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Air Dispersion Modeling

Air quality dispersion modeling is a computer simulation that predicts air quality concentrations from various types of emission sources. For pollutants emitted through a stack, it considers the emission rate, stack height, stack diameter, and stack gas temperature and velocity, as well as the effect of nearby buildings and terrain. Other emission sources like vehicle traffic or wind erosion from storage piles are represented as 2-dimensional area sources or 3-dimensional volume sources.

Air quality dispersion models use meteorological data such as temperature, wind direction, and wind speed to calculate concentrations. Five years of National Weather Service meteorological data is typically used for air quality modeling required by the US Environmental Protection Agency (EPA), Federal Land Managers (FLMs), and Minnesota Pollution Control Agency (MPCA). In some cases one year of onsite meteorological data is used.

Model results are often compared to benchmark levels such as ambient standards to determine if they meet state and federal requirements. Sometimes results are used to help site ambient monitors or they may be input into health risk assessments or ecological risk assessments.

What's New

Standardized Air Modeling (SAM) spreadsheets for regulatory air modeling (April 7, 2009):

These spreadsheets are works-in-progress. Their use is optional but recommended. We expect they will change/improve over time and we appreciate your patience as we move forward. The 5-digit version numbers resemble EPA model version numbers (i.e. 2-digit year id and 3-digit Julian date) so that “09097” is April 7, 2009, “09098” is April 8, 2009, “09099” is April 9, 2009, etc. Please direct any questions to Dennis Becker at 651-757-2217 or dennis.becker@state.mn.us.

• Standardized Air Modeling (SAM) Spreadsheet (SAM09097.XLS) (file size=1.4 MB)
• Standardized Mobile Source (SMS) Spreadsheet (SMS09097.XLS) (file size=5.9 MB)
• Standardized Data Visualization (SDV) Spreadsheet (SDV09097.XLS) (file size=12.3 MB)

Standardized Air Modeling (SAM) Goals:

• Better communicate air dispersion modeling information to all persons
• Better communicate agency expectations to air permit applicants and consultants
• Standardize and streamline our review of air dispersion modeling submittals
  • Protocols, results, revisions, modeling-related permit conditions, etc.
• Identify/fix the most common mistakes (e.g. math errors, missing sources, etc.)
• Identify/fix the most time consuming modeling steps (e.g. fugitive emissions)
• Use less paper

Modeled Background (April 7, 2009)

In Nov. 2008, MPCA gave a presentation to the Upper Midwest Section – Air & Waste Management Association on modeled background instead of monitored background for PM2.5 NAAQS modeling - sources & background paired in space & time (paired sums) is just a conceptual idea and is not for regulatory purposes at this time. Please direct any questions to Dennis Becker at 651-757-2217 or dennis.becker@state.mn.us.

• PM2.5 NAAQS Modeling – Challenges and Possible Solutions

Modeling Guidance

Listed below are links to EPA modeling guidance for federal purposes and MPCA modeling guidance for state Title V modeling.

• MPCA New Source Review Reform Modeling Guidance: Policies and Procedures (January 2004)
• EPA Support Center for Regulatory Air Models (SCRAM) has EPA modeling guidance, dispersion models, and related items.
• MPCA Guidance for Title V and PSD Air Dispersion Modeling (October 2004)
• MPCA Title V Modeling - Fugitive PM10 Emission Spreadsheet (March 2003)
• MPCA Instructions for Model Information Request permit requirement (November 2002)

Air Disperson Modeling Training from the 2006 Air, Water and Waste Environmental Conference

• Air Disperson Modeling Training (file size = 3.2 MB)
• AERMOD Update
• AERMOD Urban Source Modeling
• FAR Data for Nearby Sources
• Rochester/Olmsted County SIP (file size = 1.5 MB)
• AERA/RASS Modeling Developments
• Fugitive PM10 Modeling

PSD Minor Source Baseline Dates

Federal Prevention of Significant Deterioration (PSD) rules prescribe modeling requirements for facilities whose emissions and/or ambient impacts of certain pollutants exceed regulatory thresholds. One part of these rules regulates the level of deterioration in air quality allowed for a specified area. Deterioration of air quality is measured against specific "baseline dates" and is tracked at a county level in Minnesota. The amount of additional air pollution allowed after the baseline dates is called the "increment." The "Major Source Baseline Date" is the date after which increases in air pollution from major sources (as defined in PSD rules) "consume" increment. These dates are January 6, 1975 for PM₁₀ and SO₂, and February 8, 1988 for NO_x. The Major Source Baseline Dates are the same for every county in Minnesota. The "Minor Source Baseline Date" is the date after which increases in air pollution from all sources consume increment. The Minor Source Baseline Dates are pollutant and county specific. Minor Source Baseline dates for Minnesota counties are shown in the files below.

• NO_x Minor Source Baseline Dates
• PM₁₀ Minor Source Baseline Dates
• SO₂ Minor Source Baseline Dates
• NO_x, PM₁₀, SO₂ Minor Source Baseline Dates (text)

Who to Contact

• MPCA
  Phone: 651-296-6300 or 800-657-3864
  • MPCA Title V Modeling Policy (call to obtain a copy)
    
    
• Dennis Becker
  Phone: 651-757-2217
  e-mail: dennis.becker@pca.state.mn.us
  • MPCA Title V modeling policy
  • MPCA Title V modeling guidance
  • MPCA Standardized Air Modeling (SAM) spreadsheet
  • PSD modeling and Title V modeling
  • Status of Minnesota Environmental Review Cumulative Effects-Air Modeling
    
    
• Margaret McCourtney
  Phone: 651-757-2558
  e-mail: margaret.mccourtney@pca.state.mn.us
  • Title V information submittals (electronic information requests)
  • PM-2.5/ozone/regional haze/toxics modeling
    
    
• Greg Pratt
  Phone: 651-757-2655
  e-mail: gregory.pratt@pca.state.mn.us
  • Toxic air pollutants modeling
  • Deposition modeling
    
    
• Ruth Roberson
  Phone: 651-757-2672
  e-mail: ruth.roberson@pca.state.mn.us
• Status of Minnesota Environmental Review Cumulative Effects-Air Modeling
• Toxic air pollutants modeling
• PSD modeling and Title V modeling
  
  
• Melissa Sheffer
  Phone: 651-757-2718
  e-mail: melissa.sheffer@pca.state.mn.us
  • PSD increment files & nearby sources for PSD modeling
  • PSD minor source baseline dates by county (NOX, SO2, and PM10)
  • PSD modeling and Title V modeling
  • Pre-processed meteorological data (AERMOD)

Please contact any of the above persons if you have general modeling questions.

Additional Resources

• EPA Support Center for Regulatory Air Models (SCRAM) has EPA modeling guidance, dispersion models, and related items.
• EPA ClearingHouse for Inventory and Emission Factors (CHIEF) has information on EPA emission factors (e.g., AP-42).
• United States Geological Survey (USGS) has digital elevation model (DEM) terrain data.
• USGS topographic maps and USGS aerial photos.
• National Climatic Data Center.
• A Citizens Guide to Air Dispersion Modeling