EUROTRAC-2 Symposium 2000, 27 th
– 31 st
March 2000 Garmisch -Partenkirchen
A contribution to subproject SATURN
G. Vachon, J.-M. Rosant, P.G. Mestayer, P. Louka, J.-F. Sini
Laboratoire de Mécanique des Fluides UMR 6598 CNRS, Ecole Centrale de Nantes
BP 92101, 44321 Nantes Cedex3, France.
Abstract
Nantes’99 is a first stage of the European project URBCAP. This project aims to validate the requested models to predict local air quality and the time and space distributions of harmful pollutants within the different quarters of a city. These models are necessary to assess the urban pollution impact on citizen’s health. The experiment Nantes’99 took place in a street canyon of the city center of Nantes, in June 1999. The measurements during Intense
Observation Period (IOP) are sorted in a data base.
Two time series of CO concentration at the different locations within the street show that a reference wind direction perpendicular or oblique to the street axis induces higher concentrations at the leeward side of the street canyon.
Introduction
The experimental campaign Nantes’99 is the first French stage of the international project
URBCAP. The main aim of URBCAP is to assess the chemical and physical processes taking place within an urban canopy and affecting the local air pollution. In this context Nantes’99 has four main objectives : the wind field in the street, the production of turbulent kinetic energy induced by the motion of vehicles, the influence of the temperature distribution of the street surfaces on the structure of the flow and consequently on the pollutant dispersion within the street, the validation of the various models developed by the teams participating in the campaign.
Experimental set-up
The experiment took place in June-July 99 in the centre of the city of Nantes (France). The
“rue de Strasbourg” is a street canyon approximately North-South (332° from North). It is a high-traffic one-way street, with three lanes, and a great homogeneity in buildings

EUROTRAC-2 Symposium 2000, 27 th
– 31 st
March 2000 Garmisch -Partenkirchen construction. The width of the street is 15 m and the mean height of the buildings is 22 m
(H/W = 1.4). The wind field and the vehicle induced turbulence were measured by sonic, 3-D propeller, and hot wire anemometers at three levels on each side of the street: 1.5 m, 4 m and
12 m at the east side, 1.5 m, 4 m and 16 m at the west side of the street. The air temperature and the temperature of the walls were measured with thermocouples located at the same levels as the anemometers. CO concentration was measured at 1.5 m, 4 m, 12 m and 16 m above ground. Wind, CO concentration and temperature were also measured at 7 m over the roof of n°23 of the “rue de Strasbourg” for reference. Global, diffuse, and Infra-Red radiations were obtained at the same level. Traffic was measured by vehicle counters at eight different places within the street and within the lateral streets. During the experiment, the mobile laboratory of
Air Pays de la Loire was parked 250 m South from the measurement section on the street west side; it allowed measurements of CO, NO x
, SO
2
, dust and O
3
at 3.5 m above the ground as well as wind speed and direction at 10 m and temperature and relative humidity at 5 m
(Figure 1).
7 m
Acquisition
16 m
12 m
8 m u, v, w, CO
*7
West
H/W=1.4
CO, T u, v, w, CO
East
*7
4 m
1.50 m
*7
3.00 m
Sonic anemometer
3D propeller anemometer
Traffic counter
Hot wire anemometer u, v, w, CO, T
14.85 m
Thermocouples
Comb of x thermocouples
Radiometres u, v, w, CO, T
CO sensor inlet
Figure 1 : sketch of the experimental site
*7
1.90 m
*x

EUROTRAC-2 Symposium 2000, 27 th
– 31 st
March 2000 Garmisch -Partenkirchen
CO and NO x
were also measured permanently 150 m North from the study section at 4 m above ground, on the western wall. A detailed description of the experimental set-up was given by Vachon et al . (1999) and may be found on the Internet site of the Ecole Centrale de
Nantes (see : http://www.ec-nantes.fr/DEE/dah/dah.htm
).
Data base
A first data base comprising the measurements taken during the experiment Nantes’99 has been constructed. This data base includes only the IOP days (Intense Observation Periods) which correspond to wind speed less than 3 m.s
-1
at the reference level (30 m above the ground), sunny weather, high radiation, and dense traffic. These atmospheric conditions were chosen in order to obtain high pollution episodes and the most effective thermal situations.
Each table of this data set corresponds to one full day of measurements (0:00 to 24:00 local time) with a time step of 15 minutes. It contains wind, temperature and CO measurements and the counts of vehicle numbers.
CO concentration diurnal cycles
The measurements of CO in the street at different heights are presented in Figure 2 (22 nd of
June) and Figure 3 (24 th of June) with a reference wind above the roof oblique or perpendicular to the street axis. The distribution of CO concentration is not homogeneous in the street, the pollutant concentration is higher at the lower levels of the street than at the highest level. Moreover, in the lower part of the street we can observe higher concentration at the leeward side than at the windward side of the street. The angle between the street axis and the wind direction is more than 30°, therefore we can suppose that a skimming flow vortex develops in the street and carries the traffic emissions to the leeward side. Clearly, the concentration of the background pollution is quite small and therefore relatively “clean” air is introduced to the windward side and the re-circulation flow transports the traffic emissions to the leeward side.
Conclusion
A data base has been built using the measurements obtained during the experiment Nantes’99.
For the moment, this data base includes only IOP days. The analysis of CO measurements for two days showed the effect of a vortex developing within the street on the dispersion of pollutants. This vortex leads to a better ventilation and therefore greater pollutant dispersion at the windward side of the street.

EUROTRAC-2 Symposium 2000, 27 th
– 31 st
March 2000 Garmisch -Partenkirchen
Figure 2 : Evolution of CO concentrations during the 22 nd of June 1999 from 06:00 to 00:00
The reference wind direction varied between 30° and 150° from the street axis
The mean direction for the period was 82° from the street axis
7000,0
6000,0
5000,0
4000,0
3000,0
2000,0
1000,0
0,0
Z/H=1.33 (roof) z/H=0.69 windward side z/H=0.19 windward side z/H=0.5 close to the windward side z/H=0.5 close to the leeward side z/H=0.071 windward side z/H=0.071 leeward side
Figure 3 : Evolution of CO concentrations during the 24 th of June 1999 from 01:00 and 0:00
The reference wind direction varied between 30° and 130° from the street axis
The mean direction for the period was 80° from the street axis
Acknowledgements
The authors gratefully acknowledge support from the PRIMEQUAL-PREDIT (contrat
INERIS 1998), the Agence de l'Environnement et de la Maîtrise de l'Energie (ADEME) and
PSA Peugeot Citroën, the European Commission within the framework of the European
Research Network on 'Optimisation of Modelling Methods for Traffic Pollution in Streets'
(TRAPOS).
Reference
Vachon G., J.-M. Rosant, P.G. Mestayer, J.-F. Sini (1999). Measurements of dynamic and thermal field in a street canyon, URBCAP Nantes’99, 6 th Int. Conf. on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, 11-14 October, Rouen, France.