There is no one dispersion model that is capable of addressing all conceivable situations of an industrial source's impact on ambient air quality. Therefore, two types of modeling analysis areused: screening and refined analysis.

Screening Analysis

The purpose of the screening technique is to:

1. eliminate the need for more detailed analysis of those sources that clearly will not cause or contribute to ambient concentrations in excess of the National Ambient Air Quality Standards (NAAQS); or
2. in the case of Prevention of Significant Deterioration (PSD) permits, determine whether perform a full impact analysis is required

ADEQ adheres to the EPA's Guidelines on Air Quality Models (EPA-450/2-78-027R) as a common measure of acceptable technical analysis that ensures consistency among permit modeling analysis.

Screening techniques may be applied to both simple and complex terrain, as well as for the modeling of toxic air pollutants. Determination of land classification (urban or rural), climatology of the area and topography are necessary for analysis. On-site meteorological data is not required as the SCREEN model uses a worst-case meteorological data file.

Screen Modeling Data Requirements

The minimum source information needed for screen modeling includes:

• Pollutant emission rate
• Stack height for point source and release height for an area source Stack exit data (diameter, velocity and temperature) for plume rise calculations
• Location of the point of emission with respect to surrounding topography and the character of the topography
• A detailed description of all structures in the vicinity of, or attached to, the stack in question, including:
  • Length, width and height of buildings and adjacent buildings
  • Distances between buildings
  • Size of process area
  • Hours of operation (daily and annual)

Site Specific Problems

The following problems should be examined when performing an air quality permit analysis. Each problem is briefly described with reference to informative documentation.

  Good Engineering Practice Stack Height

• If stacks for new or existing major sources are found to be less than the height defined in EPA's refined formula for determining GEP, then the air quality impacts associated with cavity or wake effects due to nearby building structures should be determined. Detailed downwash screening procedures for both the cavity and wake regions should be followed. Refer to the Guideline on Air Quality Models (EPA-450/2-78-027R)

  Merged Parameters for Multiple Stacks

• Sources that emit the same pollutant from several stacks with similar parameters that are within 100 m of each other may be analyzed by treating all of the emissions as coming from a single stack. Refer to section 2.2 of the Screening Procedures for Estimating the Air Quality Impacts of Stationary Sources, Revised (EPA-454/R-92-019).

  Topographic Considerations

• Topographic features, through their effects on plume behavior, will sometimes be a significant factor in determining ambient ground-level pollutant concentrations." Refer to sections 4.2, 4.3 4.5.2 and 4.5.3 of the Screening Procedures for Estimating the Air Quality Impacts of Stationary Sources, Revised (EPA-454/R-92-019) for topographic modeling techniques.

  Source Building Complex

• The downwash phenomenon caused by the aerodynamic turbulence induced by a building may result in high ground-level concentrations in the vicinity of an emission source." The screening procedure for building downwash is described in section 4.5.1 of the Screening Procedures for Estimating the Air Quality Impacts of Stationary Sources, Revised (EPA-454/R-92-019).

  Cavity Region

• "Generally downwash has the greatest impact when the effluent is caught in the cavity region. The cavity is defined as when the plume centerline equals the cavity height." Refer to section 4.5.1 of the Screening Procedures for Estimating the Air Quality Impacts of Stationary Sources, Revised (EPA-454/R-92-019).

  Wake Region

• The wake region consists of the "near wake" region (that which extends to 10 L which is 10 times the lesser of the building height or width) and the "far wake" region (which extends beyond 10 L).

  Fumigation

• Fumigation occurs when a plume that was originally emitted into a stable layer is mixed rapidly to ground-level when unstable air below the plume reaches plume level." Refer to section 4.5.3 of the Screening Procedures for Estimating the Air Quality Impacts of Stationary Sources, Revised (EPA-454/R-92-019).

General modeling information can be obtained from the Guideline on Air Quality Models (EPA-450/2-78-027R). In all screening situations, the appropriate document for the model in question should be used.

Please refer to the documents listed in the references for a thorough explanation of modeling issues. When in doubt, modeling questions should be presented to the Evaluation Unit modeling team for their assistance.

Available Screen Models

A number of air quality models are used in examining an air pollution situation for a given area. Screen models are basic dispersion models which estimate pollution concentrations from a source using the most minimum of input data (i.e. meteorological data).

All EPA approved screening models (i.e., SCREEN3) are available for download and use through the EPA's Bulletin Board, now available on the Internet. The most appropriate model to use depends upon various factors, such as; location, topography, complex terrain and the amount and types of meteorological data available. One should consult the EPA's Guidelines on Air Quality Models (Revised) document for a description of EPA approved models and their uses.

Refined Analysis

All procedures, techniques and recommendations pertaining to refined modeling analysis used by ADEQ are guidelines established by the EPA; the majority of these were established from the Clean Air Act of 1977.

Before any refined modeling examination is performed, ADEQ requires that the source submit a written protocol which outlines and describes the methodologies utilized in the modeling analysis. The modeling protocol should contain the following topics which include the use of procedures and techniques recommended by the EPA:

• Description of the Region, the Facility and the Process area
• Initial Screening Results
• Air Quality and Meteorological Monitoring and Data Collection
• Techniques on how the Data is Pre-processed for Modeling
• Description of the Dispersion Models
• Assumptions Made in the Modeling Analysis

In order to complete a thorough modeling protocol, applicants are required to reference the following EPA documents:

• Guidelines on Air Quality Models (EPA-450/2-78-027R Supplements B, C and D)
• Code of Federal Registrar (Part 58, Appendix B)
• On-Site Meteorological Program Guidance for Regulatory Modeling Applications (EPA-450/4-87-013)
• Workbook Of Atmospheric Dispersion Estimates (EPA; Office of AIR Programs Research Triangle Park, NC, Revised 1970)

If an applicant prepares a modeling protocol pertaining to Prevention of Significant Deterioration (PSD), the following document is considered in addition to the documents listed above:

• PSD Workshop Manual (EPA, 1990; Volume I)

Once the protocol is approved and modeling is submitted, a formal examination is performed by ADEQ. All techniques and regulatory selections required for a complete analysis are referenced in the EPA documents mentioned above as well as in the specific modeling documents which accompany the model. Refined models used in the modeling analysis must be EPA approved or written permission must be given to the applicant if a non-regulatory model is used.

ADEQ examines modeling results for the following major pollutants: particulate matter less than 10 microns (PM10), sulfur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), lead (Pb) and ozone (O3). The modeling results are compared to the National Ambient Air Quality Standards (NAAQS) established by the EPA as required by the Clean Air Act of 1977. Other modeling parameters examined (depending upon the type of analysis performed) such as monitoring, modeling, and PSD increments were also established by the EPA as required by the Clean Air Act.

Air Toxics are examined using the Arizona Ambient Air Quality Guidelines (AAAQG's) which were established in 1984 and updated January 6, 1994. As a result of delays in development of the HAPs permit rules, the Division Director has decided that ADEQ will use the AAAQG risk evaluation methods in the same manner as the last few years. Therefore, new and modified sources will generally be required to meet the AAAQG values subject to a risk management review.

ADEQ emulates EPA regulatory requirements pertaining to siting air quality monitors, meteorological stations, and refined modeling analyses. Arizona has adopted guidelines pertaining to toxic pollutants and uses these guidelines which are at or above EPA standards.

Available Refined Models

A number of air quality models are used in examining an air pollution situation for a given area. All EPA approved refined models (i.e. ISCST3, RAM) and meteorological pre-processors (i.e., PCRAMMET) are available to download and use through the EPA's Bulletin Board, now available on the Internet. The most appropriate model to use depends upon various factors, such as: location, topography, complex terrain and the amount and types of meteorological data available. One should consult the EPA Guidelines on Air Quality Models (Revised) document for a description of EPA approved models and their uses.

Air Quality and Meteorological Monitoring Criteria Used in Modeling

The criteria used in siting any air quality or meteorological monitoring station are referenced in the following EPA documents:

• Code of Federal Regulations (Part 58, Appendix B)
• On-Site Meteorological Program Guidance for Regulatory Modeling Applications (EPA-450/4-87-013)
• Quality Assurance Handbook for Air Pollution Measurement Systems (Meteorological Measurements (EPA/600/4-90/003, Volume IV, August 1989))

The following are additional documents which are used when considering the selection of upper-air meteorological stations:

• Mixing Heights, Wind Speeds, And Potential For Urban Air Pollution Throughout The Contiguous United States (EPA; Office of Air Programs Research Triangle Park, NC, January 1972)

It should be noted that each site is unique in topography, physical or man-made obstructions, meteorology and climatology. The siting documents listed above state that if certain criteria cannot be met, substitutions or alternatives may be used. The Evaluation Unit's Air Quality Specialist or Meteorologist can be contacted for their recommendations on siting stations.

Air Quality Modeling Index · Permit Plan Modeling · SIP Modeling · EPA Regulatory Models