4 The record articles

Appendix W Revision: The New Era of Air Quality Modeling – NO2 Modeling Updates

Posted: January 17th, 2017

Author: All4 Staff 

(UPDATE 1/26/17): In accordance with the White House executive directive as expressed through a memorandum on January 20, 2017, EPA has temporarily delayed the effective date of the Revised 40 CFR 51, Appendix W until March 21, 2017.

(ORIGINAL: 1/17/17): Air dispersion modeling of nitrogen oxides (NOX) emissions and I have a love-hate relationship.  As a meteorologist, I love the process of implementing air quality modeling studies, but I hate giving bad news to clients about elevated concentrations.  Luckily, the updated 40 CFR Part 51 Appendix W (Guideline on Air Quality Models) Final Rule has shifted my feelings more towards love, due to some of the improvements made by U.S. EPA in this rulemaking, and I am looking forward to my next air quality modeling project where I have to demonstrate compliance with the nitrogen dioxide (NO2) National Ambient Air Quality Standards (NAAQS)!

Improvements, you say?

On December 20, 2016, the long awaited Appendix W Final Rule was signed, which revised the Guideline on Air Quality Models.  With it, came a great deal of updates, some impacting NO2 air quality modeling.  In July 2015, Dan Dix published a 4 The Record article detailing the proposed changes to Appendix W overall. He followed this up in August 2015, with a blog that discussed the details of 1-hour NO2 air quality modeling.  Now that U.S. EPA has finalized the rule, let’s take a look at what’s new and improved for NO2 air quality modeling using AERMOD…

Regulatory Options

AERMOD currently has several regulatory and non-regulatory options for air quality modeling of NO2 that use atmospheric chemistry to predict more realistic modeled concentrations.  These options require additional user inputs and initial data to set up and run AERMOD.  Historically, the use of non-regulatory options in AERMOD reclassified AERMOD as an “alternative model”; and therefore, required U.S. EPA Regional Office approval prior to use.  The Appendix W Final Rule has made several of these options regulatory default options in AERMOD, thus no longer requiring Regional U.S. EPA approval prior to use.

The Ambient Ratio Method 2 (ARM2) has been added as a regulatory tier 2 option.  The Minimum Ambient Ratio (MAR) is set at a default of 0.5, which will, in some instances, still overstate modeled concentrations.  Justification and approval by the appropriate reviewing authority to use a lower or alternative MAR is still required.  The ARM2 essentially multiples the modeled concentrations by a NO2/ NOX ratio [developed from U.S. EPA’s Air Quality System (AQS) data] to provide an improved prediction of NO2 concentrations.

The Ozone Limiting Method (OLM) and Plume Volume Molar Ratio Method (PVMRM) have been added as regulatory tier 3 options.  PVMRM has also been replaced with PVMRM2 (although still referred to as “PVMRM” in AERMOD).  PVMRM2 includes 3 major modifications to PVMRM: adjusts the formulation during stable conditions to avoid over predictions, assesses impacts of plume width far from the centerline, and addresses the treatment of penetrated plumes.  The OLM and PVMRM2 options use ozone data to reflect the conversion process of nitric oxide (NO) to NO2 and will improve the accuracy of modeled NO2 concentrations.

1-Hour NO2 Modeling Updates

Modeling for compliance with the 1-hour NAAQS has been tricky regarding NO2.  Maybe the most impactful update to Appendix W is that for cumulative air quality modeling analyses (i.e., when you are evaluating  your facility plus local sources), local sources can be modeled at average actual emissions based on the most recent 2-years of normal source operation.  This use of actual emissions rates for local sources is HUGE.  Compiling local source data is a long and painful process in some States, so being able to use more readily available data, such as annual emissions statements, permit limits, and operating information is a big plus.  In addition, we can now evaluate actual reported emissions for local sources, which should represent lower emissions rates and thus reduce the impact of local sources when modeling for NAAQS compliance.

So, Let’s Get Modeling!

If in the past, the effort of conducting NO2 air quality modeling was too high of a hurdle for your project due to the need to obtain U.S. EPA Regional approval of alternate air quality modeling procedures, the updates to Appendix W will be important to you.  However, the use of these options for NO2 air quality modeling still require experienced and knowledgeable staff to apply the options in a supportable and appropriate manner.  That’s where ALL4 comes in.  Our experience implementing NO2 air quality modeling options goes back to the release of AERMOD.  Give me a call (610-933-5246×129) or reach out to Dan Dix (ddix@all4inc.com, 610-933-5246×118), or the rest of ALL4’s air quality modeling team if you have any questions and want to know more.  And, stay tuned for more blogs about the Appendix W Final Rule updates, and how they could impact your projects.


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