The emissions projections used here are the same as used in the projections presented by Stedman et al, 1998 and included in the review of the NAQS (DETR et al, 1999a, DETR et al, 1999b). It is reasonable to assume that road traffic emissions of NO_x will determine the additional NO_x or NO₂ measured at the roadside in comparison to that measured at nearby urban background locations (the roadside enhancement of NO_x or NO₂). Similarly, it was assumed that traffic emissions contribute 90 % of the total low level NO_x emissions in areas with the highest urban background concentrations. It was also assumed that this remaining 10% of emissions will remain unchanged between 1996 and 2005.

The National Atmospheric Emissions Inventory (NAEI) road transport model was been used to calculate estimates of urban road traffic NO_x emissions for 1996 and other years (Salway et al, 1999, Murrells, pers comm). The emissions in future years have been calculated for a 'business as usual' scenario, designed to represent the impacts of current national and international policies. The emissions estimates listed in Table 1 show that urban road traffic emissions of NO_x are expected to fall to about 44% of 1996 levels by 2005. If the remaining 10% of emissions contributing to background concentrations of NO_x remain at 1996 levels in 2005, then this leads to a reduction in 'background' emissions to 50% of 1996 levels.

The emissions projections used both here and in the work completed for the review of the NAQS were based on the NAEI road transport model as used to compile estimates of UK emissions for 1996 and subsequently published by Salway et al (1999). Emissions projections have recently been calculated using a revised model, which will be used to calculate an NAEI inventory for 1997. Table 2 shows a comparison of these new estimates with those that have been used in previous work. The revised estimates have lower emissions for 1996 and higher emissions for 2005. As a result urban road traffic emissions of NO_x are expected to fall to about 51% of 1996 levels by 2005, as opposed to the estimate of 44% of 1996 levels used in earlier work. The site specific projections of NO_x and NO₂ concentrations presented in this report have been based on the same emissions estimates as used previously for consistency with the review of the NAQS. This enables us to assess the reliability of the prediction methods that have been used in the review of the NAQS. Projections based on the revised emission estimates have been included in section 6 as a sensitivity analysis. Site specific and map based projections of future concentrations carried out for subsequent reviews of the NAQS will be based on the most up to date emission estimates available at the time. It will also be appropriate to revisit the assumption of 10% of 1996 emissions from non-road traffic sources at this stage and to compare historical trends in non-traffic emissions with monitoring results.