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.