High urban NO x triggers a substantial chemical downward flux of ozone.

Nitrogen oxides (NO x ) play a central role in catalyzing tropospheric ozone formation. Nitrogen dioxide (NO 2 ) has recently reemerged as a key target for air pollution control measures, and observational evidence points toward a limited understanding of ozone in high-NO x environments. A complete understanding of the mechanisms controlling the rapid atmospheric cycling between ozone (O 3 )-nitric oxide (NO)-NO 2 in high-NO x regimes at the surface is therefore paramount but remains challenging because of competing dynamical and chemical effects. Here, we present long-term eddy covariance measurements of O 3 , NO, and NO 2 , over an urban area, that allow disentangling important physical and chemical processes. When generalized, our findings suggest that the depositional O 3 flux near the surface in urban environments is negligible compared to the flux caused by chemical conversion of O 3 . This leads to an underestimation of the Leighton ratio and is a key process for modulating urban NO 2 mixing ratios. As a consequence, primary NO 2 emissions have been significantly overestimated.