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Comments on 

40 CFR Part 49 

[EPA–R09–OAR–2009–0598; FRL–8950–6] 

Assessment of Anticipated Visibility Improvements at Surrounding Class I Areas and 

Cost Effectiveness of Best Available Retrofit Technology for 

Four Corners Power Plant and Navajo Generating Station: 

Advanced Notice of Proposed Rulemaking 

AGENCY: Environmental Protection Agency (EPA). 

ACTION: Advanced Notice of Proposed Rulemaking. 

Federal Register /Vol. 74, No. 166 / Friday, August 28, 2009 / Proposed Rules; Pages 44313-44334 

Submitted by 

Eladio M. Knipping, Ph.D. 

Electric Power Research Institute (EPRI) 

2000 L St NW, Suite 805 

Washington, DC 20036 

Naresh Kumar, Ph.D. and Stephanie L. Shaw, Ph.D. 

Electric Power Research Institute (EPRI) 

3420 Hillview Ave. 

Palo Alto, CA 94304 

October 26, 2009
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The following discussion is presented in response to the following sections in EPA’s Request for Public Comment 

II. Request for Public Comment 

B. Factor 5: Degree of Visibility Improvement 

1. FCPP 

b. EPA Modifications to Emissions Rate Inputs 

In addition to the estimates for PM fine described above, EPA additionally revised the modeling inputs for PM fine to 

include emissions of hydrogen chloride (HCl) and hydrogen fluoride (HF). AP–42 (1.1 Bituminous and Subbituminous 

Coal Combustion) provides a single emission factor each for HCl and HF from all coal and boiler types. APS assumed 

H2SO4 to be the only contributor to condensable inorganic PM, and the NPS raised concerns about the exclusion of HCl 

and HF and recommended these two compounds be factored into the CPM–IOR (SO4) modeling input. Method 202 for 

measuring condensable PM does not capture HCl and HF, therefore, EPA added these emissions to PM fine rather than 

SO4. [Page 44323] 

and 

II. Request for Public Comment 

B. Factor 5: Degree of Visibility Improvement 

2. NGS 

b. EPA Modifications to Emissions Rate Inputs 

For all modeling scenarios, EPA included HCl and HF emissions as PM fine modeling inputs and scaled them in a similar 

manner described for FCPP. [Page 44329] 

Our comment is the following: It is inappropriate to treat HCl and HF as PM Fine for BART Modeling 

HCl and HF are emitted as gases and will not form particles that impact visibility on a regional level. Treating HCl and 

HF as non-volatile PM Fine mass is incorrect. 

In this document, we review the physical properties of HCl and HF in detail, and also use theoretical calculations and 

measurement data to demonstrate that treating these components as PM Fine or H2SO4 is inappropriate for regional 

haze calculations. 
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Do HCl and HF emissions from Power Plants contribute to visibility degradation? 

It is instructive to first understand how emissions from power plants may contribute to visibility degradation in the 

atmosphere. Visibility degradation can occur due to primary particles emitted directly or secondary particles formed 

due to the oxidation of emitted gases such as sulfur dioxide (SO2). Primary particles from power plants are divided 

into two categories: filterable particulate matter (FPM) and condensable particulate matter (CPM). Condensable 

particulate matter forms when emissions condense upon cooling of stack gases, such as sulfuric acid (H2SO4), or when 

emissions condense upon cooling of stack gases and reaction with ambient humidity, such as sulfur trioxide (SO3). 

Secondary particles are formed by reactions in the atmosphere that lead to the oxidation of gases to condensable 

products. Those condensable products with very low vapor pressures, such as sulfuric acid, condense fairly readily. 

However, other condensable products contribute to particulate matter only when conditions thermodynamically favor 

the condensed phase. An example is the formation of ammonium nitrate (NH4NO3) which requires 

 a molar equivalent excess of ammonium relative to sulfuric acid, and 

 cool temperatures that favor the molecular stability of the salt form over its gaseous precursors, nitric acid 

(HNO3) and ammonia (NH3). 

Hydrogen halides, such as hydrogen chloride (hydrochloric acid, HCl) and hydrogen fluoride (hydrofluoric acid, HF), 

may also be emitted from power plants. There are three separate, but closely related, issues that arise regarding the 

ability of these compounds to affect visibility degradation: 

1. Can HCl and HF emissions be considered as a component of primary PM fine—either “condensable 

particulate matter” (CPM) or “filterable particulate matter” (FPM)—from industrial stacks? 

2. Are the properties of HCl and HF similar to that of H2SO4 such that the hydrogen halide emissions can be 

modeled as sulfuric acid in models such as CALPUFF? 

3. Does the presence of visible plume imply a regional haze concern? 

These questions are addressed in the following pages by inspecting the thermodynamics properties of HCl and HF 

and exploring their reactions and transformations in the atmosphere. 
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1. Can HCl and HF emissions be considered as a component of primary PM fine—either “condensable 

particulate matter” (CPM) or “filterable particulate matter” (FPM)—from industrial stacks? 

HCl and HF cannot be considered components of primary PM fine, and to understand why we need only look at their 

thermodynamic properties: these compounds are gases under atmospheric conditions. 

HCl and HF have fundamentally different thermodynamic properties compared to the properties of H2SO4, the main 

component of CPM. Regarding their potential contribution to CPM, the following differences should be noted: 

As shown in Table 1, at ambient temperatures, sulfuric acid exists either as a solid (at temperatures below 10 °C) or a 

liquid (at temperatures above 10 °C). Due to its extremely low vapor pressure (see Table 2), even when exposed to the 

open atmosphere at 25 °C, sulfuric acid will favor its condensed form over volatilization to a gas. 

Table 1. Freezing (Melting) and Boiling Points [K] 

Compound State at 298K Tf Tb

HF Gas 190 292.7 

HCl Gas 161.5 191.1 

H2SO4 Liquid 283.5 611.2 

HNO3 Liquid 231.5 356.1 

NH3 Gas 195.4 238.8 

Table 2. Pure Compound Vapor Pressures [mbar] @ 298K 

Compound Pure Compound 

Vapor Pressure 

H2SO4 <0.0014 

HNO3 62 

H2O 31.7 

HF 1213 

NH3 9,986 

HCl 47,530 

CO2 57,226 

On the other hand, HCl exists solely in the gas form at all ambient temperatures. (The vapor pressure of HCl is also 

the highest of all compounds listed in Table 2 except for CO2, which has a vapor pressure of similar order of 

magnitude.) HF can exist as a liquid at ambient temperatures below 20 °C. (In fact, it is stable in polymer forms of 

(HF)n, where n is an integer higher than or equal to 6.) However, due to its high vapor pressure it can only exist in the 

liquid form in an enclosed space where it can remain at high enough concentrations (mass per volume); upon 

exposure to the atmosphere, it will rapidly volatilize to the gas phase. 

As a reference, the pure compound (saturation) vapor pressure of water at 298 K (25 °C) is 31.7 mbar; at that 

temperature, a pool of water exposed to the open atmosphere will evaporate (volatilize) to the gas phase—with the 

rate depending on ambient RH, in an attempt to reach an ambient RH of 100%. 
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Due to these fundamental thermodynamic properties, it is impossible to consider ambient HCl and HF as 

compounds that fall into the “condensable particulate matter” (CPM) category—let alone the “filterable 

particulate matter” FPM category which represents entirely non-volatile material—of primary PM fine 

emissions. 

2. Are the properties and reactions of HCl and HF similar to that of H2SO4 such that the hydrogen halide 

emissions can be modeled as sulfuric acid in models such as CALPUFF? 

We now look at the question of whether the properties of HCl and HF are sufficiently similar to that of H2SO4 such 

that the HCl and HF emissions can be modeled as H2SO4in models such as CALPUFF.. 

Tables 1 through 7 provide the values for several key physical and thermodynamic properties of HCl, HF and H2SO4, 

including the properties for several ammonium-based and sodium-based salts of bisulfate, sulfate and chloride. These 

properties are necessary to determine the reactions and transformations that occur in the atmosphere. Inspection of 

these tables show that the properties of the chlorine and fluorine compounds —hydrochloric acid, hydrogen fluoride, 

and corresponding salts—are quite dissimilar to that of sulfuric acid and its corresponding salts. 

In contrast to H2SO4, HCl and HF are not hygroscopic at ambient conditions due to their gaseous state. However, they 

are highly soluble in water where they can dissociate into chloride and fluoride anions; thus, at least in theory, HCl 

and HF emissions have the potential to contribute to both the mass and increase in the hygroscopicity of ambient 

particles. However, in the real atmosphere, this potential is never realized in fully deliquesced, meta-stable, mixedphase, or crystallized ambient aerosol particles due to competing processes by other less-volatile acids.

We first address the case of fully deliquesced particles. One of the fundamental criteria for equilibrium in aerosol 

particles is that of charge equilibrium, i.e. cations must balance anions. In fully deliquesced, aqueous aerosol particles, 

charge equilibrium must be maintained at the same time as acid-base dissociation equilibrium is maintained for all 

dissolved electrolytic species (including any dissolved CO2), which must in turn maintain equilibrium with their gasphase concentrations as determined by their effective Henry’s Law constants (a measure of solubility that takes into 

account the dissociation of electrolytic species in water). Finally, water equilibrium:, H2O ⇋ H+ + OH–, must be 

maintained. 

Table 4 shows that effective solubility of H2SO4 is orders of magnitude higher than that of the other inorganic acids 

listed, i.e. HNO3, HCl and HF. This is in agreement with the vapor pressure property data. The solubility of acids must 

balance with the other equilibrium considerations, including charge balance. Thus, when a less-volatile (more-soluble) 

acid is present, the solubility of more-volatile (less-soluble) acids is inhibited. Thus, in industrial plumes, the presence 

of H2SO4 (emitted directly or formed from the reaction of SO3 with H2O or the oxidation of SO2) will inhibit the 

dissolution (in order of increasing strength of inhibition) the dissolution of HNO3, HCl and HF. The dissolution of 

ammonia—which can form ammonium hydroxide, a base that readily dissociates to yield ammonium (NH4

+

) cations—

allows for an increase in the amount of acid that can dissolve into the condensed medium, but not to the extent 

where HCl and HF dissolution becomes favorable in the atmosphere due to the common presence of sulfuric and 

nitric acids.

A similar case occurs with meta-stable (i.e. particles that are deliquesced but are between their deliquescence and 

crystallization points), mixed-phase (liquid and solid) particles and solid particles. In essence, a well-known 

phenomenon known as halogen displacement occurs in the atmosphere. When solid or dissolved halogen-containing 
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salts in the atmosphere are exposed to less-volatile acids, the less-volatile acids displace the halogens and new 

preferred salts are formed. In fact, this is a very common process in coastal regions, where sea-salt aerosol (which 

contains pre-existing sulfate from seawater) experiences chloride depletion. Chloride is depleted first (due to masstransfer considerations) from the smallest particles and the displacement continues in order of increasing particle size 

until all available sulfuric acid is in the particle phase. When there is no more sulfuric acid left to displace chloride, any 

available nitric acid will displace chloride from the remaining particles (again, in order of increasing particle size) until 

all available nitric acid is consumed. This leads to the commonly observed formation of coarse sodium nitrate 

(NaNO3) particles near coastal regions, such as those observed in the Tampa Bay Regional Aerosol Characterization 

Experiment (BRACE). The displaced chloride is released to the gas phase as HCl. 

The most common fate of the resulting volatile species HCl (and HF) is dry deposition and washout/rainout during 

precipitation events. Only under unusual circumstances, (e.g., dust storms) is it likely that conditions could become 

favorable for the condensation of these species into coarse mode particles (e.g., alkaline dust particles). In contrast, 

once H2SO4 condenses onto particles, it does not partition back to the gas phase.

As noted in conversation with Dr. William Malm of the National Park Service (September 10, 2009), chloride 

measurements at non-coastal IMPROVE sites show negligible, virtually zero chloride in fine particle measurements. 

This is seen throughout the network, thereby providing support to the theoretical basis for not expecting chloride 

aerosols in continental air masses except as in the form of sea-salt chloride. (Fluoride is even more negligible than 

chloride.) 

Due to differences in properties and reactions, it is inappropriate to consider ambient HCl and HF as behaving 

similar to H2SO4 and to simulate by using H2SO4 as a surrogate in models such as CALPUFF. 

In an industrial plume, the preference for sulfate-based and (if conditions are right) nitrate-based salts and 

solutions make for conditions that do not favor the existence of chloride/fluoride salts or solutions in aerosol 

particles reaching Class I areas. Even with reductions in SO2 and NOx, these salts chloride/fluoride salts or 

solutions would still be unfavorable due to other natural and anthropogenic sources leading to sulfuric and 

nitric acid and more-favorable organic acids in the particle phase. Observations support this theoretically 

based conceptual model. 

A note on conservatism: 

The potential for any compound to form a solid does not suggest that it should be treated as either primary PM or a 

surrogate compound in the search for a conservative estimate. For example, CO2 is known to form carbonate salts; a 

future version of the AIM thermodynamic model (which can be used for a variety of applications) will include 

simulation of carbonate salts. However, modeling CO2 emissions as primary PM or H2SO4 is clearly inappropriate. It is 

just as inappropriate to model HCl and HF as primary PM or H2SO4. With enabling science models and data available, 

the only proper route is to model the compounds with their appropriate properties. 
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Table 3. Acid-Base Dissociation Constants [M] @ 298 K (HA ⇋ H + A–) 

HA A–

 Ka pKa 

HCl Cl– 1.7 × 106 -6.23 

H2SO4 HSO4

–

 1.0 × 103 -3 

HNO3 NO3

–

 1.5 × 101 -1.17 

HSO4

–

 SO4

2– 1.2 × 10-2 1.92 

HF F– 3.5 × 10-4 3.45 

NH4

+

 NH3 5.6 × 10-10 9.25 

Table 4. Solubility in H2O @ 298K [M/atm] (Caq,A = pA × H*) 

Compound 

Physical 

Henry’s Law 

H 

Effective Henry’s Law 

H* 

at pH = 5.6 

Effective Henry’s Law 

H* 

at pH = 3.0 

H2SO4 2.50 × 1014 4.77 × 1025 5.50 × 1020

HNO3 2.10 × 105 1.25 × 1012 3.15 × 109

HCl 1.10 × 100 7.44 × 1011 1.87 × 109

HF 2.74 × 104 3.85 × 106 3.70 × 104

NH3 6.10 × 101 6.96 × 101 3.48 × 103

Table 5a. Equilibrium Constants (Solid to Gas) 

Salt K† a b 

NH4NO3 (s) ⇋ NH3 (g) + HNO3 (g) 5.47 × 10-17 -74.38 6.12 

NH4Cl (s) ⇋ NH3 (g) + HCl (g) 1.09 × 10-16 -71.00 2.40 

† K(T) = K exp { a (298/T – 1) + b [ 1 + ln (298/T) – 298/T ] }; units of atm2

 for these equilibrium constants 

Table 5b. Displacement Reactions 

Displacement reactions occur by first allowing for displacement of chloride/nitrate by sulfuric acid to form the (more 

hygroscopic) bisulfate salt and any remaining sulfuric acid will lead to the formation of the (less hygroscopic) sulfate 

salt. Chloride displacement occurs before nitrate displacement and displacement to form the bisulfate salt occurs 

before formation of the sulfate salt, with intermediate salts such as letovicite [(NH4)3H(SO4)2] forming during the 

process, in the case of ammonium salts. 

 K‡ a b 

NaCl (s) + HNO3 (g) ⇋ NaNO3 (s) + HCl (g) 4.01 5.63 -2.19 

‡ K(T) = K exp { a (298/T – 1) + b [ 1 + ln (298/T) – 298/T ] }; unitless for this equilibrium constant 

The rate of following displacement reactions by sulfuric acid are assumed by atmospheric models to be constrained 

only by mass transfer limitations. 
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NaCl (s) + H2SO4 (g) ⇋ NaHSO4 (p) + HCl (g)

NH4Cl (s) + H2SO4 (g) ⇋ NH4HSO4 (p) + HCl (g)

NaNO3 (s) + H2SO4 (g) ⇋ NaHSO4 (p) + HNO3 (g)

NH4NO3(s) + H2SO4 (g) ⇋ NH4HSO4 (p) + HNO3 (g)

Table 6. Dissolution Equilibrium Constants/Solubility Product at 298 K 

Salt Ksp

(NH4)2SO4 1.82 

NH4NO3 1.49 × 101

NH4Cl 2.21 × 101

NH4HSO4 2.55 × 102

Na2SO4 4.80 × 101

NaNO3 1.20 × 101

NaCl 3.77 × 101

NaHSO4 2.41 × 104

Units: M2

 except for (NH4)2SO4 which has units of M3

Table 7. Deliquescence Points [% RH] at 298 K 

Salt DRH 

Na2SO4 84.2 

NaCl 75.3 

NaNO3 74.3 

NaHSO4 52.0 

(NH4)2SO4 79.9 

NH4Cl 80.0 

NH4NO3 61.8 

NH4HSO4 40.0 

3. Does the presence of visible NH4Cl plume imply a regional haze concern? 

HCl may react with NH3 to form NH4Cl particles under some conditions (Table 5a, 5b). These particles can form from 

stack gases under some operating circumstances and lead to visible (opaque) plumes. Does this imply a regional haze 

concern? 

Visible plumes and regional haze are two distinct issues; the formation of a visible plume does not imply a regional 

haze concern as these particles are short-lived under atmospheric conditions. As the plume dilutes, the conditions for 

the formation of the condensed phase of NH4Cl become increasingly unfavorable. The ambient vapor phase 

concentrations of NH3 and HCl in equilibrium with the particle phase NH4Cl decrease due to mixing with ambient air 

of lower concentrations. As a result, the thermodynamic equilibrium of the chemical reaction shifts such that the 

condensed NH4Cl converts back to the gas phase precursors (NH3 and HCl). 
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Once the NH3 and HCl are in the gas phase again, reformation of NH4Cl requires a low temperature to overcome the 

much diluted concentrations downwind of the emissions source. Thus the fate of gas phase NH3 and HCl changes as 

compared to near the stack. As noted earlier, NH3 preferentially reacts with sulfate-containing, then nitratecontaining, and only subsequently chloride-containing chemicals. Sulfate is usually sufficiently elevated at distances 

far downwind in power plant plumes that virtually all the NH3 in the stack gases as well as that present from other 

sources will react with it. Similarly, the fate of the reformed HCl will likely be wet or dry deposition due to its affinity 

for both deposition processes. This processing means that even under favorable conditions for NH4Cl formation, the 

resulting plumes will be quite transient.

The following simple Gaussian plume calculation will demonstrate the short-lived nature of NH4Cl particles. Several 

worst-case assumptions are made to overestimate the likelihood and stability of NH4Cl formation. 

 No competitive chemistry for reaction with NH3 is included. This is accomplished by not including H2SO4 or 

HNO3 so that all NH3 is available for reaction with HCl, thereby favoring formation of NH4Cl. 

 Concentrations are calculated for the plume center-line in a neutrally stable atmosphere for the least 

possible amount of diffusion. 

 Nucleation is assumed to occur instantaneously, as long as equilibrium conditions are met. 

 The atmosphere is fairly stable, thereby reducing turbulent dispersion effects. 

Table 8 below shows the distances to which the particulate form of NH4Cl would persist. For example, given stack 

concentrations of 1 ppm NH3 and 75 ppm HCl, the NH4Cl formed would be completely removed within 7 km (or 40 

minutes) downwind. 

Table 8: Demonstration of short-lived nature of NH4Cl particles 

NH3 in 

stack 

(ppm) 

HCl in 

stack 

(ppm) 

Distance by Which NH4Cl 

Removed (km) 

NH3 in 

stack 

(ppm) 

HCl in 

stack 

(ppm) 

Distance by Which NH4Cl 

Removed (km) 

1 10 3.4 5 10 5.9 

1 75 7 5 75 11.6 

1 150 8.5 5 150 14.7 

2 10 4.4 10 10 7.4 

2 75 8.5 10 75 14.7 

2 150 10.8 10 150 18.7 

Distances by which NH4Cl would decompose and volatilize to NH3 and HCl for a “worst-case” estimation. 

Wind speed is assumed to be 3 m s-1 (10.8 km h-1 or 6.7 mi h-1). 

If the presence of H2SO4 had been considered then the resulting NH4Cl particles would begin forming further 

downwind from the stack and persist for a shorter period of time, if they form at all. 

The second attached document explains in more detail the inadequacy of considering NH4Cl from power 

plants as condensable particulate matter. 
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Summary 

We have shown that due to fundamental thermodynamic properties, it is impossible to consider ambient HCl and HF 

as compounds that fall into the “condensable particulate matter” (CPM) and due to differences in their properties and 

reactions, it is inappropriate to consider ambient HCl and HF as behaving similar to H2SO4 and to simulate by using 

H2SO4 as a surrogate in models such as CALPUFF. Thus, considering HCl and HF as filterable particulate matter (FPM, 

PM Fine) or condensable particulate matter (CPM, CPM-IOR, SO4) will only produce errors in regional haze 

calculations and cannot be supported by scientific theory or observations. 

References 

Finlayson-Pitts, B.J. and Pitts, Jr., J.N. Chemistry of the Upper and Lower Atmosphere: Theory, Experiments, and 

Applications, Academic Press, 1999. 

Lide, D.R., ed., CRC Handbook of Chemistry and Physics, 89th Edition (Internet Version 2009), CRC Press, Taylor and 

Francis, 2009. http://www.hbcpnetbase.com

Sander, R. Compilation of Henry's Law Constants for Inorganic and Organic Species of Potential Importance in 

Environmental Chemistry (Version 3), 1999. http://www.henrys-law.org 

Sander, S.P. et al., Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, NASA/JPL, 2006. 

http://jpldataeval.jpl.nasa.gov

Seinfeld, J.H. and Pandis, S.N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 2nd Edition,

Wiley-Interscience, 2006. 

Snoeyink, V.L. and Jenkins, D. Water Chemistry, Wiley, 1980.