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

Posted: January 17th, 2017

Authors: Maggie G. 

(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): I have to say, I personally was prettyyyy excited for 2017 to finally get here [and according to the internet (which is obviously never wrong), I wasn’t the only one].  Likewise, the air quality modeling team here at ALL4 has been anxiously awaiting the final revisions to 40 CFR Part 51, Appendix W (Guideline on Air Quality Models) (Guideline or Appendix W).  On December 20, 2016, U.S. EPA Administrator Gina McCarthy granted our holiday wish by signing the final rule!

For those of you entrenched in the air quality modeling world (read: fellow nerds), the Appendix W revisions have been on your radar for a while now.  You may have read Dan Dix’s article about the proposed revisions back in July 2015.  But for those who can’t be entrenched in the air quality modeling world, here is the main take away: these Appendix W revisions offer relief to regulated entities and will help you when you are required to demonstrate compliance with air quality standards.

In this blog, I’ll provide an overview of the revisions that affect AERMOD, U.S. EPA’s preferred air quality dispersion modeling system (but stay tuned… we will be publishing a series of articles in the coming weeks covering all of the impactful updates!).

Two primary goals of the Appendix W revisions were to address technical concerns within the AERMOD (American Meteorological Society/Environmental Protection Agency Regulatory Model) modeling system, and to generally improve model performance.  To accomplish this goal, U.S. EPA made the following key updates:

  1. Incorporated ADJ_U* as a regulatory option in AERMOD’s meteorological data preprocessor (AERMET).  ADJ_U* works by (yup, you guessed it) adjusting U* (the surface friction velocity) and addresses issues with AERMOD over-prediction of concentrations under stable, low wind speed conditions.  It is not uncommon for an emissions source to have its highest concentrations predicted to occur during periods of low wind speed and stable atmospheric conditions.  Previously, ADJ_U* was an alternative option (i.e., beta) that required U.S. EPA Regional Office approval; now ADJ_U* can be used without U.S. EPA Regional Office approval. Thus with ADJ_U* becoming a regulatory option, an emissions source may have more flexibility when addressing air quality concentration levels for air permitting projects.  The only caveat is that that use of site specific turbulence measurements (i.e., sigma W and sigma q) can’t be utilized with ADJ_U* because of documented issues with under predicted concentrations.
  2. Integrated the Buoyant Line and Point Source (BLP) model into AERMOD.  BLP was designed to handle unique modeling scenarios where plume rise and downwash effects from stationary line sources are important.  In this update, BLP was removed from 40 CFR Part 51, Appendix A as a preferred model and integrated directly into AERMOD for use.  This enhancement will help those sources with emissions that exhaust from roof monitors/vents/cupolas.
  3. Made modifications to address over-prediction for scenarios where tall stacks are located near small urban areas. This change was made within the model itself, so no user input is required as long as the correct version of AERMOD (i.e., v16216r) is used.  It should be noted that, while AERMOD v16216r is currently available for download on U.S. EPA’s Support Center for Regulatory Atmospheric Modeling (SCRAM) website, the official effective date of the final rule will be 30 days after publication in the Federal Register, which was published on January 17, 2017.  Until that time, use of the new version of the model should be discussed with the appropriate reviewing authority.
  4. Updated the nitrogen dioxide (NO2) Tier 2 and Tier 3 screening techniques within AERMOD.  Previously, the Guideline outlined a 3-tiered approach for assessing nitrogen oxides (NOX) sources that addressed the co-emissions of NO2 and nitrogen oxide (NO).  With the Appendix W revisions, the Ambient Ratio Method (ARM) Tier 2 option was replaced with the ARM2 option.  The ARM2 option utilizes a NO2/NOX ratio based on an evaluation of measured NO2/NOX ratios from U.S. EPA’s Air Quality System (AQS) data.  Furthermore, the Ozone Limiting Method (OLM) and updated Plume Volume Molar Ratio Method (PVMRM) were incorporated as default options in AERMOD.  This means that these options will no longer be treated as an alternative model requiring U.S. EPA regional office approval.  All emissions sources of NOX stand to benefit from availability of these air quality modeling options as regulatory options.  See Amanda’s blog for more details on NOX air quality modeling.
  5. Updated the AERMOD formulation to address plume rise for horizontal and capped stacks.  This included updating the POINTHOR and POINTCAP options from beta to default options.  The POINTHOR and POINTCAP include adjustments to account for the Plume Rise Model Enhancements (PRIME) algorithm, which accounts for entrainment of plume mass into the cavity recirculation region, for sources subject to building downwash.  Capped stacks are frequently found at smaller (i.e., non-major) emissions sources that seldom have to address air quality modeling issues.  However, as U.S. EPA tightens the National Ambient Air Quality Standards (NAAQS), some state agencies require these non-major sources to assess air quality impacts.  This AERMOD formulation change will help small and large emission sources address their capped and horizontal stacks in a more realistic manner than was available with previous versions of AERMOD.
  6. Incorporated AERSCREEN (a screening version of AERMOD) as the recommended screening model for all types of terrain and applications, replacing SCREEN3. 

Also, a notable change that U.S. EPA did not incorporate into the Appendix W revisions is the addition of LOWWIND3 as a regulatory option in AERMOD.  This, like ADJ_U*, is a model option to address issues with over-predication of concentrations under low wind speed conditions.  Although U.S. EPA had proposed this change, they determined that additional study and evaluation is still warranted for this option at this time.

This overview is not a comprehensive list of all of the AERMOD modeling system change resulting from the Appendix W revisions (for a complete summary see this model change bulletin).  If you’d like more detail about the changes within AERMOD including which pollutant(s) and source type(s) would be affected and how your facility may benefit from the changes, ALL4’s air quality modeling staff has extensive experience with the implementation of AERMOD, AERMOD regulatory and non-regulatory options, and applying AERMOD for the best possible outcome for air quality modeling demonstrations.  Do not hesitate to reach out to me at mgreene@all4inc.com or 678-460-0324 x213!  The rest of ALL4’s modeling team is here to help, too.


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