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.

Evaluation of Nitrogen Oxide Emission Inventories and Trends for On-Road Gasoline and Diesel Vehicles.

On-road vehicles continue to be a major source of nitrogen oxide (NOx) emissions in the United States and in other countries around the world. The goal of this study is to compare and evaluate emission inventories and long-term trends in vehicular NOx emissions. Taxable fuel sales data and in-use measurements of emission factors are combined to generate fuel-based NOx emission inventories for California and the US over the period 1990-2020. While gasoline and diesel fuel sales increased over the last three decades, total on-road NOx emissions declined by approximately 70% since 1990, with a steeper rate of decrease after 2004 when heavy-duty diesel NOx emission controls finally started to gain traction. In California, additional steps have been taken to accelerate the introduction of new heavy-duty engines equipped with selective catalytic reduction systems, resulting in a 48% decrease in diesel NOx emissions in California compared to a 32% decrease nationally since 2010. California EMFAC model predictions are in good agreement with fuel-based inventory results for gasoline engines and are higher than fuel-based estimates for diesel engines prior to the mid-2010s. Similar to the findings of recent observational and modeling studies, there are discrepancies between the fuel-based inventory and national MOVES model estimates. MOVES predicts a steeper decrease in NOx emissions and predicts higher NOx emissions from gasoline engines over the entire period from 1990 to 2020.