Dispersion of a passive tracer within and above an urban canopy: Birmingham, UK, experiment

The transport and dispersion of pollutants in the urban environment are important processes due to their potential impact on city inhabitants and those living in nearby communities. The release of atmospheric tracers is a powerful technique to simulate the dispersion of pollutants. The combination of the low atmospheric background concentration of perfluoromethylcyclohexane (PMCH) (4.6 ±0.8 fl/l, Straume et al. 1998) and the possibility of measuring these very low concentrations, allows the realisation of nearly ideal fluid dynamic experiments in real urban environments. Urban dispersion field experiments are still scarce and consistent data sets for the neighbourhood scale very sparse. The long-term goal of this activity, designed within the UK URGENT programme, is to fill this gap by providing appropriate data sets to evaluate dispersion models.

The first experiment took place in the city-centre of Birmingham, UK, on an overcast day in July 1999 during a collaborative field campaign that provided the detailed meteorological data. The PMCH was released through a heated chimney of height 4.5 m. The release lasted for approximately 40 minutes at an average rate of 4 g/s. Four samplers were placed in an arc at approximately 3.5 km from the release point, with a cross-wind spacing of about 500 m. The mean measured concentration was obtained using an averaging time of 15 minutes for Site 1, 2 and 5 and 9 minutes for Site 3. Secondary samplers were placed approximately 9 km from the release point in the expected plume path and a background sampler was placed outside the path of the plume. The samples were analysed using a highly sensitive, novel gas chromatography technique developed at Bristol University (Cooke et al. 2000).

The experimental results were analysed and compared with model predictions using the Atmospheric Dispersion Modelling System ADMS 3 (CERC, 2000) and simple analytical models.

The measurements show three phases in the temporal evolution of the tracer concentration at the measuring sites. At first the concentration rises very rapidly to attain a high concentration plateau. The advection time was 17 minutes which is consistent with the wind speed of 3.5 m s-1, measured at a height of 12 m, near the source position. After this first plateau, the measurements show a slow decrease in concentration (time constant ~7 min) down to an above background low concentration plateau. The rise, the first plateau and the slope of the decrease of the measured concentration are well predicted by a time dependent puff model (from ADMS3) that also takes into account the effect of the shear.

References Cambridge Environmental Research Consultants (2000). ADMS 3 Technical Specification. Available from CERC, 3 Kings Parade, Cambridge, CB2 1SJ, UK.

Cooke K.M., P.G. Simmonds, G.N. Nickless, (2000) The Development of a Highly Sensitive and Selective Technique to Monitor Tracer Dispersion within an Urban Environment. Third Symposium on the Urban Environment, University of California, Davis, California.

Straume, A. G., R. N. Dietz, E. N. Koffi and K. Nodop, (1998), Perfluorocarbon Background Concentrations in Europe, Atmospheric Environment, 32, 4109-4122,