1. Chemical Dispersion Experiments in DAPPLE Centre for Atmospheric Science School of Chemistry University of Bristol Bristol BS8 1TS

2. General Comments: Bristol will attempt to release the chemical tracers, sample and determine concentrations at specified sites within the DAPPLE zone. Our role is to provide observations using designed experiments for the theoretical aspects of the project. The rate of release, duration of the releases, the locations of the sampling equipment and the frequency of sampling will be ascertained following detailed discussions with the modelling groups and within the constraints afforded by the site.

3. Releases: We envisage being able to release 2 types of chemical tracer: 1/ Sulphur hexafluoride from a pressurised cylinder; 2/ Perfluorodimethylcyclohexane, a fairly volatile liquid by a process requiring some form of heat action to allow dispersion to take place. Other perfluorocarbons can be used if needed. Both of these chemical tracers are very inert, non-toxic and display very high sensitivities towards gas chromatography coupled to either electron capture or mass spectrometric techniques. The releases can be of the form of a PUFF type, say of duration up to 30 seconds or a CONTINUOUS steady state release of say up to 30 minutes.

4. Releases Cont.: The PUFF format would require either a pressurised cylinder being opened for a set time or the breaking of a glass ampoule of a volatile liquid. In these types of releases the total quantity of tracer would be of the order of a few grams, but would be of limited range, say up to 50 meters. In the CONTINUOUS type of release, greater quantities of chemical tracer would have to be released say up to 100 grams per release, to cover the 250 meter radius of the DAPPLE zone.

5. CO 2 Releases: We are working on the release of carbon dioxide and real- time monitoring and have had success in narrow canyons at the Reading farm site. In this case the carbon dioxide is in the form of a PUFF release from a normal fire extinguisher. The technique may prove useful in the DAPPLE context in quite restricted places because the range is of the order of a few meters. In Reading we are working over about ten meters. We only have four monitors at present so the spatial area covered is limited. However, the results are available instantly for viewing and may be useful since each release takes only a few minutes. Therefore, we can carry out a number of ‘mini’-releases in a relatively short space of time.

6. Sampling: Each set of sampling equipment consists of a 12 volt battery driven pump with an arrangement of automated valves and tubes to allow the collection of up to 5 litres of air in a series of flexible polyfluoro-plastic (TEDLAR) bags. Obviously the whole sampling process is a batch sequence, requiring a separate analysis procedure. Each sampler can collect up to 10 air samples. The pumping rate is of the order of 1.5 litres per minute and the shortest collection period is around 20 seconds. So closer to the release point we are able to collect smaller volume air samples whilst further away we can collect larger volumes.

7. Sampling cont.: The time between collection of air samples can be varied allowing ventilation or clearance type requirements - the further away the particular sampler from the release point the longer period between collections can be made. Eventually we hope to co-ordinate all samplers and hence start the sampling sequences from one spot by means of radio-control. We hope to have some 12 of these multi-bag samplers available plus several single shot samplers (which can only hold 1 TEDLAR bag) for DAPPLE. The portable nature of the samplers allows their spatial location and temporal operation to be designed for the particular experiment being undertaken. However, we found in the URGENT project that ideal locations were difficult due to obtaining necessary permissions and access to site the samplers.

8. Analyses: Analyses for both the sulphur hexafluoride and perfluorocarbons so released will have to be carried out back in Bristol. Analysis is the most expensive par of our effort necessitating expensive instrumentation both to operate and to keep maintained. But, by using GC-ECD methods for sulphur hexafluoride and NICI-GC-MS for the perfluoro-compounds we can be sure of both a highly sensitive and selective determination procedure. Each GC/MS run takes at least one hour and so one experimental release can involve many days of analysis time, involving duplicates, varying sample volumes for precision, etc. The analysis for sulphur hexafluoride is faster, but is dependent on the concentration in the particular sample.

9. Analyses Cont.: We have many difficult experimental problems to guard against, such is the sensitivity of the analytical procedures we have to take special precautions against contamination. Thus the release team must never come near, touch or handle the TEDLAR bags. The pure tracers must not be close to or stored near the samplers, etc. So, if we sometimes appear difficult, say travelling in 2 vehicles and so wanting car parking spaces in our requirements please bear with us, we are only thinking of the good of the DAPPLE project.