Pollutant dispersion close to an urban surface – the BUBBLE tracer experiment

Summary

In this publication first results of an urban tracer experiment are reported. This experiment was realized in the framework of the “Basel UrBan Boundary-Layer Experiment” (BUBBLE) in an area with abundant information on turbulence and flow conditions available. Release height was close to roof level and so was the height of the concentration samplers. The meteorological conditions during the experiments were mainly convective, but due to the rough character of the underlying surface also the mechanical turbulence was substantial.

The concentration distribution is found to be essentially Gaussian in the horizontal plane and some commonly used methods to estimate the plume widths in applied dispersion models are compared to the observations. From measurements at one site downwind of the source it is found that for a near-roof level source, only an insignificant vertical gradient in tracer concentration is present within a street canyon. Using a Lagrangian Particle Dispersion Model the tracer experiments are simulated. It is shown that the exact form of the parameterization for the flow and turbulence structure within the urban roughness sublayer is of great importance for the simulation results. Also the numerical simulation results underline the necessity (and difficulty) to describe the vertical profile of the dissipation rate of turbulent kinetic energy close to an urban surface.