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Environmental Protection Agency (EPA) published and promulgated the Guideline on Air Quality Models1 (the Guideline) which has been used since then as the primary guidance tool in EPA Regional offices and state environmental regulatory agencies for industrial air quality permitting, State Implementation Plan (SIP) development, and many other applications regarding model selection and use.  Since 2005, important modeling issues associated with regulated pollutants and standards have been addressed through Clearinghouse memoranda on EPA’s Support Center for Regulatory Air Models (SCRAM)2 web site.  Meanwhile, EPA promulgated new 1-hour probabilistic National Ambient Air Quality Standards (NAAQS) for NO2 and for SO2 for which the approved Guideline models were either inappropriate or based on physics that caused the models to err over short term periods such as one hour. 

Because of the central role of dispersion modeling in the regulation of sources and emissions in the U.S. air quality management program, there has been an increasing and significant need for an update to the Guideline.  In response to the many outstanding issues, EPA has proceeded with proposed revisions to the Guideline.  At the May 20, 2014 Regional/State/Local (R/S/L) Modelers meeting, Tyler J. Fox, Group Leader of the Air Quality Modeling Group at EPA’s Office of Air Quality Planning and Standards (OAQPS), presented a roadmap of proposed regulatory revisions to the Guideline.  This overview was presented to an audience of approximately 200 attendees from the industrial, consultant, academic, and regulatory sectors.  Mr. Fox provided a schedule for revisions to the Guideline including the following milestones:

  • Proposed rulemaking for revisions to the Guideline on Air Quality Models - Spring 2015
  • 11th Conference on Air Quality Modeling (a 2-3 day conference where EPA would address the proposed changes and the modeling community provide feedback) – late Summer 2015
  • Final rulemaking and promulgation of the revised Guideline - Spring 2016

While this schedule seemed aggressive, EPA’s published applicable interim modeling guidance, addenda to user’s guides, and clarification memoranda from over the past several years provided a great storehouse of applicable and usable information to draw from for the Guideline update.  In addition, EPA has solicited from the modeling community any studies and research activities that could contribute to better science in the regulatory models.  Roughly following the proposed schedule, on July 14, 2015, EPA released the “Revision to the Guideline on Air Quality Models: Enhancements to the AERMOD Dispersion Modeling System and Incorporation of Approaches to Address Ozone and Fine Particulate Matter; Proposed Rule”  which was subsequently published to the Federal Register on July 29, 2015.

The proposed revisions include a substantial amount of new material as well as reorganization and removal of outdated or obsolete information.  The 11th Conference on Air Quality Modeling was held on August 12-13, 2015 in Research Triangle Park, NC and served as the public hearing for the proposed revisions. Despite the slight delay in issuing the proposed revisions to the Guideline, EPA reiterated during the conference its plans to meet the timeline to issue the final rulemaking by Spring 2016.

Summary of the Proposed Guideline Revisions

The Guideline is organized with sections providing model overviews and recommendations, and sections addressing the use of meteorological data sets, actual versus allowable emissions data, model accuracy, and model options.  The proposed revisions to the Guideline use a similar organization.  The following are excerpts and summaries of EPA’s proposed components of the revised future Guideline.

Section 1 - Introduction
This section describes the purpose and applicability of the Guideline and remains essentially the same as in the 2005 version of the Guideline with any needed updates to version numbers or nomenclature. 

Section 2 – Overview of Model Use
This section was updated to clarify the modeling process flow and provide consistent definitions of commonly-used terms such as: screening technique, screening model, and refined model.

Section 3 – Preferred and Alternative Air Quality Models
This section contains the requirements or “rules of the game” for model use and approvals with reference to Appendix A which contains a list of “preferred” models that can be used in dispersion modeling applications without further justification.  Modeling tools developed for specific source types or by private companies may be proposed for use as “alternative models,” provided the approval criteria delineated in the Guideline can be met.  The last portion of Section 3 includes the most substantive proposed change—codifying the role of the Modeling Clearinghouse in the model approval process, in regulatory interpretation, and in other case-specific modeling issues that may arise.

Section 4 – Models for Carbon Monoxide, Lead, Sulfur Dioxide, Nitrogen Dioxide and Primary Particulate Matter
This section describes the models approved for use to model “inert” criteria pollutants. The approved refined model remains AERMOD.  AERSCREEN is formally codified as the screening model; CALINE3 is replaced by AERMOD for mobile source modeling; and the Buoyant Line Plume (BLP) model is integrated into AERMOD (BLP is removed from Appendix A).  In addition to the discussion of models, a revised “multi-tiered” approach for NO2 is proposed, as follows:

  • Tier 1 assumes that all emitted NOx converts to NO2.
  • Tier 2 (previously known as the Ambient Ratio Method [ARM]) proposes use of ARM2 in AERMOD, which relies on concentration-specific ambient ratios based on nationwide monitoring data, rather than the single, fixed value used previously.  ARM2 is proposed as a regulatory default and use of the fixed value ARM is no longer recommended.  EPA also proposes to change the minimum ambient ratio from 0.2 to 0.5, making the proposed regulatory default of ARM2 more conservative than the non-regulatory ARM2 in the previous version of AERMOD.
  • Tier 3 would make the refined techniques OLM and PVMRM2 (revised PVMRM formulation in AERMOD) regulatory default options in AERMOD, requiring minimal additional approval efforts beyond modeling protocol approval. 

The section also confirms that AERMOD should be used for primary PM2.5 modeling while secondary impacts are addressed in later sections.  As CALPUFF is removed as the preferred model for long range transport in Section 3, Section 4 includes a discussion of Lagrangian models for long-range modeling.  EPA now classifies CALPUFF as a “screening technique” and this section allows for the use of other Lagrangian or photochemical models to perform these long range transport analyses.

Section 5 - Ozone and Secondary Particulate Matter 
This section is completely new in the Guideline and the most substantive portion of the proposed revision.  This section addresses how individual facility impacts on atmospheric ozone and secondary particulate formation might be assessed.  There is no specific model or modeling technique required or recommended in the proposed Guideline, providing applicants flexibility for these analyses.  The concept of “Modeling Emission Rate for Precursors” or “MERP” is introduced as a way to determine if secondary impacts should be addressed through modeling or if they would “screen out.”  EPA gave no indication of what emission rates would correspond to a MERP, other than that they would be greater than the PSD Significant Emission Rates.  Instead, EPA stated that the MERPs would be proposed in a separate future rule-making.  Emission rates below the MERP would not be expected to appreciably contribute to secondary particulate or ozone formation.  Projected emissions in excess of the MERP would trigger additional requirements for applicants to address secondary formation through a two-tiered approach.  The first tier consists of a semi-quantitative approach based on historical photochemical modeling data, monitoring data, and other documented information, while a second tier would consist of full photochemical modeling analyses, when necessary.

Section 6 – Modeling for Air Quality Related Values and other Governmental Programs 
This section in the Guideline provides guidance on performing visibility modeling and analysis to determine the impacts on Air Quality Related Values (AQRVs).  AQRVs can include nitrate and sulfate deposition on surfaces as well as visibility degradation at Class I areas (national parks, wildlife refuges, monuments, etc.) in the form of plume blight and extinction coefficients.  This is the first time that EPA has consolidated this guidance into the Guideline.  Previously, most AQRV guidance has come in the form of reports and guidelines from the Federal Land Managers (e.g., FLAG 2010 guidance) in association with EPA.  Although CALPUFF has been omitted as the preferred long range transport model in Sections 3 and 4; EPA notes that the change does not affect CALPUFF’s use under the FLM’s guidance regarding AQRV assessments (FLAG 2010).  Section 6 specifically defers to the most current FLM guidance for AQRV analysis, but also goes on to provide guidance on visibility and deposition modeling that emphasizes the use of photochemical models. 

Section 7 – General Modeling Considerations
This section is designed to capture specific modeling details that are not otherwise included in the remainder of the document.  Much of the basic information in the section is addressed in the current Guideline on topics such as dispersion coefficients, complex winds, gravitational settling and deposition, and plume rise formulations.  The section also includes a discussion of mobile source modeling, namely how to use AERMOD to appropriately characterize and model roadway emissions.  Additionally, EPA removed outdated references and reworded many of the subsections in an effort to keep the Guideline as a high-level, general modeling guidance document.

Section 8 - Model Input Data
This section includes and expands on the current 2005 Guideline Section 8, describing the use of source input data for the models including the consideration of actual and allowable emissions, regional inventory requirements, and source parameterization.  The section also includes an overview on the appropriate use of background concentrations, tying together all current guidance to reflect both the old NAAQS forms of the deterministic standards as well as the new probabilistic forms of the new one-hour SO2 and NO2 standards.  The discussion on inventory sources and background concentrations reflect EPA’s attempt to discourage use of outdated and generally overly conservative approaches that applicants have relied upon in the past.  Finally this section provides the meteorological input requirements for modeling for National Weather Service data and onsite data collection.  EPA has also developed the Mesoscale Model Interface (MMIF) program that processes prognostic weather data (e.g. MM5 or WRF models) into formats compatible with AERMET and AERMOD, allowing for the use of those forecast model outputs in situations where representative NWS or onsite data are unavailable.  The use of the MMIF program is required for applications that currently require the use of prognostic meteorological data.

Section 9 - Regulatory Application of Models
This final section remains focused on recommendations on modeling requirements, development of modeling protocols, use of measured (monitored) air quality data, and emissions limits and their relationship to modeled concentrations.  The section also discusses the concept of design concentrations, NAAQS based limits, and PSD increment-based limits.  Previously, Section 9 addressed the issues of model accuracy and uncertainties, but EPA has removed those discussions in the proposed revisions.

In summary, this significant revision to EPA’s primary modeling guidance document is intended to formalize current guidance and provide insight and instruction on leading edge modeling intricacies.  Environmental professionals with a focus on air quality assessment will find the publication required reading.  EPA is accepting comment on the proposed revisions to the Guideline through October 27, 2015 and aims to complete the final rulemaking and promulgation by late Spring or early Summer 2016. 

1Guideline on Air Quality Models, 40 CFR Part 51, Appendix W, November 9, 2005.
3 40 CFR Part 51, Federal Register Vol. 80 No. 145, pp 45340-45387, July 29, 2015.
4 2005, the U.S.