8.2 Off-Road Mobile Sources

Category ID	Description	EIC
1646	Agricultural Equipment - Diesel	87089312100000
1647	Lawn and Garden Equipment - Gasoline - 4 Stroke	86088311000040
1648	Lawn and Garden Equipment - Gasoline - 2 Stroke	86088311000020
1651	Lawn and Garden Equipment - Diesel	86088312100000
1652	Transportation Refrigeration Unit - Gasoline - 4 Stroke	86088411000040
1654	Transportation Refrigeration Unit - Diesel	86088412100000
1655	Agricultural Equipment - Gasoline - 4 Stroke	87089311000040
1657	Construction & Mining Equipment - Gasoline - 4 Stroke	86088711000040
1658	Construction & Mining Equipment - Gasoline - 2 Stroke	86088711000020
1661	Construction & Mining Equipment - Diesel	86088712100000
1662	Industrial Equipment - Gasoline - 4 Stroke	86088611000040
1663	Industrial Equipment - Gasoline - 2 Stroke	86088611000020
1666	Industrial Equipment - Diesel	86088612100000
1667	Industrial Equipment - Natural Gas	86088601100000
1668	Light Commercial Equipment - Gasoline - 4 Stroke	86088511000040
1669	Light Commercial Equipment - Gasoline - 2 Stroke	86088511000020
1672	Light Commercial Equipment - Diesel	86088512100000
1673	Light Commercial Equipment - Natural Gas	86088501100000
1674	Oil Drilling Equipment - Diesel	86089112100000
1675	Other Equipment - Diesel	86099512100000
1676	Recreational Boats - Gasoline - 4 Stroke	84086411000000
1677	Recreational Boats - Gasoline - 2 Stroke	84086411000020
1680	Recreational Boats - Diesel	84086412100000
1763	Off-Road Rec. Vehicles - Off-Road Motorcycles	85087211000000
1764	Off-Road Rec. Vehicles - All-Terrain Vehicles	85087411000000
1765	Off-Road Rec. Vehicles - Minibikes	85087311003452
1766	Off-Road Rec. Vehicles - Golf Carts	85087511002152
1767	Off-Road Rec. Vehicles - Specialty Vehicles	85087711005354
1768	Off-Road Rec. Vehicles - Snowmobiles	85087011000000
2355	Airport Ground Support Equipment - Diesel - San Francisco International	86088912100000
2356	Airport Ground Support Equipment - Gasoline - San Francisco International	86088911000040
2357	Airport Ground Support Equipment - Natural Gas - San Francisco International	86088901100000
2358	Airport Ground Support Equipment - Diesel - Oakland International	86088912100000
2359	Airport Ground Support Equipment - Gasoline - Oakland International	86088911000040
2360	Airport Ground Support Equipment - Natural Gas - Oakland International	86088901100000
2361	Airport Ground Support Equipment - Diesel - San Jose International	86088912100000
2362	Airport Ground Support Equipment - Gasoline - San Jose International	86088911000040
2363	Airport Ground Support Equipment - Natural Gas - San Jose International	86088901100000
2364	Airport Ground Support Equipment - Diesel - Travis AFB	86088912100000
2365	Airport Ground Support Equipment - Gasoline - Travis AFB	86088911000040
2366	Airport Ground Support Equipment - Natural Gas - Travis AFB	86088901100000
2627	Airport Ground Support Equipment - Diesel - Sonoma	86088912100000
2628	Airport Ground Support Equipment - Gasoline - Sonoma	86088911000040
2629	Airport Ground Support Equipment - Natural Gas - Sonoma	86088901100000
2630	Cargo Handling Equipment - Gasoline	86089611005237
2631	Cargo Handling Equipment - Natural Gas	86089601105157
2632	Cargo Handling Equipment - Diesel	86089612105220

Introduction

This document outlines the methodology for estimating greenhouse gas (GHG) emissions from off-road mobile sources in the San Francisco Bay Area (SFBA). Off-road internal combustion engines are major contributors to air pollution and are significant sources of GHG emissions. The California Air Resources Board (CARB) has developed an off-road vehicle emissions inventory model to estimate criteria pollutant and GHG emissions across all counties and air basins in California. This model is specifically designed to assess emissions from non-road vehicles, which are typically not used on public roads or highways.

Version 1.0.9 of the OFFROAD2021 model (OFFROAD, 2025), was released in January 2025 and features a comprehensive inventory of equipment spanning a broader range of categories. Off-road mobile sources refer to any equipment that operates primarily off public roadways and is powered by an internal combustion engine. These include machines used in construction, agriculture, recreation, landscaping, aviation support, and freight operations. CARB categorizes this equipment based on its operational use and fuel type (CARB, 2023a). These categories include, but are not limited to, industries and equipment such as:

• Lawn and Garden Equipment
• Transportation Refrigeration Units (TRUs)
• Light Commercial Equipment
• Off-Road Recreational Vehicles
• Recreational Boats
• Agricultural Equipment
• Airport Ground Support Equipment
• Off-Road Equipment (including Construction & Mining Equipment, Industrial Equipment, Oil Drilling, Forestry Equipment, Portable Equipment, and Cargo Handling Equipment).

The OFFROAD2021 inventory model represents a significant improvement over previous iterations of the model, incorporating actual population data for years that were previously forecasted, and refining estimation methods. These improvements include a more detailed analysis of fuel usage, accounting for factors such as load levels and equipment age. Moreover, for categories like Portable and Cargo Handling Equipment, OFFROAD2021 relies on data specific to California, offering a more accurate representation of the region compared to broader data sources like those from United States Environmental Protection Agency (USEPA).

Off-road equipment emits three greenhouse gases as a result of fuel combustion: carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). CO₂ is the primary GHG emitted from off-road equipment and is released directly during the combustion of fossil fuels (e.g., gasoline or diesel). CO₂ emissions are proportional to the amount of fuel burned and are calculated using fuel-specific carbon content factors.

CH₄ is emitted in small quantities from incomplete combustion. N₂O can also be produced during combustion, particularly in high-temperature conditions associated with lean-burn engines or engines equipped with catalytic converters.

Methodology

CARB Sources

The methodology used to estimate GHG emissions from off-road mobile sources in the SFBA is based on data developed by CARB using its OFFROAD2021 emissions inventory model. These emissions are classified as area sources, meaning they are not tied to specific permitted facilities but instead represent distributed sources of pollution across the region.

The OFFROAD2021 model provides CO₂ emission estimates by equipment type, engine type (e.g., spark-ignition or compression-ignition), fuel type (e.g., diesel, gasoline), and geographic region. While the OFFROAD2021 model does not directly estimate CH₄ or N₂O emissions, these pollutants are calculated using supplemental methods, as described in the Emissions Apportionment section below. The Bay Area Air Quality Management District (BAAQMD or Air District) incorporates this data into its regional emissions inventory by aligning it with local source categories. All off-road equipment categories included in this methodology are summarized in the following table.

Off-Road Mobile Source Subsectors	Off-Road Mobile Source Categories
Lawn and Garden Equipment	Category #1647 (4-Stroke Gasoline Equipment) Category #1648 (2-Stroke Gasoline Equipment) Category #1651 (Diesel Equipment)
Transportation Refrigeration Units (TRUs)	Category #1652 (4-Stroke Gasoline Equipment) Category #1654 (Diesel Equipment)
Agricultural Equipment	Category #1655 (4-Stroke Gasoline Equipment) Category #1646 (Diesel Equipment)
Construction & Mining Equipment	Category #1657 (4-Stroke Gasoline Equipment) Category #1658 (2-Stroke Gasoline Equipment) Category #1661 (Diesel Equipment)
Industrial Equipment	Category #1662 (4-Stroke Gasoline Equipment) Category #1663 (2-Stroke Gasoline Equipment) Category #1666 (Diesel Equipment) Category #1667 (Natural Gas Equipment)
Light Commercial Equipment	Category #1668 (4-Stroke Gasoline Equipment) Category #1669 (2-Stroke Gasoline Equipment) Category #1672 (Diesel Equipment) Category #1673 (Natural Gas Equipment)
Oil Drilling Equipment	Category #1674 (Diesel Equipment)
Off-Road Recreational Vehicles	Category #1763 (Offroad Motorcycles) Category #1764 (All-Terrain Vehicles) Category #1765 (Minibikes) Category #1767 (Specialty Vehicles) Category #1768 (Snowmobiles)
Recreational Boats	Category #1676 (4-Stroke Gasoline Equipment) Category #1677 (2-Stroke Gasoline Equipment) Category #1680 (Diesel Equipment)
Airport Ground Support Equipment	Category #2355, 2358, 2361, 2364, 2627 (Diesel Equipment) Category #2356, 2359, 2362, 2365, 2628 (Gasoline Equipment) Category #2357, 2360, 2363, 2366, 2629 (Natural Gas Equipment) Note: categories are split by the airport the ground support equipment operates at
Cargo Handling Equipment	Category #2630 (Gasoline) Category #2631 (Natural Gas) Category #2632 (Diesel)
Other Offroad Equipment (Portable & Forestry Equipment)	Category #1675 (Diesel)

CARB deploys a general methodology (CARB, 2020a) to calculate CO₂ emissions for the base year(s) of off-road mobile source categories, which typically involves the following steps to calculate county-level emissions for each pollutant and off-road source category:

Base Year(s) Emissions _{category,county,pollutant} =

Population_category × Activity_category × Emission Factor_{category,pollutant} × Fraction_county × Correction Factors_category × GWP _pollutant

Where:

• Base Year: is a year for which emissions data is directly reported by CARB and available.
• Population_category: is an estimate of the total number of equipment units operating in a specific category.
• Activity_category: is an estimate of the annual usage rates (e.g., hours of operation per year) for equipment operating in a specific category.
• Emission Factor_{category,pollutant}: is a factor that allocates an amount of emissions, in mass, of a particular pollutant by unit of activity data. The emission factor in this equation is specific to the equipment type(s) operating in a specific category.
• Fraction_county : is the fraction of total regional emissions (between 0 and 1) estimated to be allocated to a particular county.
• Correction Factors_category: are additional factors that may be applied to account for differences related to fuel properties, load factors, geographic areas, environment conditions (temperature and humidity), and to reflect regulations in each source category.
  • • Fuel Usage: Incorporates data on fuel consumption.
    • Load Factors: These account for average engine load during operation.
    • Geographic Allocation: Distributes emissions spatially based on equipment usage patterns across regions.
    • Temporal Allocation: Accounts for seasonal and time-of-day variations in equipment use.
• GWP_pollutant is the Global Warming Potential of a particular GHG pollutant. The current version of the GHG emissions inventory incorporates the global warming potential (GWP) reported in the Fifth Assessment report of the Intergovernmental Panel for Climate Change (IPCC, 2014). The GWPs for the three principal GHGs are 1 for carbon dioxide (CO₂), 34 for methane (CH₄), and 298 for nitrous oxide (N₂O), when calculated on a 100-year basis with climate-carbon feedback included.

GHG emissions data for off-road mobile sources in the SFBA are extracted from the CARB OFFROAD2021 emissions inventory model. This model provides comprehensive estimates of CO₂ emissions from various types of non-road equipment. It is important to note that the OFFROAD2021 model only reports CO₂ emissions. It does not include CH₄ or N₂O emissions. To address this gap, the Air District applies additional methods to estimate CH₄ and N₂O emissions. These methods are described in detail in the Emissions Apportionment section of this document.

Once base year emissions are determined, historical backcasting and forecasting of emissions relative to the base year emissions are estimated using growth profiles as follows:

Current Year Emissions _county = Base Year(s) Emission _county × Growth Factor

Where:

• Growth Factor: is a scaling factor that is used to derive historical emissions estimates for years for which activity data and/or emissions are not reported (or available), and to forecast emissions for future years, using surrogates that are representative of activity and/or emissions trends for a specific source category.

The Growth Factor represents the estimated change in the number of off-road equipment units over time and is developed by CARB using a combination of data sources and modeling techniques, including:

• Equipment survey data collected in 2001, 2012, and 2018 to understand how different types of off-road equipment are used and how their populations have changed.
• Age distribution data for each equipment type, which CARB uses to estimate how long equipment remains in use. These estimates are often referred to as equipment lifespan models or retirement curves, and they help determine how many units from each model year are still operating in any given year.
• First-year sales and shipment data, drawn from CARB’s Production Line Testing program and 2018 Small Off-Road Engine (SORE) evaporative production volume reports (CARB, 2020a).
• Economic indicators, such as projected growth in household numbers or construction activity, which are used to forecast future demand for specific equipment types.

The OFFROAD2021 model provides emissions estimates spanning the years 2000 through 2050, encompassing both historical and projected data. All applicable regulatory controls (e.g., emission standards, fleet turnover requirements) and technological assumptions are embedded within the model’s framework, ensuring consistency with CARB’s statewide emissions inventory and regulatory planning processes.

Emissions Apportionment

CH₄ and N₂O Emissions

Although the OFFROAD2021 model provides estimates for CO₂ emissions, it does not include data for CH₄ and N₂O. To develop a complete GHG emissions profile for off-road mobile sources, the Air District supplements OFFROAD2021 data using an apportionment method based on historical emissions ratios.

This method uses CH₄-to-CO₂ and N₂O-to-CO₂ emission ratios derived from CARB’s earlier OFFROAD2007 model (CARB, 2017), which includes emissions estimates for CH₄ and N₂O. These ratios represent the typical amount of CH₄ and N₂O emitted per unit of CO₂ for specific equipment categories and fuel types. The Air District applies these ratios to the CO₂ emissions from OFFROAD2021 to estimate CH₄ and N₂O as follows:

• CH₄ = CO₂_{, OFFROAD2021} × (CH₄ / CO₂)_OFFROAD2007
• N₂O = CO₂, _OFFROAD2021 × (N₂O / CO₂)_OFFROAD2007

Where:

• CO₂_{, OFFROAD2021} = annual CO₂ emissions from OFFROAD2021 for a given equipment category and fuel type
• CH₄ / CO₂ and N₂O / CO₂ ratios = emission factors derived from OFFROAD2007 projections for the year 2022 for the same equipment category and fuel type

This approach assumes that the relationship between CO₂ and the other GHGs (CH₄ and N₂O) has remained relatively stable over time and can be reasonably applied to the updated CO₂ estimates from OFFROAD2021. While this introduces some uncertainty, it provides a practical and transparent method for estimating CH₄ and N₂O emissions where direct data are not available. By applying this method, the Air District is able to maintain consistency in estimating total GHG emissions across all major off-road mobile source categories.

County Fractions

The OFFROAD2021 model provides CO₂ emissions data at the county level across California, including all counties within the SFBA. To estimate regional GHG emissions from off-road mobile sources, the Air District uses this county-specific data to allocate emissions accurately across its jurisdiction.

The Air District includes all or portions of nine counties. For counties that are only partially within the Air District’s jurisdiction—specifically Sonoma and Solano—emissions are proportionally apportioned to reflect only the areas that fall within the District’s official boundaries. This ensures that regional totals represent emissions occurring within the SFBA, rather than the entire county.

To achieve consistency and accuracy in geographic coverage, CARB has confirmed that users should select the “Sub-Area” option within the OFFROAD2021 model when extracting data for the Air District. The “Sub-Area” setting returns emissions estimates that are pre-adjusted to reflect only the portions of each county that fall within the Air District’s jurisdictional boundaries.

ID	Description	ALA	CC	MAR	NAP	SF	SM	SNC	SOL	SON
1646	Agricultural Equipment - Diesel	0.04	0.07	0.05	0.35	0.00	0.04	0.10	0.03	0.33
1647	Lawn and Garden Equipment - Gasoline - 4 Stroke	0.22	0.14	0.04	0.02	0.14	0.10	0.24	0.04	0.07
1648	Lawn and Garden Equipment - Gasoline - 2 Stroke	0.22	0.14	0.04	0.02	0.13	0.10	0.25	0.04	0.06
1651	Lawn and Garden Equipment - Diesel	0.22	0.14	0.04	0.02	0.14	0.10	0.24	0.04	0.06
1652	Transportation Refrigeration Unit - Gasoline - 4 Stroke	0.23	0.15	0.04	0.03	0.10	0.10	0.24	0.04	0.08
1654	Transportation Refrigeration Unit - Diesel	0.46	0.14	0.02	0.02	0.01	0.02	0.23	0.07	0.02
1655	Agricultural Equipment - Gasoline - 4 Stroke	0.04	0.06	0.04	0.38	0.00	0.03	0.11	0.02	0.33
1657	Construction & Mining Equipment - Gasoline - 4 Stroke	0.20	0.16	0.04	0.02	0.16	0.10	0.21	0.04	0.07
1658	Construction & Mining Equipment - Gasoline - 2 Stroke	0.20	0.17	0.04	0.02	0.16	0.10	0.22	0.04	0.07
1661	Construction & Mining Equipment - Diesel	0.19	0.29	0.01	0.05	0.01	0.03	0.25	0.09	0.09
1662	Industrial Equipment - Gasoline - 4 Stroke	0.17	0.05	0.01	0.01	0.09	0.07	0.55	0.01	0.04
1663	Industrial Equipment - Gasoline - 2 Stroke	0.16	0.05	0.01	0.01	0.08	0.07	0.55	0.01	0.04
1666	Industrial Equipment - Diesel	0.21	0.07	0.01	0.04	0.04	0.07	0.49	0.02	0.06
1667	Industrial Equipment - Natural Gas	0.17	0.05	0.01	0.01	0.09	0.07	0.55	0.01	0.04
1668	Light Commercial Equipment - Gasoline - 4 Stroke	0.22	0.13	0.04	0.02	0.15	0.10	0.24	0.03	0.06
1669	Light Commercial Equipment - Gasoline - 2 Stroke	0.22	0.14	0.04	0.02	0.14	0.10	0.24	0.04	0.06
1672	Light Commercial Equipment - Diesel	0.21	0.12	0.05	0.02	0.17	0.11	0.23	0.03	0.06
1673	Light Commercial Equipment - Natural Gas	0.21	0.11	0.05	0.02	0.18	0.11	0.23	0.03	0.05
1674	Oil Drilling Equipment - Diesel	0.03	0.01	0.01	0.01	0.01	0.01	0.01	0.92	0.01
1675	Other Equipment - Diesel	0.20	0.22	0.01	0.03	0.07	0.07	0.29	0.05	0.06
1763	Off-Road Rec. Vehicles - Off-Road Motorcycles	0.00	0.00	0.02	0.37	0.00	0.00	0.02	0.58	0.00
1764	Off-Road Rec. Vehicles - All-Terrain Vehicles	0.46	0.03	0.16	0.01	0.00	0.01	0.07	0.03	0.22
1766	Off-Road Rec. Vehicles - Golf Carts	0.14	0.18	0.03	0.08	0.02	0.06	0.17	0.14	0.19
2355	Airport Ground Support Equipment - Diesel - San Francisco International	0.00	0.00	0.00	0.00	0.00	1.00	0.00	0.00	0.00
2356	Airport Ground Support Equipment - Gasoline - San Francisco International	0.00	0.00	0.00	0.00	0.00	1.00	0.00	0.00	0.00
2357	Airport Ground Support Equipment - Natural Gas - San Francisco International	0.00	0.00	0.00	0.00	0.00	1.00	0.00	0.00	0.00
2358	Airport Ground Support Equipment - Diesel - Oakland International	1.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
2359	Airport Ground Support Equipment - Gasoline - Oakland International	1.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
2360	Airport Ground Support Equipment - Natural Gas - Oakland International	1.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
2361	Airport Ground Support Equipment - Diesel - San Jose International	0.00	0.00	0.00	0.00	0.00	0.00	1.00	0.00	0.00
2362	Airport Ground Support Equipment - Gasoline - San Jose International	0.00	0.00	0.00	0.00	0.00	0.00	1.00	0.00	0.00
2363	Airport Ground Support Equipment - Natural Gas - San Jose International	0.00	0.00	0.00	0.00	0.00	0.00	1.00	0.00	0.00
2364	Airport Ground Support Equipment - Diesel - Travis AFB	0.00	0.00	0.00	0.00	0.00	0.00	0.00	1.00	0.00
2365	Airport Ground Support Equipment - Gasoline - Travis AFB	0.00	0.00	0.00	0.00	0.00	0.00	0.00	1.00	0.00
2366	Airport Ground Support Equipment - Natural Gas - Travis AFB	0.00	0.00	0.00	0.00	0.00	0.00	0.00	1.00	0.00
2627	Airport Ground Support Equipment - Diesel - Sonoma	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	1.00
2628	Airport Ground Support Equipment - Gasoline - Sonoma	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	1.00
2629	Airport Ground Support Equipment - Natural Gas - Sonoma	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	1.00
2630	Cargo Handling Equipment - Gasoline	1.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
2631	Cargo Handling Equipment - Natural Gas	1.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
2632	Cargo Handling Equipment - Diesel	0.81	0.09	0.00	0.00	0.04	0.06	0.00	0.00	0.00

Emission Control

Emissions from off-road mobile sources are regulated at the federal and state levels, with USEPA and CARB leading implementation. Although the Air District does not impose additional off-road controls, its planning and incentive programs support the broader regulatory framework.

Federal Regulations – USEPA

At the federal level, the Nonroad Diesel Engine Emission Standards (Tier 1–4), enacted under the Clean Air Act and phased in from 1996 through 2015, reduce emissions of nitrogen oxides (NO_x), particulate matter (PM), hydrocarbons (HC), and carbon monoxide (CO) from nonroad diesel engines. These standards improve engine efficiency and fuel combustion, indirectly reducing CO₂ emissions (USEPA, 2004). In addition, the Renewable Fuel Standard promotes the use of lower-carbon fuels such as biodiesel, which can further reduce the GHG emissions of diesel-powered equipment (USEPA, 2023).

State Regulations – CARB

CARB has implemented several key regulations that directly or indirectly drive down GHG emissions from off-road mobile sources:

• Regulation for In-Use Off-Road Diesel-Fueled Fleets
  • Applicability: This regulation applies to diesel-powered off-road vehicles with 25 horsepower (hp) or more used in California. It covers a wide range of equipment—from small skid steer loaders used in residential landscaping to heavy-duty construction and mining equipment such as dozers, forklifts, cranes, and excavators (CARB, 2023b).
  • Emission Reduction Strategies: Fleets are required to reduce emissions by retiring older, high-emission vehicles, replacing them with newer, more fuel-efficient models, re-powering engines to meet cleaner standards, or installing verified diesel emission control systems. Although designed primarily to reduce pollutants like NO_x and PM, these strategies yield additional benefits by lowering CO₂ emissions per unit of output.
  • Amendments: On November 17, 2022, CARB adopted amendments to this regulation to accelerate emission reductions by tightening compliance deadlines, removing credit provisions for low-use vehicles, and requiring all fleets to phase out the use of Tier 0 and Tier 1 engines by specific milestone years. These updated requirements began taking effect in January 2024 (CARB, 2023c).
• Small Off-Road Engine (SORE) Regulation

In December 1990, CARB adopted two levels of emission standards for small off-road engines.

• • Tier 1 Standards (Implemented in 1995)
    • Applied to newly manufactured off-road diesel engines with power ratings from 25 to 75 hp, with the goal of reducing emissions from older, less efficient engines (CARB, 2020b).
  • Tier 2 Standards (Effective 1999)
    • These standards further tightened emissions limits for a broader range of engine sizes, covering engines from 25 to 175 hp.
    • Achieving these limits required significant improvements in engine technology, such as enhanced fuel injection systems, better exhaust aftertreatment, and more efficient combustion processes (CARB, 2020b).

By driving fleet modernization, encouraging repowering, and necessitating improved engine technologies, these CARB regulations reduce GHG emissions by improving fuel efficiency and reducing fuel consumption across the off-road fleet.

Local Support – Air District

While the Air District does not directly regulate off-road mobile sources, it promotes compliance and accelerates emissions reductions through programs (BAAQMD, 2024) such as:

• Incentive Programs: Grants and financing options (e.g., the Carl Moyer Program and Climate Tech Finance) help fleet operators replace older equipment with zero- or near-zero-emission alternatives.
• Regional Planning: The Air District incorporates CARB regulations and regulatory impacts into its regional emissions inventories and climate action plans, ensuring that policy development reflects the latest advancements in emissions control.

Historical Emissions / Backcast

Emissions from off-road mobile sources in the San Francisco Bay Area for the years 1990 through 1999 are derived from CARB’s OFFROAD2007 model (CARB, 2017), which provided the earliest available estimates for GHG emissions from this category. For the years 2000 through 2021, estimates are based on the CARB OFFROAD2021 model, which incorporates substantial methodological updates and improved equipment population data.

Future Projections / Growth Factor

Projected emissions from off-road mobile sources are obtained from OFFROAD2021. CARB develops future year emissions estimates using growth factors that reflect changes in equipment population and usage over time based on survey data, economic indicators, and assumptions about fleet turnover.

Assessment of Methodology

To develop the GHG emissions inventory for off-road mobile sources, emissions estimates were derived using the most current version of the CARB OFFROAD model available at the time of inventory development (v1.0.9). The table below outlines the primary steps and references used in each base year inventory.

Base Year	Revision	Reference
2022	Retrieved county-level CO₂ emissions for years 2000-2050 from CARB’s OFFROAD2021 model (v1.0.9), using the "Sub-Area" region filter to isolate the BAAQMD portion of Solano and Sonoma counties. Estimated CH₄ and N₂O emissions using CH₄/CO₂ and N₂O/CO₂ emission ratios from the OFFROAD2007 model. Applied linear regression to CO₂ emissions from OFFROAD2021 for 2000–2022 to estimate emissions for earlier years (1990–1999). Global Warming Potential (GWP) taken from the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC, 2014)	CARB OFFROAD2021 v1.0.9 (OFFROAD, 2025) CARB, 2017 CARB, 2017 IPCC, 2014
2015	Retrieved county-level GHG emissions for the years 1990-2040 from CARB’s OFFROAD2007 model. Applied regional projections from BA-CALCAPS (Bay Area California Climate Action Planning Scenario Tool). Global Warming Potential (GWP) taken from the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC, 2014)	CARB, 2017 BAAQMD, 2017 IPCC, 2014

Sample Calculations

The table below shows a sample calculation for estimating emissions from Off-Road Mobile Lawn and Garden gasoline equipment (Categories #1647, #1648) in Alameda County for the base year 2022. Emissions are in units of metric tons of CO₂ equivalents (MTCO₂eq).

		CO2	CH4	N2O
Step 1	Obtain 2022 emissions from CARB OFFROAD 2007 for lawn and garden gasoline equipment in Alameda County (tons/day)	164.1	0.422	0.178
Step 2	IPCC AR5 GWP (Global Warming Potential) values	1	34	298
Step 3	Apply GWP and calculate CARB OFFROAD 2007 emissions in MTCO2eq	164.1 tons/day × 365 days/year × 0.9072 MT/ton × 1 = 54,338 MTCO2eq	0.422 tons/day × 365 days/year × 0.9072 MT/ton × 34 = 4,729 MTCO2eq	0.178 tons/day × 365 days/year × 0.9072 MT/ton × 298 = 17,574 MTCO2eq
Step 4	Calculate CARB OFFROAD 2007 pollutant-specific ratios using CO2 as basis	1	4,729 ÷ 54,338 = 0.0870	17,574 ÷ 54,338 = 0.3234
Step 5	Obtain 2022 emissions from CARB OFFROAD 2021 for lawn and garden gasoline equipment in Alameda County (tons/day)	78.16	NA	NA
Step 6	Apply pollutant-specific ratios to calculate 2022 emissions for lawn and garden gasoline equipment in Alameda County (MTCO2eq)	78.1635 tons/day × 365 days/year × 0.9072 MT/ton = 25,881 MTCO2eq	25,881 MTCO2eq × 0.0875 = 2,252 MTCO2eq	25,881 MTCO2eq × 0.3234 = 8,370 MTCO2eq

Emissions

The detailed breakdown of 2022 Off-Road Mobile Sources subsector GHG emissions in units of MTCO₂eq are shown in the table below.

ID	Description	CH4	CO2	N2O	Total
1675	Other Equipment - Diesel	1124.5	322420.3	0.0	323544.8
1661	Construction & Mining Equipment - Diesel	1053.6	240040.8	0.0	241094.4
1667	Industrial Equipment - Natural Gas	10864.9	193452.3	0.0	204317.2
1668	Light Commercial Equipment - Gasoline - 4 Stroke	3734.7	141258.6	25196.4	170189.7
1662	Industrial Equipment - Gasoline - 4 Stroke	993.7	134143.4	13253.0	148390.1
1647	Lawn and Garden Equipment - Gasoline - 4 Stroke	2806.4	98661.7	28883.1	130351.2
1646	Agricultural Equipment - Diesel	769.2	128780.8	0.0	129550.0
1654	Transportation Refrigeration Unit - Diesel	2226.1	107981.5	0.0	110207.6
1666	Industrial Equipment - Diesel	360.3	68886.4	0.0	69246.7
2632	Cargo Handling Equipment - Diesel	2.2	54230.1	3.2	54235.5
2356	Airport Ground Support Equipment - Gasoline - San Francisco International	17.5	35971.0	9.6	35998.1
1648	Lawn and Garden Equipment - Gasoline - 2 Stroke	5550.0	19080.6	8190.1	32820.7
1672	Light Commercial Equipment - Diesel	142.8	31908.2	0.0	32051.0
1764	Off-Road Rec. Vehicles - All-Terrain Vehicles	2171.3	12772.1	9515.3	24458.7
1673	Light Commercial Equipment - Natural Gas	440.7	19601.6	0.0	20042.3
2355	Airport Ground Support Equipment - Diesel - San Francisco International	1.1	11498.1	1.0	11500.2
1657	Construction & Mining Equipment - Gasoline - 4 Stroke	155.4	6982.7	1407.2	8545.3
2359	Airport Ground Support Equipment - Gasoline - Oakland International	4.0	8267.8	2.2	8274.0
1763	Off-Road Rec. Vehicles - Off-Road Motorcycles	1487.1	3644.7	2172.5	7304.3
2362	Airport Ground Support Equipment - Gasoline - San Jose International	3.0	6212.3	1.7	6217.0
1655	Agricultural Equipment - Gasoline - 4 Stroke	115.4	3502.8	625.7	4243.9
1651	Lawn and Garden Equipment - Diesel	14.1	4028.4	0.0	4042.5
2630	Cargo Handling Equipment - Gasoline	3.1	3848.2	0.2	3851.5
1674	Oil Drilling Equipment - Diesel	0.0	3406.1	0.0	3406.1
2357	Airport Ground Support Equipment - Natural Gas - San Francisco International	0.2	3180.3	0.3	3180.8
1669	Light Commercial Equipment - Gasoline - 2 Stroke	113.2	2048.3	744.4	2905.9
2358	Airport Ground Support Equipment - Diesel - Oakland International	0.3	2642.7	0.2	2643.2
1652	Transportation Refrigeration Unit - Gasoline - 4 Stroke	36.2	1654.4	397.4	2088.0
2361	Airport Ground Support Equipment - Diesel - San Jose International	0.2	1985.7	0.2	1986.1
1766	Off-Road Rec. Vehicles - Golf Carts	265.8	481.6	102.6	850.0
2631	Cargo Handling Equipment - Natural Gas	0.1	746.1	0.1	746.3
2360	Airport Ground Support Equipment - Natural Gas - Oakland International	0.1	731.0	0.1	731.2
2363	Airport Ground Support Equipment - Natural Gas - San Jose International	0.0	549.3	0.1	549.4
2365	Airport Ground Support Equipment - Gasoline - Travis AFB	0.1	235.2	0.1	235.4
1663	Industrial Equipment - Gasoline - 2 Stroke	17.7	209.9	0.0	227.6
1658	Construction & Mining Equipment - Gasoline - 2 Stroke	0.0	92.0	0.0	92.0
2364	Airport Ground Support Equipment - Diesel - Travis AFB	0.0	75.2	0.0	75.2
2628	Airport Ground Support Equipment - Gasoline - Sonoma	0.0	52.6	0.0	52.6
2366	Airport Ground Support Equipment - Natural Gas - Travis AFB	0.0	20.8	0.0	20.8
2627	Airport Ground Support Equipment - Diesel - Sonoma	0.0	14.2	0.0	14.2
2629	Airport Ground Support Equipment - Natural Gas - Sonoma	0.0	4.7	0.0	4.7

Summary of Base Year 2022 Emissions

The relative contribution of off-road mobile source GHG emissions to region-wide and sector-level GHG emissions totals are highlighted in the table below.

Contribution of Off-Road Mobile Sources Emissions by Sector

Subsector	Sector	Subsector GHG Emissions (MMTCO2eq)	Sector GHG Emissions (MMTCO2eq)	% of Sector
Off-Road Mobile Sources	Agriculture	0.13	1.26	10.60%
Off-Road Mobile Sources	Commercial + Residential	0.63	12.85	4.91%
Off-Road Mobile Sources	Industrial	1.06	17.90	5.91%
Off-Road Mobile Sources	Transportation	0.11	22.60	0.50%

Contribution of Off-Road Mobile Sources Emissions to Regional Total

Subsector	Subsector GHG Emissions (MMTCO2eq)	Regional Total GHG Emissions (MMTCO2eq)	% of Regional Total
Off-Road Mobile Sources	1.93	65.68	2.95%

Trends

The emission trends from 1990 to 2050 for different Off-Road Mobile source subsectors are presented in the figure below.

Summary of Trends

GHG emissions from off-road mobile sources in the SFBA are projected to decline steadily between 2023 and 2050. The anticipated decline in emissions is primarily due to CARB regulations targeting off-road diesel engines, such as the In-Use Off-Road Diesel-Fueled Fleets Regulation and Zero-Emission Off-Road Forklift Regulation. These policies aim to reduce emissions by requiring fleet operators to:

• Retire older, higher-emitting equipment,
• Retrofit engines with exhaust after-treatment technologies (e.g., diesel particulate filters),
• Replace combustion equipment with newer, cleaner-burning engines, or
• Transition to zero-emission equipment (e.g., battery-electric).

The construction and industrial sectors are expected to show the most significant reductions due to relatively fast equipment turnover rates (i.e., how quickly older equipment is replaced with newer models) and strong regulatory enforcement.

However, emission trends vary significantly by equipment category:

• Forklifts powered by gasoline or liquefied petroleum gas (LPG) may experience a slower decline due to longer lifespans and higher upfront costs of electric alternatives. While CARB mandates a phase-out of internal combustion forklifts for certain users by 2035, full turnover may lag.
• Portable equipment, such as diesel-powered generators or pumps, may remain in service longer due to their relatively low usage hours and fewer direct regulatory requirements, resulting in slower emission reductions.
• Gasoline-fueled cargo handling equipment used at airports or distribution centers may see temporary increases in emissions if usage grows faster than turnover or regulatory compliance.

Uncertainties

One major source of uncertainty arises from the off-road equipment activity data used in the OFFROAD2021 model. These models estimate equipment populations, activity levels (e.g., hours of operation), and fuel usage based on statewide surveys, registration records, and other indirect sources. While the models incorporate region-specific surrogates to allocate emissions geographically (e.g., construction spending, agricultural output), they do not fully capture local variations in equipment usage patterns or fleet turnover rates.

Additional uncertainty arises from the method used to estimate CH₄ and N₂O emissions, which are not directly provided by OFFROAD2021. To produce a complete GHG profile, the Air District applies CH₄-to-CO₂ and N₂O-to-CO₂ emission ratios from CARB’s OFFROAD2007 model to the CO₂ emissions from OFFROAD2021. This approach assumes that the historical ratios from 2007 remain applicable to more recent data, despite potential shifts in technology, emission control standards, and fleet composition between the two model versions. If these ratios no longer reflect current equipment characteristics, the resulting CH₄ and N₂O estimates may be biased. This introduces some uncertainty into the totals for non-CO₂ GHGs, though their contribution to overall GHG emissions is relatively small compared to CO₂.

Finally, there is some uncertainty related to fuel-specific assumptions, particularly for non-diesel fuels such as LPG, CNG, and electricity. The OFFROAD models rely on fuel-type distributions that may not be current or representative of all sectors, especially as zero-emission equipment becomes more prevalent. However, due to the relatively small contribution of alternative fuel types to total off-road GHG emissions, this uncertainty is considered secondary to those associated with activity and emission factors.

Contact

Author: Michael Nguyen

Reviewer: Abhinav Guha

Last Update: 06/26/2025

References

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