5.3. N₂O Emissions Update

N₂O emissions for medium heavy-duty and heavy heavy-duty trucks were updated using two different data sources. EMFAC2021 previously assumed N₂O emissions to be 1.6 g/gal of diesel for all model year trucks, for both idle and running operation modes.

For chassis model year 2011 and newer trucks, which are equipped with selective catalytic reduction (SCR) aftertreatment systems, EMFAC2025’s running emission factors were updated based on CARB’s Truck and Bus Surveillance Program data, since the TBSP dataset included trucks that were all equipped with SCRs. Data was used from vehicles tested on a chassis dynamometer using an array of cycles, including the Urban Dynamometer Driving Schedule (UDDS), Creep, Cruise, HS Cruise, Modified HS Cruise, Local Drayage, and Near Dock Drayage. The average emission rate for all cycles was 0.4 g/mi, which was used to represent all trucks with a model year of 2010 or later that were assumed to have been equipped with SCRs.

Older trucks (i.e., chassis model year before 2011) not equipped with SCR aftertreatment systems have a lower N₂O emission rate. The SCR catalyst effectively reduces NO_x species, but incomplete reduction of NO_x increases N₂O emissions, as explained in the literature (Quiros et al., 2017). CARB staff therefore used the average N₂O emission rate (0.05 g/mi) from a model year 2007 Conventional Diesel not equipped with SCR, as reported in Quiros et al. (2017), for all pre-2011 chassis model years in EMFAC2025. The pre-2010 model year emission rate was an average of multiple on-road cycles including Hill Climb, Interstate, Regional, Local, and Near-Dock routes. These updates have the counterintuitive effect of older trucks (pre-2011 model years), having lower N₂O emissions than the newer trucks.

The updates described above were only applied to running emissions. EMFAC2025 continues to use the 1.6 g/gal for HD diesel idle emissions. N₂O emissions for NG fuel in EMFAC2025 are treated identically to diesel, and emissions from gasoline fuels are estimated by correlations to NO_x emissions from VSP data, as documented in (CARB, 2018). Table 5.6 summarizes the N₂O emission rates used in EMFAC2021 and EMFAC2025.

Table 5.6: HD Diesel Emission Rates of Nitrous Oxide in EMFAC2021 and EMFAC2025

Operation Mode, Model Year	EMFAC2025	EMFAC2021
Start Emissions, All Model Years	1.6 g/gal	1.6 g/gal
Running Emissions, Pre-2010 Model Years	0.05 g/mi	1.6 g/gal
Running Emissions, Post-2009 Model Years	0.4 g/mi	1.6 g/gal