Limitation of O3 Dry Deposition by Elevated-Source NO Emissions and Surface Dryness in Urban Environments.

Tropospheric ozone (O3) is a major air pollutant that adversely affects human health, climate and ecosystems. However, its removal in urban environments remains poorly represented in air-quality and climate models, largely due to limited flux-based observations of urban O3 deposition. Using eddy-covariance flux measurements from a meteorological tower in Beijing, we show─for the first time to our knowledge─that surface uptake plays a central role in O3 removal in urban environments but is strongly limited under dry conditions by evaporation and by higher-above-measurement nitrogen monoxide (NO) concentrations (HAMNC), inferred from downward NO fluxes and likely linked to elevated-source NO emissions. These limitations led to significantly more frequent positive ozone-flux events under low relative humidity (RH < 30%) and downward NO flux, reducing downward O3 flux by ∼5% and ∼3%, respectively. We provide explicit functional relationships describing the response of downward O3 velocity (Vdw,O3) to both near-surface dryness (RH) and HAMNC, accounting for processes missed by routine ground-based measurements that can lead to misinterpretation of O3 removal in the urban environment and beyond. Incorporating the RH-dependent limitation of Vdw,O3 into air-quality and climate models is critical, given that ∼40% of the global land surface is arid or semiarid.