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In 2005, the U.S. 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 permitting, State Implementation Plan (SIP) development, and many other applications regarding model selection and use. Meanwhile, EPA promulgated new 1-hour probabilistic National Ambient Air Quality Standards (NAAQS) for NO2 and for SO2 for which the Guideline models were either inappropriate or issues were inherent in the models’ physics that caused the models to err over short term periods such as one hour. Important modeling issues associated with the regulated pollutants and standards have been addressed in an interim fashion through Clearinghouse memoranda on EPA’s Support Center for Regulatory Air Models (SCRAM)2 web site.

Because of the central role of dispersion modeling in the manner that sources and emissions are regulated in the U.S. air quality management program, the need for an update to the Guideline is of paramount concern to the modeling community. In response to the clamor for a revised Guideline, EPA has responded. 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, 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 will address the proposed changes and the modeling community will provide feedback) – late Summer 2015
  • Final rulemaking and promulgation of the revised Guideline - Spring 2016

While this schedule may seem aggressive, EPA has been publishing applicable interim modeling guidance, addenda to user’s guides, and clarification memoranda for the past several years and thus, has a great storehouse of applicable and usable information to draw from for the Guideline updates. In addition, EPA has solicited from the modeling community any studies and research activities that could contribute to better science in the regulatory models.

Summary of 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 revised Guideline will be similar. The following are excerpts and summaries of EPA’s planned and initially proposed components of the revised future Guideline.

Sections 1-3 - Introduction, Overview, and Recommendations

These sections will remain essentially the same as in the 2005 version of the Guideline with any needed updates to version numbers or nomenclature. These sections will address why modeling is an important tool in air quality management, discuss the suitability of models, review the levels of sophistication from screening to refined models, and describe the availability of models on today’s computer and internet platforms. The recommended or “preferred” EPA modeling techniques and models will be described along with the requirements to substantiate alternative model use.


Section 4 – Criteria Pollutant Specific Models 

This section will outline recommended models for particulate matter, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. The specific calling out of PM10 and PM2.5 models and modeling methods will be new but will follow closely many of the guidance documents currently available unless additional methods become available. Most models will be industrial source-based in terms of applicability, with CO model recommendations more mobile source oriented. EPA is not expected to indicate any other preferential treatment of pollutant and source type.

Section 5 - Ozone and Secondary Particulate Matter

In this revised section, newer ozone models will be introduced as well as models and modeling techniques for addressing the impacts of secondary particulate matter. In recent years, EPA and the modeling and monitoring communities have become aware of the large burden of secondary PM on ambient monitors in the form of nitrates and sulfates. With this knowledge and the mandate to bring areas into attainment as well as to regulate new sources of PM under the requirements for Prevention of Significant Deterioration (PSD), the consideration of modeling these components has become important. This section will elucidate and echo other EPA guidance related to secondary PM consideration.

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Section 6 - Visibility and AQRV Models

This new section in the Guideline will provide 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 will be the first time that EPA has consolidated this guidance into the Guideline. Previously, most of the AQRV guidance has come in the form of reports and guidelines from the Federal Land Managers in association with EPA. Recommendations will be provided for visibility modeling, long range transport (50-300km from the source), and modeling for other governmental programs.

Section 7 - Stationary Source Models

This section will concatenate the 2005 Guideline sections on preferred stationary source models (old Section 4) and the section on general modeling considerations (old Section 7). Screening and refined models will be presented along with an overview of design concentrations, critical receptors, atmospheric stability and dispersion, Good Engineering Practice (GEP) stack height, plume rise, chemical transformation, deposition, and complex wind flow situations. EPA updates to these general considerations will be included; for example, the consideration of building downwash, new techniques and changes to building configuration consideration, and use in generating affecting building influence areas.

Section 8 - Mobile Source Models

This new section will greatly expand on the short preferred methods section for models dealing with sources of carbon monoxide. The continued use of CAL3QHCR is expected but with more emphasis on using AERMOD for project level modeling analysis (intersections and roadway segments near new infrastructure projects in mass transit as well as typical arterial construction).

Section 9 - Model Input Data

This section will be comparable to 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 will also include an overview of applicable use of background concentrations, tying in 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. Finally this section will provide the meteorological input requirements for modeling for National Weather Service data and onsite data collection. A new subsection will address the use of prognostic weather data, possibly with respect to longer range transport models such as CALPUFF, CAMx, and CMAQ.

Section 10 - Regulatory Application of Models

This section will remain focused on recommendations on the analysis requirements, use of measured (monitored) air quality data, and emissions limits and their relationship to modeled concentrations. The latter section will discuss design concentrations, NAAQS based limits, and PSD increment-based limits.

No information was provided at the R/S/L meeting regarding the current 2005 Guideline, Section 9 which addresses the accuracy and uncertainties in models. Given the changes to the models over the past several years and the need for additional model evaluation studies to test the appropriateness and representativeness of added features and options, the inclusion of this section seems imperative. Further information on the upcoming revised Guideline will be provided as it becomes available.

In summary, this significant revision to EPA’s primary modeling guidance document will 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.

1 Guideline on Air Quality Models, 40 CFR Part 51, Appendix W, November 9, 2005. 2