1 Introduction

1.1 BACKGROUND
The UK Government is taking active measures to improve air quality through the National Air Quality Strategy (NAQS). This Strategy defines Air Quality Standards and Objectives for eight pollutants and identifies their major sources. The NAQS gives the following objectives for nitrogen dioxide (NO2) to be achieved by the end of 2005:
In addition the European Union 'Daughter Directive' gives the following proposed limit values for NO2 to be achieved by 1 January 2010:
The annual mean objective and limit value are likely to be the most stringent of the targets, particularly at the roadside.

The Government has recently published a consultation document on the Review of the NAQS (DETR et al, 1999). This report sets out proposals for amending the NAQS following a review carried out during 1998.

The national modelling of roadside NO2 concentrations (Stedman et al, 1998), carried out as part of this review, indicated that policies currently in place or to take effect before 2005 will lead to the annual average objective being achieved at all background locations, except inner London, and at most roadside locations by 2005. However, the national modelling identified a number of major urban road links where concentrations at the roadside may exceed the objective.

One of the key uncertainties within the national modelling of NO2 is the question of public exposure. The approach adopted in the Strategy is to apply the objectives where members of the public are likely to be exposed over the averaging time of the objective. This includes roadsides in the case of annual averages, but only where there are houses, schools, hospitals etc. along the road, and then only when the building facade is close to the kerb - the location thought to best represent long term exposure. The national models used for the review of the NAQS simply predicted NO2 at the roadside and did not consider the location of housing. NO2 concentrations also decrease significantly with distance away from the immediate vicinity of the road. If housing is located more than about 10 m from the edge of the road, then NO2 levels there are likely to be lower than the roadside levels predicted by the model. A preliminary assessment of a sample of road links outside London predicted to exceed the annual average NO2 objective indicated that a significant proportion have no people living close to the road.

1.2 STUDY OBJECTIVE
The main objective of this pilot study was to provide a semi-quantitative estimate of the population likely to be exposed to roadside NO2 concentrations greater than 21 ppb annual mean in 2005 as a result of housing adjacent to major roads (within 10 m of the edge of the road) in UK cities outside London. Additional work has been carried out within London, consisting of a case study area along the North Circular Road (A406).

A secondary objective of the study was to consider if the national scale traffic flow data are consistent with local inventories and whether those road links identified in the national modelling are consistent with those considered to be an issue by local authorities.

1.3 REPORT STRUCTURE
This report describes the methodology used in the field survey and in the analysis of the survey data and additional GIS data (in section 2). The results are presented in section 3. Conclusions are provided in section 4.

Executive Summary          Methodology
Report and site prepared by the National Environmental Technology Centre, part of AEA Technology, on behalf of the UK Department of the Environment, Transport and the Regions