Overall, the backwards projections of concentrations at urban background sites show good agreement with the measurements, which gives confidence in the conclusions reached in the review of the NAQS. These conclusions were that inner London background sites are likely to be at risk of exceeding 21 ppb in 2005 but all other background sites should have concentrations lower than 21 ppb. There are however indications that one site in Scotland has not seen the expected decline in measured annual mean NO₂ concentrations in recent years.

Measured annual mean concentrations of both NO_x and NO₂ were generally higher in 1997 than in 1996, particularly set against the expected decline in emissions, presumably due to unusually poor dispersion of primary pollutants due to meteorological conditions during 1997. Backwards projections based on these two years therefore provide reasonable estimates of the range of mean values in earlier years. Measured concentration in 1998 were generally lower than would be expected due to the falling trend in emissions, again presumably due to the prevailing meteorological conditions which lead to unusually efficient dispersion of pollutants.

The backwards projections at roadside sites generally show reasonable agreement with the limited amount of monitoring data that is available. This confirms the conclusions in the review of the NAQS that many of the roadside sites included in the analysis are likely to be at risk of exceeding the 21 ppb objective in 2005.

Chapter 6         Chapter 8