Introduction
Monitoring of nitrogen dioxide (NO₂) concentrations is undertaken on a wide scale basis in the UK by means of diffusion tube samplers. This includes the UK Nitrogen Dioxide Tube Survey¹, which is co-ordinated by AEA Technology plc's National Environmental Technology Centre (NETCEN) on behalf of DETR, and involves some 300 local authorities, and more than 1200 monitoring sites. Additional diffusion tube monitoring is also undertaken on an independent basis by many local authorities, and on an ad hoc basis in support of many development and infrastructure projects.

Diffusion tubes have the advantage of being a low cost, convenient way of mapping spatial distributions and investigating long term trends of NO₂. One shortcoming of the method is that it can only provide a concentration which is averaged over the period of exposure (typically 1 to 4 weeks), and it is not possible to measure short-term (eg hourly) concentrations. The method is, however, extremely useful for the assessment of long term concentrations, such as the annual average. The introduction of an annual mean standard within the UK National Air Quality Strategy² ensures that diffusion tubes will continue to play a significant role in air quality monitoring across the UK.

This study, which has been undertaken jointly between Stanger Science & Environment (SSE) and the National Environmental Technology Centre (NETCEN) is intended to provide a comprehensive validation of diffusion tube monitoring in urban environments.

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Study undertaken jointly by Stanger Science and Environment and the National Environmental Technology Centre.
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