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MEMORANDUM 

SUBJECT: Reconsidering Excess Nitric Oxide (NO) requirements for Nitrogen 

Dioxide (NO2) audits contained in 40 CFR part 58 Appendix A 

FROM: Lewis Weinstock and Mike Papp (EPA, OAQPS) 

TO: Ambient Monitoring Rule Docket (EPA-HQ-OAR-2013-0619) 

In the course of considering potential changes to quality assurance requirements as part 

of the proposed rule - Revisions to Ambient Monitoring Quality Assurance and Other 

Requirements – OAQPS solicited input from several parties experienced in the conduct 

of NO2 audits and calibrations. The goal of the communications was to gather input on 

changes being considered to a specific section in the requirements dealing with an issue 

known as “excess NO”. The comments that were received were used to develop proposed 

language that is contained in the proposal noted above. Additional details are provided in 

this memo. 

BACKGROUND 

The current draft of 40 CFR part 58 Appendix A includes very specific language 

prescribing procedures to be used as part of gas phase titration method audits for NO2 

analyzers. The language reads as follows: 

3.2.2.2 (a) NO2 audit gas for chemiluminescence-type NO2 analyzers must also contain 

at least 0.08 ppm NO. NO concentrations substantially higher than 0.08 ppm, as may 

occur when using some gas phase titration (GPT) techniques, may lead to evaluation 

errors in chemiluminescence analyzers due to inevitable minor NO-NOX channel 

imbalance. Such errors may be atypical of routine monitoring errors to the extent that 

such NO concentrations exceed typical ambient NO concentrations at the site. These 

errors may be minimized by modifying the GPT technique to lower the NO 

concentrations remaining in the NO2 audit gas to levels closer to typical ambient NO 

concentrations at the site.

We are considering changes to this requirement because of the lack of such explicit 

instructions in the CFR based NO2 calibration procedure1, the burden associated with 

making many adjustments to NOX/NO levels when conducting procedures in accordance

with section 3.2.2.2(a), and due to the availability of modern calibration equipment that 

can automate NO2 calibrations if the technique is simplified. 

 

1

 See Appendix F to Part 50—Measurement Principle and Calibration Procedure for the Measurement of 

Nitrogen Dioxide in the Atmosphere (Gas Phase Chemiluminescence) - http://www.ecfr.gov/cgi-bin/textidx?SID=039e21f86e8bfeaf8bdb162fa3eb1121&node=40:2.0.1.1.1.0.1.19.7&rgn=div9
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FEEDBACK RECEIVED 

To help inform the impact of potential revisions to this requirement, EPA contacted 

several representatives from state and local agencies that are experienced with NO2

procedures, and also scientists from EPA’s Office of Research and Development and a 

Regional Office. Their responses are summarized below: 

State Agency: 

“Thanks for the opportunity to comment on this issue. We feel that the best course at this 

point would be to remove any language suggesting constraints on excess NO during GPT 

from 3.2.2.2. We think that the audit procedures should be consistent with the calibration 

procedures. The reference method calibration procedures in Part 50 App F, 1.5.9.4 (and 

our Thermo manual) do not restrict the NO during the GPT calibration to >80 ppb or 

even 20-200 ppb. They just say to start the GPT with an NO2 span point with NO not less 

than 10% of NO orig (about 9% of the NO FSR), then tweak the ozonator to generate 

additional equally spaced NO2 verification points throughout the rest of the NO2 range.” 

Local Agency: 

“Prior to the change in audit levels (after 2006), we lowered NO flow to obtain a new 

NO orig1 prior to each GPT point to maintain excess NO at a level between 80 ppb - 120 

ppb at each audit point. Here is the wording for the remaining points in our SOP 

procedure from 2007: 

"8.7.1.19 Repeat steps 8.7.1.15 - 8.7.1.18 for the two remaining NO2 concentrations, 

lowering the NO flow with each point so when ozone is added to obtain the appropriate 

NO2 point, NO rem2 is 0.080-0.120 ppm NO. For example, for a 0.175 ppm NO2 point 

NO orig should be 0.255-0.295 and for the 0.075 ppm NO2 point the NO orig should be 

0.155-0.195. The NO rem should not be less than 0.080. A revised reduced NO orig is 

run for each audit point (See 40 CFR 58 Appendix A Section 3.2). See section 8.7.1.1. for 

appropriate NO2 points." 

When doing calibrations of the analyzer we did not use this procedure. We simply 

generated NO orig near 450 ppb and changed the GPT ozone concentrations for each 

calibration point. This procedure followed our SOP and the manufacturer’s instruction 

manual. 

Having used these two procedures, one for calibrations and one for audits, each with 

results within specifications, would suggest that it may not be necessary to maintain the 

80 - 120 ppb NO level we used in our audit procedure. Because we seemed to be able to 

operate within specifications using both procedures, strictly maintaining the level of 

 

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 In this memo, the term NO orig refers to the concentration of NO that is used in a calibration or audit 

point before the ozone lamp is turned on to begin the titration process that creates NO2

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 The term NO rem refers to the concentration of NO that is remaining in a calibration or audit point after 

the ozone lamp is turned on and stabilized to generate a concentration of NO2 utilizing the titration process 
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excess NO may not be critical when working with a Thermo 42, 42c or 42i. I would 

caution that this may not be the case with other analyzers. This is the only monitor I 

have used extensively. 

When the NO2 audit levels were changed after 2006, we changed our audit procedure so 

that we wouldn't have to run subsequent NO orig points during the audit. We limited our 

audits to the lower end of the audit level ranges. By doing this we were able to maintain 

excess NO in the 65 ppb - 130 ppb level without having to rerun NO orig. All the audit 

points are less than 100 ppb, levels 4, 5 and 6. Using NO orig nominally around 140 we 

can get: 

140 NO orig - 65 ppb excess NO rem = 75 ppb NO2

140 NO orig - 97 ppb excess NO rem = 43 ppb NO2

140 NO orig - 122 ppb excess NO rem = 18 ppb NO2

This seems to work and saves some time. We purchased a Teledyne API T200UP for our 

near-road site. I have calibrated it in our lab. Its factory manual specifically states that 

excess NO should be approximately 20 ppb at each GPT point. I've calibrated it using 

this procedure and it works. I have not performed an audit on the analyzer. I have run 

points with higher concentrations of excess NO just to see how it performs and it seems to 

work. However, these results are very limited and not well documented. I think it will 

work with higher excess NO points. We have not written our procedure so I'm not sure 

which route we'll take.”

Office of Research and Development: 

“In our development/evaluation work, the methods we use are such that they allow us to 

calibrate, operate and audit/check the monitoring methods we are working in order to 

provide a scientifically defensible routine method. Often times we follow what is in 40 

CFR parts 50, 53, and 58 but other times we alter our approach for practical and 

scientific reasons. As to the question below, it is not necessary to maintain 0.080 ppm 

excess NO when doing NO2 by GPT. In our experience, we generate a constant NO 

concentration (i.e., 180 ppb) and then change the NO2 concentration by adding ozone in 

varying concentrations. As a result when we do a high span point (80-90% full scale) we 

generate 160 ppb of NO2 and only have 20 ppb of excess NO. In this instance we would 

not meet what is given in section 3.2.2.2 but we have shown our method to be very robust 

and produce excellent results. However, at our low precision point we still maintain the 

180 ppb NO level but generate 5--20 ppb of NO2 by adding ozone. In this case our 

excess NO is 160-175 ppb which also may not be in accordance with section 3.2.2.2 due 

to the NO being "substantially higher than 0.080 ppm". In our research however, we 

have observed no measurement errors using this excess NO levels.”

Regional Office: 

“I was a field operator/auditor at a local for 12 years calibrating and auditing a variety 

of air monitoring instruments including the Thermo 42 and the 42c. Whenever I had 
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a problem with an NO2 instruments response, it was never due to too little or too much 

NO during GPT. In fact I was not even aware of this restriction on the NO rem when 

I was a local and only became aware after arriving at EPA. Since then I’ve struggled to 

understand the reasoning for it given my experience and also relaying the rationale to 

our region. However we craft the new language I would be in favor of avoiding having 

auditors reset the NO orig or at least minimize it as much as possible.”

SUMMARY 

The feedback noted above provides some evidence that successful GPT procedures can 

be completed without a strict adherence to section 3.2.2.2(a). While this evidence is 

somewhat anecdotal in nature, it provides EPA with sufficient initial support for 

proposing changes to this specific requirement while soliciting comment on the approach 

and possible alternatives. Accordingly, we are proposing the following revised language 

for this (renumbered) section: 

3.1.2.2 The NO2 audit techniques may vary depending on the ambient monitoring method. 

For chemiluminescence-type NO2 analyzers, gas phase titration (GPT) techniques should 

be based on EPA guidance documents and monitoring agency experience. The NO2 gas 

standards may be more appropriate than GPT for direct NO2 methods that do not employ 

converters. Care should be taken to ensure the stability of such gas standards prior to 

use.