Validation of nitrogen dioxide diffusion tube methodology in the UK

Measurement of ambient NO2 concentrations using diffusion tube samplers is widespread in many countries, particularly in the UK. A National Network of NO2 diffusion tube samplers has been operational at over 1200 sites in the UK for over 5 years. Some previous studies have indicated that NO2 diffusion tube samplers may overestimate NO2 concentrations by up to 30%, whereas others have shown an underestimation. Hence, the UK Department of Environment, Transport and the Regions commissioned this large-scale validation study. In this study diffusion tubes were exposed at 17 urban background monitoring sites equipped with chemiluminescent NO2 monitors within the UK Automatic Urban Monitoring Network. Over a one year period, diffusion tubes were exposed for 2- and 4-week periods, blacked out or clear and sheltered (from the wind) or unsheltered, in order to investigate the effect of a number of possible variables. The results of the study show that overall average NO2 concentrations calculated from diffusion tube measurements are likely to be within 10% of chemiluminescent measurement data. The uncertainty on this average difference is +/- 24-38% for individual diffusion tube measurements, but reduces to +/- 10-18% for annual averages. Differences due to the exposure period and exposure procedure were found, but these were not large. (C) 2000 AEA Technology. Published by Elsevier Science Ltd. All rights reserved.