Does California's EMFAC2017 vehicle emissions model underpredict California light-duty gasoline vehicle NOx emissions?

On-road remote sensing measurements of light and medium-duty gasoline vehicles collected within California's South Coast Air Basin since 1999 generally fall within the range of observed summer ambient molar NOx/CO measurements collected during morning rush hours. Compared with ambient and on-road emissions, the California Air Resources Board EMFAC model underpredicts 2018 gasoline vehicle NOx emission factors by more than a factor of 2.6. Contributing to these differences is that vehicles older than model year 2006 have NOx emission deterioration rates that are up to four times higher on-road than predicted by the EMFAC model. A fuel-based inventory using the 2018 on-road gasoline emission factors for CO and NOx results in total CO emissions similar to the basin inventory but NOx emissions that are 74% higher than the inventory. The higher NOx emission estimates from on-road gasoline vehicle measurements make their contribution to the inventory slightly larger than heavy-duty diesel vehicles. We have found LEV I (1994-2003) gasoline vehicles are a major source of these on-road emissions and that significant NOx reductions in the South Coast Air Basin are being overlooked by not targeting the high emitters for removal.Implications: A comparison of ambient and on-road vehicle molar NOx/CO ratios collected in California's South Coast Air Basin with those predicted by California's EMFAC2017 vehicle emissions model shows that the model significantly underpredicts NOx emission factors for gasoline vehicles. This results in a 74% underestimate of the contribution of gasoline vehicles to the basin's NOx inventory, with the contribution from gasoline vehicles comparable to that from heavy-duty diesel trucks. This likely means that current projections for future NOx emission reductions from mobile sources in the basin will not be realized unless additional NOx reductions are obtained from older gasoline vehicles.