4.4 California-submitted onroad emissions

California is the only state agency for which an onroad emissions submittal was used in the 2023 NEI. California uses their own emission model, EMFAC2021, (EMFAC2021 Volume III Technical Document), which uses Emission Inventory Codes (EICs) instead of SCCs. For the 2017 NEI, EPA and California worked together to develop a code mapping to better match EMFAC’s EICs to EPA MOVES’ detailed set of SCCs that distinguish between off-network and on-network and brake and tire wear emissions. This level of detail is needed for modeling but not specifically for the NEI, because the NEI uses simplified/more aggregated SCCs than used in modeling. The mapping file was updated for the 2020 NEI by the California Air Resource Board (CARB) and applied to the EMFAC outputs prior to providing the data to EPA.

California provided CAP emissions, excluding NH3, by county using EPA SCCs after applying the EIC to SCC mapping. For the 2023 NEI, we needed to add NH3, CO2, N2O, methane, PAHs, and also onroad refueling emissions. Methane was added for onroad sources in California using MOVES-based scaling factors – for example, the ratio of emissions for methane compared to VOC from MOVES, for each county and SCC in California. For PAHs, due to differences in pollutants included in MOVES and those provided by CARB, PAH emissions were taken from MOVES rather than from CARB. Onroad refueling emissions are not part of the CARB submittal and were based on running MOVES with vehicle miles traveled (VMT) and vehicle population data provided by CARB.

NH3, CO2, and N2O were added to the California onroad emissions by setting the state-wide total of emissions to the value obtained using MOVES and then distributing the emissions to counties and SCCs using California-provided data from another pollutant. For NH3, CO from California was used, while CO2 and N2O were based on the distribution of SO2 from California. This way, the overall magnitude of emissions is based on MOVES, but the distribution of those emissions between counties and vehicles is based on California data. The factors used for these pollutants are computed by taking MOVES state total emissions divided by the CARB state total for CO or SO2. The emissions for these pollutants are computed as follows:

CO2 = SO2 × 116326.5
N2O = SO2 × 5.844
NH3 = CO × 0.0341

Like methane, manganese (MN) is an exception as it cannot be matched to speciation in MOVES and is therefore ratioed as follows:

MN = MOVES MN × CARB PM2.5/MOVES PM2.5

Hexavalent chromium was computed from CARB-provided total chromium as:

Chromium VI = CARB Total Chromium × 0.00294840295

Another facet of the CARB data is that the SCC distributions are different in places from the original CARB submission. For instance, if the CARB data had emissions but no activity, or if they had emissions for non-MOVES fuel+vehicle type combinations (electric transit buses). In those cases, the emissions were apportioned to SCCs that could be mapped to SMOKE-MOVES.

Table 4.13 illustrates the data source used for California non-refueling onroad emissions by pollutant for the 2023 NEI.