Site specific projections for urban background sites are shown in Figures 2 to 22.

Figure 2   Comparison of measured NOx and NO2 concentrations at West London with projections based on both 1996 and 1997 monitoring data.
Figure 3    Comparison of measured NOx and NO2 concentrations at Glasgow City Chambers with projections based on both 1996 and 1997 monitoring data.
Figure 4   Comparison of measured NOx and NO2 concentrations at Manchester Town Hall with projections based on both 1996 and 1997 monitoring data.
Figure 5   Comparison of measured NOx and NO2 concentrations at Walsall Alumwell with projections based on both 1996 and 1997 monitoring data.
Figure 6   Comparison of measured NOx and NO2 concentrations at Billingham with projections based on both 1996 and 1997 monitoring data.
Figure 7   Comparison of measured NOx and NO2 concentrations at Sheffield Tinsley with projections based on both 1996 and 1997 monitoring data.
Figure 8   Comparison of measured NOx and NO2 concentrations at London Bridge Place with projections based on both 1996 and 1997 monitoring data.
Figure 9   Comparison of measured NOx and NO2 concentrations at London Bloomsbury with projections based on both 1996 and 1997 monitoring data.
Figure 10   Comparison of measured NOx and NO2 concentrations at Edinburgh Centre with projections based on both 1996 and 1997 monitoring data.
Figure 11   Comparison of measured NOx and NO2 concentrations at Cardiff Centre with projections based on both 1996 and 1997 monitoring data.
Figure 12    Comparison of measured NOx and NO2 concentrations at Belfast Centre with projections based on both 1996 and 1997 monitoring data.
Figure 13   Comparison of measured NOx and NO2 concentrations at Birmingham Centre with projections based on both 1996 and 1997 monitoring data.
Figure 14   Comparison of measured NOx and NO2 concentrations at Newcastle Centre with projections based on both 1996 and 1997 monitoring data.
Figure 15    Comparison of measured NOx and NO2 concentrations at Leeds Centre with projections based on both 1996 and 1997 monitoring data.
Figure 16   Comparison of measured NOx and NO2 concentrations at Bristol Centre with projections based on both 1996 and 1997 monitoring data.
Figure 17   Comparison of measured NOx and NO2 concentrations at Hull Centre with projections based on both 1996 and 1997 monitoring data.
Figure 18   Comparison of measured NOx and NO2 concentrations at Hull Centre with projections based on both 1996 and 1997 monitoring data.
Figure 19   Comparison of measured NOx and NO2 concentrations at Hull Centre with projections based on both 1996 and 1997 monitoring data.
Figure 20   Comparison of measured NOx and NO2 concentrations at Leicester Centre with projections based on both 1996 and 1997 monitoring data.
Figure 21   Comparison of measured NOx and NO2 concentrations at Southampton Centre with projections based on both 1996 and 1997 monitoring data
Figure 22   Comparison of measured NOx and NO2 concentrations at London Bexley with projections based on both 1996 and 1997 monitoring data.

Projections based on both 1996 and 1997 annual mean data have been calculated for sites for which at least five years of monitoring data are available for the period from 1990 to 1998. Sites classified as Urban Background, Urban Centre and Suburban (Broughton, et al, 1998) have been included in this analysis. The graphs show the following information:

• projected annual mean NO₂ based on 1996 NO_x measurements, calculated from NO_x projections using the non-linear relationship between NO_x and NO₂ derived from 1997 UK national monitoring data at background sites.
  
• measured annual mean NO₂.
  
• projected annual mean NO_x, based on 1996 NO_x measurements.
  
• measured annual mean NO_x.
  
• projected annual mean NO₂ based on 1997 NO_x measurements, calculated from NO_x projections using the non-linear relationship between NO_x and NO₂ derived from 1997 UK national monitoring data at background sites.
  
• projected annual mean NO_x, based on 1997 NO_x measurements.
  
• NAQS linear projected NO₂, based on 1996 NO₂ measurements, these are the values included in Table A9 of the review of the NAQS report. A linear NO₂ based method was used for consistency with the mapped projections of background NO₂ in the same report.
  
• NAQS linear projected NO₂, based on 1996 NO₂ measurements, these are the values included in Table A9 of the review of the NAQS report.
  
• 21 ppb line, the NAQS objective for the end of 2005.
  

The non-linear NO_x based projections of NO₂ are generally equal to or lower than the NO₂ based linear projections included in the review of the NAQS (DETR et al, 1999a, DETR et al, 1999b). The non-linear projections are lower at sites where the current measured NO₂/NO_x ratio is unusually high. The predictions of current NO₂ (derived from the current NO_x measurements) are lower than the measured values for these sites.

Overall, the backwards projections 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 (Glasgow City Chambers, see below) 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.

Projected NO₂ concentrations at London Bridge Place and London Bloomsbury are not consistent with the measurements. The projections consistently under predict NO₂ concentrations at London Bridge Place due to the unusually high NO₂/NO_x ratio at this site. Preliminary results from a detailed examination of measured annual mean NO_x and NO₂ concentrations at a range of monitoring sites over the period from 1990 to 1998 indicate that a linear NO₂ to NO_x may provide a better fit with changes in measured concentrations over time. This linear relationship is very similar to the non-linear relationship adopted here at all sites except those with the highest measured NO_x concentrations, such as London Bridge Place. Both NO_x and NO₂ concentrations show little reduction in concentrations at London Bloomsbury compared with that seen at other sites. These two inner London sites are therefore expected to be at risk of exceeding the annual mean NO₂ objective in 2005.

Measured concentrations at Glasgow City Chambers show no reduction in NO₂ as NO_x levels decline. Preliminary results from a detailed examination of measured NO_x and NO₂ concentrations at this monitoring site over the period from 1990 to 1998 show that this unusual behaviour is confined to the winter months, with summer concentrations declining as expected with reductions in measured NO_x. The lack of response to changes in NO_x during the winter is sufficient to cause the annual mean concentrations to remain approximately constant from year to year. This unusual behaviour of winter mean NO₂ concentrations is also seen at Edinburgh Centre but the effect is smaller at this site and annual mean concentrations are seen to decline with reductions in measured NO_x as expected between 1993 and 1998.

Sites such as Sheffield Tinsley, Walsall Alunwell and Southampton Centre show projected NO₂ higher than measurements, due to the greater influence of nearby roads on NO_x than on NO₂.

There is excellent agreement between projections and measurements at West London, Cardiff Centre, Belfast Centre, Leeds Centre, Liverpool Centre, Birmingham East, Hull Centre, Leicester Centre and London Bexley.

Chapter 3         Chapter 5