Dispersion of Air Pollution & Penetration into the Local Environment

Tracer Release Experiments 2007

The choice of trace gases, PFCs or SF6?

Both tracers are inert, harmless gases and, especially in the quantities to be used, have no local environmental impacts. However, they are gases with large greenhouse warming potentials and this aspect of their use had to be considered. Analysis shows that there are no significant concerns with the use of PFCs but possibly some for SF6. Only PFC tracers were in fact used in the field-work.

The following calculations of Greenhouse warming potential were based on "Guidelines for company reporting on greenhouse gas emissions", DEFRA 2005, and "Passenger transport emissions factors", DEFRA, 2007.

SF6 3xPMCH Notes
CO2 equivalence 23,900 10,000 upper value in range for PMCHs
Lifetime in the atmosphere 3,200 yr very long
Release per experiment 100 g 500 mg
Number of experiments 30 30
Total tracer release 3,000 g 15 g
Total CO2 equivalent 71,700 kg 150 kg
Typical off-set cost 717 1.50 @ 10/T
Equivalent car travel 398,000 km 833 km @ 0.18 kg/km

An estimate of the total CO2 emissions from travel (by road and rail) associated with the field work gave a figure of order 2,000 kg.

Recent EU and UK regulations seek to "contain, prevent and thereby reduce emissions of F gases covered by the Kyoto Protocol". This includes SF6 and some PFCs, but not those used in our experimental work. See "EC Regulation No 842/2006 on Certain Fluorinated Greenhouse Gases", defra, September 2006 for further details.

Tracer and meteorological experiments

Experimental procedures have been further developed since the previous trials and now enable dispersion experiments to take place in whatever wind conditions arise. The basic meteorological requirements for experiments to proceed are dry conditions with a reasonably well defined wind speed and direction at roof level. During a dose experiment, a single sample covering the passage time of the whole tracer cloud is taken at each sampling point, whereas during a time-resolved experiment a series of ten samples is taken during the passage time. Up to twenty sampling units can be deployed in any experiment, limited by manpower and sample bag numbers (a total of 200 being available).

Reference ultra-sonic anemometers were installed in October 2006 on the BT Tower (at 189m above street level) and on the roof of the Westminster Council House (WCC, at 18m above street level). Both are fully operational and will continue to operate for at least a year from the date of installation, gathering important information of the wind conditions over central London.

Five additional anemometers were installed on lampposts and two on masts at the Marylebone Road - Gloucester Place intersection on May 20th. These were all brought into service during the following week, marking the beginning of the intensive wind field monitoring and tracer dispersion study period.

PFC tracer release experiments were successfully carried out on May 31st, June 7th, 8th and 28th (details below). The timing was constrained by the availability of tedlar sample bags, of which there were 200 in all. On each of these days more than twenty workers were present on-site to operate emission and sampling equipment and oversee experimental activities. Altogether, 36 volunteers helped obtain 360 air samples over the four days.

Anemometers will now be removed from the lampposts as they are required for other work but will later be re-installed for the second phase of tracer studies. A longer operational period can be planned for that period if analysis of all available data reveals inadequacies of cover. To date, we have 8 weeks in 2004 and 6 weeks in 2007 with the BT Tower, WWC roof-top and lamppost mounted anemometry operating. The seasonal cover is, however, rather heavily biased towards springtime.

31st May - 28th June, 2007, Tracer Studies

The map below presents the site lay-out for the tracer experiments, showing the available release and sampling locations. Different tracers (PMCP, C6F12; PMCH, C7F14; PDCH, C8F16) were released from three release locations in each experiment, the locations being Point X (in front of WCC House) and two others chosen according to wind direction.

The experimental conditions are summarised below. The experiments on May 30th, June 7th and 8th were dose experiments, in which the total dose was measured as the tracer passed the sampling points; each sampling point returned one sample per experiment. The experiment on June 28th was time-resolved, each sampling point collecting 10 sequential samples during a 30 minute period. These were timed so that the arrival and departure of the tracer could be clearly detected. Tests were included in most experimental periods to check that tracer had indeed cleared the area during the sampling period; in all cases, this was confirmed. Initial analysis shows that the concentration levels in the samples were in the range anticipated. Complete analysis of all samples is expected towards the end of July.

The programme summarised above provided 39 (i.e. 3x4x3+3) sets of dispersion data. These can be combined with the output from the original EPSRC funded DAPPLE work, which total 12 sets of dispersion data plus 2 from a moving source. This gives a total of over 50 dispersion experiments with associated meteorological observations, which provides a very comprehensive set of data for characterising dispersion behaviour and examining the performance of dispersion models.

Wind tunnel work

Recent wind tunnel work has concentrated on: