The issue of over sampling by diffusion tubes in the outdoor environment as a result of wind turbulence, has been previously investigated. Under windy conditions turbulent eddies can be set up in the mouth of the tube, introducing ambient air further into the tube and effectively shortening the diffusion pathway (Gair and Penkett, 1995). The possibility of wind speeds typical of the indoor environment, causing diffusion tubes to over sample is investigated here.
Brown bromine vapour was placed in inverted test tubes and observed under various wind speeds. Even at low wind speeds common in indoor environments, the vapour was pushed up into the tube, away from the mouth, suggesting that turbulent eddies were becoming established.
Both in an outdoor environment and indoors at the Ashmolean museum in Oxford, sets of five diffusion tubes were exposed in the usual way, alongside sets of tubes with turbulence inhibiting devices. A polystyrene mesh or a Whatman filter paper was placed on the open end of the tube, in the hope that it would allow free air exchange, but reduce the turbulent nature of the air entering the tube. A cylinder was also used, into which the tubes were placed. At either end of the cylinder were discs, with a configuration of holes, which would hopefully allow air to pass through the cylinder but reduce its turbulent nature.
It was found that of the tubes exposed outdoors, those with turbulence inhibiting devices gave lower readings with less variability. In the indoor museum environment, the tubes in the cylinder measured a lower concentration than the other tubes. The tubes deployed in the normal way and those with polystyrene meshes and Whatman filters, all measured the same concentration. The results indicate that the meshes and filters do not create a starvation effect, although the cylinder does and needs to be modified perhaps using less discs or discs with bigger holes. They confirm that turbulence is an issue outdoors and suggest that the wind speeds present in a museum environment do not lead to diffusion tube over sampling. If tubes are deployed both inside and outside a museum building, in order to ascertain the ability of the building to buffer against outdoor pollution, turbulence inhibiting devices should be used with the outdoor tubes. In the future, we plan to test the turbulence inhibiting devices alongside a chemiluminescent NO2 monitor and to repeat the above experiment numerous times in various locations.
Gair, A. and Penkett, S. (1995). The effects of wind speed and turbulence on the performance of diffusion tube samplers. Atmospheric Environment Vol. 29, No. 18, pp. 2529-2533.
There are a number of sources of error with all passive samplers. At the analysis end, need to consider the chemistry, the calibration and the precision. At the trapping end, we need to consider Fick's law, validation, adsorption and non-Ficks's law - turbulence. If eddys occur in the mouth of the diffusion tube, there is a reduction in the diffusion length and the pollutants are forced into the tube. The effective diffusive length is then reduced leading to an overestimation of the pollutant concentration.
To determine the effect of this turbulence, the evaporation of ether from a diffusion tube was studied at different wind speeds. Also used bromine to measure the size of the convection cell that occurred at the mouth of the tube. Wind speeds under 1m/s seemed to give considerable oversampling, at 0.5 m/s there was about 20 % oversampling. Turbulence has previously been reported to lead to oversampling in outdoors (Campbell 1991).
To investigate ways of reducing the turbulence effect different methods were used:
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