Urban Tracer Dispersion and Infiltration into Buildings Over a 2-km Scale

Field experiments were undertaken in the summer of 2015 in Manchester, UK, to investigate the dispersion behaviour and infiltration into buildings of gas-phase pollutants over horizontal distances of 1–5 km. Inert cyclic perfluorocarbon tracers were released for 15 min at either one or three release points and samples taken in locations indoors and outdoors up to 2 km downwind. Background measurements of these cyclic perfluorocarbons range between 5.6 and 12.6 parts per quadrillion (ppq). On most occasions, tracer concentrations are higher on the sixth floor than at ground level. Tracer concentrations persist in the least well-ventilated rooms after concentrations return to background levels outdoors. The highest tracer concentrations, 329 ppq above background, occur at dawn on 23 July from a sixth-floor sampling position during thermally stable conditions. At low wind speeds, tracer is detected upwind of the prevailing wind direction; on 24 July, tracer is detected to the north-west of the release point for north-north-east wind direction. A simple street network model does not predict tracer concentrations at low wind speeds over the km scales in this investigation due to tracer likely escaping the urban canopy. Predictions from a simple correlation model overestimate concentrations originating from distant sources, which is believed to be due to infiltration into buildings along the journey from source to receptor. A Gaussian plume model predicts the highest tracer concentrations for most receptor points on 23 July when the lowest Obukhov length of – 26 m was measured, agreeing with tracer measurements.