Direct observations of NO_x emissions over the San Joaquin Valley using airborne flux measurements during RECAP-CA 2021 field campaign

Abstract. Nitrogen oxides (NO_x) are principle 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 NO_x emissions is a pressing need, yet quantifying current emissions is complicated due to a mixture of mobile and agriculture sources. We performed airborne eddy covariance flux measurements 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. On average we find emissions of 2.95 mg m^-2 h^-1 over highways, 1.24 mg m^-2 h^-1 over urban areas and 0.79 mg m^-2 h^-1 over croplands. The calculated NO_x emissions using flux observations are utilized to evaluate anthropogenic emission inventories and soil NO_x emission schemes. We show that two anthropogenic inventories for mobile sources, EMFAC (EMssion FACtor) and FIVE (Fuel-based Inventory for Vehicle Emissions), yield similar agreement with emissions derived from measured fluxes over urban regions with 24 % and 22 % low bias, respectively. Three soil NO_x schemes, including MEGAN v3 (Model of Emissions of Gases and Aerosols from Nature), BEIS v3.14 (Biogenic Emission Inventory System) and BDISNP (Berkeley Dalhousie Iowa Soil NO Parameterization), show substantial underestimates over the study domain. Compared to the cultivated soil NO_x emissions derived from measured fluxes, MEGAN and BEIS are lower by more than one order of magnitude and BDISNP is lower by a factor of 2.7. Despite the low bias, observed soil NO_x emissions and BDISNP present a similar spatial pattern as well as temperature dependence. We conclude that soil NO_x is a key feature of the NO_x emissions in the SJV and that a state-of-the-science model of these emissions is needed to simulate emissions for modeling air quality in the region.