Direct Observations of NOx Emissions over the San Joaquin Valley using Airborne Flux Measurements during RECAP-CA 2021 Field Campaign

Nitrogen oxides (NOx) are principal components of air pollution and serve as important ozone precursors. As the San Joaquin Valley (SJV) experiences some of the worst air quality in the United States, reducing NOx emissions is a pressing need. Specific actions to reduce NOx are complicated by the lack of knowledge of the balance between anthropogenic and biogenic sources. We performed airborne eddy covariance flux measurements of NOx during the Re-Evaluating the Chemistry of Air Pollutants in CAlifornia (RECAP-CA) field campaign in June 2021. Combining footprint calculations and land cover statistics, we disaggregate the observed fluxes into component fluxes characterized by three different land cover types, highways, urban areas and croplands/grasslands. We then build emission inventories based on two anthropogenic mobile sources, EMFAC (EMssion FACtor) and FIVE (Fuel-based Inventory for Vehicle Emissions), and three soil NOx schemes, including MEGAN v3 (Model of Emissions of Gases175 and Aerosols from Nature), BEIS v3.14 (Biogenic Emission Inventory System) and BDISNP (Berkeley Dalhousie Iowa Soil NO Parameterization as implemented in WRF-Chem). We show that the emission inventory constructed using EMFAC and MEGAN underestimates the emission over the study domain by 60%. In contrast, we demonstrate that the emission inventory using EMFAC and BDISNP yields the best agreement with the inferred emission from the flux analysis. We conclude that biogenic NOx is a key feature of the NOx emissions in the SJV and that a state-of-the-science model of these emissions is needed to describe air quality in the region.