Modelling the influence of road elevation on pollutant dispersion

Local urban air quality models must be able to account for complex road geometries if they are to predict near-road concentrations accurately. This includes flyovers, which are often used to improve flow at busy junctions or to take traffic through urban greenspace. We present a new methodology for modelling elevated roads in which the plume is only allowed to grow downwards once it has left the downwind road edge, thus accounting for road shielding. This new approach has been implemented in the operational dispersion model ADMS-Urban. The updated model is validated against monitoring data from two sites located next to busy flyovers-one in London, UK, the other in Antwerp, Belgium. It is shown to perform very well compared with simulations in which the flyovers are modelled at ground level, and slightly better than simulations when the traditional approach to modelling elevated roads (no road shielding) is used. Near-ground concentrations are significantly reduced with road elevation due to (i) increased vertical source-receptor distance, (ii) greater dispersion from the source where wind speeds are higher, and (iii) reduced impact of ground-level plume reflections. Pollutant trapping in street canyons is also minimised in cases where a flyover is elevated above the local building level. A sensitivity analysis is also presented in which multiple road elevations are tested; these results can be used by urban planners when designing new flyovers or by modellers in deciding whether it is important to account for road elevation near sensitive receptors.