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SO2 NAAQS Designations Modeling
Technical Assistance Document
Office of Air and Radiation
Office of Air Quality Planning and Standards
Air Quality Assessment Division
This document is one of two technical assistance documents being provided by the EPA to assist
state, local, and tribal air agencies in the characterization of ambient air quality in areas with
significant sulfur dioxide (SO2) emission sources either through ambient monitoring or dispersion modeling as outlined in the data requirements rule (79 FR 27446). The primary purpose of this SO2 national ambient air quality standard (NAAQS) Designations Modeling Technical Assistance Document (TAD) is to provide recommendations on how an air agency might appropriately and sufficiently model ambient air in proximity to an SO2 emission source to establish air quality data for comparison to the SO2 NAAQS for the purposes of designations.
This TAD provides recommendations on several aspects of dispersion modeling in this context including the use of temporally varying actual emissions, source characterization, meteorological data, model selection, and background concentrations. Recommendations discussed in this TAD supersede the guidance set forth in the March 24, 2011 memorandum, “Area Designations for the 2011 Revised Primary Sulfur Dioxide National Ambient Air Quality Standards” (U.S. EPA, 2011a).
This TAD does not impose binding and enforceable requirements or obligations on any person, and is not final agency action. It is intended to provide recommendations for others to consider as they develop information to be used in future separate final actions, such as area designations and other NAAQS implementation actions. The TAD is subject to change and does not represent the culmination of any agency proceeding or a final interpretation by the EPA of any pre-existing statutory or regulatory requirements.
ii Table of Contents Page
1. Introduction 1
1.1 Background 1
1.2 Purpose 1
2. Guidance on Air Quality Models 3
3. Model Selection 5
4. Modeling Domain 6
4.1 Determining Sources to Model 7
4.2 Receptor Grid 8
5. Emissions Inputs 9
5.1 Years to Model 10
1.1 Background The traditional NAAQS implementation process begins with the area designations process described in section 107 of the Clean Air Act (CAA), which generally relies on air quality concentrations to be characterized by ambient monitoring data collected by the air agency to identify areas that are exceeding the relevant standard. The preamble to the final 2010 SO2 NAAQS rulemaking (75 FR 35520) noted that although the current SO2 ambient monitoring network included 400+ monitors nationwide, the scope of the network had certain limitations, and approximately two‐thirds of the monitors are not located to characterize maximum concentration source‐oriented impacts. It was observed that some areas without monitoring likely have concentrations violating the NAAQS. To address these potential public health impacts, the 2010 SO2 NAAQS preamble to the final rule and subsequent draft guidance issued in September 2011 recommended that air agencies submit substantive attainment demonstration state implementation plans (SIPs) based on air quality modeling by June 2013 [under Clean Air Act section 110(a)(1)] that would show how areas expected to be designated unclassifiable and have sources emitting over 100 tons of SO2 per year would attain and maintain the NAAQS in the future.
A number of stakeholders expressed concern with this suggested implementation approach, particularly with the number of sources to be modeled (more than 1680 sources had emissions exceeding 100 tons in 2008), and the fact that the recommended SIP submission date for areas without monitoring data being before the SIP due date for violating areas with monitoring data.
In response, EPA Assistant Administrator Gina McCarthy sent letters to state Environmental Commissioners on April 12, 2012, indicating that the EPA wanted to further consult with stakeholders regarding how to best implement this standard and protect public health in an effective manner. The letters also stated that the agency would not expect air agencies to submit attainment demonstrations by June 2013 for areas not designated as “nonattainment” based on ambient monitoring data. The EPA developed a white paper on possible implementation approaches and proceeded to convene three stakeholder meetings in May‐ June 2012 with environmental group representatives; state, local, and tribal air agency representatives; and industry representatives. On July 27, 2012, the EPA also announced that it was extending the deadline for SO2 NAAQS area designations by an additional year, to June 3, 2013, based on the unavailability of data.
On May 14, 2014, the EPA proposed the Data Requirements Rule for the 1-Hour Sulfur Dioxide Primary Ambient Air Quality Standard (DRR) (79 FR 27446), taking comment on where ambient SO2 air quality around SO2 emission sources should be required to be characterized. On Friday, August 21, 2015, the Agency finalized and promulgated the SO2 DRR (80 FR 51052). In that final rulemaking, the EPA required the characterization of ambient SO2 air quality around SO2 emission sources emitting 2,000 or more tons per year of SO2. The air quality characterization that was prescribed to be carried out could be done through ambient monitoring or through dispersion modeling. The modeling recommendations presented in this document are appropriate for use in satisfying source-oriented SO2 modeling requirements, such as those required by the SO2 DRR.
1.2 Purpose This TAD is one of two being provided by the EPA to assist state, local, and tribal air agencies in the characterization of ambient air quality in areas proximate to or impacted by significant SO2 emission sources 1. The EPA issued an SO2 Strategy Paper in February 2013 describing the agency’s plan for addressing public health concerns in areas outside the initial areas intended for nonattainment designation in June 2013. The SO2 Strategy Paper recognized the need to further characterize current air quality across the country to address important public health impacts caused by exposure to ambient SO2 concentrations, with an understanding that there are limitations in the current SO2 ambient monitoring network. The paper described the EPA’s plans to develop a future data requirements rule which would require states to characterize air quality in areas around and impacted by the nation’s larger SO2 emission sources (including areas in which maximum one-hour emissions levels are not adequately characterized by the current ambient monitoring network). Under this intended approach, state, local, and tribal air agencies would be able to characterize ambient air quality conditions either through the use of additional ambient monitoring or through dispersion modeling. The planned data requirements rule would provide criteria for identifying the sources for which air quality should be characterized. It would also provide a schedule defining when states would need to decide whether to conduct monitoring or modeling for key sources, and when the resultant findings would need to be submitted to the EPA for use in the designations process.
In the final data requirements rule published in 2015 (FR 51052-51088), the EPA established requirements for characterizing SO2 air quality in priority areas, focusing on areas with sources that have emissions greater than 2,000 tons per year. States have the flexibility to characterize air quality using modeling of actual emissions or using appropriately sited existing and new monitors. These data would be used in two future rounds of designations in 2017 (based on modeling) and 2020 (based on new monitoring).
This TAD presents recommendations on how an air agency might appropriately and sufficiently model ambient air in proximity to or impacted by an SO2 emission source to assess compliance with the SO2 NAAQS for designations purposes only. The primary objective of the modeling would be to determine whether an area currently meets the SO2 NAAQS, and thereby indicate the designation process for the area. This TAD presents recommended steps to characterize current air quality based on actual emissions for the most recent years for a source of interest, and for any nearby sources, which may influence the air quality of the area. The TAD also presents recommendations on proper use of modeling inputs, such as emissions, meteorological data, and background concentrations.
For historical use of modeling in SO2 designations see 75 FR 35551-35552.
Many sections of this TAD are consistent with the EPA’s Guideline on Air Quality Models (U.S.
EPA, 2005), or Appendix W to 40 CFR part 51, and other relevant modeling guidance issued to characterize projected future air quality based on allowable emissions to support attainment planning and permitting programs. However, an important difference between the attainment planning and permitting guidance and is the guidance in this TAD regarding characterization of current air quality based on recent emissions data. Users of this TAD should consult with the appropriate EPA Regional Modeling Contact 2 to discuss any specific questions they may have.
This TAD is primarily for use by air agencies to assess likely areas of attainment and nonattainment with the 1-hour SO2 NAAQS. We recognize that other stakeholders may wish to use modeling to assess ambient SO2 air quality, and this TAD provides the EPA’s expectations for modeling for those other stakeholders as well. 3 We also note that states—or other parties— may wish to assess SO2 compliance on a schedule that is quicker than what we included in our strategy paper or in the final data requirements rule, and that credible modeling information submitted that indicates potential violations of the NAAQS would need to be evaluated by states and, potentially, the EPA.
Throughout this document, ambiguous terms such as “nearby”, “representative” will appear.
While this TAD offers recommendations on modeling techniques, every situation is unique, and modelers should also rely on their best professional judgement and past experience when deciding on certain issues, such as inclusion of sources to model, background concentration estimates, and representativeness of meteorological data. Terms such as “nearby” and “representative” are left to interpretation because every situation is unique and the reviewing authority is often the best judge on such issues within their local jurisdictions.
2. Guidance on Air Quality Models The starting point for this TAD is the EPA’s Guideline on Air Quality Models, also published as Appendix W of 40 CFR part 51 4. Appendix W is the primary source of information on the regulatory application of air quality models for SIP revisions for existing sources and for New Source Review (NSR) and Prevention of Significant Deterioration (PSD) permitting programs.
Air quality modeling in support of this designations process would need to employ air quality dispersion models 5 that properly address the source-oriented nature of SO2 and, thus, should rely upon the principles and techniques in Appendix W when applicable.
List of Regional Modeling Contacts by EPA Regional Office is available from SCRAM website at:
Please note that this guidance supersedes the EPA’s March 2011 air quality modeling guidance, intended for the designations process, which recommended the use of allowable emissions only to characterize air quality.
Hereafter, referred to Appendix W in the TAD.
Dispersion modeling uses mathematical formulations to characterize the atmospheric processes that disperse a pollutant emitted by a source. Based on emissions and meteorological inputs, a dispersion model can be used to predict concentrations at selected downwind receptor locations.