A new parameterisation of motorway-induced turbulence and its application in a numerical model

A parameterisation of motorway-induced turbulence has been developed and has been integrated in a diffusion model to simulate the transport and the chemical reactions of air pollutants in the vicinity of a motorway. Turbulence and therefore diffusion is influenced by the motorway in two different ways, namely traffic-induced turbulence and production of turbulence due to the flow over the dam on which the motorway is located. Both effects are parameterised. By comparing measured vertical profiles of CO of the BAB II project with simulated vertical profiles of CO the parameterisation is tested. Using the new parameterisation, the plume height and the maximum concentration near the surface are modelled correctly. With the extended model concentration distributions of reactive species in the vicinity of the motorway are simulated. The strong influence of the motorway-induced turbulence on the concentrations of nitrogen oxides is shown. With increasing the distance to the motorway, concentration of NO decreases much faster than the NO2 concentration. When the motorway-induced turbulence is taken into account the plume height increases up to 200% at a distance of 60 m on the leeside of the motorway. At the same distance motorway-induced turbulence reduces the concentration of CO by 50–70%, the concentration of NO by about 50%, and the concentration of NO2 by about 40%, respectively. The percentage changes depend on the atmospheric stability. Since the motorway is also a dominant source of coarse mode particles produced by abrasion processes of tyres, brake linings and street surface and by resuspension, concentration distributions of coarse mode particles are simulated. The maximum range of particles decreases with increasing particle diameter and is clearly increased by the motorway-induced turbulence.