Title 40

PART 51 APPENDIX W



Appendix W to Part 51 - Guideline on Air Quality Models

40:2.0.1.1.2.25.11.20.38 : Appendix W

Appendix W to Part 51 - Guideline on Air Quality Models Preface

a. Industry and control agencies have long expressed a need for consistency in the application of air quality models for regulatory purposes. In the 1977 Clean Air Act (CAA), Congress mandated such consistency and encouraged the standardization of model applications. The Guideline on Air Quality Models (hereafter, Guideline) was first published in April 1978 to satisfy these requirements by specifying models and providing guidance for their use. The Guideline provides a common basis for estimating the air quality concentrations of criteria pollutants used in assessing control strategies and developing emissions limits.

b. The continuing development of new air quality models in response to regulatory requirements and the expanded requirements for models to cover even more complex problems have emphasized the need for periodic review and update of guidance on these techniques. Historically, three primary activities have provided direct input to revisions of the Guideline. The first is a series of periodic EPA workshops and modeling conferences conducted for the purpose of ensuring consistency and providing clarification in the application of models. The second activity was the solicitation and review of new models from the technical and user community. In the March 27, 1980, Federal Register, a procedure was outlined for the submittal to the EPA of privately developed models. After extensive evaluation and scientific review, these models, as well as those made available by the EPA, have been considered for recognition in the Guideline. The third activity is the extensive on-going research efforts by the EPA and others in air quality and meteorological modeling.

c. Based primarily on these three activities, new sections and topics have been included as needed. The EPA does not make changes to the guidance on a predetermined schedule, but rather on an as-needed basis. The EPA believes that revisions of the Guideline should be timely and responsive to user needs and should involve public participation to the greatest possible extent. All future changes to the guidance will be proposed and finalized in the Federal Register. Information on the current status of modeling guidance can always be obtained from the EPA's Regional Offices.

Table of Contents List of Tables 1.0 Introduction 2.0 Overview of Model Use 2.1 Suitability of Models 2.1.1 Model Accuracy and Uncertainty 2.2 Levels of Sophistication of Air Quality Analyses and Models 2.3 Availability of Models 3.0 Preferred and Alternative Air Quality Models 3.1 Preferred Models 3.1.1 Discussion 3.1.2 Requirements 3.2 Alternative Models 3.2.1 Discussion 3.2.2 Requirements 3.3 EPA's Model Clearinghouse 4.0 Models for Carbon Monoxide, Lead, Sulfur Dioxide, Nitrogen Dioxide and Primary Particulate Matter 4.1 Discussion 4.2 Requirements 4.2.1 Screening Models and Techniques 4.2.1.1 AERSCREEN 4.2.1.2 CTSCREEN 4.2.1.3 Screening in Complex Terrain 4.2.2 Refined Models 4.2.2.1 AERMOD 4.2.2.2 CTDMPLUS 4.2.2.3 OCD 4.2.3 Pollutant Specific Modeling Requirements 4.2.3.1 Models for Carbon Monoxide 4.2.3.2 Models for Lead 4.2.3.3 Models for Sulfur Dioxide 4.2.3.4 Models for Nitrogen Dioxide 4.2.3.5 Models for PM2.5 4.2.3.6 Models for PM10 5.0 Models for Ozone and Secondarily Formed Particulate Matter 5.1 Discussion 5.2 Recommendations 5.3 Recommended Models and Approaches for Ozone 5.3.1 Models for NAAQS Attainment Demonstrations and Multi-Source Air Quality Assessments 5.3.2 Models for Single-Source Air Quality Assessments 5.4 Recommended Models and Approaches for Secondarily Formed PM2.5 5.4.1 Models for NAAQS Attainment Demonstrations and Multi-Source Air Quality Assessments 5.4.2 Models for Single-Source Air Quality Assessments 6.0 Modeling for Air Quality Related Values and Other Governmental Programs 6.1 Discussion 6.2 Air Quality Related Values 6.2.1 Visibility 6.2.1.1 Models for Estimating Near-Field Visibility Impairment 6.2.1.2 Models for Estimating Visibility Impairment for Long-Range Transport 6.2.2 Models for Estimating Deposition Impacts 6.3 Modeling Guidance for Other Governmental Programs 7.0 General Modeling Considerations 7.1 Discussion 7.2 Recommendations 7.2.1 All sources 7.2.1.1 Dispersion Coefficients 7.2.1.2 Complex Winds 7.2.1.3 Gravitational Settling and Deposition 7.2.2 Stationary Sources 7.2.2.1 Good Engineering Practice Stack Height 7.2.2.2 Plume Rise 7.2.3 Mobile Sources 8.0 Model Input Data 8.1 Modeling Domain 8.1.1 Discussion 8.1.2 Requirements 8.2 Source Data 8.2.1 Discussion 8.2.2 Requirements 8.3 Background Concentrations 8.3.1 Discussion 8.3.2 Recommendations for Isolated Single Sources 8.3.3 Recommendations for Multi-Source Areas 8.4 Meteorological Input Data 8.4.1 Discussion 8.4.2 Recommendations and Requirements 8.4.3 National Weather Service Data 8.4.3.1 Discussion 8.4.3.2 Recommendations 8.4.4 Site-specific data 8.4.4.1 Discussion 8.4.4.2 Recommendations 8.4.5 Prognostic meteorological data 8.4.5.1 Discussion 8.4.5.2 Recommendations 8.4.6 Treatment of Near-Calms and Calms 8.4.6.1 Discussion 8.4.6.2 Recommendations 9.0 Regulatory Application of Models 9.1 Discussion 9.2 Recommendations 9.2.1 Modeling Protocol 9.2.2 Design Concentration and Receptor Sites 9.2.3 NAAQS and PSD Increments Compliance Demonstrations for New or Modified Sources 9.2.3.1 Considerations in Developing Emissions Limits 9.2.4 Use of Measured Data in Lieu of Model Estimates 10.0 References Appendix A to Appendix W of Part 51 - Summaries of Preferred Air Quality Models List of Tables
Table No. Title
8-1 Point Source Model Emission Inputs for SIP Revisions of Inert Pollutants.
8-2 Point Source Model Emission Inputs for NAAQS Compliance in PSD Demonstrations.