EPA Webinar Seeks to Provide Clarity on Guidelines for PSD Ozone and PM2.5 Demonstrations

August 14, 2017
Richard Hamel

NSR Law Blog is pleased to welcome Richard Hamel, Principal Consultant, at ERM’s Air Quality and Climate Change Practice, as a contributing author. Richard can be reached at ERM’s Boston, MA office, at richard.hamel@erm.com or 617-648-7826. NSR Law Blog thanks Richard for his contribution!

EPA’s Office of Air Quality Planning and Standards recently hosted a webinar on “Tier 1 and Tier 2 Demonstrations for Ozone and PM2.5 under the PSD Permitting Program.” The purpose of the webinar was to provide an update on EPA’s current thoughts on how these demonstrations should be accomplished.


The overall seminar presented EPA’s current thoughts on addressing ozone and the secondary formation of PM2.5 as part of a PSD permitting action. EPA reiterated its preference for photochemical grid models, while noting that Lagrangian models (e.g., SCICHEM) might have promise for single source ozone or PM2.5 assessments.

Once again, EPA stressed that the first tier does not necessarily involve modeling, but can use “technically credible relationships between emissions and ambient impacts based on existing modeling results or studies deemed sufficient for evaluating a project source’s impacts.” This may involve use of the Model Emission Rates for Precursors (MERPs), which EPA believes can be developed for VOCs and NOx for ozone and SO2 and NOx for PM2.5. MERPs can then be used to demonstrate that an individual source’s contribution will, in many cases, not be significant. EPA reiterated that MERPs are not the only way to make this demonstration. EPA suggested close coordination with the EPA Regional office during development of a MERP or alternative technical approach.

The following were the major new points made by EPA during the call:

1. The Critical Air Quality Threshold used in MERPs comparison as described in the draft MERPs guidance issued on December 2nd, 2016 is expected to be the SIL (for ozone and 24-hour and annual PM2.5) in all cases unless some other threshold is proposed and cleared through EPA. EPA was advised by its legal counsel not to use the word “SIL” specifically in that part of the guidance, but that is the intent.

2. In the draft MERPs guidance, when assessing secondary PM2.5 formation against the MERPs, the one scenario that includes primary PM2.5 implies (without addressing the matter) that if the primary modeling is over the SIL already, then using MERPs to avoid assessing secondary PM2.5 directly is not an option and thus would require the applicant to either find another Tier-1 approach or proceed to Tier-2 which could require photochemical grid modeling. I asked for clarification on this point since there could easily be a scenario where a source exceeds the PM2.5 SIL at the fence line, potentially due to some sort of fugitive emission, but has miniscule NO2 and SO2 emissions that amount to only a very small percentage of their respective MERPs, not to mention that any secondary formation is likely to happen far downwind, well away from the fence line impacts. EPA confirmed that MERPs could not be used in this case to avoid assessing secondary PM2.5 in the cumulative analysis, but that there might be alternative approaches other than a Tier-2 analysis available on a case-by-case basis.

3. The question of the use of SCICHEM as an alternative to one of the photochemical grid models came up. EPA indicated that as yet there have been no proposals to use SCICHEM for a regulatory application, but that the model has “promise”.

4. EPA will be releasing updated guidance for ozone and PM2.5 permit modeling, and expects to have the draft out for public comment in Fall of 2017. The currently expected guidance and its approximate release dates are as follows:

1. The day after the webinar EPA released a memorandum titled: “Use of Photochemical Grid Models for Single-Source Ozone and secondary PM2.5 impacts for Permit Program Related Assessments and for NAAQS Attainment Demonstrations for Ozone, PM2.5 and Regional Haze.” The memorandum provides an alternative model demonstration for two photochemical transport models, CAMx and CMAQ, to establish their applicability for PSD compliance demonstrations for ozone and PM2.5, and in NAAQS demonstrations for ozone, PM2.5, and Regional Haze.

2. Guidance for Ozone and PM2.5 Permit Modeling (Projected for Draft/Comment release in Fall 2017)

3. Guidance on Significant Impact Levels for Ozone and Fine Particles in the Prevention of Significant Deterioration Permitting Program (Currently in review by senior management)


The requirement to assess single-source contributions of ozone and secondary formation of PM2.5 in the revised Appendix W is perhaps the area of greatest concern to the modeling community in the new guidance. Thus, this webinar was timely and resulted in a lively questions and answer segment after the presentation. Additionally, since the decision as to which photochemical model to use has been left as a case by case determination, there are many questions regarding how to approach such modeling. Hopefully the additional guidance and rulemaking coming during the remainder of 2017 will provide the information necessary to allow for these demonstrations to be completed with a minimal impact to project timelines and budget.