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MEMORANDUM

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

Dioxide (NO 2 ) 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-2 013-0619)

In the course of considering potential changes to q uality assurance requirements as part of the proposed rule - Revisions to Ambient Monito ring Quality Assurance and Other Requirements – OAQPS solicited input from several p arties experienced in the conduct

of NO 2 audits and calibrations. The goal of the communica tions was to gather input on changes being considered to a specific section in t he requirements dealing with an issue

known as “excess NO”. The comments that were receiv ed were used to develop proposed language that is contained in the proposal noted ab ove. Additional details are provided in

this memo.

BACKGROUND

The current draft of 40 CFR part 58 Appendix A incl udes very specific language prescribing procedures to be used as part of gas ph ase titration method audits for NO2 analyzers. The language reads as follows:

3.2.2.2 (a) NO 2 audit gas for chemiluminescence-type NO 2 analyzers must also contain at least 0.08 ppm NO. NO concentrations substantial ly higher than 0.08 ppm, as may occur when using some gas phase titration (GPT) tec hniques, may lead to evaluation errors in chemiluminescence analyzers due to inevit able minor NO-NO X channel imbalance. Such errors may be atypical of routine m onitoring errors to the extent that such NO concentrations exceed typical ambient NO co ncentrations at the site. These errors may be minimized by modifying the GPT techni que to lower the NO concentrations remaining in the NO 2 audit gas to levels closer to typical ambient NO

concentrations at the site.

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

instructions in the CFR based NO 2 calibration procedure 1 , the burden associated with making many adjustments to NO X /NO levels when conducting procedures in accordance with section 3.2.2.2(a), and due to the availabilit y of modern calibration equipment that

can automate NO 2 calibrations if the technique is simplified.

1 See Appendix F to Part 50—Measurement Principle an d Calibration Procedure for the Measurement of Nitrogen Dioxide in the Atmosphere (Gas Phase Chemi luminescence) - http://www.ecfr.gov/cgi-bin/textidx?SID=039e21f86e8bfeaf8bdb162fa3eb1121&node=40:2. 0.1.1.1.0.1.19.7&rgn=div92

FEEDBACK RECEIVED

To help inform the impact of potential revisions to this requirement, EPA contacted several representatives from state and local agenci es that are experienced with NO 2 procedures, and also scientists from EPA’s Office o f Research and Development and a Regional Office. Their responses are summarized be low:

State Agency:

“Thanks for the opportunity to comment on this issu e. We feel that the best course at this point would be to remove any language suggesting co nstraints on excess NO during GPT from 3.2.2.2. We think that the audit procedures s hould be consistent with the calibration procedures. The reference method calibration proced ures in Part 50 App F, 1.5.9.4 (and our Thermo manual) do not restrict the NO during th e GPT calibration to >80 ppb or even 20-200 ppb. They just say to start the GPT wi th an NO 2 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 NO 2 verification points throughout the rest of the NO 2 range.”

Local Agency:

“Prior to the change in audit levels (after 2006), we lowered NO flow to obtain a new NO orig 1 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 t he 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 NO 2 concentrations, lowering the NO flow with each point so when ozone is added to obtain the appropriate

NO 2 point, NO rem 2 is 0.080-0.120 ppm NO. For example, for a 0.175 p pm NO 2 point NO orig should be 0.255-0.295 and for the 0.075 ppm NO 2 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 calibrati ons and one for audits, each with results within specifications, would suggest that i t may not be necessary to maintain the 80 - 120 ppb NO level we used in our audit procedur e. Because we seemed to be able to operate within specifications using both procedures , strictly maintaining the level of

1 In this memo, the term NO orig refers to the conce ntration of NO that is used in a calibration or aud it point before the ozone lamp is turned on to begin t he titration process that creates NO 2

2 The term NO rem refers to the concentration of NO that is remaining in a calibration or audit point a fter the ozone lamp is turned on and stabilized to gener ate a concentration of NO 2 utilizing the titration process3

excess NO may not be critical when working with a T hermo 42, 42c or 42i. I would caution that this may not be the case with other an alyzers. This is the only monitor I have used extensively.

When the NO 2 audit levels were changed after 2006, we changed o ur audit procedure so that we wouldn't have to run subsequent NO orig poi nts 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 hav ing to rerun NO orig. All the audit points are less than 100 ppb, levels 4, 5 and 6. U sing NO orig nominally around 140 we

can get:

140 NO orig - 65 ppb excess NO rem = 75 ppb NO 2 140 NO orig - 97 ppb excess NO rem = 43 ppb NO 2 140 NO orig - 122 ppb excess NO rem = 18 ppb NO 2

This seems to work and saves some time. We purchase d 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 GP T 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 wri tten 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 m ethods 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 o ur approach for practical and scientific reasons. As to the question below, it i s not necessary to maintain 0.080 ppm excess NO when doing NO 2 by GPT. In our experience, we generate a constant NO concentration (i.e., 180 ppb) and then change the N O 2 concentration by adding ozone in varying concentrations. As a result when we do a h igh span point (80-90% full scale) we generate 160 ppb of NO 2 and only have 20 ppb of excess NO. In this instan ce we would not meet what is given in section 3.2.2.2 but we ha ve 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 NO 2 by adding ozone. In this case our excess NO is 160-175 ppb which also may not be in a ccordance with section 3.2.2.2 due to the NO being "substantially higher than 0.080 pp m". In our research however, we have observed no measurement errors using this exce ss NO levels.”

Regional Office:

“I was a field operator/auditor at a local for 12 y ears calibrating and auditing a variety of air monitoring instruments including the Thermo 42 and the 42c. Whenever I had4

a problem with an NO 2 instruments response, it was never due to too litt le or too much NO during GPT. In fact I was not even aware of thi s 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 wou ld 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 tha t 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 whil e soliciting comment on the approach and possible alternatives. Accordingly, we are prop osing the following revised language for this (renumbered) section:

3.1.2.2 The NO 2 audit techniques may vary depending on the ambient monitoring method. For chemiluminescence-type NO 2 analyzers, gas phase titration (GPT) techniques sh ould be based on EPA guidance documents and monitoring a gency experience. The NO 2 gas standards may be more appropriate than GPT for dire ct NO 2 methods that do not employ

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

use.