- Conclusions
Measurements of nitrogen dioxide concentrations using six different diffusion tube exposure techniques have been made and compared with measurement data from co-located automatic chemiluminescent analysers at 17 locations in the UK.
The main conclusions from this study are summarised below:
- Overall correlations between diffusion tube and automatic chemiluminescent measurement data were excellent, and ranged from 0.97 - 0.99 depending upon exposure type.
- Highly significant correlations (at P=0.01) were found between all diffusion tube exposure types and corresponding chemiluminescent measurement data.
- The overall differences between diffusion tube measurements and chemiluminescent measurements of NO2
were well within the estimated uncertainty associated with chemiluminescent monitors.
Overall, unsheltered tube exposures were shown to overestimate automatic chemiluminescent data by 8-9% (normal tube exposure) and 2-6% (blacked out tube exposure). Sheltered tube exposures were shown to underestimate automatic chemiluminescent data by 9-10%. Observed differences in tube performance have been attributed to the interference of wind on unsheltered tubes, resulting in increased uptake rates.
Evidence of a small systematic difference between 2 and 4-week tube exposure was identified. 2-week tube exposures are shown to overestimate automatic chemiluminescent measurements by 1-4% more than 4-week tube exposures. No substantial evidence was found linking observed systematic differences between 2 and 4-week exposures with photodegradation of chemical absorbent in NO2 diffusion tubes.
Estimates of uncertainty (at P=0.05) for individual measurements and annual average diffusion tube measurements have been calculated. Comparisons of uncertainty estimates for annual average diffusion tube measurements and the chemiluminescent technique showed good agreement
Estimates of the precision of diffusion tube measurements were consistent with those made in previous studies. The average coefficient of variation for duplicate tube exposures was calculated as 10.3%.
The results of 2 and 4-weekly normal exposure of diffusion tubes were found to be well within the estimated overall uncertainty of the results from automatic chemiluminescent measurements. Therefore, although the correlation of 4-week exposure of blacked out tubes was even closer than for normal exposure, the authors do not considered that any change from the current procedures for diffusion tube exposure used in the DETR's UK NO2 Diffusion Tube Survey is justified. Indeed, changes at this stage in the survey may result in considerable disruption to the survey's datasets and the possibility of compromising the identification of trends in local, regional and national datasets. The results of this study also clearly justify the continued use of diffusion tubes for assessment of NO2 concentrations on both local and national scales.Strictly, however, the results presented will only relate to the analysis of the diffusion tubes by the single operating laboratory used in this study. However, within the current UK National NO2 Diffusion Tube Survey a quality assurance scheme operates to check harmonisation of analyses between participating laboratories.
A note of caution is offered regarding the use of scaling factors to correct for systematic differences between diffusion tube and chemiluminescent measurement data. This practise is not recommended for the UK NO2 Survey which collates data from 38 laboratories. The results presented in this report only relate to the analysis of diffusion tubes by the single operating laboratory used in this study. Increased harmonisation between operating laboratories within the UK NO2 Survey by an enhanced quality assurance scheme is therefore a priority.
Study undertaken jointly by Stanger Science and Environment and the National Environmental Technology Centre.
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