Cornwall Air Quality Forum

In the Review of the National Air Quality Strategy (NAQS January 1999) the Government has proposed a new hourly mean objective for NO2 of 104.6 ppb to be achieved by 2005, (as opposed to the existing objective of 150 ppb), and an annual mean of 21 ppb to be achieved by 2005. These objectives apply in non-occupational, near-ground level, outdoor locations where people might reasonably be expected to be exposed over the relevant averaging period.

An extensive NO2 diffusion tube survey was undertaken at 48 sites in Truro with the aim of producing an isolevel concentration maps of the pollutant over time. The technique of high resolution spatial monitoring yields detailed information about the distribution of airborne pollutants over an urban territory, supplying the authority with a powerful tool for town planning at a very low cost. A result usually associated with more traditional and expensive monitoring techniques.

NO2 diffusion tubes were exposed at 48 sites in and around Truro for four two-week periods (see Appendix 2). The sites were distributed along the knots of a grid drawn over a city map. While maintaining the number of sampling sites consistent with the available financial resources, the application of the grid enabled the study to cover a wide and varied area of the city and also provided a spread of sampling sites suitable for modelling. Two blank tubes were allocated for each 2-week programme in order to ensure the integrity of the tube batch. Diffusion tubes were supplied by Gradko International Ltd, Winchester, Hants. Gradko International Ltd also analysed the tubes (see Appendix 3)

  • Diffusion tubes can be used for spatial and temporal assessments.
  • They are portable, easily sited and cheap.
  • Results are directly comparable to environmental standards (21 ppb as an annual objective).
  • Precision is usually
    • There is no standard method or validation for diffusion tubes.
    • The results are averaged over the monitoring period and give no indication of short-term peaks.
    • The reliability of diffusion tubes and of diffusion tube monitoring has been questioned.

    Over the entire monitoring period 9 tubes were lost due to vandalism.

    Programme 1 Site 15. Station Rd Site 29. St Georges Rd Site 31. Thurlby
    Programme 2 Site 10. Redsnick Cr Site 14. Chapel Hill Site 44. Kenwyn Hill
    Programme 3 Site 44. Kenwyn Hill
    Programme 4 Site 31. Thurlby Site 44. Kenwyn Hill

    2.3. Sites The sites comprised the following:
    26% background
    28% hotspots
    45% periurban

    • The background sites are open spaces (squares or parks) or streets apart from intense traffic.
    • The hotspots coincide with road crossings or roads with intense or slowed down traffic.
    • The periurban sites were chosen in peripheral areas with scarce or very flowing traffic.

    The diffusion tube data was modelled on to four maps and the results shown as colour contour maps detailing NO2 levels.

    NAQS objectives of 104.6 ppb as an hourly mean may well have been exceeded at some of the sites monitored. It is also probable that the annual mean of 21 ppb would also be exceeded.

    As mentioned in Section 2.2.3. one of disadvantages of using diffusion tubes is that results are averaged over the whole of the monitoring period, in this case over a two-week period and any hourly peaks of NO2 levels will not be shown. As some average levels were greater than 60 ppb it is probable that hourly peaks were in excess of the NAQS hourly objectives.

    Table 1 shows tabulated results for all monitoring sites for all four programmes and these results are displayed on maps 1 – 4. Inspection of Table 1 shows that high levels occurred at the same sites for all four programmes.

    All four programmes also showed levels of NO2 > 30 ppb at city centre sites. This suggests that not only is traffic related pollution responsible for high levels at sites of high traffic density but that levels remain high within the confines of the city centre.

    Although some studies concerning the reliability of diffusion tube monitoring have suggested that tubes overestimate NO2 levels (Heal, 1999) more recent European studies (Lebret, 2000) have found that there is good agreement between samplers and that hot spots picked up in one survey tended to be reliably picked up in following surveys. These findings are reinforced by the Truro study where similar levels were recorded at all sites for consecutive weeks.

    Although highest levels of NO2 were recorded at sites of greatest traffic density it is worth noting that levels of 30-40 ppb were consistently recorded at town centre sites. These data should be considered when traffic planning decisions are being made.

    It is recommended that further surveys be undertaken during the winter months when traffic flow is not greatly influenced by influxes of tourist traffic. Traffic related pollution may also be influenced by the greater levels of pollution caused by cold starting of cars and the prevailing weather conditions. It is also recommended that the area being surveyed be extended to the south-east and south-west to encompass areas of high traffic flow – particularly around the Royal Cornwall Hospital Trust at Trelisk.

    In order to assess hourly NO2 levels a continuous monitoring programme would be required. This would involve the use of real-time continuous monitoring equipment at selected sites.

    Heal, M et al. (1999). Overestimation of urban nitrogen dioxide by passive diffusion tubes: a comparative exposure and model study. Atmospheric Environment. 33, 513 – 524.

    Lebret, E. et al. (2000). Small variations in ambient NO2 concentrations in four European areas. Atmospheric Environment. 34, 177 – 185.

    Mukala, K. et al. (1999). Personally measured weekly exposure to NO2 and respiratory health among pre-school children. European Respiratory Journal. 13, 1411-1417.

    All sampling results are the property of Carrick District Council, and are subject to strict confidentiality and cannot be disclosed to any third party without prior formal permission from Carrick District Council. Cornwall College cannot accept any responsibility for the use to which the information is put nor for decisions, inferences or conclusions that are made on the basis of the information provided. No responsibility is taken for the accuracy of the sampling unless this is done under our own supervision.

    Prog 1 Prog.2 Prog.3 Prog.4
    Site No Location mg/m3 mg/m3 mg/m3 mg/m3
    1 Treyew/ Daniell Rd 23.18 17.26 26.88 18.43
    2 Green Close 10.98 11.52 17.10 12.67
    3 Park View 14.68 11.50 14.66 14.98
    4 Tregolls Road (a) 42.82 37.41 46.49 31.11
    5 Malpas Road (a) 9.76 8.10 13.48 10.37
    6 Trennick Lane 9.76 9.24 11.03 9.22
    7 Malpas Road (b) 20.74 19.64 22.06 21.92
    8 Tregolls Road (b) 31.72 38.02 41.47 36.87
    9 Lemon Street/ Strangways Terr 17.08 18.43 20.74 17.28
    10 Redsnick Crescent 9.82 N/a 13.44 10.37
    11 Treview Road 35.38 22.97 30.63 25.35
    12 Treyew Mills Rd 17.13 12.69 15.88 13.82
    13 Highertown/Chapel Hill 22.02 26.50 47.64 28.80
    14 Chapel Hill 19.58 N/a 21.99 20.74
    15 Station Road n/a 26.50 41.54 33.41
    16 The Crescent 8.54 10.34 12.22 10.37
    17 Richmond Hill 18.30 20.74 31.72 19.59
    18 Kenwyn Street 18.30 17.28 18.30 18.43
    19 Lemon Quay 21.96 25.35 31.76 31.11
    20 St Clements Hill/ Trennick Lane 13.42 12.71 13.50 14.98
    21 St Clements Close 14.64 8.09 12.25 10.37
    22 St Clements Hill / RFC 30.50 25.42 25.73 26.65
    23 Trefusis 8.54 8.09 12.25 9.22
    24 Trevidden Close 8.54 11.56 13.48 13.82
    25 Tregolls Rd ( c) 67.09 64.90 59.77 60.00
    26 Campfield Hill 19.40 16.13 25.69 23.04
    27 Pydar Street 37.81 35.71 39.09 35.71
    28 Stokes Road 11.01 10.35 14.68 13.82
    29 St Georges Road N/a 8.06 14.66 10.37
    30 Barton Meadow 9.76 8.06 12.22 9.22
    31 Thurlby N/a 10.37 13.44 N/a
    32 Higher Trehaverne 9.79 11.52 9.82 11.52
    33 Rosedale Road 8.56 9.22 12.23 9.22
    34 Moresk Rd 8.56 10.31 12.27 11.52
    35 Trevithick Lane 8.54 10.37 9.76 10.40
    36 Midway Drive 7.32 9.23 12.25 9.22
    37 Tregolls Road (d) 35.38 32.35 29.36 32.26
    38 Tregolls Road (e) 53.67 55.46 46.49 44.93
    39 Cornubia Close 4.88 8.08 13.48 9.22
    40 Tregurra Lane 12.20 16.20 18.41 12.67
    41 Allot Gardens 9.79 9.22 9.79 9.22
    42 Bishops Close 4.91 6.91 8.58 8.06
    43 Chainwalk Dr/ Catherdral View 6.12 9.22 9.79 8.06
    44 Kenwyn Hill 17.13 N/a N/a N/a
    45 Bishops Temple Road 8.54 11.54 11.03 10.37
    46 Tregollis Road (f) 40.25 33.66 45.13 42.69
    47 Boscowen Street 24.20 21.89 28.10 20.74
    48 Newham Rd, (past Tesco) 17.08 15.04 20.83 18.43

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