4 The record articles

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

Posted: January 26th, 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): The long awaited revisions to the 40 CFR Part 51, Appendix W (also known as the Guideline on Air Quality Models referred to as Guideline hereafter) were finally published in Federal Register (FR) on January 17, 2017.  The final Guideline, which was signed by U.S. EPA Administrator Gina McCarthy on December 20, 2016, will become effective on February 16, 2017 for all of the covered regulatory applications with the exception of transportation conformity.  The respective reviewing authorities such as state, local, and regional permitting authorities will have until January 17, 2018 to integrate the Guideline in the regulatory process.  However, if you have already submitted a protocol for air quality modeling analysis, the approval of the protocol will be at the discretion of the appropriate reviewing authority.  As for the transportation conformity purposes, the final Guideline will set a 3-year transition period ending on January 17, 2020.

This revision to the Guideline comes more than a decade after the previous update and sets the stage for the new era in the world of air dispersion modeling.  In this new era, that the focus will be on improving the modeling techniques and incorporating the new science to address the secondary chemical formation of fine particles and ozone pollution.  The new Guideline also comes with a revised version of U.S. EPA’s AERMOD modeling system v16216.  One of the two primary objectives of the Guideline revisions was to address technical concerns within the AERMOD modeling system to generally improve model performance.  The second primary objective was to finalize the modeling techniques to address secondary chemical formation of fine particle (PM2.5) and ozone (O3) pollution from direct, single source emissions of sulfur dioxide (SO2), oxides of nitrogen (NOX) and volatile organic compounds (VOCs).  In addition to the Guideline revisions, the final rule will also include guidance on single source modeling, Model Emission Rates for Precursors (MERP) guidance, and guidance on use of Model and Mesoscale Model Interface Program (MMIF) for use of prognostic meteorological data in the AERMOD modeling system.  All of these revisions could impact the obligations that facilities must meet for upcoming Prevention of Significant Deterioration (PSD) permitting projects.

In order to help you navigate through all parts of this complex regulation, the ALL4 modeling team is publishing a blog series entitled “Appendix W Revision: The New Era of Air Quality Modeling”.  The first two blogs of this series are already published and available for you.  Brief information on the blog series is as follows:

  • The first blog by Maggie Greene on the revisions to the AERMOD Modeling System provides a brief summary of key updates to model codes including incorporation of ADJ_U*, Buoyant Line and Point Source (BLP) model, bug fixes, incorporation of horizontal and capped stacks, and AERSCREEN.
  • The second blog published by Amanda Essner focuses on the NO2 Modeling Updates and the impact of these changes on your upcoming air quality modeling projects.
  • The future blogs in this series will include:
    • Introduction to MMIF;
    • Review of Single Source Secondary Formation Modeling Guidance;
    • SO2 SIP Attainment Demonstration; and
    • Long Range Transportation Modeling Updates.
    • In addition to these blogs, you can also read Meghan Barber’s blog on introduction to the MERP Guidance.

The new era in air quality modeling is going to be exciting and adventurous.  But at the same time, this new era will have its own set of challenges.  The ALL4 modeling team is here to help you navigate through these challenges.  For all your modeling needs, feel free to give us a call or reach out to any of the ALL4 modeling team members.


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