Investigating the role of urban shade on ground-level ozone pollution in cities

Ground-level ozone is a major urban pollutant posing increasing health risks. As cities reduce NO_x emissions, ozone concentrations can temporarily increase due to reduced NO titration. This study investigates whether urban shade produced by buildings and trees is associated with street-scale ozone variability. Using high-resolution mobile air quality data and detailed shade modelling across Dublin and Hamburg, this study shows that increased shade coverage is consistently associated with lower ozone concentrations. To evaluate whether this relationship persists under comparable atmospheric conditions, a stratified contrast analysis was performed balancing observations on NO₂, time of day, temperature, and wind speed. Across both cities, shaded locations exhibited lower ozone and total oxidant (O_x = O₃ + NO₂) concentrations, while matched NO₂ differences remained small. This indicates that the observed ozone reduction is not readily explained by concurrent NO₂ variation alone. The shade-ozone contrast was strongest under low wind conditions and attenuated under higher wind speeds, indicating sensitivity to atmospheric mixing. These findings highlight urban shade as a spatial factor associated with ozone heterogeneity at the street scale and motivate further work to evaluate its role alongside emissions and meteorology in shaping urban air quality.