The 1997 National Air Quality Strategy (DoE et al, 1997) included the following specific objectives for benzene and 1,3-butadiene, to be achieved by the end of 2005:

benzene: 5 ppb, measured as a running annual mean

1,3-butadiene: 1 ppb, measured as a running annual mean

The recently completed review of the NAQS (DETR, et al, 1999) included a proposal to bring the date for achievement of these objectives forward to the end of 2003.

The Commission of the European Communities has published a proposal for a Directive in relation to limit values for benzene and carbon monoxide. This is the second Daughter Directive to be brought forward by the European Commission under the Framework Directive on Ambient Air Quality Assessment and Management. The proposed limit value for benzene is 5 m gm^-3 (1.542 ppb), as an annual mean, to be achieved by 1 January 2010.

The projections of roadside benzene and 1,3-butadiene that have been published in the review of the NAQS (DETR et al, 1999, Stedman and Dore, 1998) were based on emissions projections derived from the 1996 National Atmospheric Emissions Inventory (NAEI) road transport model, details of which can be found in Salway et al, 1999. Projections of roadside concentrations for comparison with the proposed EU Daughter Directive limit value for benzene (Stedman, 1999) were based on the same emissions projections. Emissions projections based on the 1997 NAEI road transport model are now available and estimates of roadside concentrations have been re-calculated using these revised projections. The 1997 NAEI road transport model incorporates an improved treatment of cold start and evaporative emissions and the likely changes in emissions over time. Tables 1 and 2 show comparisons of UK urban road traffic emissions projections for both benzene and 1,3-butadiene. These tables show that emissions in 2005 as a percentage of 1996 emissions are very similar for 1,3-butadiene for the two emissions projections but a rather smaller decline in benzene emissions is predicted according to the 1997 NAEI. Both the 1996 NAEI and 1997 NAEI based emissions projections are designed to represent the impact of current national policies.

The estimates of roadside benzene and 1,3-butadiene that were published in the review of the NAQS did not include roads in Northern Ireland because the assignment of roads as built up or non-built up was not available. The same model has been used to derive the estimates presented in this report and the results are tabulated for direct comparison with earlier work. Analysis of roadside concentrations of benzene, 1,3-butadiene and other pollutants for subsequent reviews of the NAQS will include roads in Northern Ireland.