Trends in NOx and NO2 emissions from road traffic in Great Britain

Abstract In this paper we have compared detailed road traffic NO x emissions with the equivalent trends in ambient roadside NO x measurements. This was undertaken separately in Great Britain and London, for all of the major roads, and by road type and location. The emissions trends were created using different emissions factors, those used in UK emissions inventories (Base case), the Swiss-German handbook (HBEFA), and those created from recent remote sensing measurements in the UK (RSD). An alternative assumption for use of Selective Catalytic Reduction (SCR) in the articulated Heavy Goods Vehicle (HGV) fleet was also tested. For all scenarios traffic flows, speeds and vehicle age were kept constant. Comparison between the emissions scenarios in Great Britain showed that by 2009, NO x emissions estimates from road traffic could be as much as 25% greater than current UK estimates and 31% greater in London. The RSD emissions inventory gave the smallest downward trend in NO x emissions, from 2004 to 2009, of between 3 and 4%/year and this compared with a reduction of ∼6%/year from the Base Case and HBEFA scenarios. All scenarios compared poorly with roadside NO x measurement trends from UK sites, which typically reduce by between 1% and 2%/year. We have shown that the differences in NO x emissions trends were driven, partially at least, by the relative contribution from light duty diesel vehicles. An analysis from 2700 NO x measurement sites throughout Europe has shown that this problem is unlikely to be limited to the UK, and identifies a difficulty in meeting EU limit values for NO 2 , obligations under the National Emission Ceilings Directive ( NECD, 2001 ) and the Gothenburg Protocol ( UNECE, 1999 ) and for forecasting future changes in PM 2.5 .