Keywords: air quality monitoring, physical modelling, pollution dispersion, urban site field measurements
Near-field geometry effects on urban street canyon measurements for model validation
Selection of numerical urban pollution dispersion models for regulatory purposes entails reliable prior validation against either physical modelling or field measurements. It is shown that an asymmetric geometry of the urban street canyon, in which measurement takes place, induces persistent gradients in pollution dispersion within it, irrespective of the wind vector meandering above the roofs. This influences field measurements taken there and, depending on canyon aspect ratio and roof geometry, favourable or unfavourable ventilation regions develop, on an annually-averaged basis. Therefore, inappropriate placement of the sampling probe can lead to systematic under or over-predictions of actual air quality. Proper consideration of the potential influence of the near-field on pollution dispersion guarantees that the campaign's results remain free of systematic bias and, therefore, appropriate for regulatory model testing and validation. The repercussions of these findings for urban air quality monitoring are identified and discussed.