 0 15 23 19 83 19%

Arsenic 55 0 1 1 57 200 120 62%

Beryllium 2 0 0 0 2 10 13 44%

Cadmium 3 0 0 0 3 25 38 39%

Chromium 236 0 0 4 240 120 430 22%

Cobalt 10 0 0 7 18 54 60 47%

Lead 43 0 2 3 48 21 1,194 2%

Manganese 137 0 0 12 149 270 1800 13%

Nickel 238 0 1 161 399 320 840 28%

Selenium 230 0 0 1 231 580 120 83%

Total 962 0 4 204 1170 1619 4698 26% 1

2005 NATA: http://www.epa.gov/ttn/atw/nata2005/ EGU emissions were extracted from the total using the MACT code 

field (1808)

2

IGCC = integrated gasification combined cycle

3

PC=petroleum coke

4 The estimate of the current base emissions for metals may underestimate EGU metal emissions due to targeting of the 

2010 ICR on the best performing EGUs.

As shown by the table, U.S. EGUs are estimated to be a significant source of emissions nationally for 

these metals.
10

For the final rule, average emission factors for the untested units were recomputed using improved 

methods as compared to the proposed rule. The approaches and underlying data are available in “Emission 

Factors Supporting Documentation for the Final Mercury and Air Toxics Standards,” which is available in 

the docket. For Chromium, Nickel, and Arsenic, the data and calculations used for the average emission 

factors are available in the spreadsheet “EPA_Ar_Cr_Ni_Bin_EFs.xlsx”.

3.3 Expected Impact on Non-Hg HAP Emissions from Existing Clean Air Act (CAA) and Other 

Regulations and Programs

It is expected that acid gas and metal emissions will be reduced in the future due to existing CAA and 

other regulations and programs. Acid gases and metals are emitted by a number of other industries already 

regulated or scheduled for regulation under the National Emissions Standards for Hazardous Air Pollutants 

(NESHAP) or the Risk and Technology Review (RTR) program. In addition, because acid gases respond 

somewhat to the same types of controls used for SO2, it is expected that acid gas emissions will be reduced at 

both EGU and non-EGU sources as a result of national, state, and local control programs to control SO2. 

Likewise, metals respond to the same types of controls used for PM, and as a result it is expected that metal 

emissions will be reduced at both EGU and non-EGU sources due to national, state, and local control programs 

to control direct PM.

4. Hg and non-Hg HAP Emissions Projections 

This section summarizes the base and future-year emissions for Hg and HCl used as part of the public 

record for this rule. Future projections of other HAPs were not developed; therefore, they are not included in 

this section. The current base emissions estimate has been described in Section 2.2. This section includes 

further information about the emissions created for (a) the Hg Risk TSD conducted to inform the appropriate 

and necessary finding, and (b) the base and final policy cases using the final version of IPM for the final cobenefit estimates in the final RIA25

.

For the 2016 emissions estimates, different phases of analysis resulted in slightly different emissions 

estimates for the appropriate and necessary finding and the final base and policy cases. The purpose of this 

section is to explain the source of these differences and document the different Hg and HCl emissions values. 

The subsections below provide additional information on emissions for the Hg Risk TSD and the final RIA. 

Table 7 summarizes the origin information about all of the emissions cases, and Table 8 summarizes the 

emissions estimates of Hg and HCl for each, including speciated Hg.

4.1 Emissions for the Hg Risk TSD

The Hg Risk TSD is based on modeling Hg deposition associated with total 2005 Hg emissions and 

total 2016 projected Hg emissions, as well as modeling Hg deposition resulting from scenarios that zeroed 

out Hg emissions from U.S. EGUs. While the Hg Risk TSD was revised in response to comments received 

during the peer review, EPA did not remodel Hg deposition since the MATS proposal.

The “2005 base year” emissions case in Table 7 uses emissions of criteria pollutants, Hg, HCl, and 

Cl2 from all source categories. All emissions except Hg come from the 2005 version 4.1 modeling platform, 

 

25 While the final MATS included an updated RIA to capture the final co-benefits of SO2 and PM2.5 changes, the Hg emissions 

and benefits for that analysis were kept identical to those used for the MATS proposal.
11

which is derived from the 2005 National Emissions Inventory (NEI). The HCl and Cl2 emissions have a 

slight impact on the modeled ozone and are therefore included in the standard modeling configuration used 

for this effort. 2005 Hg emissions for all sources (EGU and non-EGU) come from the 2005 National Air 

Toxics Assessment (NATA) inventory, a revised version26 of the 2005 NEI. EPA further revised the 2005 

NATA inventory for non-EGU point and nonpoint sources based on data collected from the NESHAP for 

industrial, commercial and institutional boilers. The current base case contains emissions only for U.S. 

EGUs, and the Hg, HCl, and metal emissions in this inventory are based on the 2010 ICR data. As 

previously described, initial and boundary conditions for the photochemical and deposition modeling were 

based on a GEOS-CHEM simulation using a 2000 based global inventory that includes approximately 7,000 

mt/yr of mercury emissions27. 

Also summarized in Table 7, the “Proposal interim 2016 base case” emissions projections were 

developed for use in modeling based on the latest available (at the time of the proposed rule) projection 

techniques and they account for emissions reductions of all modeled pollutants from federal measures. In 

addition, the “Proposal interim 2016 policy case” emissions projections are an interim version of the 

expected emissions from U.S. EGUs after reductions associated with the proposed CSAPR and interim 

proposed MATS. The interim 2016 EGU emissions for both the base and MATS policy cases were created 

using an interim version of the IPM because the modeling needed to be completed in advance of the IPM 

revisions associated with the proposed MATS. EPA concluded that the deposition modeling for the Hg Risk 

TSD was sufficient; therefore, all of the emissions reflected below for the proposal interim 2016 base case 

are unchanged from proposal. The change in the 2016 base Hg emissions estimate for U.S. EGUs of two 

tons of Hg between proposal and final would not have a significant impact on risk results. The non-EGU 

source emissions were created for the interim 2016 base projected emissions, and these are described in the 

Emission Inventory TSD for the proposed MATS. These emissions in Table 8 were used for all other 2016 

emission cases.

The emissions used for the Hg Risk TSD are described in more detail in the three TSDs associated 

with this rule: (1) the 2005 version 4.1 Platform TSD, (2) the Emission Inventory TSD for the proposed

MATS, and (3) the Air Quality Modeling TSD: EGU Mercury Analysis. These documents and the IPM results 

are available in the MATS docket. 

4.2 Emissions for the Regulatory Impact Analysis (RIA) for the final rule

EPA updated the emissions estimates of the 2016 base and policy case emissions for the final rule. In 

Tables 7 and 8, the resulting emissions are labeled as the “Final 2016 base” and “Final 2016 policy” cases.

The final IPM emissions are cited in the preamble for the MATS and in any summaries stating the final 2016 

emissions associated with the rule generally. The final base and policy case emissions were not used in the 

modeling. Instead, the PM-related health co-benefits estimated for the final interim scenario were scaled 

using benefit-per-ton estimates, which is fully described in the RIA for the final rule.

 

26

 Mercury revisions for NATA primarily affected non-EGU sources such as cement kilns and hazardous waste combustors.

27 Selin, N.E., Jacob, D.J., Park, R.J., Yantosca, R.M., Strode, S., Jaegle, L., Jaffe, D., 2007. Chemical cycling and deposition of 

atmospheric mercury: Global constraints from observations. Journal of Geophysical Research-Atmospheres 112.
12

Table 7: Emission sources and basis for current and future-year emissions HAP inventories.

Purpose Emissions Case Pollutants Sector Emissions source Hg Deposition modeling for the Appropriate and Necessary Finding and Hg benefits analysis

2005 base year

Hydrogen 

chloride, and 

chlorine

EGU

Non-EGU point

Non-point

Mobile

2005 emissions from the version 4.1 

modeling platform, derived from the 2005 

National Emission Inventory (NEI) data

Mercury

EGU

Mobile

2005 National Air Toxics Assessment 

(NATA) data

Non-EGU point

Non-point

2005 NATA data, adjusted to account for 

Boiler MACT ICR data

Proposal Interim 

2016 Base Case 

projections

mercury and 

hydrogen chloride

EGU Emissions from IPM v.4.10, including the 

Transport Rule Proposal

Non-EGU point

Nonpoint

Mobile

Emissions from the version 4.1 modeling 

platform: projected 2005 emissions to 

account for all known national rules, 

consent decrees, and plant closures

Proposal Interim 

2016 Policy 

Case projections

Criteria pollutants, 

mercury and 

hydrogen chloride

EGU

Emissions from IPM v.4.10, including the 

Transport Rule Proposal and interim MATS 

Proposal

Non-EGU point

Nonpoint Same as interim 2016 base projections

Current 

significance of 

EGUs

Current base 

Emissions

Mercury, acid 

gases, and metals EGU

EGU emissions inventory based on unitspecific emission factors for tested units and 

average emission factors for untested units. 

2007-2009 average CEM heat input data 

used to represent current conditions where 

available, and ICR-based 2007-2009 heat 

input estimates otherwise.

Final impact of the rule

(no modeling done with 

these data)

Final 2016 Base 

Case 

Criteria pollutants, 

mercury and 

hydrogen chloride 

EGU IPM 4.10_MATS, including the final CrossState Air Pollution Rule

Non-EGU point

Nonpoint

Mobile

Interim 2016 Base Case projections

Final 2016 

Policy Case

Criteria pollutants, 

mercury and 

hydrogen chloride 

EGU IPM 4.10_MATS, including the final CrossState Air Pollution Rule

Non-EGU point

Nonpoint

Mobile

Interim 2016 Base Case projections

Table 8 summarizes the Hg and HCl emissions for the anthropogenic sources of these emissions 

across the various cases. All EGU emissions presented in Table 8 represent units greater than 25 MW. The 

2005 base case includes 105 tons of Hg and 430,000 tons of HCl from all sources, of which 53 tons of Hg 

and 350,000 tons of HCl are from EGUs. The current base emissions include the Hg and HCl emission 

estimates from U.S. EGUs are lower than in 2005, at 29 tons and 100,000 tons respectively. Speciated Hg 

emissions were not computed for the current base case. The interim 2016 base total Hg emissions from all 

sources are 64 tons and HCl emissions are 140,000 tons, with 29 tons of Hg and 74,000 tons of HCl from 

EGUs. For the interim 2016 policy case, total Hg was estimated to be 42 tons, with 6.8 tons from EGUs. 

For HCl, the interim 2016 policy case shows a total of 75,000 tons from all sources with EGU emissions 

emitting 8,800 tons. 
13

The final 2016 base case EGU emissions were 27 tons of Hg out of a total of 62 tons, and 45,000 tons 

of HCl out of a total of 115,000 tons. In the final 2016 MATS policy case, EGU emissions of Hg are 

estimated to be reduced to 6.6 tons and emissions of HCl reduced to 5,500 tons.

Table 8. Summary of Hg and HCl emissions for emissions cases.

Hg0

(tons)

Hg2+

(tons)

Hgp

(tons)

Total Hg 

(tons)

Hydrogen 

chloride (tons)

2005 Base Case Emissions (tons)

EGU *30 21 1.6 53 350,000

NonEGU Point 30 11 6.2 46 49,000

Nonpoint 3.1 1.06 0.65 4.8 29,000

Mobile 0.79 0.29 0.13 1.2

All sources 64 33 8.5 105 430,000

Current Base Emissions (tons)

EGU n/a n/a n/a 29 100,000

Proposal interim 2016 base case projections (tons)

EGU**21.1 6.9 0.67 28.7 74,000

NonEGU Point 16.9 7.9 4.4 29.2 38,000

Nonpoint 3.1 1.06 0.65 4.8 29,000

Mobile 0.79 0.29 0.13 1.2

All sources 42 16 5.9 64 140,000

Proposal interim 2016 policy case projections (tons)

EGU**4.8 1.6 0.36 6.8 8,800

NonEGU Point 16.9 7.9 4.4 29.2 38,000

Nonpoint 3.1 1.06 0.65 4.8 29,000

Mobile 0.79 0.29 0.13 1.2

All sources 26 11 5.6 42 75,000

Final 2016 base case projections (tons)

EGU 20.6 5.5 0.37 26.5 45,300

NonEGU Point 16.9 7.9 4.4 29.2 41,000**

Nonpoint 3.1 1.06 0.65 4.8 29,000

Mobile 0.79 0.29 0.13 1.2

All sources 41 15 5.5 62 115,300

Final 2016 policy projections (tons)

EGU 5.2 1.2 0.15 6.6 5,500

NonEGU Point 16.9 7.9 4.4 29.2 41,000**

Nonpoint 3.1 1.06 0.65 4.8 29,000

Mobile 0.79 0.29 0.13 1.2

All sources 26 10 5.3 42 75,500
14

* For this table, the EGU Sector is defined as follows: (1) For all pollutants other than mercury (Hg): 2005 NEI v2 point source 

EGUs mapped to the Integrated Planning Model (IPM) model using the National Electric Energy Database System (NEEDS) 

2006 version 4.10 database. A few revisions were made to the 2005 NEI v2 annual emission estimates as discussed in the 2005-

based, Version 4.1 platform document. (2) For Hg: 6/18/2010 version of the inventory used for the 2005 National Air Toxics 

Assessment (NATA) mapped to IPM using NEEDS version 4.10. The NATA inventory is an update to the 2005 NEI v2 and was 

divided into EGU and non-EGU sectors consistent with the other pollutants. We additionally removed Hg from sources from the 

NESHAP for Industrial, Commercial, and Institutional Boilers and Process Heaters (aka “Boiler MACT”) Information Collection 

Request (ICR) database because we included these emissions in the non-EGU sector. For more information, see the 2005-based 

Platform Documentation, version 4.1.

** EGU Sector for 2016 is consistent with the 2005 base case discussed above. The future year emissions were generated by the 

Interim version of IPM4.10 and adjustments were made to remove Hg associated with the Boiler MACT ICR (which is accounted 

for in the Non-EGU sector), and to apply the impact of the final Boiler MACT for the other pollutants.

Since proposal, EPA has compiled additional documentation on the speciation approach applied to 

U.S. EGUs. The speciation approach applied for the final MATS was developed as part of the Clean Air 

Mercury Rule (CAMR) based on a 1999 ICR. Eighty mercury emissions tests were collected from 69 

facilities. These data include inlet and outlet speciated mercury concentration measurements (μg/dscm) for 

Hgp, Hg2+, and Hg0 mercury. Only the outlet data (measurements after the last control device) were used to 

develop the mercury speciation profiles. Forty-three mercury speciation profiles were developed from these 

data based on the same bins developed for the Emission Reduction Factors used in IPM. Additional 

information on these profiles is available in the “Electric Generating Utility Mercury Speciation Profiles for 

the Clean Air Mercury Rule” and the workbook “Hg_speciation_data_CAMR.xlsx”, which are available in 

the docket.

Table 9 highlights the U.S. EGU emissions for the various cases shown in Table 8. It also includes 

the average percent of divalent gaseous and particulate Hg. The divalent gaseous Hg ranges from 20% to 

40% of the total Hg emissions, and the particulate Hg ranges from 1% to 5% of total Hg emissions.

Table 9: EGU emissions of Hg and HCl for emissions cases in support of the final MATS.

Hg0

(tons)

Hg2+

(tons)

Hgp

(tons)

Total Hg 

(tons)

Hydrogen 

chloride 

(tons)

Hg2+

Percent of 

total Hg

Hgp

Percent of 

total Hg

2005 Base Case 

Emissions 30 21 1.6 53 350,000 40% 3%

Current base

Emissions N/A* N/A* N/A*

29 100,000

Proposal interim 

2016 base case 

projections 

21.1 6.9 0.67 28.7 74,000 24% 2%

Proposal interim 

2016 policy case 

projections

4.8 1.6 0.36 6.8 8,800 24% 5%

Final 2016 base case 

projections 20.6 5.5 0.37 26.5 45,300 21% 1%

Final 2016 policy 

case projections 5.2 1.2 0.15 6.6 5,500 18% 2%

*

Speciation data were not collected for Hg in the 2010 ICR and CAMR-based speciation profiles were not applied to the current 

base emissions estimate.

Publication No. EPA-454/R-11-014

November, 2011

Office of Air Quality Planning and Standards

Air Quality Assessment Division

Research Triangle Park, NC

United StatesEnvironmental ProtectionAgency