Mean flow and turbulent exchange characteristics in real urban street canyons: the Lazzaretto case study

In the context of the European iSCAPE project (Improving the Smart Control of Air Pollution in Europe) an experiment was set up in order to test photocatalytic coatings effectiveness in reducing the concentration of pollutants. Further analysis on various aspects of the urban environment have been possible thanks to the experiment. The measurements collected during the experimental field campaign have been used to study the differences in the atmospheric phenomena due to the different morphology of two street canyons. Data in time periods characterized by non-synoptic conditions have been selected and then compared. The first part of the study is dedicated to the characterization of the mean flow. Subsequently, turbulent fluxes have been studied for both canyons with the Eddy Covariance method, in order to appreciate the different behaviour of the two canyons. In the end, the data collected have been elaborated with the Quadrant Analysis. This latest study has made possible to compare the results obtained by the measurements in a real urban canopy layer with the results obtained in a controlled environment and in a vegetated canopy layer. The study has examined the intensity of sweep and ejection effects in an urban canopy layer. The results show that in both canyons sweep effects prevail for the downwind case. Instead, for an upwind flow the dominant effects are those of ejection. For parallel wind directions the two canyons behave differently, due to their different morphology. Furthermore, the intensity of both effects is not as strong as in the perpendicular cases. The behaviour of the fluctuations of CO2 and H2O concentration with respect to the temperature fluctuations has been compared to the results obtained in a vegetated canopy layer. The analysis has shown that in the downwind and parallel cases the trends are in agreement with the ones obtained in the vegetated environment. For an upwind flow, however, the expected trends are not found.