Dispersion of Air Pollution & Penetration into the Local Environment

Tracer Release Experiment

PFC and SF6 tracer

Tracer experiments were undertaken using perfluorocarbons and SF6 - clear, inert, non toxic gases with very high sensitivities towards gas chromatography coupled to either electron capture or mass spectrometric techniques. Releases were from a pressurized (25bar) cylinder (50 litre) within a parked van with only a small tube exiting so as not to cause alarm, or from portable boxes. Experiments conducted during DAPPLE 2004 included:

Automated air samples were collected in the "blue sampling boxes" each consisting of a 12 volt battery driven pump with an arrangement of automated valves and tubes to allow the collection of up to 10 litres of air in a series of flexible polyfluoro-plastic (TEDLAR) bags. Each sampler collected up to 10 air samples and their deployment locations depended on wind direction. Manual air samplers were also used, operated by University of Bristol staff.

Particle and dust tracer

Two particle dispersion experiments were carried out in Gloucester Place on May 26th using road grit spread over a section of the road as a source. Measurements included observations of particle spread along the road and concentrations of airborne particles. The work was supported by SF6 dispersion experiments and local anemometry.

Additional tracer studies

Four tracer dispersion experiments were carried out on the 2nd and 11th November 2004 as additional work associated with the DAPPLE field campaign. These involved the release of cyclic perfluorocarbons from both ground and roof level sources, as well as from a moving vehicle. Two different sources, each releasing a different tracer, were used in each experiment. Two of these experiments involved releases from a car moving along Marylebone road and these are believed to be the first examples of such an experiment in Europe and, possibly, anywhere. Integrated Tedlar bag samples were taken at 13 or 14 sampling points within about 250 m of the sources; measurements were made at a height of 1.5 m above ground level. The samples were analysed by pre-concentration on carbon based adsorbents and analysis by gas chromatography with detection by a mass spectrometer in the negative ion chemical ionisation mode. Sonic anemometry measurements within the streets and at the WCC Library roof reference position were made during each release period to characterise the wind flow. On November 2nd the wind was light and from the SE and on November 11th light and from the N. Accompanying wind tunnel dispersion experiments were also carried out - later these were greatly extended to give a full velocity and dispersion data set that is suitable for evaluating the performance of advanced computational methods.