Executive Summary

The UK National Air Quality Strategy gives the following objectives for benzene and 1,3-butadiene, to be reached by 2005:

Benzene:	The maximum annual running mean must not exceed 5 ppb
1,3-Butadiene:	The maximum annual running mean must not exceed 1 ppb

National measures are likely to deliver significant reductions in the emissions of these two pollutants from road traffic sources between now and 2005. It is therefore important to examine current concentrations of these pollutants at roadside locations and to determine whether current policies are likely to reduce concentrations at these locations sufficiently to meet the objectives by 2005.

An empirical model for predicting roadside concentrations of benzene and 1,3-butadiene in the UK has been developed. This model uses previously published maps of estimated background concentrations as it's starting point, to which a "roadside enhancement" of concentrations is added. The empirical relationships between this "roadside enhancement" and individual road link emissions estimates have been derived from an analysis of both automatic monitoring data from the Marylebone Road monitoring site and diffusion tube monitoring data from a more extensive group of sites.

Our estimates of roadside concentrations for 2005 indicate that current policies are likely to lead to roadside concentrations well within the objectives. Concentration estimates for intermediate years have also been calculated in order to provide an indication of the earliest date at which the objective concentrations are likely to be achieved. The benzene objective is likely to be achieved by 2002 and the 1,3-butadiene objective by 2004. Our calculations also show that, at most, only one or two roads will be at risk of exceeding the benzene and 1,3-butadiene objectives in 2001 and 2003 respectively.

Urban background concentrations of benzene and 1,3-butadiene have also been calculated for 2005 at the locations of sites on the UK Hydrocarbon Monitoring Network. Several emission reduction scenarios have been used. The highest concentrations of benzene and 1,3-butadiene, 0.78 and 0.14 ppb respectively, are arrived at using a "business as usual" scenario.

Concentration predictions have also been made with respect to the 1 ppb target value for benzene, set by EPAQS. Estimates indicate that over 500 road links will exceed this target value in 2010, although the highest concentration is estimated to be 1.3 ppb with concentrations rapidly falling away to 1.0 ppb. Some 80% of the 500 road links are located in the London area. Results from hypothetical emission reduction scenarios have been investigated, and it is postulated that a national reduction in traffic emissions of 30% is not adequate to attain the target value for benzene in 2010- the majority of exceedances occurring in the London area.