Background to Study
If diffusion tube samplers are to be used with confidence in air quality surveys, then it is important that their performance with regard to the EC reference method (the chemiluminescence analyser) is understood. An intercomparison between the two methods was carried out by Warren Spring Laboratory some years ago3, and indicated that the diffusion tubes tended to over-read relative to the chemiluminescence instruments by around 30%. However, the survey was limited in terms of the number of sites investigated, and the factor was extremely variable between the different site locations. It was observed that the greatest differences occurred at the more exposed sites, suggesting that this could be an effect of wind turbulence leading to an effective shortening of the length of the tube.
More recently, NETCEN have reported that diffusion tubes appear to under-read with respect to chemiluminescent analysers4, although this conclusion is based upon an analysis of annual average data from equivalent site locations in the UK NO2 Survey and the AUN, as opposed to co-located sites.
In addition, several workers have reported a systematic effect between the measured NO2 concentration and the period of tube exposure. Results obtained from the London Wide NO2 survey5, co-ordinated by SSE, indicated a significant decline in mean concentrations in London, between 1992 and 1993. Further more, the decline in concentrations was much greater than that shown at London sites equipped with chemiluminescence analysers. At this time, there was a change from a 2-week exposure period, to a 4/5 week exposure period in order to synchronise the data with the National Survey. Although changes in meteorological conditions and vehicle emission reductions may have accounted for some of the variation, it was hypothesised that the length of exposure period may have played a significant role.
Further unpublished studies have tended to support this conclusion. A six month study carried out by SSE compared 2-week and 4-week exposure periods at 14 sites in East London. A clear seasonal effect was identified, with 4-week exposure concentrations being much lower, particularly during the summer months. It was suggested that this effect may be caused by the photodegredation of the triethanolamine (TEA), which might be expected to occur to a greater extent in the warm sunny months. Averaged over the six month period, the 4-week tubes produced measured concentrations which were 18% lower than the 2-week tubes.
These results are consistent with a study carried out by Bristol City Council, which demonstrated a 5 to 10% difference between 1 and 4-week exposures. These data also showed a clear seasonal effect.
Study undertaken jointly by Stanger Science and Environment and the National Environmental Technology Centre.
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