Direct observations of NO.sub.x emissions over the San Joaquin Valley using airborne flux measurements during RECAP-CA 2021 field campaign

Nitrogen oxides (NO.sub.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.sub.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 0.95 mg N m.sup.-2 h.sup.-1 over highways, 0.43 mg N m.sup.-2 h.sup.-1 over urban areas, and 0.30 mg N m.sup.-2 h.sup.-1 over croplands. The calculated NO.sub.x emissions using flux observations are utilized to evaluate anthropogenic emissions inventories and soil NO.sub.x emissions schemes. We show that two anthropogenic inventories for mobile sources, EMFAC (EMission FACtors) and FIVE (Fuel-based Inventory for Vehicle Emissions), yield strong agreement with emissions derived from measured fluxes over urban regions. Three soil NO.sub.x schemes, including the 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.sub.x emissions derived from measured fluxes, MEGAN and BEIS are lower by more than 1 order of magnitude, and BDISNP is lower by a factor of 2.2. Despite the low bias, observed soil NO.sub.x emissions and BDISNP present a similar spatial pattern and temperature dependence. We conclude that soil NO.sub.x is a key feature of the NO.sub.x emissions in the SJV and that a biogeochemical-process-based model of these emissions is needed to simulate emissions for modeling air quality in the region.