4. Monitoring Methodology
   
   4.1   Selection of Sampling Locations
   A total of 17 monitoring stations within the UK Automatic Urban Network (AUN) were selected for the study as follows:
   
   • Belfast Centre
     
   • Birmingham Centre
     
   • Bristol Centre
     
   • Cardiff Centre
     
   • Edinburgh Centre
     
   • Leeds Centre
     
   • Leicester Centre
     
   • London Bexley
     
   • Glasgow Centre*
     
   • Hull Centre
     
   • Manchester Centre
     
   • Middlesbrough Centre
     
   • Newcastle Centre
     
   • Southampton Centre
     
   • Sheffield Centre
     
   • Swansea Centre
     
   • Wolverhampton Centre
     
   Note: Glasgow Centre replaced Southampton Centre due to vandalism problems
   
   The sites were all at urban background locations, selected on the basis of uniform criteria within each city.
   
   Routine exchange of the diffusion tubes was carried out by the Local Site Operators for each AUN station. Each LSO was provided with an operation manual which provided background information to the survey and detailed instructions on the tube exposure programme.
   
   4.2   Sampling Techniques
   The diffusion tubes were mounted at the manifold inlet of each AUN station by means of two specially manufactured holders. These were held in place at the manifold by means of cable ties.
   The first mounting device consisted of a long arm, capable of holding 5 diffusion tubes in the vertical plane. The second mounting consisted of an inverted 'bucket', which was capable of holding 2 tubes in a sheltered position (see Figure A).
   
   Diffusion tubes manufactured to standard specifications were used throughout the survey, and were prepared by the AEA Technology laboratory. Tubes manufactured from black opaque perspex, supplied by Gradko Ltd, were used to represent the 'blackened' exposures.
   
   Within any given 4 week period, the following tube exposures were used;
   
   • 2-week exposures (Weeks 1 and 2, and Weeks 3 and 4)
     
   • One 4 week exposure (Weeks 1 to 4)
     
   • Two 2-week exposures with blackened tubes
     
   • One 4 week exposure with blackened tubes
     
   • Two 2-week exposures with tubes in sheltered locations
     
   • One 4 week exposure with tubes in sheltered locations
     
   
   A detailed system of tube numbering was used, with each tube assigned a unique code which also acted as guide for LSOs to aid in positioning the tube in the correct clip on the mountings. The coding system took into account the site location, the type of tube exposure, the number of weeks the tube was to be exposed for, and the month of monitoring. For example the code [MA B 1 1] relates to the Manchester AUN station (MA). The exposure type is Normal for a 4-week period (B). The month code is January (1), and the week code is 1, ie exposure started on Week 1.
   
   A tube code chart and a site code chart were contained within each LSOs manual to enable operatives to interpret the codes.
   
   A total of 9 tubes would have been exposed at each site in January, and as an example would therefore have been labelled as follows for the Manchester station;
   
   MA/A/1/1Normal 2-week exposure (weeks 1 & 2)
   MA/A/1/3Normal 2-week exposure (weeks 3 & 4)
   MA/B/1/1Normal 4-week exposure
   MA/C/1/1Blacked-out 2-week exposure (weeks 1 & 2)
   MA/C/1/3Blacked-out 2-week exposure (weeks 3 & 4)
   MA/D/1/1Blacked-out 4-week exposure
   MA/E/1/1Sheltered 2-week exposure (weeks 1 & 2)
   MA/E/1/3Sheltered 2-week exposure (weeks 3 & 4)
   MA/F/1/1Sheltered 4-week exposure
   
   4.3   Exposure Sheets
   With each monthly batch of tubes the site operators were provided with an exposure sheet which was designed in such a way as to act as a prompt for the positioning of tubes. This was used to record the date and time the tube first exposed and then recovered. The LSOs also used the sheet to record any additional notes that they thought relevant.
   
   4.4   Duplicate Tubes & Travel Blanks
   
   Four duplicate tubes were sent out each month to 4 sites, selected on a random basis, for exposure alongside the 4-week exposure tubes. In addition to the monthly duplicate tubes, travel blanks were sent out on a quarterly basis to all sites. These tubes were not exposed and were designed to detect any contamination which may occur to the tubes during transit.
   
   4.5   Analysis of Nitrogen Dioxide Concentrations
   
   Following exposure the tubes were returned to Stanger Science & Environment where they were processed and then passed for analysis to the AEA Technology laboratory. Any tubes which had been exposed for longer than the specified period were discounted at this stage. The nitrite concentrations were reported to SSE who then calculated the corresponding NO₂ concentrations.
   
   4.7   Operational Difficulties
   
   The main operational difficulty encountered throughout the survey was theft of the tubes. The problem was most acute in Southampton and Middlesborough, and these sites were eventually withdrawn from the survey due to continuing vandalism at the sites. The Southampton site was replaced by the Glasgow AUN station. Unfortunately, it was not possible to find a replacement for the Middlesbrough site.
   
   

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Study undertaken jointly by Stanger Science and Environment and the National Environmental Technology Centre.
Site prepared by the National Environmental Technology Centre, part of AEA Technology, on behalf of the UK Department of the Environment, Transport and the Regions