Effects of inhomogeneous ground-level pollutant sources under different wind directions

Highlights: • Pollutant concentrations in the near field are sensitive to the source distribution. • The extent of the near field can be estimated using a simple statistical method. • Near-field dispersion strongly reflects velocity statistics near the source. • Optimal source allocations are determined for different external wind directions. • Velocity and near-field pollutant statistics change dramatically around 30°. Effects of source inhomogeneity on pollutant dispersion from a cubic building array are investigated as a function of the external wind direction. Using building-resolving large-eddy simulation, it is found that the results depend strongly on the source location and source uniformity inside a near-field region defined by a radius of homogenisation (RAD) based on the spatial autocorrelation of the pollutant concentration. The sensitivity of the RAD to the source location changes abruptly around 30° and is greatly reduced for wind angles between 30 and 45°, in agreement with velocity statistics and the mean horizontal streamlines. The optimal source allocation, which is a proxy for emissions from time-dependent traffic, also changes around 30°. This work clarifies the relationship between inhomogeneous velocity and pollutant statistics and may be applied to the formulation of traffic control policy.