Introduction

1. Introduction#

EMFAC2025 is the latest version of the California Air Resources Board’s (CARB) EMission FACtors (EMFAC) model. It provides state-of-the-science estimates of on-road motor vehicle emissions for regulatory and air quality planning in California. EMFAC quantifies emissions from all on-road motor vehicles in California, ranging from motorcycles and passenger cars to motorhomes and heavy-duty trucks. The model supports CARB’s efforts to control greenhouse gases, criteria pollutants, and air toxics. EMFAC serves the basis of CARB’s motor vehicle regulations and supports key programs such as the Scoping Plan and State Implementation Plans. The model is regularly updated to incorporate the latest data, regulations, and scientific advancements.

EMFAC is developed through rigorous data collection and analysis, including laboratory and on-road emissions testing, detailed vehicle population and activity studies, and integration of emerging “Big Data” sources. The model reflects the most current regulations from both California and the federal government. CARB collaborates with state agencies, Air Districts, Metropolitan Planning Organizations (MPO), community stakeholders, U.S. EPA, and researchers to ensure EMFAC remains accurate and relevant.

EMFAC is updated every three to four years to ensure it provides the most recent planning assumptions for transportation conformity determinations and State Implementation Plan (SIP) development, as required by the Clean Air Act (U.S. EPA and U.S. DOT, 2008). Each EMFAC version is submitted to the U.S. EPA for approval and is authorized for use in California-specific planning. EMFAC2025 builds on the advancements of EMFAC2014, EMFAC2017, and EMFAC2021, introducing updated methodologies, new data sources, and the latest regulatory requirements to improve emissions inventories for on-road vehicles statewide.

1.1. Major Updates and Features#

EMFAC2025 introduces several important updates and enhancements:

  • Improved accounting for emissions from vehicles aged 45 years and older, which, though a small population, contribute a significant share of emissions.

  • Enhanced estimates of vehicle miles traveled (VMT) and emissions at higher speeds, reflecting real-world driving behavior.

  • Updated emission rates and speed correction factors for both light-duty and heavy-duty vehicles by incorporating the latest in-use emissions testing data from chassis dynamometer and Portable Emissions Measurement System (PEMS) tests.

  • More granular modeling of Zero-Emission Vehicle (ZEV) adoption at the county level, with projections through 2035.

  • More sophisticated forecasting light-duty vehicle sales and vehicle activity, integrating recent socioeconomic, demographic, and regulatory data.

  • Use of Big Data sources, or large quantities of vehicle activity data from telematics providers, to better characterize heavy-duty truck operations and activity patterns.

  • Electric fuel types are now separated into Battery Electric and Fuel Cell Electric fuel types, with improved reporting of electricity consumptions for battery vehicles and hydrogen consumptions for fuel cell vehicles.

  • Refined methodologies for estimating VMT, vehicle population, and age distributions using the latest DMV registration and other relevant datasets.

  • Integration of new federal and state rulemakings, such as Clean Truck Check (CTC), Advanced Clean Fleets (ACF), and the Federal Clean Trucks Plan.

1.2. Modeling Architecture#

EMFAC2025 is provided as a PC application developed using a modular, Python and MySQL-based computational framework. Over the years, this framework has been substantially refined to improve model stability and leverage high performance computing resources, enabling efficient data processing, transparency, and flexibility for future updates. These enhancements allow users to generate on-road emissions inventories more efficiently, accommodating the increasing complexity of California’s vehicle fleet and regulatory environment.

1.3. EMFAC Features#

EMFAC2025 supports three main modes of operation to accommodate various user needs. Figure 1.1 provides an overview of these modes:

  • Default Activity Mode: Generates emissions inventories using CARB’s default activity data and methodologies.

  • Custom Activity Mode: Allows users to input project-specific activity data, including VMT from planning agencies, for conformity and SIP work.

  • Project-Level Assessment (PL) Mode: Supports detailed, project-specific emission rate analyses using user-defined meteorological and activity inputs.

        flowchart TD
   A["<b>New EMFAC Run</b>"] -->B["<b>Emissions Mode</b>"]
   A -->C["<b>Emission Rate Mode</b>"]
   B -->D["<b>Default Activity</b>"]
   D -->E["<b>Default Emissions Inventory</b>"]
   B -->F["<b>Custom Activity</b>"]
   F -->G["<b>Generate Template</b>"]
   G -->H["<b>Custom Activity Template</b>"]
   F -->I["<b>Load Custom Activity</b>"]
   H -->I
   I -->J["<b>Custom Emissions Inventory</b>"]
   C -->K["<b>Project-Level Assessment</b>"]
   K -->L["<b>Emission Rates</b>"]
   classDef default stroke-width:3px,fill:#ffffff
   classDef carbnavy color:#1b577d,stroke:#1b577d
   classDef carbblue color:#327cac,stroke:#327cac
   classDef carbgreen color:#2fa596,stroke:#2fa596
   classDef carbyellow color:#fcbc00,stroke:#fcbc00
   class A,B,C carbnavy
   class D,E carbblue
   class F,G,H,I,J carbgreen
   class K,L carbyellow

Figure 1.1: Workflow for EMFAC’s Three Main Modes: Default Activity, Custom Activity, and Project-Level Assessment#

1.4. EMFAC Web Platform#

With EMFAC2021, the EMFAC Web Platform (https://arb.ca.gov/emfac/) was introduced to provide EMFAC emissions inventories generated using the Default Activity Mode, along with the functionality of the Custom Activity Mode and Project-Level Assessment Mode, through an easy-to-use web interface. With EMFAC2025, the EMFAC Web Platform has been further refined to help users process large numbers of EMFAC runs more efficiently.

Because the EMFAC Web Platform provides the core features of the PC application, most users can generate EMFAC outputs more efficiently using the web platform without running the computationally expensive model locally. Users are encouraged to use the EMFAC Web Platform for EMFAC2025 as well as previous versions.

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