Emission Factors Model
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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:
- Fleet turnover effects
- Electrification policies
- Clean-vehicle standards
- Fuel changes[4]
Model structure
EMFAC produces emissions based on vehicle population, activity, and emission rates, using three core components:
Vehicle activity module
Determines:
Emission rate module
Emission factors (grams per mile, gram per start, etc.) are developed through:
- Laboratory vehicle testing
- Portable emission measurement systems (PEMS)
- Fuel and technology trends
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:
- Light-duty cars and trucks
- Heavy-duty diesel vehicles
- Motorcycles
- Transit buses and school buses[1]
Technology and fuel modeling
Includes modeling for:
Emission processes accounted for
- Running exhaust
- Start emissions
- Evaporative emissions
- Tire and brake wear
- Fuel vapor venting
- GHG emissions from fuel combustion[5]
Scenario and policy analysis
Frequently used to evaluate:
- Zero-Emission Vehicle (ZEV) standards
- Low Carbon Fuel Standard (LCFS) impacts
- Clean truck regulations (ACT, ACF rules)[6]
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 |
See also
- Clean Air Act
- State Implementation Plans
References
- ↑ 1.0 1.1 1.2 1.3 "Volume I – User's Guide" (PDF). EMFAC2021 – via California Air Resources Board.
- ↑ 2.0 2.1 "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.
- ↑ 3.0 3.1 "2020 Mobile Source Strategy | California Air Resources Board". ww2.arb.ca.gov. Retrieved 2025-12-07.
- ↑ "Emissions and Health Impact of Electric Vehicle Adoption on Disadvantaged Communities". UC Davis Institute of Transportation Studies.
- ↑ 5.0 5.1 "Volume III Technical Document" (PDF). EMFAC2021.
- ↑ "AB 32 Climate Change Scoping Plan | California Air Resources Board". ww2.arb.ca.gov. Retrieved 2025-12-07.
- ↑ "MOVES3 Introduction & Overview" (PDF). MOVES Review Work Group. December 10, 2020.
- ↑ "Preparation of Emissions Inventories for the 2020 North American Emissions Modeling Platform" (PDF). Technical Support Document (TSD).
- ↑ "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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