How does MOVES Calculate CO2 and CO2 Equivalent Emissions?

MOVES calculates the energy it takes to operate an onroad vehicle based on energy consumption rates as explained in the technical reports Greenhouse Gas and Energy Consumption Rates for Onroad Vehicles MOVES3 (pdf) (November 2020, EPA-420-R-20-015) and Exhaust Emission Rates for Heavy-Duty Onroad Vehicles in MOVES3 (pdf) (November 2020, EPA-420-R-20-018).

Then using the fuel carbon content, MOVES calculates the CO₂ emitted from burning the fuel in the vehicle. The CO₂ equation used by MOVES and the carbon content of each fuel are documented in Section 4.1 of the Greenhouse Gas and Energy Consumption Rates for Onroad Vehicles MOVES3. MOVES estimates only the direct emissions from the vehicle and does not model the upstream emissions or energy consumption from producing the fuel.

While small portion of the fuel used in the vehicle is initially released to the atmosphere as unburned fuel or as carbon monoxide (CO), we assume that eventually all of the released carbon will convert to CO₂ in the atmosphere (i.e., the “oxidation fraction” equals one). We refer to this CO₂ as "atmospheric" and calculate it based on the total carbon in the fuel.

In addition, MOVES models onroad vehicle emissions of methane (CH4) and nitrous oxide (N₂O), which also contribute to climate change. MOVES does not estimate hydrofluorocarbon (HFC) emissions. If selected, MOVES will estimate total "CO₂ equivalent" emissions for onroad vehicles by multiplying CH₄ and N₂O emissions by their global warming potential (GWP) and adding them to the atmospheric CO₂ estimate to calculate the CO₂ equivalent. In MOVES:

CO₂ has a GWP value of 1 (the reference).
Methane (CH₄) has a GWP value of 25.
Nitrous oxide (N₂O) has a GWP value of 298.

Note, these GWP values have not been updated recently. For more information on GWPs, see EPA’s website, Understanding Global Warming Potentials. The equation and GWP values used by MOVES to calculate CO₂ equivalent emissions are documented in Section 4.2 of the Greenhouse Gas and Energy Consumption Rates for Onroad Vehicles MOVES3.

For nonroad equipment, fuel consumption is modelled as “Brake Specific Fuel Consumption” (BSFC). The input for this value is in lbs/bhp-hr, but engine population and activity are applied within MOVES, so MOVES outputs the total mass of fuel per day for the given engine category in the nation or selected county. Whether this is grams, kilograms, pounds, or U.S. tons will depend on the units selected on the “General Output” panel.

Nonroad atmospheric CO₂ is computed from BSFC in the same way as onroad CO₂ is computed from onroad fuel consumption. Methane emissions are modeled as a function of total hydrocarbons. However, nitrous oxide and CO₂ equivalent are not modelled for nonroad equipment. If desired, the nonroad CO2 equivalent methane emissions may be estimated outside the model by multiplying the CH₄ output by its GWP.