Draft:Emission Factors Model

EMFAC (Emission Factors Model) is a motor vehicle emissions modeling system developed by the California Air Resources Board (CARB) to estimate pollutant emissions from on-road vehicles operating in California. EMFAC produces emissions inventories for criteria pollutants, air toxics, greenhouse gases (GHGs), and particulate matter across a wide range of vehicle types, model years, and operating conditions.[1]

The United States Environmental Protection Agency (EPA) has formally approved the use of EMFAC for State Implementation Plans (SIPs) and transportation conformity analyses in California regions.[2]

Purpose and applications

EMFAC is primarily used for:

Statewide and regional emission inventories

Government agencies use EMFAC to calculate emissions of:

Transportation conformity analyses

Metropolitan planning organizations (MPOs) use EMFAC outputs to determine whether future transportation plans meet federal Clean Air Act requirements.

Air quality planning and rulemaking

EMFAC estimates inform:

  • Local air district regulations
  • SIP development
  • State climate programs (e.g., Scoping Plan)[3]

Research and scenario modeling

Universities, consultants, and transportation agencies use EMFAC for evaluating:

Model structure

EMFAC produces emissions based on vehicle population, activity, and emission rates, using three core components:

Vehicle activity module

Determines:

  • Miles traveled
  • Vehicle speeds
  • Starts
  • Idling and soak periods
  • Fleet age distribution

Emission rate module

Emission factors (grams per mile, gram per start, etc.) are developed through:

Post-processing and aggregation

EMFAC integrates activity and emissions across the vehicle fleet, outputting total emissions for:

  • Geographic regions
  • Roadway types
  • Time periods (hourly, daily, annual)

Model versions

EMFAC2011 / EMFAC2014

Earlier versions used for SIP development and conformity through the mid-2010s.

EMFAC2017

Approved by EPA for federal conformity in 2019.[2]

EMFAC2021

The most recent model, incorporating:[1]

  • Updated vehicle activity data
  • Electrified vehicle fleet projections
  • New emissions testing results
  • Enhanced greenhouse gas modeling

Key features and capabilities

Detailed fleet categorization

29+ vehicle classes, including:

Technology and fuel modeling

Includes modeling for:

Emission processes accounted for

Scenario and policy analysis

Frequently used to evaluate:

Limitations

  • California-specific: Not intended for nationwide application (outside California, EPA's MOVES model is used).[7]
  • Data-intensive: Requires substantial input on fleet composition and activity.
  • Model lag: Regulatory approval can trail behind new technology adoption, requiring periodic model revisions.[5]

Comparison to MOVES (EPA)

EMFAC and MOVES share similar goals but differ in methodology and geographic focus.

Feature EMFAC MOVES
Primary Use California SIP & conformity National emissions modeling
Developer CARB U.S. EPA
Vehicle Activity California-specific data National multi-state datasets
ZEV Modeling ZEV Modeling Broader national assumptions

[8][9]

See also

References

  1. ^ a b c d "Volume I – User's Guide" (PDF). EMFAC2021 – via California Air Resources Board.
  2. ^ a b "Official Release of EMFAC2017 Motor Vehicle Emission Factor Model for Use in the State of California". Federal Register. 2019-08-15. Retrieved 2025-12-07.
  3. ^ a b "2020 Mobile Source Strategy | California Air Resources Board". ww2.arb.ca.gov. Retrieved 2025-12-07.
  4. ^ "Emissions and Health Impact of Electric Vehicle Adoption on Disadvantaged Communities". UC Davis Institute of Transportation Studies.
  5. ^ a b "Volume III Technical Document" (PDF). EMFAC2021.
  6. ^ "AB 32 Climate Change Scoping Plan | California Air Resources Board". ww2.arb.ca.gov. Retrieved 2025-12-07.
  7. ^ "MOVES3 Introduction & Overview" (PDF). MOVES Review Work Group. December 10, 2020.
  8. ^ "Preparation of Emissions Inventories for the 2020 North American Emissions Modeling Platform" (PDF). Technical Support Document (TSD).
  9. ^ "TECHNICAL EVALUATION OF THE GREENHOUSE GAS EMISSIONS REDUCTION QUANTIFICATION FOR FRESNO COUNCIL OF GOVERNMENTS' SB 375 2018 SUSTAINABLE COMMUNITIES STRATEGY" (PDF). CARB.


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