The Commission of the European Communities has recently published a proposal for a Directive relating to limit values for benzene and carbon monoxide (CO). 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. A common position was reached on the first Daughter Directive, which will set limit values for sulphur dioxide, nitrogen dioxide, lead and particles.

The proposed limit value for CO is 10 mgm^-3 as an 8-hour running mean to be achieved by 1 January 2005. This is equivalent to 8.59 ppm at the standardisation of 293 K and 101.3 kPa specified in the proposal. This can be compared with the both the UK National Air Quality Strategy (NAQS) objective of 10 ppm as an 8-hour running mean to be achieved by 2005 (DoE, 1997) and the proposal to amend this objective of 10 ppm to be achieved by 2003 (DETR et al, 1999).

The analyses of roadside CO concentrations carried out as part of the review of the NAQS were limited to a 'worst case' analysis of roadside concentrations in Central London based on a simple addition of monitoring data from roadside and background monitoring sites and emissions projections. This analysis showed that policy measures already in place should lead to 8-hour CO concentrations at all roadside locations falling below 10 ppm by 2003 (DETR et al, 1999).

The proposal of a Daughter Directive limit value for CO of 8.59 ppm means that it is now appropriate to carry out a more detailed analysis of likely roadside CO concentrations in the UK. This report describes the development of such a method for calculating maps of predicted roadside CO concentrations in the UK. The method makes use of emissions estimates from the National Atmospheric Emissions Inventory and monitoring data from roadside monitoring sites within the UK national air quality monitoring networks. The empirical model of roadside CO presented here is the first attempt to estimate maximum CO concentrations at the roadside of urban major roads throughout the UK. The model is clearly subject to a number of uncertainties, including those due to the limited number of roadside monitoring sites in the UK national networks for which data are available. Maps are presented for both 1998 and 2004 for comparison with the proposed limit value (to come into force 1 January 2005).

A related report on 'Estimated benzene concentrations in the UK and proposed EU limit value' (Stedman, 1999) has already been prepared.

Executive Summary         Chapter 2