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• Laws and Regulations
• Policy and Guidance
• Research
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• Models & Methodologies
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  • Dispersion Models
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• Training

Dispersion Models

A dispersion model is a mathematical formulation which:

• Accepts data on transportation variables, source emissions (usually from emissions modeling), meteorological conditions, geographic boundaries, etc as inputs;
• Computes the dispersion of pollutants in the atmosphere; and
• Produces output data on the concentration of pollutants over the area of interest for specified time periods.

With the dispersion model, temporal and spatial estimates of pollutants from transportation and other sources, along with estimates of background pollution and weather conditions, are combined. When this analysis is completed, comparisons of estimated pollution concentrations with the National Ambient Air Quality Standards (NAAQS) are made to determine whether control action is needed to reduce emissions.

Preferred dispersion models are published in Appendix W (PDF) of 40 CFR Part 51.

Software

• AERMOD - A steady-state plume model that incorporates air dispersion based on planetary boundary layer turbulence structure and scaling concepts, including treatment of both surface and elevated sources, and both simple and complex terrain.
• CAL3QHCR is a refined version of CAL3QHC that requires local meteorological data. CAL3QHC is a CALINE3 model with queuing and hot spot calculations and with a traffic model to calculate delays and queues that occur at signalized intersections.
• User Friendly Version of CAL3QHC - The FHWA Resource Center is designing a new, easy-to-use interface for two air quality models: EPA's California Line Source Dispersion Model (CALINE3) for freeways and the CAL3QHC model for signalized intersections. With the new interface, the models will run on the Microsoft Windows® operating system and will include interactive graphical forms for entering data.

Policy & Guidance

• Transportation Conformity Guidance for Quantitative Hot-spot Analyses in PM2.5 and PM10 Nonattainment and Maintenance Areas (EPA, December 2010) - Guidance for modeling the local air quality impacts of certain transportation projects on the PM2.5 and PM10 national ambient air quality standards (NAAQS). This guidance is to be used by state and local agencies to conduct quantitative PM "hot-spot analyses" for new highway and transit projects that involve significant diesel emissions.
  • Main document only
  • Appendices only
  • Fact Sheet (HTML) or ( PDF)

Laws & Regulations

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

Research

• Implications of the Implementation of the MOBILE6 Emissions Factor Model on Project-Level Impact Analyses Using the CAL3QHC Dispersion Model (August 2004) - Prepared by ICF Consulting for FHWA, this report discusses how revisions to the MOBILE model affect the estimation of localized CO concentrations, including problems and difficulties in the interaction of MOBILE6 with CAL3QHC, as well as recommendations for addressing these issues. In addition, this report discusses how the implementation of MOBILE6 impacts screening assessment procedures.

Training

• FHWA Air Quality Resource Center Training

PDF files can be viewed with the Acrobat® Reader®