2 Emission projections

It is reasonable to assume that road traffic emissions of NOx will determine the roadside enhancement of NOx and NO2 concentrations. Similarly, we have assumed that traffic emissions contribute 90 % of the total low level NOx emissions in areas with the highest urban background concentrations. We have also assumed that the remaining 10% of emissions will remain unchanged between now and 2005 or 2009.

The National Atmospheric Emissions Inventory (NAEI) road transport model has been used to calculate estimates of urban road traffic NOx emissions for 1996 and for future years (Murrells, pers comm). The emissions estimates listed in Table 2.1 show that urban road traffic emissions of NOx are expected to fall to about 44% of 1996 levels by 2005. If the remaining 10% of emissions contributing to background concentrations NOx remain at 1996 levels in 2005, then this leads to a reduction in 'background' emissions to 50% of current levels. Urban road traffic emissions are expected to fall to about 33% of 1996 levels by 2005, leading to a reduction in 'background' emissions to 40% of current levels.

Table 2.1 UK urban road traffic emissions of NOx and emission reduction factors relative to 1996

	1996	2005	2009
UK urban road traffic emissions (kTonnes per year)	370	162	123
urban road traffic (roadside enhancement), relative to 1996	-	0.44	0.33
background, relative to 1996	-	0.50	0.40