4 The record articles

Draft MERP Guidance Has Arrived

Posted: December 12th, 2016

Authors: Meghan B. 

MERP…no I’m not referring to the word used to indicate an awkward conversation or making a sound you may hear from the likes of Kermit the Frog. I’m referring to Modeled Emission Rates for Precursors – MERPs.  On December 2, 2016, U.S. EPA published highly anticipated (at least in the air quality dispersion modeling world) guidance for the development of MERPs as part of a Tier 1 demonstration for ozone and PM2.5 Prevention of Significant Deterioration (PSD) permitting. Sounds a little scary, right? Don’t worry, we’re here to help.

What the heck is a MERP?

In the preamble of the soon to be finalized Appendix W, U.S. EPA made mention to a new fine particulate (PM2.5) and ozone precursor tool known as MERPs. MERPs were originally planned to be generally applicable thresholds to determine if a permitting project would contribute significantly to the PM2.5 and/or ozone National Ambient Air Quality Standards (NAAQS).  After a review of the available data, U.S. EPA determined that variations in source location and source characteristics make generally applicable MERPs a bad idea.  So with this shift in U.S. EPA thinking, a new formula for calculating a MERP on a case-by-case basis was developed: 

I know what you’re probably thinking – “Cool, now I have a million more questions.” Let’s break it down.

  • MERPs: Units are evaluated in tons per year (tpy).
  • Critical Air Quality Threshold: The critical air quality threshold will be determined by each permitting authority and will be used to indicate that a value above this threshold number will contribute to a violation of the appropriate NAAQS.  For ozone, the critical air quality threshold is provided in units of either parts per million (ppm) or parts per billion (pbb).  For PM2.5, the critical air quality threshold is provided in units of micrograms per cubic meter (µg/m3).
  • Modeled Emission Rate from Source: The emissions rate of precursor emissions for ozone or PM2.5 of the source you are evaluating as part of the PSD permitting analysis.  This is evaluated in tpy.
  • Modeled Air Quality Impact from Source:  The result of the air dispersion modeling analysis for the source that you are evaluating as part of the PSD permitting analysis.  This is evaluated in the same units as the critical air quality threshold.

And I have to use a MERP when?

We can make this short. A facility is required to evaluate a MERP when an emissions analysis determines that emissions increases from a proposed project will exceed the PSD significance thresholds for ozone precursors (i.e., 40 tpy increases for either VOC and NOX) and/or PM2.5 (i.e., 10 tpy) and its precursors (i.e., 40 tpy increases for either SO2 and NOX).  

Ok – So what are my MERP’ing options?

Now that we have an explanation for what a MERP actually is and why you potentially may need one, let’s take a look at what a facility’s options are for developing a MERP. The way we view it, you have three potential paths to take:

1.      Demonstrate Equivalency with a Source Provided in Appendix A of the MERP Guidance

As part of the guidance provided, U.S. EPA conducted PM2.5 and ozone photochemical analyses of a large dataset of hypothetical sources, which varied in the following characteristics:

  • Areas of the country (i.e., north, south, east, west, central),
  • Release height (i.e., high, low), and
  • Emissions rates of PM2.5 and ozone precursors (i.e., 500 tpy, 1000 tpy, 3000 tpy).

Facilities have the option to demonstrate that the source at their facility that they are evaluating under PSD is equivalent to one of the sources included in Appendix A of the MERP Guidance.  This equivalency will be based upon the characteristics listed above.  If a facility can demonstrate equivalency, the MERP can be developed using the values in Appendix A.  This would mean no air quality modeling, a shorter time frame for agency review and less resistance in the development of the MERP.

2.      Conduct Photochemical Modeling

If a facility cannot make the argument that their source is equivalent to one of the sources included in Appendix A, the facility will be required to conduct its own single source apportionment or brute force photochemical modeling analysis.  This approach will increase the resources and timing needed to develop the MERP.  However, once the analysis is completed, it can be used again in future PSD permitting applications, as required.

3.      Follow State Provided Guidance

If a facility is located in a state that is very active in the modeling realm, there is a possibility that the state agency may:

  • Elect to deem U.S. EPA’s analysis as representative for the state’s sources;
  • Expand on U.S. EPA’s analysis to provide additional sources for facilities to demonstrate equivalence to; or
  • Develop its own state-specific photochemical modeling analysis.

We have a feeling that not many states will elect to go with any of the provided guidance scenarios listed, which would limit your options for determining a MERP to demonstrating equivalency with Appendix A of the MERP Guidance or conducting photochemical modeling.

Whatever your facility decides to do, keep in mind that addressing MERPs qualitatively or quantitatively requires a higher level of technical understanding of atmospheric chemistry and air quality modeling than what both you and the state agencies have had to consider before.  Fortunately, ALL4 has experienced staff who can explain in easy to understand terms what the MERP guidance means to you and how you should consider the options for conducting photochemical modeling.    First step in developing those skills? Reading this blog. Second step? Providing comments to U.S. EPA on the draft guidance by February 3, 2017. If you would like additional information regarding the guidance, or help with commenting, please reach out to me at mbarber@all4inc.com. Make sure to stay tuned to ALL4’s blog – something tells me this is only the beginning of exciting modeling changes (HINT HINT final Appendix W updates).


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