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

Abstract. Nitrogen oxides (NO_x ≡ NO + NO₂) are involved in most atmospheric photochemistry, including the formation of tropospheric ozone (O₃). While various methods exist to accurately measure NO_x concentrations, it is still a challenge to quantify the source and flux of NO_x emissions. We present airborne measurements of NO_x and winds used to infer the emission of NO_x 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 NO_x emission inventory from the California Air Resources Board (CARB). We find that the NO_x 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 NO_x 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.