Does California's EMFAC2017 vehicle emissions model underpredict California light-duty gasoline vehicle NO 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 NO/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 NO emission factors by more than a factor of 2.6. Contributing to these differences is that vehicles older than model year 2006 have NO 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 NO_x results in total CO emissions similar to the basin inventory but NO_x emissions that are 74% higher than the inventory. The higher NO 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 NO_x 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 NO_x/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 NO_x emission factors for gasoline vehicles. This results in a 74% underestimate of the contribution of gasoline vehicles to the basin's NO_x inventory, with the contribution from gasoline vehicles comparable to that from heavy-duty diesel trucks. This likely means that current projections for future NO_x emission reductions from mobile sources in the basin will not be realized unless additional NO_x reductions are obtained from older gasoline vehicles.