Tracer release experiments to interpret pollutant dispersion in Bristol, UK.

Urban air flow is affected by topography, built environment and meteorology in a way that can be difficult to predict. The deliberate release of inert, non-toxic tracers can provide measurements of air flow. A series of experiments in Bristol, UK, have used inert perfluorocarbon tracers to investigate the dispersion effects. Tracer data can be used to validate wind tunnel models and large eddy simulations of the test area.
The River Avon, flows from the Severn Estuary through a gorge to the west of the city of Bristol into the Harbour. Hills of up to100 m elevation surround this location and tracer campaigns have been conducted in the Cotham area uphill and to the north. Modelling has shown the gorge to be important for the circulation of winds in the city, and a vehicle fire in the South East in January 2021 provided an example of pollutants being recirculated through the city.
Tracers were released for 15 minutes and air sampled for 30 minutes in Tedlar bags in multiple locations for offline analysis by GCMS. Three releases from the North West in June 2021 with wind speeds between 1 and 4 m s-1 were detected 1.5 km downwind from the release point, but measurements from a similar distance from the South, North and South West in January 2022 at ~1 m s-1 were more disperse. Tracer releases with a southerly wind of up to 2 m s-1 in the Cotham area in September 2024 have shown that over the short distance scale, tracers follow street networks in the predominant wind direction. Perfluoromethylclohexane released on a street at the bottom of a hill was measured at 1741 ppq 320 m up the hill, but less than 550 ppq on side streets 400 m up the hill in the predominant wind direction.