Nantes '99 is a first experimental campaign of the URBAP project which aims at assessing the importance of the pollutant transformation processes within the urban canopy and validating the models allowing to predict local air quality within the different quarters of a city. The objectives of Nantes '99 are the determination of wind field in a street canyon, the study of the traffic influence on turbulence, the valuation of thermo-radiative aspects and the validation of different models. The experiment took place during the whole month of June 1999 in a section of the Rue de Strasbourg, a 3-lane, one-way, highly-trafficked, straight street of the city centre of Nantes. A first data base concerning the measurements during IOP (Intense Observation Period) days throughout Nantes '99 experiment has been built. This paper presents CO concentrations measured at different heights within the rue de Strasbourg in relation to traffic density and reference wind speed and direction. It is shown that high pollution episodes are associated with increased traffic. Furthermore wind direction perpendicular to the street leads to high concentrations at the leeward side of the street. It also appears that the skimming flow vortex is not observed for wind speeds lower than a threshold between 0.9 and 1.2 m.s-1. Finally, it is pointed out that the background pollution levels influence the concentration of pollutants within the street canyon. (A)
Abstract