Project-Level Air Quality Analysis

Air quality analysis is a complex process that must follow established regulatory procedures and requirements. The information and tools described below comprise of materials developed by Caltrans, as well as materials developed by external agencies including the U.S. Environmental Protection Agency (EPA), the Air Resources Board (ARB), and local air districts.

Travel Activity Data

Travel Forecast
In general, the basis of transportation project air quality analysis is directly attributed to how a transportation project affects travel activity and traffic flow. Assembling travel activity data is one of the first step of an air quality analysis.
  • DataBridge
    DataBridge is a tool for California Department of Transportation (Caltrans) users to process travel activity data for project-level emissions assessments and PM hot-spot analyses. DataBridge can be used to convert data derived from a travel demand model (TDM) to the data formats required by CT-EMFAC, a project-level modeling tool for obtaining emissions estimates from on-road mobile sources in California.

Emission Modeling

Criteria Pollutants

The EPA sets National Ambient Air Quality Standards (NAAQS) for six common air pollutants, also known as criteria pollutants. These pollutants are particulate matters (PM10 and PM2.5), ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. In addition to the NAAQS, there is also the California Ambient Air Quality Standards.

    The EMFAC is the mobile source emission inventory that ARB developed to assess emissions from on-road vehicles in California. The latest version, EMFAC2021(v1.0.2), was approved for use in transportation conformity analyses by the EPA on 11/15/2022, under the federal register docket number FRL-10366-01-R9.

    CT-EMFAC is the California Department of Transportation (Caltrans) Emission FACtors model, and is based on data derived from California Air Resources Board’s (CARB) Emission FACtors (EMFAC) model and ancillary data sets.

    CT-EMFAC can be used to estimate project-level emissions from on-road mobile sources; travel activity data (supplied by the user) are convolved with emission factors and vehicle fleet information (from an underlying database) to obtain total fleet emissions at the link level. Fundamentally, CT-EMFAC is a tool that calculates EMFAC-based emissions estimates at the data aggregation (resolution) level of typical transportation projects. By organizing project information and input data, project analysts can use CT-EMFAC to directly calculate project-level emissions and fuel usage.

    CT-EMFAC is not available for download from this site. Available by request from project AQ analysis please contact the project air quality staff.

Mobile Source Air Toxics (MSAT)

In addition to the criteria pollutants, the EPA identified nine air toxic compounds with mobile sources as the considerable contributors. These are 1,3-butadiene, acetaldehyde, acrolein, benzene, diesel particulate matter (diesel PM), ethylbenzene, formaldehyde, naphthalene, and polycyclic organic matter.

FHWA considers these as priority mobile source air toxics to be considered in NEPA documents. The FHWA issued an Updated Interim Guidance on Mobile Source Air Toxic Analysis in NEPA Documents on January 18, 2023. The guidance describes three categories for analyzing MSAT in NEPA documents, depending on specific project circumstances:

  1. No analysis for projects with no potential for meaningful MSAT effects;
  2. Qualitative analysis for projects with low potential MSAT effects; or
  3. Quantitative analysis to differentiate alternatives for projects with higher potential MSAT effects.

Construction Emission Analysis

Construction can result in fugitive dust emissions and engine exhaust emissions from various types of activities. While construction emissions are typically considered as temporary impacts, in areas that are non-attainment or attainment (maintenance) for CO or PM, construction emissions must be included in the conformity hot-spot analysis if construction will last more than five years at any individual site (40 CFR 93.123(c)(5)).

Analysis for shorter construction periods may be considered for NEPA and CEQA if sensitive receptors, such as schools, hospitals, elderly-care facilities, or child-care facilities, are nearby, or if construction emissions are anticipated to be high. Consult with the project's Caltrans Headquarters Environmental Coordinator if a construction emission analysis is being considered for the project.

  • Caltrans Construction Emissions Tool (CAL-CET)

    The Caltrans Construction Emissions Tool (CAL-CET) was developed to use Caltrans-specific equipment activity data and the best available equipment emissions information to improve estimates of transportation-related construction emissions, fuel consumption, and electricity consumption and support transportation and air quality planning. CAL-CET is not available for download from this site.  Available by request from

  • Road Construction Emission Model (RCEM)
    The RCEM was developed by the Sacramento Metropolitan Air Quality Management District (Sac Metro) as a part of their CEQA Guidance & Tools to analyze construction emissions for roadway projects within the Sacramento region. For projects outside of the Sac Metro jurisdiction, please be aware that RCEM utilizes emission factors specific to the Sacramento region.
  • California Emissions Estimator Model (CalEEMod)
    The CalEEMod was developed for the California Air Pollution Officers Association in collaboration with several local air districts. This is a statewide land-use emission model that include a construction emission module. CalEEMod's primary focus is on land development projects more so than roadway projects. It is more viable for projects that involve building structures or parking lots facilities, such as offices, safety roadside rest areas, park and rides, or transit hubs.
  • ARB Off-road Diesel Analysis & Inventory
    Construction equipment is classified as off-road. The predominant fuel type for construction equipment is diesel. ARB OFFROAD model provides emission factors for analysis of off-road equipment.

Dispersion Modeling

Carbon Monoxide Dispersion Modeling

  • CO Protocol (PDF)
    The CO Protocol provides procedures and guidelines to conduct a project-level CO analysis. *Do Not use Appendix A of the CO Protocol. Appendix A is no longer applicable. Where the CO Protocol mentions EMFAC or CT-EMFAC, refer to the criteria pollutants estimate guidance above for the latest required model.
    CALINE4 manual and background information about the development of the model. The CALINE4 dispersion model is currently only applicable for CO hot-spot analysis, as referenced in the CO Protocol. *DO NOT use CALINE4 to analyze any other pollutant.

Particulate Matter Dispersion Modeling

  • EPA Guidance on PM Hot-Spot Analysis
    The EPA "Transportation Conformity Guidance for Quantitative Hot-spot Analyses in PM2.5 and PM10 Nonattainment and Maintenance Areas" provides procedures and guidelines to conduct a project-level PM hot-spot analysis. Where the guidance mentions EMFAC or CT-EMFAC, refer to the criteria pollutants estimate guidance above for the latest required model.

Other References

  • Attainment Designation Maps
    EPA Region IX designation maps for criteria pollutants under the NAAQS. Conformity rules apply to areas federally designated as Non-Attainment or Attainment (Maintenance).