Abstract: Air pollution remains a major environmental and health concern in urban environments, especially in street canyons that show increased pollution levels due to a lack of natural ventilation. Previous studies have investigated the relationship between street canyon morphology and in-canyon pollution levels. However, these studies are typically limited to the scale of a single street canyon and city-wide assessments on this matter are scarce. In 2018, NO2 concentrations were measured in 321 street canyons in the city of Antwerp (Belgium) as part of the large-scale citizen-science project “CurieuzeNeuzen”. In our research, this data was used to study the correlation between morphological indices (e.g. aspect ratio (AR), lateral aspect ratio (LAR), presence of trees) and the traffic volumes on a city-wide scale. The maximum hourly traffic volume (TVmax) and AR correlated significantly with the measured NO2 values, making them useful indicators for air quality in street canyons. For street canyons with AR > 0.65, a TVmax of 300 vehicles/hour was found as a threshold value to guarantee acceptable air quality. No significant correlations were found for the other parameters. Finally, a number of typical street canyon types were defined, which can be of fundamental interest for further research and spatial policy making.

Abstract: Computational Fluid Dynamics (CFD) methods are widely used to investigate wind flow and dispersion in urban environments. Validation with field experiments that represent the full complexity of the problem should be performed to assess the predictive capabilities of the computations. In this context it will be necessary to quantify the effect of uncertainties in simulations of the full-scale problem. The present study aims at quantifying the uncertainty related to the variability in the inflow boundary conditions for Reynolds-averaged Navier-Stokes (RANS) simulations of the flow in downtown Oklahoma City to address validation with the Joint Urban 2003 field measurements. Three uncertain inflow parameters were defined: the wind speed and wind direction at a reference height, and the aerodynamic roughness in the logarithmic velocity inlet profile. An ensemble of 729 RANS simulations were performed to determine the polynomial chaos expansion coefficients that define the response surfaces for the velocity magnitude and direction at 13 field measurement stations, and the results are compared to the experimental data. For the velocity magnitude the mean experimental velocity magnitude is encompassed within the 95% confidence interval for the magnitudes predicted by the Uncertainty Quantification study in all stations. For the velocity direction this holds in 11 out of 13 locations. The study demonstrates the significant potential of applying advanced uncertainty quantification methods to address validation with field measurements and to develop a more realistic approach to the definition of inflow boundary conditions in atmospheric CFD simulations. (C) 2014 Elsevier Ltd. All rights reserved.