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Amendments to the Guideline on Air Quality Models, Where Are We Now and Other Recent Updates

Posted: December 10th, 2018

Authors: Dan D. 

We’ve been living with the amendments to 40 CFR Part 51 Appendix W – Guideline on Air Quality Models (The Guideline) for a little over a year and a half now so I thought it would be a good time to update the regulated community on how the changes have affected modeling projects that ALL4 has been involved with over the last year and a half.  The Guideline amendments went into effect after a “presidential-requested” delay on May 22, 2017, followed by a 1-year grace period to utilize pre-amendment guidance (if desired) which is now also over.  A side note for those involved with transportation conformity projects; the grace period continues for 3-years until May 22, 2020.

Most of the Guideline amendments either streamlined air quality modeling and/or allowed for the use of less conservative more accurate air quality modeling approaches.  I’d rank the advantages with respect to air quality modeling for permitting of the Guideline amendments as follows:

  1. Nitrogen dioxide (NO2) Tier 2 (i.e., ambient ratio method) and Tier 3 [i.e., ozone limiting method (OLM) and plume volume molar ratio (PVMRM)] options becoming default options.
  2. Use of actual emissions rates when modeling local sources as part of National Ambient Air Quality Standards (NAAQS) demonstrations.
  3. Modeled Emissions Rates for Precursors (MERPs) Guidance.

Tier 2 and 3 NOX to NO2 Conversion Options

Due to the stringent 1-hour NO2 NAAQS, it has become a necessity to use less conservative, more accurate approaches for quantifying the conversion of oxides of nitrogen (NOX) to NO2 in the atmosphere.  The Tier 2 and Tier 3 options do this by incorporating measured amounts of ozone (O3) to quantify this conversion.  The Tier 1 method of assuming that all NOX emitted converts to NO2 is overly conservative.  The Guideline amendment streamlined the process by incorporating the Tier 2 and Tier 3 methods as default options in AERMOD.  Before the Guideline amendments, the Tier 2 and 3 methods where non-default, which required alternative model review approval by both the state and regional EPA jurisdiction. The additional review and approval steps often slowed down the permitting process.

While these options are now default, the Guideline amendments established a default Tier 2 minimum ambient ratio (AR) of 0.5 and a Tier 3 NO2/NOX in-stack ratio (ISR) of 0.5 which are still conservative.  I’ve been preaching this to clients my entire career and will continue to recommend documenting ISR information during any stack test and/or as part of a continuous emissions monitoring system (CEMS).  For fossil fuel combustion sources, I typically see NO2/NOX ISR of 0.1.  Having ISR documentation of your emissions sources and/or similar emissions sources is the easiest way to justify the use of a lower than default AR or ISR which can make a substantial impact during your next construction permit application that could include air quality modeling requirements.  This may not be practical for all your emissions sources depending on your permit requirements.  Where actual ISR values are unavailable, the next best thing is to utilize documented ISR from similar sources.  However, it has been my experience that your local permitting jurisdiction may add a level of conservativism to this value that will fall between the measured ISR and the default ISR.  Be aware the U.S. EPA has an ISR database, though, not too many types of emissions sources have been certified since its inception (it’s mostly emergency generators that are included in the list).

Use of Actual Emissions

It has been my recent (18 months) experience that the use of local source actual emissions instead of potential-to-emit (PTE) emissions when conducting particulate matter less than 2.5 microns (PM2.5), sulfur dioxide (SO2), and NO2 NAAQS modeling demonstrations has removed a level of significant conservatism.  This is particularly true, when conducting a cumulative impact NAAQS analysis as part of the Prevention of Significant Deterioration (PSD) permitting requirements, where applicants are required to include their facility emissions sources, local facility emissions sources, and background concentrations from representative ambient monitoring stations.  The Guideline amendments allow for the use of the average actual emissions from the two most recent years of operation (if the two most recent years are representative of normal operation) when modeling local sources as part of a NAAQS demonstration.  This approach is typically less conservative than the use of PTE emissions.  In addition, because of yearly state required emissions reporting, development of a local source emissions inventory can be a simpler process for those states that publish facility emissions reports online.

MERPs for ozone and secondary PM2.5

As outlined in my recent blog post, the U.S. Environmental Protection Agency (U.S. EPA) Office of Air Quality Planning and Standards (OAQPS) finalized guidance which updated the PM2.5 significant impact levels (SILs) and proposed a SIL for O3.  One of the additions to the Guideline amendments is a new requirement to evaluate O3 precursors, NOX and volatile organic compounds (VOC).  In preparation for the Guideline amendments and the O3 SIL, U.S. EPA released guidance in December 2016 outlining a screening approach for evaluating O3 and secondary PM2.5 precursors impacts through the development of modeled emissions rates for precursors (MERPs).  While the MERPs guidance was not part of the Guideline amendments, U.S. EPA did plan well by providing a screening approach to evaluate O3 and PM2.5 precursor which was part of the new Guideline amendments.

It has been my experience that use of the MERPs screening approach has been a straightforward streamlined approach to evaluate O3 and secondary PM2.5 precursors.  Essentially the MERPs are regionally based (e.g., eastern, central, and western) de-minimis emissions thresholds that are used to demonstrate that a project’s emissions won’t exceed the O3 or PM2.5 SILs and therefore, not adversely affect air quality and the need to conduct a quantitative O3 or PM2.5 precursor NAAQS analysis.  Be aware that some states have developed state specific MERPs (Georgia is the best example) based on conducting photochemical modeling for facilities in their state.

The Guideline amendments were generally favorable for permit applicants from the perspective of providing less conservative default options and more streamlined air quality dispersion modeling approaches.  However, air quality modeling remains a cumbersome process that continues to rely on inherently conservative guidance and approaches.  You might be asking if there will be more Guideline amendments soon to incorporate the current “state of the science.”  To answer that I’d point to the last time the Guideline amendments were made in 2005 as an indicator that it will likely be some time until another round of amendments is made.  That being said U.S. EPA OAQPS modeling staff have released a series of white papers prioritizing the planned areas of science updates to the AERMOD modeling system. They include:

  1. LOWWIND Options
  2. NO2 Modeling Techniques
  3. Downwash Algorithms
  4. Mobile Source Modeling
  5. Overwater Modeling
  6. Saturated Plumes


White papers have been released for stakeholders to participate in improvements that would be expected to be proposed during the next Conference on Air Quality Models (Conference).  The Conference, held by U.S. EPA, is a unique requirement specified in the Guideline that must take place every three years, and is included as part of the process for making amendments to the Guideline.  Based on communication from U.S. EPA, the 12th Conference on Air Quality Modeling is likely to occur in spring of 2019 at U.S. EPA OAQPS headquarters in RTP, NC.

If there is a project that your facility was considering before the Guideline amendments that was sidelined because of air quality modeling or not evaluated because of air quality modeling concerns, I’d recommend revisiting it because the Guideline amendments may positively impact the issues associated with your project.  Also, if any of the White Paper topics have been issues for your facility, consider engaging ALL4 to assist in proposing updates that could be incorporated in the next round of Appendix W amendments.


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