Experimental investigation of pollutant dispersion within a street 
in low wind conditions, the experiment Nantes'99

Gaëlle VACHON1, Jean-Michel ROSANT1, Patrice MESTAYER1, Petroula LOUKA1, 
Jean-François SINI1, Didier DELAUNAY3, Marie-Joëlle ANTOINE2, François DUCROZ4, 
Joël GARREAU5, Richard GRIFFITHS6, Chris JONES7, Yves LORIN1, Fabrice MOLLE4, 
Jean-Pierre PENEAU2, Yves TETARD3, Michel VIOLLEAU1
1Laboratoire de Mécanique des Fluides UMR 6598 CNRS - Ecole Centrale de Nantes, 1 rue de la Noë,
BP 92101, F-44321 Nantes Cedex 3 – Fax : 02-40-37-25-56 – email : Gaelle.Vachon@ec-nantes.fr
2CERMA UMR 1563 CNRS, Ecole d’Architecture de Nantes, rue Massenet, F-44300 Nantes
3CSTB, 11 rue Henri Picherit, BP 82341, F-44323 Nantes Cedex 3
4Air Pays de la Loire, 2 rue Alfred Kastler, BP 30723, F-44307 Nantes Cedex 3
5Service Environnement Urbain, Mairie de Nantes, 23 rue de Strasbourg, F-44000 Nantes
6UMIST, Environmental Technology Center, P.O. Box 88, Sackville Street, Manchester M60 1QD, England ; 7DERA Porton Down, Salisbury, SP4 OJQ, England

Abstract
Nantes‘99 is a first experimental campaign of the URBCAP 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 evaluation 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 is 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.
Keys-words: street canyon, full-scale experiment, traffic, wind direction, pollutant dispersion

Résumé
L’expérience Nantes’99 est une première étape du projet URBCAP dont le but est de déterminer l’importance des processus de transformation des polluants dans la canopée urbaine et valider les modèles requis pour calculer la qualité de l’air locale dans les différents quartiers d’une ville. Les objectifs de Nantes'99 sont d'étudier la dynamique du vent dans une rue canyon, caractériser la turbulence induite par les véhicules, évaluer les aspects thermo-radiatifs et valider les différents modèles.
La campagne expérimentale Nantes’99 a eu lieu au mois de Juin 1999 dans une rue du centre-ville de Nantes (France) qui peut être considérée comme une rue de type canyon. Dans ce papier sont analysées les concentrations en CO mesurées au cours de la journée à différents niveaux de hauteur dans la rue en fonction du trafic et de la direction du vent au-dessus des toits. La relation 
directe entre trafic et concentration de polluants est mise en évidence. Les champs de concentration sont également modifiés par la direction du vent ; trois situations sont plus particulièrement étudiées : deux vents transversaux à l’axe de la rue, l’un d’Est et l’autre d’Ouest et un vent presque longitudinal.
Mots-clefs : rue canyon, expérience in-situ, trafic, direction du vent, dispersion de la pollution

Introduction

The determination of air pollution within urban streets is based on several factors such as the local (fixed and mobile) sources, the local dispersion (transport and pollutants by wind, dispersion by turbulence, chemical transformations) and the local immissions (from the neighbourhood streets). The numerous parameters that influence the dispersion of pollutants within the street makes the analysis of the developing airflow often complex. In particular, traffic increases the concentration of pollutants within the street, but at the same time the motion of the vehicles generates additional turbulence that modifies the concentration fields in the street and possibly enhances their dispersion (Kastner-Klein et al., 1998, Delaunay and Houseaux, 1997, DePaul and Sheih, 1986). Wind speed and direction also affect the dispersion of pollutants. It has been observed that one or more vortices may develop in the street depending on the wind speed and wind direction (Berkowicz et al. 1996, Dabberdt et al. 1973) as well as on the geometry of the street e.g. its aspect ration Height/Width, H/W (Oke, 1988). Radiation may cause photochemical reactions of the primary pollutants emitted by the vehicles (Harrison et al., 1999). It also changes the fields of temperature and the flow regimes in the street (Berkowicz et al., 1997, Sini et al., 1996).

The experimental campaign Nantes’99 is the first French stage of the project URBCAP. The main aim of URBCAP is to determine the chemical and physical processes taking place within an urban canopy and affecting the local air pollution. 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. For this purpose, Nantes’99 is the first street-scale experiment combining measurements of traffic, turbulence, temperature, pollutant tracers at several levels and on both street sides.

1 - 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 construction. The width of the street is 15 m and the mean height of the buildings is 22 m (H/W = 1.4). The section chosen to install the instruments is situated approximately at the centre of the street (n°23). The street part is separated by two cross-sections and is 58 m in length (Figure 1). 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. Carbon monoxide CO was chosen as the pollutant emissions tracer; this gas is a good indicator of the dispersion and the dilution of pollutants within the street as its chemical transformations are very slow. Concentration of CO was also measured at 1.5 m, 4 m, 12 m and 16 m from the 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 (Figure 1). During the experiment, an Air Pays de la Loire mobile laboratory was parked 250 m South from the measurement on the street west side; it allowed measurements of CO, NOx, SO2, dust and O3 at 3.5 m from the ground as well as wind speed and direction at 10 m and temperature and relative humidity at 5 m. CO and NOx were also measured permanently 150 m North from the study section at 4 m from the ground, on the western wall. A detailed description of the experimental set-up is given in Vachon et al. (1999) and may be found on the Internet site of Ecole Centrale de Nantes (http://www.ec-nantes.fr/DEE/dah/dah.htm).

2 - Data base

A first data base comprising the measurements taken during the experiment Nantes’99 has been constructed with Microsoft® Access. This data base includes only the IOP days (Intense Observation Periods) which correspond to days with wind speed less than 3 m.s-1 at the reference level (30 m above the ground), sunny weather, large radiation, and dense traffic. These atmospheric conditions were chosen in order to obtain high pollution episodes and the most effectives thermal situations. During these IOP, all the sensors were kept operational. Two parts were defined within this data base: one data set called “Nantes’99” and another one called “Trafic’99”. 

Data set “Nantes’99”

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 the measurements of wind (direction, wind speed and the fluctuations of the three wind components) at the different levels within the street, temperature and CO concentration in the study section, the measurements of pollution at the surrounding stations and the global, diffuse and Infra-Red radiation. In addition, the meteorological data obtained at the position of the mobile laboratory are also included.

Data set “Trafic’99”

Flux of vehicles : all the counters.
Total and Heavy-duty vehicles : 4, 6, 9, 10.
Velocity of the vehicles : 12, 13, 14.

Each table corresponds to a full day of measurements with a time step of 15 minutes (except for the measurements of the velocity of the vehicles for which the time step is 1 hour), and at a particular position of the sensors (see Figure 1). The flux of vehicles per lane, the total flux of vehicles and, according to the type of the counter, the mean velocity of the vehicles or the number of light and heavy-duty vehicles are included in each table. The data set is still not complete. Some more days will be added, particularly, the days for which the sensors were operating, but the atmospheric conditions were different from those defined as IOP, in order to identify the effects of the different atmospheric conditions on traffic pollution dispersion.

Figure 1 : Location of the traffic counters during Nantes’99 experiment

Hereunder is presented the initial analysis of the relationships between traffic, reference wind direction and the CO concentration within the “rue de Strasbourg”.

3 – Results

Evolution of traffic within the “rue de Strasbourg”

Figure 2 represents the averaged variation of traffic, accumulated in the three lanes, during one full day. The Figure shows the mean values for the period of 9 June to 2 July 1999, without the weekends, with their standard deviation. From this period, the 21st of June has also been excluded as the traffic was exceptionally high after 19:00 due to a music festival. Three main peaks in the traffic are observed during the day indicating the rush hour; the first between 7:30 and 9:00, the second between 12:00 and 13:30 and finally at approximately 16:00. Traffic remains relatively constant between 9:00 and 11:30 and between 16:00 and 19:00. After 19:00 a progressive decrease of the traffic is observed. Furthermore, the daily values show a small deviation from the mean, especially during the periods when there is a sharp increase of the traffic.
The percentage of heavy-duty vehicles in the study section during the day is shown in Figure 2. This percentage is on the average of 4% and it is relatively constant during the day with a decrease in the night and an increase in the early morning (4:00 – 5:30) that can be attributed to the circulation of the vehicles used by professionals.

Figure 2 : Evolution of the traffic in the rue de Strasbourg and percentage of heavy-duty vehicles

Evolution of CO concentration 

Here, results from two days are shown in Figures 3 and 4, for which the wind direction varied slightly:
•	The 14th of June, the reference wind was from the NE with a daily mean direction of 34°±6° from the axis of the street.
•	The 24th of June, the reference wind was nearly normal to the street axis, with a daily mean direction of 80°±32° from the axis.
Also, a period with approximately constant wind direction, quasi perpendicular to the street, was observed on the 1st of July from 0:00 to 19:00. The mean direction of the reference wind was –119°±36° from the axis of the street.
Figures 3 to 5 represent the evolution of traffic and CO concentration at the different levels of the street section for each of the selected days: at 1.5 m from the ground at the east and west sides of the street, at 4 m at the east side, and at 12 m at the east side and 16 m at the west side. The values of CO concentration at 4 m are not shown in Figure 4 (24th of June) as they are similar to those at 12 and 16 m.
The general distribution of CO during the day follows the pattern of the distribution of traffic. Consequently, an increase of the concentration is observed starting at 7:00 and a decrease at 20:00. The peaks in the concentration directly correspond to the peaks in the traffic. An increase of traffic leads to an increase in CO and accordingly, a less dense traffic is followed by a lower concentration.

Situations of winds normal to the street axis (24th of June and 1st of July)

On the 24th of June (Figure 4) the CO concentration at 1.5 m is clearly greater than at the high levels within the street; they can reach 6000 µg.m-3 at 1.5 m while the maximum at 16 m is 1500 µg.m-3. Moreover, the increase of the concentration due to traffic is larger at the lowest level than at the higher ones showing that the pollutants are better dispersed at the higher levels. 

North

Wind

Traffic

A pronounced feature of the distribution of the concentration within the street is that it reaches its greater values at the east side of the street at 1.5 m than at the west side. This is probably due to the 
Figure 3 : Evolution of traffic and CO concentration in the street canyon, the 14th of June 1999

Wind

North

Traffic

Figure 4 : Evolution of traffic and CO concentration in the street canyon, the 24th of June 1999

North

Traffic

Wind

Figure 5 : Evolution of traffic and CO concentration in the street canyon, the 1st of July 1999

fact that the ambient wind being perpendicular to the street, a vortex develop within the street. This vortex has been referenced by several authors (Berkowicz et al., 1996, Croxford and Penn, 1998) who studied its effect on the dispersion of pollutants in the street; they stressed the better ventilation and therefore greater pollutant dispersion at the windward side of the street. Clearly, the CO concentration of the surroundings is quite small (580 µg.m-3 on the roof) and therefore relatively “clean” air is introduced to the windward side (west side in this case) and, the re-circulation flow transports the traffic emissions to the leeward (east) side.

On the 1st of July (Figure 5) the difference in the concentrations at 1.5 m on the east and west sides of the street is less significant than in the previous case. In addition, the concentration at 4 m is greater than at 12 m whereas on the 24th of June the values at 4 and 16 m were comparable. The wind was coming from the west and the concentration could be expected greater at the west side than at the east side due to the influence of the main vortex. However, this was not the case: the pollutant dispersion was weak, maybe due to an absence of vortex. DePaul and Sheih (1986) did not observe the vortex within their street for wind less than 1.5 m.s-1. In the “rue de Strasbourg”, which has about the same aspect ratio as the street canyon of DePaul and Sheih (H/W = 1.4), on the 1st of July the mean wind speed was 0.9 m.s-1 and maybe there was no vortex, while on the 24th of June there is clear evidence of a vortex with an ambient wind of 1.2 m.s-1. Therefore it is possible that there is a threshold value of wind speed between 0.9 m.s-1 and 1.2 m.s-1 above which a vortex develops within the street.

Situation of oblique wind (14th of June)

An oblique wind direction within the “rue de Strasbourg” would form a spiral vortex (Nakamura and Oke, 1988) leading to larger concentration of pollutants at the leeward side of the street as it is shown in Figure 3. Comparing a perpendicular with an oblique wind situation (Figure 3, 4 and 5) it is obvious that the CO concentration is greater on the 14th of June that on the 24th of June or the 1st of July (maximum of 8000 µg.m-3 at 1.5 m of the east side at 8:00). A possible reason of this behaviour is that for the oblique wind direction there is an influence from the closest cross street (“rue du Maréchal Leclerc”) upstream the study section. The two streets have similar geometries and comparable traffics. Therefore the rates of pollutants generated in both streets should be comparable. Consequently, oblique wind direction of 34° from the axis of the street, as was measured on the 14th of June, could lead to the entrainment of air and pollutants from the “rue du Maréchal Leclerc” to the “rue de Strasbourg”. 

Pollution concentration of the surroundings

Values of pollutant concentration of the surroundings are available for two out of these three days of measurements : the 14th of June (Figure 6) and the 24th of June (Figure 7). The concentration measured by the mobile laboratory are comparable with those measured in the study section at the west side, at similar heights (3.5 m and 4 m, respectively) and at the same distance from the traffic lanes (Figure 6). Moreover, the measurements at the permanent station show smaller concentrations than those of the mobile laboratory. The distance from the lane centre (approximately 8 m for the permanent station and 3.5 m for the mobile laboratory) may explain this difference. The concentration measured by the mobile laboratory on the 24th of June (Figure 7) is weaker than that measured by the sensor positioned at the study section at the same height and side of the street (West). This is probably due to the different pollution levels originating from the surrounding environment and reaching the two sites. In particular, the wind comes from the east and is perpendicular to the axis of the street: at the east of the study section there is “La place du Maréchal Foch” where the traffic is rather significant, whereas at the east of the mobile laboratory there is “Le Château des Ducs de Bretagne” where traffic is close to zero. The same analysis may also be true for the values measured by the permanent station on the 24th of June, which are similar to the values taken at the position of the mobile laboratory.
Other parameters such as the state of the traffic and/or the thermal convection may influence the concentration field in the street and should also be included in the further study.

Figure 6 : Measurements of the surrounding pollution in the street canyon, the 14th of June 1999

Figure 7 : Measurements of the surrounding pollution in the street, the 24th of June 1999

Conclusion

This paper presented the different relationships between traffic and CO concentration in the “rue de Strasbourg” during Nantes’99. The analysis of the measurements for three days showed that the pollutant concentration within the street canyon depends not only on the density of the traffic but also on the wind direction and speed. In addition, attention has been drawn to the importance of the location of the sensor on the pollution measurement. The concentration value obtained by a sensor is associated to the reference wind direction and is representative of the pollution at the particular height and the particular position of the sensor. Therefore, measurements taken by the permanent stations of the air quality network may be different from the concentrations breathed by the pedestrians. For example, measurements of CO at the pavement at 1.5 m is almost ten times greater than the value measured at the permanent station at 4 m on the 24th of June at 17:00.

Acknowledgements

The authors gratefully acknowledge support from PRIMEQUAL-PREDIT (contrat INERIS 1998), 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). The authors would also like to acknowledge DERA for funding the participation of R. Griffiths and C. Jones.

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