Measurement report: Airborne measurements of NOx fluxes over Los Angeles during the RECAP-CA 2021 campaign

Nitrogen oxides (NOx ≡ NO + NO2) are involved in most atmospheric photochemistry, including the formation of tropospheric ozone (O3). While various methods exist to accurately measure NOx concentrations, it is still a challenge to quantify the source and flux of NOx emissions. We present airborne measurements of NOx and winds used to infer the emission of NOx across Los Angeles. The measurements were obtained during the research aircraft campaign RECAP-CA (Re-Evaluating the Chemistry of Air Pollutants in CAlifornia) in June 2021. Geographic allocations of the fluxes are compared to the NOx emission inventory from the California Air Resources Board (CARB). We find that the NOx fluxes have a pronounced weekend effect and are highest in the Eastern part of the San Bernardino valley. The comparison of the RECAP-CA and the modeled CARB NOx fluxes suggest the modeled emissions are too high near the coast and in downtown Los Angeles and too low further inland in the Eastern part of the San Bernardino valley.