Executive Summary#
The EMissions FACtor (EMFAC) model is California’s state-of-the-science emission inventory model designed to quantify emissions from on-road motor vehicles, including cars, trucks, and buses in California. EMFAC2025 is the latest iteration of EMFAC, building on the decades of on-road emissions inventory development efforts. The model will continue to support the California Air Resources Board’s (CARB) planning and policy development to improve air quality and reduce greenhouse gas emissions in California.
CARB released the first version of EMFAC2025, v2.0.0, on May 14, 2025. However, unlawful Congressional Resolutions adopted after the release of EMFAC2025 purported to disapprove the U.S. Environmental Protection Agency’s (U.S. EPA) actions to grant California’s Clean Air Act waiver requests for the following regulations: Advanced Clean Cars II (ACC II), Advanced Clean Trucks (ACT), Heavy-Duty Omnibus (Omnibus), Zero-Emission Airport Shuttle, and Heavy-Duty Vehicle and Engine Emission Warranty and Maintenance Provisions (Warranty Phase 1). While the congressional actions are being contested, EMFAC2025 Version 2.1.0 was released on March 4, 2026 to reflect the regulatory conditions, removing the emissions benefits of the challenged regulations. In addition, EMFAC2025 v2.1.0 removes the benefits of federal greenhouse gas (GHG) regulations that have been repealed, including the Heavy-Duty GHG Phase 1, Phase 2, and Phase 3 standards, as well as the Multi-Pollutant Emissions Standards for Model Years 2027 and Later Light-Duty and Medium-Duty Vehicles.
CARB staff have improved and updated the EMFAC Web Platform (https://arb.ca.gov/emfac/) to serve EMFAC2025 and previous versions back to EMFAC2017. The Web Platform, first introduced in 2021 with the release of EMFAC2021, provides all functionalities of the EMFAC PC application in an easy-to-use web interface. Users can access emissions inventories generated using EMFAC’s default vehicle activity, generate emissions inventories using custom activity inputs, and produce emission factors using user-defined ambient temperature and relative humidity for project-level conformity assessment. By relying on pre-generated data in a queuing system, the EMFAC Web Platform reduces the burden of processing computationally expensive EMFAC model runs for users.
This Technical Documentation provides a detailed account of major changes and updates in EMFAC2025 and summarizes differences from the previous version, EMFAC2021. For instructions on how to use EMFAC2025, including installation and navigation of the user interface, refer to the EMFAC2025 User’s Guide (CARB, 2025). Components, methodologies, data, and logic retained from prior versions are not covered in this document.
Structure of This Document#
The Introduction (Chapter 1) provides an overview of EMFAC2025, including major updates, features, architecture, and the web platform. Chapter 2 details new EMFAC2025 features: light-duty vehicles aged 45 years and older, high-speed driving, and the split of light heavy-duty trucks into public and other categories. Chapters 3 through 8 present comprehensive updates to vehicle populations, activity data, emission rates, fuel properties, and energy consumption in EMFAC2025. Chapter 9 describes new regulations and their implementation in EMFAC2025, along with California’s recent regulatory challenges posed by the federal government. Chapter 10 presents EMFAC2025 statewide results for vehicle populations, activity, and total emissions, with comparisons to EMFAC2021.
New Features#
To improve and refine California’s on-road mobile source emissions inventory and in response to stakeholder input, EMFAC2025 introduces several new features:
Light-Duty Vehicles of Age 45 Years and Older: EMFAC2025 introduces a new “Age45+” module to account for light-duty vehicles aged 45 years and older. Although these relatively small but old vehicles were not included in previous versions of EMFAC, they account for a sizable amount of emissions in EMFAC2025, as such vehicles are retained longer than expected and newer vehicles are much cleaner. The module uses DMV registration data, vehicle activity information from a survey CARB conducted among Age45+ vehicle owners, and EMFAC’s emission factors to integrate these old vehicles into both running-exhaust and start-emission calculations, improving the accuracy of light-duty inventories. See Section 2.1 for details.
High-Speed Driving: EMFAC2025 introduces a light-duty high-speed driving update that extends activity and emission factors up to 90 mph; previously, speeds were capped at 70 mph. Speed distributions of vehicle miles traveled (VMT) are also updated to better account for the high speeds using the national emissions inventory (NEI) dataset. New speed correction factors (SCF) for HC, NOx, CO, and CO2 are derived from dynamometer testing of in-use vehicles and applied to relevant light-duty technology groups. See Section 2.2 for details.
Light Heavy-Duty Trucks Split to Public and Other Categories: Beginning with EMFAC2025, light heavy-duty truck categories LHD1 and LHD2 are split into Public and Other. Public fleets are subject to State and Local Government Fleet requirements under the Advanced Clean Fleets regulation. See Section 2.3 for details.
Overview of Major Updates#
Fleet Characterization#
Light-Duty Fleet Characterization: EMFAC2025 updates light-duty fleet population and new sales using DMV registration data from calendar years 2000 through 2022 (compared to 2000 through 2019 in EMFAC2021).
Heavy-Duty Fleet Characterization: EMFAC2025 reflects updated vehicle population, new vehicle sales, and retention rates for heavy-duty fleets for calendar years 2020–2022. Primary data sources include processed DMV vehicle registration data, International Registration Plan (IRP) Clearinghouse data, and International Fuel Tax Agreement (IFTA) data. Additional sources include Automated License Plate Reader (ALPR) data for age distributions of out-of-state trucks, vehicle lists from major ports (T7 POLA and T7 POAK), California Highway Patrol (CHP) school bus inspection records, and the Truck Regulations Upload, Compliance, and Reporting System (TRUCRS) data for diesel Truck and Bus Rule exemptions.
Transit Buses and Natural Gas Vehicles: EMFAC2025 uses the same methodology as EMFAC2021 for transit bus and natural gas vehicle inventory development, with updated data.
Heavy-Duty Vehicle Miles Traveled Reallocation: EMFAC2025 reallocates heavy-duty VMT across sub-areas using telematics data and Caltrans Annual Average Daily Traffic (AADT) data, replacing older survey-based approaches. This process adjusts population distribution while keeping statewide totals fixed and is iterated until VMT per sub-area matches telematics-based percentages.
Vehicle Population and VMT Forecasting#
Accrual Rate Update: EMFAC2025 updates mileage accrual rates using 2001–2022 Bureau of Automotive Repair (BAR) Smog Check data by vehicle age, class, and region, assuming diesel rates equal to gasoline. Electric vehicle accruals are set at 70% of those for internal combustion vehicles in 2017, increasing linearly to 100% by 2025, as in EMFAC2021; however, the accruals vary by vehicle class and region in EMFAC2025.
Retention Rate Update: EMFAC2025 updates retention rates using 2005–2022 DMV data, excluding inconsistent years. Rates are computed from the proportion of vehicles retained by model year across consecutive calendar years, averaged by age, and fitted with regression curves.
Vehicle Start Adjustment: EMFAC2025 retains baseline light-duty vehicle starts from EMFAC2017 but applies an adjustment scalar to scale vehicle starts based on VMT per vehicle.
Light-Duty VMT and New Vehicle Sales Forecasting: EMFAC2025 uses the same forecasting scheme that has been used since EMFAC2014, with VMT projected under two regimes: a near-term forecast (2023–2027) using multivariate regression of socioeconomic variables, and a long-term forecast (2028–2050) assuming constant VMT per capita and applying human population growth rate. For new vehicle sales, EMFAC2025 employs a new modeling method of running a short-run multivariable regression (calendar year \(<\) 2025) and a long-run equilibrium model (calendar year \(\ge\) 2025), with results showing decreased forecasted sales compared to EMFAC2021 due to slower economic growth, lower forecasted human population, and stagnant VMT trends.
Zero-Emissions Vehicle Regional Allocation: EMFAC2025 incorporates projections from the California Energy Commission’s (CEC) 2025 Integrated Energy Policy Report (IEPR) and improves regional forecast accuracy using county-level Zero-Emission Vehicle (ZEV) market share projections from the Automobile and Technology Lifecycle-Based Assignment-Lite (ATLAS-Lite) model. Unlike EMFAC2021’s uniform share assumption across the State, ATLAS-Lite enables spatial disaggregation based on household vehicle choice, synthetic populations, and vehicle attributes. Results were validated against DMV data and show regional differences in early ZEV adoption.
Heavy-Duty Retention Rates, New Sales, and VMT Forecasting: EMFAC2025 updates heavy-duty retention rates by incorporating fleet size (\(\ge\)50 vs. \(<\)50 vehicles) using DMV and Dun & Bradstreet (D&B) data, capturing differences in scrappage patterns. New vehicle sales forecasting continues the EMFAC2021 method but now uses a 3-year average (2020–2022) to smooth out irregular purchase patterns. VMT forecasting also follows EMFAC2021 methodology.
Emission Rates#
Heavy-Duty Emission Rate Update: EMFAC2025 updates diesel heavy-duty vehicle emissions using chassis dynamometers and portable emission measurement systems (PEMS) data from the In-Use Testing Program for Heavy-Duty Diesel Engines and Vehicles (HDIUT), the Heavy-Duty Truck and Bus Surveillance Program (TBSP), and a 200-vehicle emission test study. Start emissions are re-identified using standardized criteria. Base emission rates and deterioration rates were updated using results from a combination of dynamometer and PEMS data. Speed correction factors are now based on detailed PEMS data by weight class and model year. A micro-trip method aggregates PEMS data into 5-mph bins to better reflect real-world running emissions. To update emission rates of medium heavy-duty vehicles, EMFAC2025 used additional in-use test data and excludes engines with corrective action through CARB’s in-use compliance program as of 2022. Natural gas (NG) heavy-duty vehicle emission rates were updated using reanalyzed PEMS data from the 200-vehicle study, applying new binning and micro-trip methods. The start emissions are included in running exhaust for NG heavy-duty vehicles.
Light-Duty Emission Rate Update: EMFAC2025 retains the methodology from EMFAC2021 to determine base emission rates (BER) using emission regimes (low, normal, moderate, and high) and regime fractions. LEV III groups without sufficient data use the Ratio of Standards (ROS) approach. BERs are based on U.S. EPA’s In-Use Vehicle Program (IUVP) and CARB’s Vehicle Surveillance Program (VSP) data. Light-duty HC, CO, and NOx SCFs were updated using new dynamometer data from arterial, freeway, and Unified Cycles. Best-fit equations were derived from data covering Pre-LEV, LEV I, LEV II, and LEV III vehicles.
Fuel Properties#
Reid Vapor Pressure Update: Reid vapor pressure (RVP) values in EMFAC2025 were updated using 2008-–2022 CARB service station data to reflect current fuel formulations.
Fuel Sulfur Content Update: EMFAC2025 updates fuel sulfur content using 2008-–2022 CARB service station data by RVP control region. Weighted averages yield 6.5 ppm for gasoline and 5.2 ppm for diesel, replacing the 15 ppm standard used in EMFAC2021.
Fuel Mix Update: EMFAC2025 updates fuel mix assumptions for gasoline and diesel using data from the Low Carbon Fuel Standard (LCFS) regulation, including renewable gasoline and renewable diesel to better account for GHG emissions.
Electric Vehicle Energy Consumption#
In EMFAC2021, heavy-duty ZEVs were assumed to be entirely battery electric vehicles (BEV). In EMFAC2025, the ZEV population is split into BEVs and fuel cell electric vehicles (FCEV) to support hydrogen demand assessment.
Other updates include revised BEV and plug-in hybrid electric vehicle (PHEV) energy consumption rates and electric VMT (eVMT) speed distributions using real-world data, resulting in higher energy consumption rates at low speeds and lower rates at high speeds compared to EMFAC2021. EMFAC2025 provides electricity (kWh) and hydrogen (kg) consumption estimates.
Regulations and Policies#
Policies and regulations covered in this version of EMFAC are listed below.
Clean Truck Check: Applying from 2023 to all on-road, non-gasoline heavy-duty vehicles over 14,000 lbs, Clean Truck Check requires owners to demonstrate that their emissions control systems are properly functioning, thereby reducing excess NOx and PM from poor maintenance or tampering.
Advanced Clean Fleets: EMFAC2025 reflects the Advanced Clean Fleets regulation’s state and local government fleet requirements, which phase in ZEVs for public fleets: 50% for model year 2024–2026 and 100% for model year 2027 and newer. EMFAC2025 also includes the requirement that 100% of California-certified truck sales be ZEVs beginning in 2036.
Federal Clean Trucks Plan: In 2022, U.S. EPA adopted the Clean Trucks Plan, setting stricter emission standards for federally certified heavy-duty engines over 10,000 lbs. starting in 2027. The regulation is expected to significantly reduce NOx and PM emissions.