A case study of aerosol (4.6 nm < D_p < 10 μm) number and mass size distribution measurements in a busy street canyon in Manchester, UK

The Air Quality Management community is increasingly turning its attention to urban 'hot-spots' where localised high concentrations of pollutants can arise. One such location is the urban street canyon where dispersion is poorly understood or described by regulatory models because of the complexity of the airflow, turbulence and local influences. Similarly, simple metrics such as PM₁₀ fail to describe the range of sizes, composition, sources and behaviours encompassed by the term 'particle'. A 2-week experimental case study to measure size-segregated aerosol in the size range 4.6nm-10μm at a fine time scale (10min resolution) was undertaken in a typical street canyon in Manchester. The wind direction incident to the canyon, and hence the vortex flow within the canyon, was found to have a large influence on the number concentrations, with values typically 2-10 times greater in perpendicular flow than the estimated inner-urban background. Concentrations were also inversely related to wind speed and directly related to traffic flow. Coarse mode mass concentrations were generally found to follow urban background PM₁₀ concentrations except with a 0-5μgm^-3 enhancement related to traffic-induced re-suspension within the canyon. A small pollution episode consisting of coarse material re-suspended by high winds was extended in time within the canyon.