Improved Spatial Resolution in Modeling of Nitrogen Oxide Concentrations in the Los Angeles Basin

The extent to which emission control technologies and policies have reduced anthropogenic NOx emissions from motor vehicles is large but uncertain. We evaluate a fuel-based emission inventory for southern California during the June 2021 period, coinciding with the Re-Evaluating the Chemistry of Air Pollutants in CAlifornia (RECAP-CA) field campaign. A modified version of the Fuel-based Inventory of Vehicle Emissions (FIVE) is presented, incorporating 1.3 km resolution gridding and a new light-/medium-duty diesel vehicle category. NOx concentrations and weekday-weekend differences were predicted using the WRF-Chem model and evaluated using satellite and aircraft observations. Model performance was similar on weekdays and weekends, indicating appropriate day-of-week scaling of NOx emissions and a reasonable distribution of emissions by sector. Large observed weekend decreases in NOx are mainly due to changes in on-road vehicle emissions. The inventory presented in this study suggests that on-road vehicles were responsible for 55-72% of the NOx emissions in the South Coast Air Basin, compared to the corresponding fraction (43%) in the planning inventory from the South Coast Air Quality Management District. This fuel-based inventory suggests on-road NOx emissions that are 1.5 +/- 0.4, 2.8 +/- 0.6, and 1.3 +/- 0.7 times the reference EMFAC model estimates for on-road gasoline, light- and medium-duty diesel, and heavy-duty diesel, respectively.

Automated summary

The reduction in anthropogenic NOx emissions from motor vehicles due to emission control technologies and policies is significant but uncertain. This study evaluates a fuel-based emission inventory for southern California, finding that on-road vehicles are the main source of NOx emissions in the South Coast Air Basin. The fuel-based inventory provides different estimates of NOx emissions for different vehicle types compared to the planning inventory.