High urban NOx triggers a substantial chemical downward flux of ozone.

Nitrogen oxides (NOx) play a central role in catalyzing tropospheric ozone formation. Nitrogen dioxide (NO2) has recently reemerged as a key target for air pollution control measures, and observational evidence points toward a limited understanding of ozone in high-NOx environments. A complete understanding of the mechanisms controlling the rapid atmospheric cycling between ozone (O3)-nitric oxide (NO)-NO2 in high-NOx 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 O3, NO, and NO2, over an urban area, that allow disentangling important physical and chemical processes. When generalized, our findings suggest that the depositional O3 flux near the surface in urban environments is negligible compared to the flux caused by chemical conversion of O3. This leads to an underestimation of the Leighton ratio and is a key process for modulating urban NO2 mixing ratios. As a consequence, primary NO2 emissions have been significantly overestimated.