Date: December 16, 2009

Subject: Using EPA Method 7E for Measuring NOX Emissions from Nitric Acid 

Production Facilities

Nitric Acid NSPS Review

EPA Contract No. EP-D-06-118; Work Assignment No. 3-22 

RTI Project No. 0210426.003.022

From: Melissa Icenhour

To: Bill Neuffer

OAQPS/SPPD/MMG (D243-02)

U. S. Environmental Protection Agency

Research Triangle Park, NC 27711

Introduction

The EPA is required by section 111(b)(1)(B) of the Clean Air Act (CAA) to review and, if 

appropriate, revise existing New Source Performance Standards (NSPS) at least every 8 years.

The nitric acid NSPS (part 60, subpart G) has been reviewed in the past but the last review was 

more than 8 years ago. As a result of this, EPA has received a citizen suit from Sierra Club and 

Environmental Integrity Project prompting review of the standards before the end of the year 

2010.

NSPS (40 CFR part 60) Promulgation date Previous review dates

Nitric Acid (subpart G) December 23, 1971 

(36 FR 24876)

Reviews dated June 19, 1979; April 5, 

1984. No changes. Minor testing/ 

monitoring changes made -October 6, 

1975 [40 FR 46258], April 22, 1985 [50 

FR 15894], and February 14, 1989 [40 

FR 6666]
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Options for NSPS review

The purpose of an NSPS review is to determine if the standards reflect the “degree of emissions 

limitation achievable through application of the best system of emission reduction which (taking 

into account the cost of achieving such reduction and any nonair quality health and 

environmental impact and energy requirements) the Administrator determines has been 

adequately demonstrated.”

Potential outcomes of the review process include one or more of the following: 

-No changes 

-Clarifying amendments 

-Changes to emissions limits or other rule requirements

-Add emission limits for additional pollutants (i.e., N2O)

EPA will collect and validate data to determine what changes to the NSPS may be needed. In 

discussions with industry, most of the NOX data collected by industry has been analyzed using 

Method 7E, “Determination of Nitrogen Oxides Emissions from Stationary Sources 

(Instrumental Analyzer Procedure).”

Using Method 7E for Measuring NOX Emissions from Nitric Acid Production Facilities

The nitric acid NSPS (Subpart G) specifies that Method 7, 7A, 7B, 7C, or 7D be used to 

determine the NOX concentration. Method 7E is not offered as an alternative method.

According to the Applicability Determination Index (ADI) 

[http://www.epa.gov/compliance/monitoring/programs/caa/adi.html], Method 7E was approved 

for use at the DuPont nitric acid plant in Victoria, Texas, in 2000 under Control Number 

0000006. The method was approved by Texas Natural Resource Conservation Commission

(TNRCC). 

The alternative method was approved under the conditions that the chemiluminescent nitrogen 

oxides analyzer be calibrated with nitrogen dioxide and that the analyzer pass a converter 

efficiency test. The facility submitted data “demonstrating that its nitrogen oxides CEMS, when 

calibrated with nitrogen dioxide, is positively biased with respect to reference method 7.” John 

Hepola (Chief of Air Toxics and Inspection Coordination Branch of TNRCC) responded to the 

facility, “this bias would be removed if reference Method 7E were used for the certification of 

relative accuracy of the nitrogen oxides CEMS, since reference Method 7E is an instrumental 

analyzer procedure using a chemiluminescent nitrogen oxides analyzer.”

When the Metals and Minerals Group (MMG) became aware of the use of Method 7E by the 

nitric acid industry, Bill Neuffer contacted Bill Grimley of the Measurement Technology Group 

(MTG) for guidance. The ramifications of Method 7E being used throughout the industry 

brought up two issues: should Method 7E be explicitly allowed for NOX testing by adding it to 

Subpart G and could emissions data collected using Method 7E be used to set a new NOX

emission limit for new sources under Subpart G?
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According to members of MTG and the Measurement Policy Group (MPG), the alternative 

method request should have been reviewed and approved through their groups; TNRCC did not 

have the authority to grant the approval. However, after being published in the ADI, most of the 

nitric acid production plants in the United States began using Method 7E for NOX testing. 

In response, the following comment was added by Bill Grimley to the ADI under Control 

Number 0000006: 

This letter should not be used as precedent. Only the Office of Air Quality 

Planning and Standards or the appropriately delegated authority is authorized to 

approve alternative test methods. Additionally, an incorrect test method is 

described in this letter. Parties interested in an alternative to the test methods 

listed in 40 CFR Subpart G for compliance determination and relative accuracy 

testing of NOx CEMS may send a source-specific request to the Administrator's 

delegated authority as explained in guideline GD 22 at 

http://www.epa.gov/ttn/emc/guidlnd.html and 40 CFR 60.8(b)(3).

Additional Method 7E Provisions for Nitric Acid Production Facilities

June 2009

EPA’s Measurement Technology Group and Measurement Policy Group issued a statement, 

dated June 15, 2009, that Method 7E would no longer be allowed for NOX testing in the nitric 

acid production industry. The statement is included as Attachment 1 and outlines the reasons 

that Method 7E may not be applicable for use by nitric acid production facilities.

July 2009

During discussions with industry representatives from The Fertilizer Institute (TFI) and 

American Chemistry Council (ACC), EPA asked for further information on the use of Method 

7E for NOX testing within the nitric acid production industry.

During a telephone conversation with Jim Schellhorn of Terra Industries (a member company of 

TFI), Bill Neuffer summarized why TFI preferred to use Method 7E to measure NOX from nitric 

acid plants. Jim explained that:

• Method 7E is the standard stack method for conducting NOX analyzer 

performance tests and RATAs, most of which are conducted by stack test 

companies for fuel combustion sources;

• Method 7E is the method of choice for stack testers because it doesn’t rely on wet 

chemistry;

• All stack emissions testing companies are prepared to use Method 7E; and

• The cost for Method 7E is $3,000 compared to $4,500 for Method 7D.
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Also, Jim estimated that 20 – 30% of the NOX from typical NSPS nitric acid plants operating at 

steady state conditions is NO2 and there is little moisture (0.5-2.0%) in the stack of nitric acid 

plants. He said the industry would be willing to do concurrent Method 7D and 7E tests if that 

would be helpful to demonstrate the accuracy of Method 7E for nitric acid performance tests.

Later, MTG and MPG discussed the possibility of allowing the nitric acid production industry to 

use Method 7E if certain additional provisions were followed. These include performing

dynamic spiking procedures using NO2 spike gas and ensuring that the gas stream does not come 

in contact with any condensate.

August 2009

On August 11, 2009, MTG and MPG issued the first version of the provisions:

Title: Use of Method 7E for NOx Measurements - NSPS review for Nitric Acid 

Plants

To be suitable for use by EPA, NOx emissions measurements must be of verified 

quality. Because of the propensity of NO2 to be absorbed by water, the sampled 

gas must not contact any condensate, and the efficiency of any NO2 to NO 

converter (if needed by the detection principle of the instrument) must be 

determined and documented as described in the method. For wet basis 

measurement systems, the tester must measure and report temperature data to 

show that the sample line and all sample transport components up to the inlet of 

the analyzer (and, for hot-wet extractive systems, also the analyzer) were 

maintained above the stack gas dew point. Use of a dilution probe to reduce the 

dew point is an acceptable procedure, if documented. Section 8 of Method 7E 

describes the measurement system performance tests and documentation required 

by the method, both prior to and following sampling. NO2 calibration gas must 

be substituted for NO calibration gas in the performance of system bias checks. 

The alternative mentioned in Section 8.6 to follow the dynamic spike procedure 

in Section 16.1 is acceptable, but only with NO2 spike gas. The dynamic spiking 

procedure is strongly recommended.
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September 2009

On September 16, 2009, Bill Herz of TFI responded with the following, via email:

In response to our call on September 9, 2009 regarding EPA Test Method 7E, as used 

at nitric acid plants, and concerns raised by EPA regarding NO2 absorption in water 

during this test procedure, we discussed this issue with some emissions testing 

companies and TFI member company personnel. EPA's concerns are expressed in 

the attached June 15, 2009 memorandum [Attachment 1] and one paragraph Word 

document [September 16, 2009 paragraph above].

As we understand it, and as set forth in the June 15, 2009 memorandum, the premise 

set forth by EPA for concerns with the use of Method 7E at nitric acid plants is that 

"Nitrogen dioxide is very soluble in water." This conclusion, however, is very 

misleading and, from a technical standpoint, is incorrect. The uptake into water of 

NO2 from a gas stream is predominantly a bulk-phase process that is governed by 

Henry’s Law. See Chemicals in the Atmosphere-Solubility, Sources and Reactivity, 

P.G.T. Fogg, J.M. Sangster, John Wiley & Sons Ltd. (2003) at 190, 191. The 

maximum concentration of NO2 in an aqueous solution should be governed not by 

solubility, but by the partial pressure of the NO2 in the gas stream.

Given the very low partial pressure of NO2 in the exhaust gas from a nitric acid plant, 

along with the low moisture content available to condense out in the conditioning 

apparatus (based on discussions with TFI members, the highest moisture content 

we’ve encountered is 1.5%, with many below 1.0%), losses of NO2 in the condensate 

should be insignificant and should not form the basis for invalidating Method 7E for 

use at nitric acid plants. For any given stack test run, given the known quantities of 

condensate recovered, losses of NO2 during Method 7E sampling can be estimated 

using well established and published Henry’s Law constant data.

During our call, you indicated that EPA has several emissions test results from tests 

run using Method 7E. These data should enable one to calculate NO2 losses to 

conditioning apparatus condensate using Henry’s Law constant data. To the extent 

that EPA has performed this evaluation already, we would appreciate the opportunity 

to review the Agency’s results. Currently, we are aware of EPA’s conclusion, but 

have not seen the underlying data to support this conclusion.

If the data support EPA’s conclusion, we believe the next step should be utilization of 

a dynamic spiking procedure performed during a Method 7E test to quantity NO2

losses to condensate, or use of an indirect electric chiller to limit the quantity of NO2

that comes in contact with any condensate.. Such a test would allow EPA to verify 

the results of empirical calculations and, as necessary, determine what controls may 

be necessary (if any) to ensure that Method 7E used at nitric acid plants produces 

reliable results.

To presume that (1) NO2 is absorbed in condensate, and (2) such absorption results in 

inaccurate performance test results using Method 7E, without calculations and testing 

to confirm same, is arbitrary and not based on sound science.
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Please advise what work has been performed by EPA to date regarding this issue. If 

you think that a call would assist in advancing the evaluation, please let me know.

October 2009

The Method 7E provisions were updated as follows on October 8, 2009:

Emissions Testing and Continuous Emissions Monitoring for Nitric Acid 

NSPS (Subpart G) Review

Limitations for Use of Method 7E for (1) Performance Test or CEMS RATA in 

Amendments to Subpart G and (2) Current Data Request

• Must conduct system bias checks in Sections 8.2.5 and 8.5 with NO2 gas, 

not NO gas; and

• Must use one of the following approaches to effectively eliminate loss of 

NO2 in water:

§ For a wet-basis measurement system, measure and report 

temperature of sample line and components (up to analyzer inlet) 

to demonstrate that the temperatures remain above the sample 

gas dew point at all times during the sampling,

• A dilution probe may be used to reduce the dew point of 

the sample gas;

§ Use a permeation-type sample conditioning device to 

continuously remove condensate from sample gas while 

maintaining minimal contact between condensate and sample 

gas; or

§ Conduct dynamic spiking with NO2 gas according to Section 

16.1 in Method 7E.

CEMS Requirements in Amendments to Subpart G

• Must use one of the following approaches to mitigate loss of NO2 in 

water in CEMS:

§ For a wet-basis measurement system, measure and report 

temperature of sample line and components (up to analyzer inlet) 

to demonstrate the temperatures remain above the stack gas dew 

point at all times during the sampling; or

• A dilution probe may be used to reduce the dew point of 

the stack gas;

§ Use a permeation-type sample conditioning device to 

continuously remove condensate from sample gas while 

maintaining minimal contact between condensate and sample 

gas.
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The above draft was sent to TFI via email on October 8, 2009. Bill Herz responded that day with 

the following (also via email):

This imposes requirements on the use of Method 7E without scientific or 

engineering justification for same. Regarding the e-mail we sent two weeks ago 

describing why we believe 7E to be comparable to 7; your response does not 

address the issues we raised.

Until we better understand EPA’s rationale; we cannot recommend to a member 

that they perform a concurrent test to validate 7E v. Method 7.

On October 20, 2009, EPA (Bill Neuffer, Peter Westlin, Robin Segall, Bill Grimley and Ed 

Brown) responded via email to the issues from Bill Herz as follows:

Comment byTFI - NO2 absorption in water during Method 7E procedure.

As we understand it, and as set forth in the June 15, 2009 memorandum, the 

premise set forth by EPA for concerns with the use of Method 7E at nitric acid 

plants is that "Nitrogen dioxide is very soluble in water." This conclusion, 

however, is very misleading and, from a technical standpoint, is incorrect. The 

uptake into water of NO2 from a gas stream is predominantly a bulk-phase 

process that is governed by Henry’s Law. See Chemicals in the AtmosphereSolubility, Sources and Reactivity, P.G.T. Fogg, J.M. Sangster, John Wiley & 

Sons Ltd. (2003) at 190, 191. The maximum concentration of NO2 in an aqueous 

solution should be governed not by solubility, but by the partial pressure of the 

NO2 in the gas stream. Given the very low partial pressure of NO2 in the 

exhaust gas from a nitric acid plant, along with the low moisture content 

available to condense out in the conditioning apparatus (based on discussions 

with TFI members, the highest moisture content we’ve encountered is 1.5%, with 

many below 1.0%), losses of NO2 in the condensate should be insignificant and 

should not form the basis for invalidating Method 7E for use at nitric acid plants. 

For any given stack test run, given the known quantities of condensate recovered, 

losses of NO2 during Method 7E sampling can be estimated using well 

established and published Henry’s Law constant data.

EPA Response

The discussion of Henry's Law, while thought-provoking, is not applicable in this 

case. The chemical engineering textbook, Elementary Principles of Chemical 

Processes, (Felder & Rousseau, 1st edition) clearly states that Henry's Law is 

valid only for dilute solutions where the solute does not dissociate, ionize, or 

react in the liquid phase. If this were the case for NO2, it wouldn't be used to 

make nitric acid. Essentially, once chemical reactions and equilibria enter into 

the picture, all bets are off on trying to model or estimate NO2 losses, even when 

"using well established and published Henry’s Law constant data" (which vary 

from 0.007 M/atm to 0.041 M/atm, depending on which publication you read).
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The argument about low stack moisture doesn't fit with what we've seen (and 

done) in the field. The "NOx condensers" that we used back in the 1990s appear 

to still be in use today. They're essentially an ice bath with a coil of TeflonTM 

and some knockout jars. The knockout jars usually have dip-tubes in them, and 

quite often start out with some water in them (essentially a small impinger); in 

fact, the water improves the moisture removal, just as it does in impinger trains. 

These condensers were never used for SO2 because of the knockouts; instead, 

there was a similar condenser with one or two small knockout traps that were 

continuously drained by a peristaltic pump. This was used to maintain minimal 

contact between the condensate and the sample gas" as dictated in the original 

Method 6C.

Comment by TFI 

If the data support EPA’s conclusion, we believe the next step should be 

utilization of a dynamic spiking procedure performed during a Method 7E test to 

quantity NO2 losses to condensate, or use of an indirect electric chiller to limit 

the quantity of NO2 that comes in contact with any condensate. Such a test 

would allow EPA to verify the results of empirical calculations and, as necessary, 

determine what controls may be necessary (if any) to ensure that Method 7E used 

at nitric acid plants produces reliable results. 

EPA Response

We agree. As stated in our Summary of Procedures (to be used when applying 

Method 7E at nitric acid plants), which was emailed to you on August 11, 2009, 

dynamic spiking is to be conducted with NO2 gas according to section 16.1 in 

Method 7E each time it is used at a nitric acid plant.
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Comment by TFI

During our call, you indicated that EPA has several emissions test results from 

tests run using Method 7E. These data should enable one to calculate NO2 losses 

to conditioning apparatus condensate using Henry’s Law constant data. To the 

extent that EPA has performed this evaluation already, we would appreciate the 

opportunity to review the Agency’s results. Currently, we are aware of EPA’s 

conclusion, but have not seen the underlying data to support this conclusion. To 

presume that (1) NO2 is absorbed in condensate, and (2) such absorption results 

in inaccurate performance test results using Method 7E, without calculations and 

testing to confirm same, is arbitrary and not based on sound science.

EPA Response

We do not agree with the "not based on sound science" statement; see comments 

above about the applicability of Henry’s law in testing NOx emissions from nitric 

acid plants.. EPA’s reference method development strategy is not to presume that 

a problem always exists, but rather to employ performance-based methodology 

so as to preclude such problems. In this case, EPA is not presuming that a 

significant amount of NO2 gas is always present to be absorbed in condensate, or 

that such absorption always results in significantly inaccurate performance test 

results using Method 7Ebut, instead, that the test method perform in such a 

manner to minimize the potential for any such interference. As to the test data 

you reference, we have evidence that appreciable amounts of NO2 gas can be 

present in nitric acid plant emissions. For purposes of our reviewing Method 7E 

test data that could be used to establish a national emission standard, and that has 

been generated by client-focused testing firms, that possibility means that 7E 

requirements for those tests must have been modified, especially for nitric acid 

plants, to insure a full and accurate accounting of all NOx emissions.
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On October 28, 2009, Peter Westlin made a few additional changes to the Method 7E provisions.

Emissions Testing and Continuous Emissions Monitoring for Nitric 

Acid NSPS (Subpart G) Review

Limitations for Use of Method 7E for (1) Performance Test or CEMS 

RATA in Amendments to Subpart G and (2) Current Data Request

• Must conduct system bias checks in Sections 8.2.5 and 8.5 with 

NO2 gas; and

• Must use one of the following approaches to mitigate loss of 

NO2 in water:

§ For a wet-basis measurement system, measure and report 

temperature of sample line and components (up to analyzer inlet) 

to demonstrate that the temperatures remain above the sample 

gas dew point at all times during the sampling,

• A dilution probe may be used to reduce the dew point of 

the sample gas;

§ Use a refrigerated-type condenser or similar device (e.g., 

permeation dryer) to continuously remove condensate from 

sample gas while maintaining minimal contact between 

condensate and sample gas; or

§ Conduct dynamic spiking with NO2 gas according to Section 

16.1 in Method 7E.

CEMS Requirements in Amendments to Subpart G

• Must use one of the following approaches to mitigate loss of 

NO2 in water in CEMS:

§ For a wet-basis measurement system, measure and report 

temperature of sample line and components (up to analyzer inlet) 

to demonstrate the temperatures remain above the stack gas dew 

point at all times during the sampling; or

• A dilution probe may be used to reduce the dew point of 

the stack gas;

§ Use a refrigerated-type condenser or similar device (e.g., 

permeation dryer) to continuously remove condensate from 

sample gas while maintaining minimal contact between 

condensate and sample gas.

November 2009

On November 18, 2009, EPA met with representatives from TFI to discuss emission data from TFI 

members. Method 7E and its additional provisions were not further discussed.
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December 2009

EPA has decided that all NOX emission data used to determine any potentially updated emission standards 

under the reviewed NSPS must either be conducted using Method 7, 7A, 7B, 7C, and 7D or a modified 

Method 7E with the above provisions. 

As part of the data gathering effort, EPA received test data from Nathan Frank of Region V (see 

memorandum entitled “Using EPA Method 7E for Measuring NOX Emissions from Nitric Acid 

Production Facilities” dated December 16, 2009).. Most of the test data were gathered using Method 7E 

before the Method 7E provisions were issued. EPA is planning to conduct an Information Collection 

Request (ICR) under Section 114 authority in order to gather a sufficient amount of NOX data for use in 

potentially updating the NOX emissions standards in the NSPS.
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Attachment 1

Memorandum: Use of Method 7E at Nitric Acid Plants and Method 

6C at Sulfuric Acid Plants