The 1997 UK National Air Quality Strategy (NAQS) gives the following provisional objectives for nitrogen dioxide (NO₂) to be achieved by the end of 2005:

Annual mean: The annual mean must not exceed 21 ppb.

Hourly mean: The hourly mean must not exceed 150 ppb.

The recently published consultation document, The Air Quality Strategy for England, Scotland, Wales and Northern Ireland proposes the retention of the provisional annual mean objective and the replacement of the hourly mean objective by the following:

Hourly mean: 104.6 ppb, not to be exceeded more than 18 times a year.

The annual mean objective is likely to be the most stringent of the objectives, particularly at the roadside.

Site specific projections of annual mean concentrations for NO₂ and NO_x for all years between 1990 and 2005 are presented here by projecting both forwards and backwards from 1996 and 1997 monitoring data. Projecting backwards as well as forwards provides an indication of the reliability of the prediction methods that have been used in the review of the National Air Quality Strategy. These site specific projections have the advantage that they do not have the additional uncertainty associated with mapping methods.

The site specific projections presented in this report clearly illustrate the impact of emissions reductions on ambient NO_x concentrations and the correspondingly smaller reduction in annual mean NO₂.

Overall, the backwards projections 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.

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. This was presumably caused by unusually poor dispersion of primary pollutants during 1997 due to the meteorological conditions. 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 are 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.

Contents page         Chapter 1