113

Sulfuric Acid Mist: Regulating 

Uncertainties

By Matthew Thurlow*

Sulfuric acid mist, also known as H2SO4 or SO3,1 is one of the least 

publicized air pollutants associated with emissions from coal-fired power 

plants. Long overshadowed by nitrogen oxides, sulfur dioxide, and carbon 

dioxide, sulfuric acid mist is typically not emitted in the boundary-crossing and 

globe-altering quantities of the more frequently discussed air pollutants. In the 

whirlwind of the United States Environmental Protection Agency’s (EPA)

recent air regulations of coal-fired power plants including the Mercury and Air 

Toxic Standards for power plants (MATS), the New Source Performance 

Standards and the Tailoring Rule for greenhouse gases, and the recently 

vacated Cross-State Air Pollution Rule, sulfuric acid mist has remained 

relatively untouched.2 But EPA’s regulations, which have imposed dramatic

 

Copyright © 2012 Matthew Thurlow

* Former trial attorney, United States Department of Justice, Environmental Enforcement Section. 

Special thanks to Meghan Keck, Tracy Perkins, and Alexandra Pressman and the staff of Ecology Law 

Currents. The opinions expressed below are my own and do not represent the views of my current or 

former employers or clients.

1. Sulfuric acid mist is often referred to either as H2SO4 (sulfuric acid) or SO3 (sulfur trioxide). 

Sulfur trioxide forms in power plants as the result of oxidation of sulfur dioxide; as it cools, sulfur 

trioxide rapidly binds with water vapor in the stack to form H2SO4, a liquid aerosol. See U.S. ENVTL.

PROT. AGENCY, OFFICE OF POLLUTION PREVENTION & TOXICS, Emergency Planning & Community 

Right-to-Know Act Section 313, Guidance for Reporting Sulfuric Acid, EPA.GOV.,

http://www.epa.gov/tri/guide_docs/pdf/1998/sulfuric.pdf (last visited Aug. 17, 2012). Sulfuric acid mist 

is an air pollutant regulated pursuant to the Clean Air Act (CAA) as well as the Emergency Planning and 

Community Right-to-Know Act (EPCRA). 42 U.S.C. § 11001–50 (1986). Although industry contends 

there are no documented health impacts from dilute sulfuric acid mist, sulfuric acid is known to irritate 

and damage the eyes, skin, nose, and lungs. EPRI, Chemical Profile: Sulfuric Acid,

AEPSUSTAINABILITY.COM, http://www.aepsustainability.com/ourissues/envperformance/wastemgmt/tri/

chemicalprofiles/SulfuricAcid.pdf (last visited Aug. 13, 2012).

2. The Cross-State Air Pollution Rule, which was recently struck down by the D.C. Circuit, 

would have required dramatic reductions in sulfur dioxide and nitrogen oxides. See EME Homer City 

Generation, L.P. v. EPA, No. 11-1302 (D.C. Cir. Aug. 21, 2012); U.S. ENVTL. PROT. AGENCY, CrossState Air Pollution Rule, EPA.GOV, http://www.epa.gov/airtransport (last updated Aug. 21, 2012); 

Federal Implementation Plans: Interstate Transport of Fine Particulate Matter and Ozone and 

Correction of SIP Approvals, 76 Fed. Reg. 48208, 48296 (Aug. 8, 2011); Federal Implementation Plans 

for Iowa, Michigan, Missouri, Oklahoma, and Wisconsin and Determination for Kansas Regarding 

Interstate Transport of Ozone, 76 Fed. Reg. 80760 (Dec. 27, 2011); Revisions to Federal 

Implementation Plans To Reduce Interstate Transport of Fine Particulate Matter and Ozone, 77 Fed 

Reg. 10324 (Feb. 21, 2012); Revisions to Federal Implementation Plans to Reduce Interstate Transport 
114 ECOLOGY LAW CURRENTS [Vol. 39

new emission limits on sulfur dioxide, nitrogen oxides, greenhouse gases, 

mercury, and hydrochloric acid, are likely to have a significant impact on 

sulfuric acid mist emission control strategies at coal-fired power plants.3

Although sulfuric acid mist has been recognized as an air pollutant for 

decades, it only emerged as a significant problem for the utility industry in the 

early 2000s.4 In 2001, after the General James M. Gavin Power Plant installed 

a type of nitrogen oxide controls called selective catalytic reduction devices 

(SCRs), sulfuric acid mist emissions unexpectedly spiked from 9000 to 11,000 

pounds per day to allegedly more than 64,000 pounds per day.5 In Cheshire, 

Ohio, a small village of 200 people in the shadow of the Gavin plant, residents 

reported asthma-like symptoms and noted corrosion and discoloration of paint 

on cars and houses, as blue plumes of sulfuric acid periodically drifted through 

the village.6 The owner of the plant, American Electric Power (AEP),

 

of Fine Particulate Matter and Ozone, 77 Fed. Reg. 103242 (Feb. 21, 2012). Sulfur dioxide (SO2) 

reductions, which can be achieved through installation of scrubbers or use of lower sulfur coals, are 

likely to also reduce sulfuric acid mist. Nitrogen oxide reductions, including installation of selective 

catalytic reduction devices (SCRs), are likely to exacerbate sulfuric acid mist emissions. The new utility 

hazardous air pollutant rule, the Mercury and Air Toxics Standards (MATS), may also result in 

reductions of sulfuric acid mist because it imposes lower emissions limits on mercury and acid gases, 

which may be controlled using the same controls currently used to reduce sulfuric acid mist emissions. 

See National Emission Standards for Hazardous Air Pollutants from Coal and Oil-Fired Electric Utility 

Steam Generating Units and Standards of Performance for Fossil-Fuel-Fired Electric Utility, 

Industrial-Commercial-Institutional, and Small Industrial-Commercial-Institutional Steam Generating 

Units, Final Rule, 77 Fed. Reg. 9304 (Feb. 16, 2012), available at https://www.federalregister.gov/

articles/2012/02/16/2012-806/national-emission-standards-for-hazardous-air-pollutants-from-coal—andoil-fired-electric-utility. Power plant operators must also consider the impacts of new and proposed 

regulations relating to greenhouse gas emissions. See Standards of Performance for Greenhouse Gas 

Emissions for New Stationary Sources: Electric Utility Generating Units, Proposed Rule, 77 Fed. Reg. 

22392 (April 13, 2012); Prevention of Significant Deterioration and Title V Greenhouse Gas Tailoring 

Rule, 75 Fed. Reg. 31514 (June 3, 2010).

3. See Mark Pastore, Continuous SO3 Monitoring Can Reduce Sorbent Consumption, COAL 

POWER MAGAZINE (Feb. 1, 2011), http://www.coalpowermag.com/ops_and_maintenance/308.html 

(“SCRs serving another 150 GW are expected to be installed on U.S. coal-fired power plants by 

2020 . . . “).

4. See U.S. ENVTL. PROT. AGENCY, OFFICE OF AIR & WASTE MANAGEMENT, Final Guideline 

Document: Control of Sulfuric Acid Mist Emissions from Existing Sulfuric Acid Production Units, 

EPA.GOV, available at http://www.epa.gov/ttn/naaqs/ozone/ctg_act/197709_sox_epa450_2-77-

019_sulfuric_acid_mist.pdf. In June 1995, EPA modified the reporting requirements for sulfuric acid, 

but continued to require reporting of sulfuric acid aerosols including sulfuric acid mist. See EPCRA 

Guidance, supra note 1.

5. See John Amos Power Plant emitted more sulphuric acid than reported, HERALD-DISPATCH,

(Feb. 17, 2008), http://www.herald-dispatch.com/x1657941966.

6. See Health Consultation, Gavin Power Plant, Agency for Toxic Substances and Disease 

Registry 3 (Aug. 9, 2007), http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=

3&ved=0CDIQFjAC&url=http%3A%2F%2Fwebapp.psc.state.md.us%2FIntranet%2FCasenum%2FNe

wIndex3_VOpenFile.cfm%3Ffilepath%3DC%3A%255CCasenum%255C9000-9099%255C9085%

255CItem_56%255C%255CSWA-ExhibitC-1.pdf&ei=CqVCULWbAcL7iwLoooDQDg&usg=

AFQjCNFmFSWvTB9JqEUhVy_pLu8NlVzANg. Sulfuric acid mist can also cause severe corrosion to 

ductwork and stack liners, or anywhere else the sulfuric acid mist vapor condenses. See EPRI, SO3

Mitigation Guide Update 1-9 (Mar. 2004); Citizens Against Pollution v. Ohio Power Co., 484 F. Supp. 

2d 800 (S.D. Ohio 2007); Walter G. Wright, Does Flue Gas Constitute a RCRA Imminent and 

Substantial Endangerment? ENVIRONMENTAL, ENERGY, AND WATER BLOG (Mar. 7, 2012), available at

http://www.mitchellwilliamslaw.com/does-flue-gas-constitute-a-rcra-imminent-and-substantial-
2012] SULFURIC ACID MIST: REGULATING UNCERTAINTIES 115

eventually paid $20 million to buy out most of Cheshire. A decade later, the 

village remains mostly empty.7

The utility industry responded to the Gavin incident by investing 

significant time and money to study the sulfuric acid mist problem.8 EPA has 

also responded by paying closer attention to sulfuric acid mist from power 

plants and bringing a handful of sulfuric acid mist enforcement actions.9

Current and future enforcement cases involving sulfuric acid mist pose a 

number of challenges. In cases brought under the Clean Air Act’s (CAA) 

Prevention of Significant Deterioration of Air Quality (PSD) Program, utility 

companies, regulators, and courts may struggle to determine what emissions 

limits and controls are required for sulfuric acid mist. This struggle is based on 

uncertainties about the precise conditions under which sulfuric acid mist forms, 

how it can be controlled, how emissions can be monitored, and most 

importantly, at what emissions levels it poses a threat to human health and the 

environment.

The Gavin incident and subsequent studies have dramatically improved

the utility industry’s understanding of sulfuric acid mist. Sulfuric acid mist 

emissions strongly correlate with the sulfur content of coal: the higher the 

sulfur content, the higher the sulfuric acid mist emissions.10 But the precise

circumstances that result in the formation of sulfuric acid mist have been much 

more difficult to unravel. Experts believe that vanadium and other constituents 

in coal may increase sulfuric acid mist formation.11 In addition, boiler design 

and oxygen levels in the flue gas appear to influence sulfuric acid mist 

formation. High temperatures in boilers increase the formation of sulfuric acid 

 

endangerment (“CAP’s members experienced watery eyes, burning throats, headaches, and breathing 

problems during plume touchdowns.”); EPRI, Chemical Profile: Sulfuric Acid, supra note 1 (“Sulfuric 

acid fumes can irritate eyes, skin, and breathing passages, and concentrated fumes can permanently

damage the nose and lungs.”).

7. See Nicole Cohen, A Disappearing Town in the Shadow of Big Coal, NATIONAL PUBLIC 

RADIO, (Apr. 4, 2012), http://www.npr.org/blogs/pictureshow/2012/04/04/149874232/a-disappearingtown-in-the-shadow-of-big-coal.

8. American Electric Power pioneered the use of dry sorbent controls for sulfuric acid mist. See 

Douglas P. Ritzenthaler, SO3 Control: AEP Pioneers and Refines Trona Injection Process for SO3

Mitigation, COAL POWER MAGAZINE (Mar. 1, 2007), http://www.coalpowermag.com/

plant_design/29.html.

9. See U.S. v. Hoosier Energy Rural Elec. Coop., Inc., 2010 WL 4250427 (S.D. Ind. July 23, 

2010); In re E.ON U.S. & Ky. Util. Co., Notice of Violation, Mar. 19, 2009, available at

http://psc.ky.gov/pscscf/2011%20cases/2011-00161/20110616_Application%20Exhibits%20and%20

Appendices/GHR%20Exhibits%201%20-%204/Exhibit%20GHR-2%20-%20Ghent%202009%20NSRPSD%20NOV.pdf; In re E.ON U.S., Notice of Violation, Sept. 26, 2007, available at

http://psc.ky.gov/pscscf/2011%20cases/2011-00161/20110616; In re Duke Energy Corporation, Notice 

and Finding of Violation, Mar. 10, 2008, available at http://yosemite.epa.gov/r5/r5ard.nsf/

6a2817f3f71298e28625759a0045ba96/e47a671416849529862574c8006fd2ac/$FILE/ard-013713.pdf.

10. See EPRI, SO3 Mitigation Guide Update, supra note 6 (“The reaction mechanism discussed 

above for SO3 formation implies that higher sulfur fuels will always produce higher flue gas SO2 and 

SO3 concentrations than lower sulfur fuels.”).

11. See id. at 1-3, 1-4; see also James T. Murphy, SO3 Control: How Many Coal Plants Might 

Have Opacity Issues Due to SO3 Emissions?, COAL POWER MAGAZINE (Mar. 1, 2007), 

http://www.coalpowermag.com/plant_design/31.html (discussing various factors that influence sulfuric 

acid mist formation).
116 ECOLOGY LAW CURRENTS [Vol. 39

mist with the mist forming at the highest levels in a temperature band above 

approximately 800 degrees.12 Finally, ambient conditions, including wind and

water content in the air, also influence sulfuric acid mist formation and its

impacts.13 This means that even if all other factors remain constant, weather 

conditions may result in higher or lower ambient concentrations and can 

increase the risk of human exposure to sulfuric acid mist.

The uncertainties and complexities associated with sulfuric acid mist are 

further compounded by its relationship to other pollutants. Most troubling, as 

discovered at the Gavin plant, there is a clear relationship between the use of 

SCRs to reduce nitrogen oxide emissions and increases of sulfuric acid mist.14

A study of power plants equipped with SCRs found that 98 percent of the 

plants were expected to emit sulfuric acid mist at levels above 5 ppm, a level 

that might result in environmental impacts.15 But enforcement actions brought

against plants that have installed SCRs raise the troubling specter of potentially 

penalizing utilities for their efforts to reduce their environmental impact. In 

exercising enforcement discretion, regulators may be forced to balance the need 

to reduce nitrogen oxides and their regional impacts with the need to protect 

communities from the more localized impacts of sulfuric acid mist.16 And as 

with any pollutant from power plants, industry, government, and the public 

must weigh the environmental and health benefits of sulfuric acid mist control 

on one side, versus energy supply and demand and the potential increased cost

of electricity on the other. This Article briefly outlines the scientific and legal 

complexities facing the utility industry and environmental regulators in 

developing sulfuric acid mist control strategies. Next, it compares the economic 

and environmental tradeoffs of different control strategies. Finally, it

recommends control strategies that provide the utility industry operational 

flexibility while ensuring that human health and the environment are protected 

from sulfuric acid mist and recognizes that there may not be a one-size-fits-all 

solution to reduce sulfuric acid mist emissions at power plants.

 

12. See EPRI, SO3 Mitigation Guide Update, supra note 6, at 1-4, 1-5.

13. See Nenad Sarunac, Power 101: Improving the Performance of Boiler Auxiliaries, Part II, 

COAL POWER MAGAZINE (Feb. 1, 2011), http://www.coalpowermag.com/ops_and_maintenance/

300.html (providing that “atmospheric conditions (the direction of sunlight, temperature, humidity, and 

wind speed)” can influence opacity from sulfuric acid mist).

14. See EPRI, SO3 Mitigation Guide Update, supra note 6, at 1-5, 1-6; EPRI, Estimating Total 

Sulfuric Acid Emissions from Stationary Power Plants 3-4–3-5 (2010), available at http://my.epri.com/

portal/server.pt?space=CommunityPage&cached=true&parentname=ObjMgr&parentid=2&control=Set

Community&CommunityID=405 (last visited Aug. 13, 2012) (discussing SCR’s introduction of 

ammonia into flue gas, which can result in formation of sulfuric acid mist).

15. See Murphy, SO3 Control, supra note 11.

16. Facilities that installed SCRs prior to 2005 and otherwise complied with the applicable 

provisions can argue that the pollution control project exemption to New Source Review applies. See 40 

C.F.R. § 52.21. Although the pollution control project exemption has been vacated by the D.C. Circuit, 

the decision does not apply retroactively. See New York v. EPA, 413 F.3d 3 (D.C. Cir. 2005).
2012] SULFURIC ACID MIST: REGULATING UNCERTAINTIES 117

I. REGULATING SULFURIC ACID MIST UNDER THE CLEAN AIR ACT

A. Prevention of Significant Deterioration Provisions

One of the most important avenues for regulating sulfuric acid mist under 

the CAA is through the New Source Review (NSR) program.17 Under NSR, 

major new and modified stationary sources in areas that are unclassifiable or 

that meet the National Ambient Air Quality Standards (NAAQS) are subject to 

PSD permitting.18 Notwithstanding certain exceptions, NSR and PSD are 

triggered either by new construction, or a physical change or change in the 

method of operation of an existing facility that results in a significant net 

increase of emissions of a pollutant. The threshold for “significant increase”

varies by pollutant. For sulfuric acid mist, the threshold is an increase of 7 tons 

per year.19 Facilities subject to the PSD program must submit a PSD permit to 

the permitting authority and implement Best Available Control Technology 

(BACT) for each regulated pollutant. Thus, unless an exception applies,20 any 

power plant that makes a physical or operational change to its plant resulting in 

 

17. Unlike the criteria air pollutants, there are no NAAQS for sulfuric acid mist. Because sulfuric 

acid mist always falls within an unclassifiable area, the prevention of significant deterioration (PSD) 

permitting program applies to new and modified major sources that emit sulfuric acid mist. See

Coalition for Responsible Regulation, Inc. v. EPA, 684 F.3d 102; 2012 U.S. App. LEXIS 12980 (D.C. 

Cir. 2012). In Coalition, the D.C. Circuit rejected Petitioners’ arguments that greenhouse gases were not 

subject to PSD because they are not air pollutants emitted from major emitting facilities. Id. at *64, 72–

85 (“EPA’s interpretation of the CAA requires PSD and Title V permits for stationary sources whose 

potential emissions exceed statutory thresholds for any regulated pollutant—including greenhouse 

gases. . . .[G]iven both the statute’s plain language and the Supreme Court’s decision in Massachusetts 

v. EPA, we have little trouble concluding that the phrase “any air pollutant” includes all regulated air 

pollutants, including greenhouse gases.”).

18. In the Gavin matter, the non-profit Citizens Against Pollution (CAP) brought a case against 

the Ohio Power Company (a subsidiary of AEP) under the Resource Conservation and Recovery Act

(RCRA), Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), and 

Emergency Planning and Community Right to Know Act (EPCRA). It appears that the non-profit 

brought these claims because U.S. EPA and the Ohio EPA entered into a memorandum of agreement 

with the power plant to reduce sulfuric acid mist emissions and CAA claims were barred. See Opinion 

and Order, Citizens Against Pollution v. Ohio Power Co., No. C2-04-CV-371 (S.D. Ohio 2007) (“OPC 

took measures to correct the situation, including, inter alia, implementing air testing and entering into an 

Memorandum of Agreement with the United States Environmental Protection Agency and the Ohio 

Environmental Protection Agency.”).

19. 40 C.F.R. § 51:166(23)(i).

20. While there is little doubt that a significant project like the installation of nitrogen oxide 

pollution controls can be construed as a physical modification of a power plant, it is less clear if minor 

tweaks to operations trigger PSD. Power plant owners would likely contend that these tweaks were 

routine maintenance and that the statute’s exception for “routine maintenance, repair, and replacement”

applies. 40 C.F.R. § 51:166(b)(2)(iii)(a). A more interesting scenario is a situation in which the power 

plant has merely switched fuel sources from a low sulfur coal source to a high sulfur coal source. 

Making this type of switch, for instance, from 1-2 lb/MMBtu coal source to a coal source three to four 

times higher in sulfur content, would likely dramatically increase emissions of sulfuric acid mist (as well 

as sulfur dioxide). This scenario might seem an obvious trigger of EPA’s PSD program because it would 

appear to be a change in the method of the plant’s operation, but there is a specific carve-out in the PSD 

program for fuel switches. A fuel switch exception applies to the use of an alternative fuel that a source 

“was capable of accommodating before January 6, 1975.” 40 C.F.R. § 51.166(b)(2)(iii)(e)(1); see also 

Hawaiian Elec. Co., Inc. v. U.S. EPA., 723 F.2d 1440, 1448 (9th Cir. 1984) (citing 1979 EPA 

determination that “an increase in sulfur content does not constitute use of an ‘alternative’ fuel”).
118 ECOLOGY LAW CURRENTS [Vol. 39

an increase of sulfuric acid mist emissions of more than 7 tons a year must 

obtain a PSD permit and apply BACT to limit sulfuric acid mist emissions.

B. Best Available Control Technology

BACT is an emissions limit “based on the maximum degree of reduction 

of each pollutant subject to regulation under” the CAA “on a case-by-case 

basis, taking into account energy, environmental, and economic impacts and 

other costs,” that the permitting authority “determines is achievable.”21 EPA 

recommends, and most permitting agencies apply, a top-down BACT analysis 

that ranks all available control technologies for a regulated pollutant in 

descending order of effectiveness.22 Following this approach, the most 

stringent control alternative is selected unless technical considerations, energy, 

environmental, or economic impacts lead the permitting authority to conclude 

that it is not “achievable.”23 Because BACT is assessed on a case-by-case basis, 

it may differ significantly from one power plant to another power plant. 

Geography, fuel sources and types, and plant configuration can impact the 

technical feasibility of controls and the cost effectiveness of controls.24 Not 

surprisingly, permitting authorities, the utility industry, and the public, which 

can comment on PSD permits,25 may disagree over how technical, 

environmental, economic, and site-specific factors should influence BACT 

determinations.

C. Control Technology Available for Sulfuric Acid Mist

One of the most effective control options for sulfuric acid mist is a wet 

electrostatic precipitator (WESP), a particulate control device that removes 

particles, including sulfuric acid mist, from flue gas by using an electrostatic 

charge.26 WESPs are extremely efficient at removing sulfuric acid mist, but 

 

21. 42 U.S.C. § 7479(3).

22. See Alaska Dep’t of Envtl. Conservation v. EPA, 540 U.S. 461, 475–76 (2004). As the 

Supreme Court held in Alaska, “[n]othing in the Act or its implementing regulations mandates top-down 

[BACT] analysis.”).

23. Id.; see also U.S. ENVTL. PROT. AGENCY, New Source Review Workshop Manual B2 (Oct. 

1990), available at http://www.epa.gov/ttn/nsr/gen/wkshpman.pdf. EPA maintains a clearinghouse of 

best available technologies for power plants and other stationary sources. See RACT/BACT/LAER 

Clearinghouse, U.S. ENVTL. PROT. AGENCY, http://cfpub.epa.gov/rblc/index.cfm?action=Home.Home.

24. Determining the cost-effectiveness of BACT controls includes an assessment of the capital 

costs and annual operation and maintenance costs of controls and the difference between baseline 

emissions and controlled emissions. This allows for an approximation of the cost per ton of emissions 

reductions. Regulated entities also review the incremental cost of compliance, or a comparison of the 

costs of compliance between the best available control method and the next best method. U.S. ENVTL.

PROT. AGENCY, PSD and Title V Permitting Guidance for Greenhouse Gases K-1 (Mar. 2011), 

available at http://www.epa.gov/nsr/ghgdocs/ghgpermittingguidance.pdf.

25. See U.S. ENVTL. PROT. AGENCY, EPA Region 9’s PSD Permitting Process, EPA.GOV,

http://www.epa.gov/region9/air/permit/psd-issuing.html (last updated Apr. 3, 2012).

26. See U.S. ENVTL. PROT. AGENCY, OFFICE OF AIR & RADIATION, Technical Support Document: 

Impact on CAIR Analyses of D.C. Circuit Decision in New York v. EPA 5, (Dec. 2005), available at

http://www.adeq.state.ar.us/ftproot/Pub/commission/p/Closed%20Permit%20Dockets%202006-

2012/08-006-P%20AEP%20Service%20Corp.%20%26%20SWEPCO-
2012] SULFURIC ACID MIST: REGULATING UNCERTAINTIES 119

they can cost $50 million to $200 million and require a significant amount of 

energy, up to 0.5 percent of the plant’s gross output, to operate.27 Because 

BACT analyses require economic assessment of control options, including a 

calculation of removal costs on a per ton basis, utilities may effectively argue

that a WESP is not required under BACT if the amount of sulfuric acid mist 

removed is less than several thousand tons per year. There is no bright-line rule 

for per ton removal costs under BACT, but costs above five or six thousand 

dollars per ton for controls have been referenced as approaching the upper limit 

of the threshold of economically feasible technology required under BACT.28

The current preference of the utility industry for sulfuric acid mist control 

appears to be sorbent injection, an option that is much more economical than 

WESPs in the short term. Dry sorbent injection uses nozzles to spray a dry 

powder, typically magnesium, lime, or trona (a sodium-based mixture), into the 

flue gas.29 The sorbent binds with the sulfuric acid mist and removes it from 

the flue gas stream. But there are limits on the use of sorbent control. Excessive 

use of sorbents can clog equipment, so power plant engineers may need to

experiment with different levels of sorbent injection. They may also need to 

balance sorbent injection with other control methods, including configuration 

changes that increase the amount of time sulfuric acid mist remains in the 

 

Sierra%20Club%20%26%20Audubon(Consolidated)/2009-04-24_SWEPCO-BH-EX.22.pdf (noting that 

WESP can remove approximately 95 percent of sulfuric acid mist from flue gas).

27. The costs of a WESP are highly plant-specific and depend on plant size and configuration.

The cost of capital projects for power plants is typically expressed as a function of cost per kilowatt of 

energy. The estimated capital costs for WESP’s range from $20 to $45 per kilowatt, which, for a 2500 

megawatt power plant, would translate to a cost of $50 million to $112 million. See John Caine & 

Hardik Shah, Membrane WESP – A Lower Cost Technology to Reduce PM 2.5, SO3 & HG+2 Emissions

(2006), http://www.netl.doe.gov/technologies/coalpower/ewr/mercury/control-tech/pubs/AQIVCaine.pdf (last visited Aug. 17, 2012); see also Gary M. Blythe, et al., Economic Comparison of SO3 

Control Options for Coal-Fired Power Plants, NETL.DOE.GOV. (Nov. 25, 2003), 

http://www.netl.doe.gov/technologies/coalpower/ewr/pm_emissions_control/pubs/SO3%20Control%20

Options%20for%20AQ%20IV.pdf (last visited Sept. 1, 2012). Blythe estimates that the capital costs to 

retrofit a plant and install a WESP would be $40 to $90, for a total cost of $100 to over $200 million for 

a 2500 megawatt power plant.

28. Because BACT determinations are made on a case-by-case basis, there is no bright line rule 

regarding the economic feasibility of per ton pollutant removal costs. See Brandon A. Mogon, The 

BACT Analysis Guide: Cost Analysis Considerations, THE BACT ANALYSIS GUIDE, (Oct. 23, 2009), 

http://bactanalysis.com/bact-analysis/the-bact-analysis-guide-cost-analysis-considerations (“Each 

regulatory agency has a different opinion about the maximum economically feasible cost effectiveness 

value, and many (e.g., CTDEP) will not tell you what that value is.”). But some states have provided 

guidance that the general rule of thumb for the upper bound of economic feasibility for per ton reduction 

of pollutants approaches $4,000 to $6,000. See, e.g., MASS. DEP’T OF ENVTL. PROT., Best Available 

Control Technology Guidance 6 (June 2011), available at http://www.mass.gov/dep/air/approvals/

bactguid.pdf; NEB. DEP’T OF ENVTL. QUALITY, Best Available Control Technology,

http://www.deq.state.ne.us/Publica.nsf/c4afc76e4e077e11862568770059b73f/0949822f884b8ce186257

3bd007da0e9?OpenDocument; UTAH DEP’T OF ENVTL. QUALITY, Best Available Control Technology, 

http://www.airquality.utah.gov/Permits/FORMS/Form01b.pdf. Maximum removal costs should, in 

theory, relate to pollutants’ proportional threat to the environment and human health with more harmful 

pollutants having a higher cost per ton threshold of feasibility under BACT than more innocuous air 

pollutants.

29. See EPRI, Estimating Total Sulfuric Acid Emissions from Stationary Power Plants, supra 

note 14, at 3-6.
120 ECOLOGY LAW CURRENTS [Vol. 39

stack.30 The longer the residence time for sulfuric acid mist in the stack, the 

more opportunity the sulfuric acid mist has to bind with sorbent.31 Facilities 

can also maximize sulfuric acid mist control if they mill sorbent into smaller 

particles that increase the surface area of sorbent and improve its potential to 

capture sulfuric acid mist. However, even with these measures, there remains a 

saturation point beyond which increasing the amount of sorbent injected will 

not further reduce the amount of sulfuric acid emissions. Other plant 

improvements, including installation of low catalyst SCRs,32 which reduce, but 

do not eliminate the impacts of SCRs on sulfuric acid mist formation, or 

switching or blending fuel with low or medium sulfur content coal,33 may be 

used to supplement sorbent injection.

Baghouses, which are large filters designed to capture soot and other 

particulates,34 can also reduce sulfuric acid mist emissions. One study indicates 

that baghouses can remove up to 90 percent of sulfuric acid mist.35 As with 

WESPs, however, installing baghouses can be a significant capital 

expenditure.36 Utilities may balk at the expense and argue that the technology 

is not economically feasible under a BACT analysis.

Improvements in sorbent control may be increasing regulators’ and 

industry’s confidence that sorbent injection, while not achieving the same 

reductions in sulfuric acid mist as WESPs, can reduce sulfuric mist emissions 

to levels that are sufficient to protect human health and the environment at a 

fraction of the cost.37 The effectiveness of sorbent injection, however, may 

 

30. See EPRI, SO3 Mitigation Guide Update, supra note 6, at 1-7, 2-13, 4-2; see also EPRI, SO3

MITIGATION: CURRENT UTILITY OPERATING EXPERIENCE A19 (2006).

31. Residence time is the amount of time that sorbent is present in the gas stream and has an

opportunity to bind with sulfuric acid mist and form a precipitate. Injecting the sorbent before the 

electrostatic precipitator also increases residence time. See Douglas Ritzenthaler, SO3 Control: AEP 

Pioneers and Refines Trona Injection for SO3 Mitigation, COAL POWER MAGAZINE (Mar. 1, 2007), 

http://www.coalpowermag.com/plant_design/SO3-Control-AEP-Pioneers-and-Refines-Trona-InjectionProcess-for-SO3-Mitigation_29.html; EPRI, Estimating Total Sulfuric Acid Emissions from Stationary 

Power Plants, supra note 14, at 3-6, 4-19.

32. See EPRI, SO3 Mitigation: Current Utility Operating Experience, supra note 30, at 3-19 

(“Performance testing completed by the manufacturer of the catalyst actually showed less than 0.1 

percent SO2 conversion to SO3 from three different reactor tests.”).

33. See EPRI, SO3 Mitigation Guide Update, supra note 6, at 2-19 (noting that the effectiveness 

of fuel blending can be difficult to predict, but one facility reduced its sulfuric acid mist emissions to 

zero by switching to low sulfur, Powder River Basin coal). The economics and cost effectiveness of 

switching coal can be very site-specific because switching or blending fuels depends on long-term coal 

contracts, coal availability, and other plant-specific factors. Id.; see also Gary M. Blythe et al., SO3 

Control Options for Coal-Fired Power Plants, supra note 27, at 2.

34. Baghouses are air pollution control devices that remove particulates from flue gas streams. 

Baghouses typically use fabric filters to remove and collect dust from the flue gas stream. See John H. 

Turner, et al., EPA Air Pollution Control Cost Manual, U.S. ENVTL. PROT. AGENCY (Jan. 2002),

available at http://www.epa.gov/ttncatc1/dir1/c_allchs.pdf.

35. See EPRI, Estimating Total Sulfuric Acid Emissions from Stationary Power Plants, supra

note 14, at 4-19.

36. See EPA Air Pollution Control Cost Manual supra note 34, at 1-1.

37. Despite their high operation and maintenance costs, WESPs may compare more favorably 

with sorbent injection over the long-term, especially if used year-round. See EPRI, SO3 Mitigation Guide 

Update, supra note 6, at 3-37.
2012] SULFURIC ACID MIST: REGULATING UNCERTAINTIES 121

depend on proper calibration and maintenance of sorbent injection rates over 

time and a consistent fuel source. If these inputs are not constant, sulfuric acid 

mist emissions could spike. To mitigate the possibility of fluctuations in

sulfuric acid mist emissions, operators need to build in some compliance 

headroom by ensuring that day-to-day emissions of sulfuric acid mist are 

marginally lower than levels that could result in opacity problems or violate 

permit limits. Once a control strategy has been adopted, power plants and 

enforcement authorities need to monitor the effectiveness of the controls over 

time and in different operating scenarios. Some power plants may need to

continue to experiment with a variety of controls to find a solution that 

provides the best balance of sulfuric acid mist reduction, control of other 

pollutants, and power plant performance.

D. Opacity Violations

The appearance of the tell-tale blue plume of sulfuric acid mist from the 

stack of a power plant often indicates a different violation of the CAA. In 

addition to, or in lieu of, claims brought under the CAA’s PSD provisions, EPA 

may bring claims against power plant owners and operators for opacity 

violations under the Act’s New Source Performance Standards (NSPS) (Section 

111 of the Act) or the applicable State Implementation Plan.38 “Opacity means 

the degree to which emissions reduce the light and obscure the view of an 

object in the background.” 40 CFR § 63.2. At 100 percent opacity, no light is 

visible through a plume. At zero percent opacity, a plume is completely 

transparent. While opacity is not itself a pollutant, it serves as a surrogate for 

particulate matter pollution, including sulfuric acid mist pollution, from power 

plants.39

The NSPS for fossil-fuel-fired steam generators provide that, for power 

plants constructed after August 17, 1971, gases emitted from the facility cannot 

“exhibit greater than 20 percent opacity except for one six-minute period per 

hour of not more than 27 percent opacity.”40 For facilities not subject to the 

NSPS (built prior to August 1971), opacity limits can vary depending on the 

applicable State Implementation Plan. In Kentucky, for example, facilities are 

not to exceed 40 percent opacity except for one six-minute period per hour of 

not more than 60 percent opacity.41 In Texas, older facilities cannot exceed 30

percent opacity averaged over a six-minute period.42

Opacity problems associated with sulfuric acid mist can occur with 

emissions as low as 3-4 ppm.43 The utility industry reports that sulfuric acid 

 

38. 40 C.F.R. § 60.42; 40 C.F.R. § 52.01 et seq.

39. See Sierra Club v. Georgia Power Co., 443 F.3d 1346, 1350 n.4 (11th Cir. 2006).

40. 40 C.F.R. § 60.42(a)(2).

41. 401 KAR 61:015(4); 40 C.F.R. § 52.920.

42. 30 TEX. ADMIN. CODE § 111.111(a)(1)(A) (2011); 40 C.F.R. § 52.2270(c).

43. See Pastore, Continuous SO3 Monitoring Can Reduce Sorbent Consumption, supra note 3, at 

1 (“SO3 also creates a visible blue-white plume at concentrations as low as 3-4 ppm and is often 

detectable on an opacity monitor.”); but see Sarunac, Power 101, supra note 13 (“Flue gas SO3
122 ECOLOGY LAW CURRENTS [Vol. 39

mist concentrations of only 10 ppm can result in opacities greater than 40

percent, the upper opacity limit for many older power plant units.44 Other 

industry guidance indicates that opacity problems can occur at sulfuric acid 

mist concentrations above 5-15 ppm.45 The impacts of sulfuric acid mist 

emissions on stack opacity can fluctuate depending on operating conditions. 

Opacity monitors can provide accurate opacity data and send an immediate 

warning to plant operators if limits are exceeded. But many power plants have 

had to install wet scrubbers to address sulfur dioxide emissions, which makes it 

difficult or impossible to monitor opacity within their stacks. As a result, many 

plants have either removed their opacity monitors or replaced them with 

particulate monitors. Continuous sulfuric acid mist monitors, which would best 

address the current monitoring problem, are still under development.46

As a result of the unavailability of effective monitoring devices for 

sulfuric acid mist, in many instances the only method to determine opacity at a 

power plant is through visual observation of the plume. Method 9 readings,

which rely on the judgment of a trained inspector, are the most common 

method of visual opacity assessments.47 Although inspectors are typically 

experienced and well trained, Method 9 is subject to judgment, memory, and 

the human eye.48 Inspectors record 24 consecutive observations (typically in 

six-minute increments) and average the results. An accurate opacity test 

requires ideal weather conditions because the plume cannot be clearly observed 

on cloudy days. The difficulty and uncertainty associated with visual opacity 

readings are a cause for concern for both regulators and the industry.49 On the 

one hand, regulators have to undertake opacity readings onsite and in clear 

 

concentrations of about 10 ppmv can result in plume opacities greater than 50 percent in some cases; at 5 

ppmv, the opacity is about 20 percent. The specific SO3 concentration at which a blue plume can be seen 

is a function of atmospheric conditions and stack characteristics. However, it is generally accepted that 

if the SO3 concentration is less than 5 ppmv, there are no visible discoloration effects.”).

44. See EPRI, SO3 Mitigation Guide Update, supra note 6, at 1-3.

45. See EPRI, SO3 Mitigation: Current Utility Operating Experience, supra note 30; EPRI, 

Estimating Total Sulfuric Acid Emissions, supra note 14, at 3-7 (“The alkali injection system usually is 

operated to reduce SO3 emissions to between 5 and 15 ppm, an optimal range to prevent formation of a 

visible plume”); SO3 Mitigation Guide Update, supra note 6, at 1-8 (providing that flue gas 

concentrations of sulfuric acid mist of 10 ppm can result in plume opacities above 40 percent).

46. See Pastore, Continuous SO3 Monitoring Can Reduce Sorbent Consumption, supra note 3.

47. U.S. Envtl. Prot. Agency, Stationary Source Compliance Division, Visible Emissions Field 

Manual EPA Methods 9 and 22, EPA.Gov, available at http://www.epa.gov/ttn/emc/methods/

VEFieldManual.pdf.

48. Method 9 inspectors are trained and recertified every six months at “smoke school,” in which 

they observe white and black smoke plumes from a stack with opacity monitoring equipment. As part of 

their training, inspectors are tested on their ability to recognize different opacity levels of smoke plumes. 

See, e.g., SMOKE SCHOOL, INC., http://smokeschoolinc.com (last visited Aug. 13, 2012); EASTERN 

TECHNICAL ASSOCIATES, VISIBLE EMISSIONS OBSERVER TRAINING MANUAL (Aug. 2004), 

http://www.eta-is-opacity.com/ETA_VE_manual.pdf.

49. See National Parks Conservation Assoc., Inc. v. TVA, 175 F. Supp. 2d 1071, 1079 (E.D. 

Tenn. 2001) (“Obviously, monitoring the smokestack emissions continuously with equipment capable of 

reliably measuring the opacity will identify many more exceedances than will be identified by an 

operator ‘eyeballing’ the smokestack emissions once a day, or less.”); Sierra Club v. Public Service Co 

of Colorado, 894 F. Supp. 1455, 1459-60 (D. Colo. 1995) (citing the “relative reliability of CEM data 

over Method 9 data”).
2012] SULFURIC ACID MIST: REGULATING UNCERTAINTIES 123

weather to document opacity violations. On the other hand, an opacity reading 

made by a regulator visually assessing real time emissions from memory is 

difficult for a defendant to refute.

If a pattern of opacity violations at a power plant can be established, EPA 

or state environmental authorities may argue that significant controls are 

required to eliminate the visible sulfuric acid mist plume. Enforcement 

authorities may obtain civil penalties for each day a plant exceeds opacity 

limits, or obtain significant injunctive relief that, in some cases, approaches a 

BACT-like remedy.

II. HUMAN HEALTH AND THE ENVIRONMENT AND EMISSIONS LIMITS

Perhaps the thorniest issue with regard to regulating sulfuric acid mist is 

determining at what levels emissions of sulfuric acid mist threaten human 

health and the environment. Studies indicate that sulfuric acid mist can impact 

the health of children with asthma at 70 micrograms per cubic meter, and can 

impact normal adult lung function at 100 micrograms per cubic meter.50 The 

impact on health, however, is a function of exposure duration, individual 

sensitivity, and exposure to other air contaminants.

In the early 2000s, toxicologists from the Agency for Toxic Substances 

and Disease Registry evaluated the impacts of sulfur dioxide and sulfuric acid 

mist on the village of Cheshire, Ohio, near the Gavin power plant, and 

concluded that emissions posed a public health hazard to some residents.51 The 

highest officially recorded levels of sulfuric acid mist in Cheshire were 

approximately 120 micrograms per cubic meter of air, but there were unofficial 

reports of levels as high as 200 micrograms per cubic meter of air.52 It was 

difficult for investigators to determine the duration of individual exposure to 

sulfuric acid mist, but exposures ranged from several minutes to several

hours.53 Residents in Cheshire were also exposed to high levels of sulfur 

dioxide and metal oxide particulates and investigators indicated that the 

presence of these and other co-contaminants might also have had health 

impacts.54

Following the lawsuit brought by Citizens Against Pollution, AEP agreed 

to emissions limits of 14 ppm of sulfuric acid mist at the Gavin plant. By 

comparison, in the only settled case to date in which EPA has directly 

addressed sulfuric acid mist, involving the Hoosier Energy Company, the 

parties agreed to limits of approximately 2.5 ppm (.007 lb/mmBTU).55 But this 

 

50. See AGENCY FOR TOXIC SUBSTANCES AND DISEASE REGISTRY, HEALTH CONSULTATION,

GAVIN POWER PLANT 12 (Apr. 2007).

51. Id. at 16.

52. Id. at 5–6.

53. Id. at 5.

54. Id. at 13.

55. The Hoosier settlement included a limit of .007 lb/mmBTU, but provided Hoosier with the 

option to petition for a lower limit. However, the limit could not be lower than .009 lb/mmBTU. See 

U.S. ENVTL. PROT. AGENCY, Consent Decree, U.S. v. Hoosier Energy Rural Elec. Coop., Inc. (S.D. Ind. 
124 ECOLOGY LAW CURRENTS [Vol. 39

limit was only one part of a larger settlement that required a number of 

significant improvements to the facility at a total cost of $250 million to $300 

million.56 Additionally, Hoosier had a lengthy compliance period, almost two 

years, to meet the sulfuric acid mist emissions limit, and an additional year 

before it was subject to stipulated penalties to meet these limits.

Although there appears to be some consensus in the utility industry that 

emissions of sulfuric acid mist above 5 ppm may result in opacity problems, 

many power plants may be disinclined to agree to similarly low limits. This 

hesitation is in part a function of the difficulty of measuring sulfuric acid 

mist.57 Although there is an accepted EPA methodology for stack testing for 

sulfuric acid mist, the industry has expressed concern that this method does not 

provide accurate results.58 Sulfuric acid mist emissions also may fluctuate 

between stack tests. While sorbent controls may ensure compliance with a 5 

ppm limit most of the time, ambient conditions or variations in fuel could result 

in higher emissions.59 Both EPA and the industry are concerned that no 

continuous emissions monitoring device is commercially available for sulfuric 

acid that has the requisite sensitivity to detect changes of less than 1 ppm in the 

stack.

In the absence of recurrent, visible opacity problems, power plants may 

remain unaware of the potential significance of sulfuric acid mist emissions 

until stack testing can be performed, at a relatively high cost, on a quarterly or 

biannual basis. Infrequent stack tests threaten both utility operators and 

regulators. Utility operators may worry that one high stack test could be used as 

evidence of continuous non-compliance with emissions limits, while regulators 

may be concerned that stack tests do not provide adequate monitoring of 

sulfuric acid mist emissions and could fail to identify non-compliant facilities.

III. UNCERTAINTY AND BALANCING

As EPA seeks more stringent regulation of other air pollutants, the utility 

industry and regulators will need to keep close tabs on sulfuric acid mist 

emissions. Perhaps the most vexing problem for regulators and industry alike is 

the uneasy relationship between sulfuric acid mist control and control of 

nitrogen oxides. A significant sulfuric acid mist problem first emerged, at 

Gavin and elsewhere, with the adoption of SCRs used to reduce nitrogen oxide 

 

July 26, 2010), EPA.GOV, available at http://www.epa.gov/compliance/resources/decrees/civil/

caa/hoosier-cd.pdf.

56. See U.S. ENVTL. PROT. AGENCY Hoosier Energy Agreement Marks 20th Settlement Under 

EPA’s Power Plant Enforcement Initiative, EPA.GOV (July 23, 2010); available at 

http://yosemite.epa.gov/opa/admpress.nsf/f0d7b5b28db5b04985257359003f533b/9dee83d4b674b70385

257769005d7b1e!OpenDocument.

57. See EPRI, Estimating Total Sulfuric Acid Emissions from Stationary Power Plants, supra

note 14, at 2-1–2-2 (discussing sulfuric acid mist measurement uncertainties).

58. Id. EPA uses EPA Method 8 to test for sulfuric acid mist. The utility industry prefers the 

controlled condensate system method because it claims EPA Method 8 results in a positive bias for 

detection of sulfuric acid mist. See id. at 2-1; 3-7.

59. See Murphy, SO3 Control, supra note 11.
2012] SULFURIC ACID MIST: REGULATING UNCERTAINTIES 125

emissions. When SCRs are combined with high-sulfur coal, as they were at the 

Gavin plant, sulfuric acid mist emissions can increase dramatically.

In response to the now decade-old problem with SCRs, the utility industry 

developed low-acid conversion catalysts that reduce the sulfuric acid 

conversion rate.60 But SCRs still involve a trade off with generally higher 

emissions of sulfuric acid mist.61 Power plants have sought to compensate for 

the higher sulfuric acid mist emissions with many of the control technologies 

described above, but several of these strategies, including WESPs and 

baghouses, may not be economical if used solely to control sulfuric acid mist. 

Sorbent injection, while typically the most economically feasible control 

method for sulfuric acid mist in the short term, may be insufficient to control 

very high levels of sulfuric acid mist emissions.

Following the recent regulations for hazardous air pollutants, including 

mercury and acid gases, power plants may increasingly turn to sorbent 

injection.62 The need to control mercury and acid gas may have a positive 

impact on sulfuric acid mist control because power plants may not only invest 

in superior sorbent injection systems, but may also install baghouses that

increase the effectiveness of sorbent controls.63 Additional controls that target 

sulfur dioxide, including scrubbers, should also serve to reduce sulfuric acid 

mist emissions.

In the current regulatory landscape, optimizing the performance of coalfired power plants and ensuring that they run efficiently within permit limits is 

increasingly difficult. Finding the operational “sweet spot” at power plants may 

require a significant investment of time and money. In order to reduce sulfuric 

acid mist emissions and balance other regulatory requirements, enforcement 

authorities and the utility industry must develop control strategies tailored to 

individual power plants. Regulators must allow time for calibration of sulfuric 

acid mist and other air pollutant control strategies and stack testing. For their 

part, plant operators must think ahead and consider the impacts of 

implementing control schemes for multiple pollutants. In an environment of 

regulatory uncertainty, it may be worthwhile for the utility industry to consider

adopting conservative control strategies with multi-pollutant control benefits 

that increase operational flexibility, reduce the risk of future non-compliance, 

and anticipate the possibility of stricter, long-term emissions limits. The high 

costs of control equipment, complex maintenance and operation schedules of 

 

60. See Sarunac, Power 101, supra note 13 (indicating that high oxidation catalysts in an SCR 

can double the concentration of sulfuric acid mist in flue gas; low conversion catalyst significantly 

reduce sulfuric acid mist conversion).

61. See EPRI, SO3 Mitigation Guide Update, supra note 6, at 1-6.

62. The MATS for power plants sets numerical limits for mercury emissions, other hazardous 

metal emissions, and hydrochloric acid emissions.

63. 77 Fed. Reg. at 9411 (“[T]he EPA agrees that DSI [dry sorbent injection] technology is 

proven and ready for commercial uses in controlling acid gases from coal combustion.”). As described 

above, baghouses permit the injection of additional sorbent into the flue gas stream and increase the 

amount of residence time in which sorbent can bind with air pollutants. WESPs also can be used to 

control mercury. See John Caine & Hardik Shah, Membrane WESP, supra note 27, at 9.
126 ECOLOGY LAW CURRENTS [Vol. 39

power plants, and threat of future enforcement actions leave little room for trial 

and error.