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OAQPS: New Appendix W Section 8 Overrides "Overly Conservative" Aspects of the 1990 NSR Wo

EPA’s Office of Air Quality Planning and Standards recently hosted a webinar on “Appendix W – Section 8: Modeling Domain, Source Data, and Background Concentrations.” The purpose of the webinar was to provide an update on EPA’s current thoughts, after the recently approved Appendix W update, on how the modeling domain, source data, and background concentrations should be submitted for both SIP and PSD modeling purposes. George Bridgers of EPA’s Office of Air Quality Planning and Standards led the presentation.


The overall seminar presented, in a nice nutshell, EPA’s current thoughts about modeling and the modeling domain and data requirements. Probably the most interesting element of Mr. Bridgers’ presentation was on the first substantive slide, reproduced here:

The EPA continues to caution again the literal and uncritical application of very prescriptive procedures for conducting NAAQS and PSD increments modeling compliance demonstrations as described in Chapter C of the 1990 draft New Source Review Workshop Manual (a.k.a. the draft Puzzle Book).

OAQPS was clear that the Puzzle Book must be used in conjunction with the revised Appendix W and the various Implementation Rules, so while Mr. Bridgers didn’t quite say “burn it,” he did state that it does not reflect current agency policy in several areas. OAQPS also noted that it has released a new Air Quality Analysis Checklist, which it hopes to maintain as a living document, and encouraged modelers to use the checklist when preparing modeling submissions.

Modeling Domain. OAQPS began with an analysis of the changes to modeling domain encompassed within new Appendix W, Section 8. Mr. Bridgers emphasized the following points:

  • EPA no longer endorses the overly conservative practices in the 1990 NSR Manual, Chapter C, section IV.B.

  • The modeling domain extends to the most distant location where air quality modeling predicts a significant ambient impact, or 50 km, whichever is less. Most modeling should not extend beyond 10-20 km from a typical source.

  • A source-by-source analysis of nearby sources is still needed, but if those sources are “adequately represented” by monitoring data, they should not be modeled.

  • EPA no longer encourages multiple iterations of refining of the receptor grid and does not recommend the practices in Puzzle Book Chapter C, section IV.D.2. Instead, EPA recommends using a two step iterative process as outlined in Appendix W, section Sometimes some additional work may be required, but multiple rounds is not needed.

  • EPA recommends a progressive approach to receptors, with a tighter spacing nearer the source and less intensive at greater distances.

Source Data. Source data for the project source is discussed in Appendix W, Section 8.2. Tables 8-1 and 8-2 address all sources, so it is important not to get confused on what requirements apply to the project, nearby and more distant sources.

  • SIP demonstrations are addressed in Appendix W, Section 8.2.2(a). Base year emissions should be reflective of actual emissions in that year; future year emissions should consider projected future growth (increase/decrease in activity), expected emissions controls, fuel switches and other relevant information.

  • PSD and SIP revisions demonstrating NAAQS compliance for a “project source” or specific source, requires the input of allowable emissions.

Background Concentrations. Background concentrations are used in the cumulative impacts analysis, not the first step source impact analysis.

  • OAQPS notes that background ambient data may be modified when the project source is impacting the monitor. Appendix W allows for this adjustment.

  • Nearby sources need only be modeled when monitoring data does not address or where they cause a significant concentration gradient in the significant impact area. OAQPS cautioned that “nearby” does not mean “close,” it means “not represented by monitoring data”.

  • “Nearby sources” are now based on “annual level when actually operating, averaged over the most recent 2 years,” but footnote allows consideration of whether those years are “representative” and EPA encourages states, local authorities and sources to address representativeness if there is an issue. Appendix W has replaced the approach set forth in the 1990 NSR Manual Chapter C, section IV.D.4. OAQPS clarified that Appendix W:

  • Does not require annualization; may look at hourly variation.

  • Allows use of CEMS data, in appropriate circumstances, to determine short term data.

  • Uses “typical maximum” for short term rather than allowables.

Design Concentrations. The discussion of design concentrations in Appendix W was substantially updated and is now presented in a unified section 9.2.2. OAQPS noted that in general it does not recommend hourly or daily pairing of monitored background and modeled concentrations except in “rare” cases and that seasonal (or quarterly) pairing of monitored and modeled concentration should sufficiently address situations where modeled and monitored values are not temporally correlated.

Question and Answer. OAQPS provided some insight on intermittent sources. George Bridgers noted that if an intermittent source is controllable – such as the decision to test the source on a schedule -- that should be addressed in the modeling protocol, whereas if the source is not controllable – such as when an emergency generator will run in an emergency – that may be best handled through annualized emissions.


The EPA webinar provided a good overview of the agency’s thinking now that the Appendix W changes have gone final. The most significant elements of the webinar were OAQPS’ statement that several Appendix W changes specifically overrule aspects of the 1990 NSR Workshop Manual and its caution that the Workshop Manual should not be applied “literally or uncritically.”

The other major changes that EPA highlighted: (1) the express disavowal of modeling beyond 50 km; (2) the disavowal of multiple runs for receptor grid “optimization,” (3) the emphasis that nearby sources should be few in number because monitoring data should address most such sources;, and (4) that those few nearby sources that are modeled should be modeled using representative actual maximums instead of allowable, all reflect an encouraging emphasis on pragmatism and modeling judgment in lieu of mechanical application of guidance. It will be interesting to see how this emphasis on pragmatism and modeling judgment will work with the new Appendix W restrictions on regional office authority to approve alternatives.

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