1 Introduction

AEAT-3133

1.1 Background
This is the third in a series of reports describing the high resolution air quality mapping work carried out at AEA Technology's National Environmental Technology Centre (NETCEN) on behalf of the Department of the Environment, Transport and the Regions (DETR). In previous reports we have presented maps of estimated background air pollutant concentrations for the UK for 1991 (Stedman, 1995) and for 1994 (Stedman et al, 1997b). This report contains a new set of maps for 1996 and describes the significant enhancements to the mapping procedure that have been made since the last report.

Maps of ambient background air quality are required for the following:

• Assessment of local air quality as part of the United Kingdom National Air Quality Strategy (DoE, 1997). Local urban background concentrations can be considered as a combination of regional background concentrations onto which contributions from sources in the more immediate area are added. Results from dispersion or other models can be used to estimate the impact of individual sources on local air quality. Ambient air quality near to sources, such as at the edge of a busy road or near to an industrial chimney, can be estimated by calculating the sum of this local impact and the background concentration.
  
• Air Quality assessments for the EU Framework and forthcoming Daughter Directives. Maps of background concentrations are valuable in assessing the exposure of the population in different areas to particular levels of air pollutants. This type of analysis may assist with compliance with the EU Framework and forthcoming Daughter Directives on Air Quality.
  
• Quantification of health impacts. Estimates of population exposure can be combined with dose-response relationships for the health impact of air pollutants to provide estimates of the magnitude of health impacts across the country (Stedman et al, 1997a, Department of Health, 1997).
  
• Comparison with critical levels. Maps showing estimated pollutant concentrations in rural and urban areas are also useful for comparison with critical levels for the effects of pollutants on materials and vegetation.
  

Many of the maps presented in this report are available on-line from the following web sites
/netcen/airqual/
http://www.environment.detr.gov.uk/airq/aqinfo.htm

1.2 General Approach to Mapping
Our approach to mapping air pollutant concentrations has been described in detail in our previous report (Stedman et al, 1997b). A brief outline is presented here. Measured annual mean air pollutant concentrations can be considered to be made up of two parts:

• A contribution from relatively distant major source areas such as power stations or large conurbations. Measurements from monitoring sites well away from local sources, from sites within the DETR rural networks, for example, provide good indications of the spatial variation of concentrations due to distant sources.
  
• A contribution from more local emissions. In earlier work on the estimation of air pollutant concentrations from emission related parameters (Stedman et al, 1997c) we have studied the spatial scale at which local emissions seem to influence ambient air quality. We found that estimates of emissions in an area of 25 km² centred on a background monitoring site provide the most robust relationships. (Air quality at the roadside or very near to a major industrial source will be strongly influenced by this local emission and measured concentrations will reflect a combination of background and nearby influences).
  

The difference, diff, between measured ambient pollutant concentrations at urban automatic monitoring sites (not roadside or industrial sites) and an underlying rural concentration field is calculated where monitoring data are available. A regression analysis is then performed to find a coefficient, k, for the relationship between diff and estimated emissions in the vicinity of the monitoring sites:

diff = k.emissions

This coefficient, which is the equivalent of an empirical box model coefficient, can then be used to derive a map of annual mean concentrations from a combination of a rural map and emissions inventory estimates. Thus automatic monitoring data is used to calibrate the relationship between ambient air quality and emissions inventories.

Section 2 describes the important changes to the mapping methods since our last report. Maps of estimated annual mean concentrations of a range of air pollutants are presented in section 3. The input data and coefficients used to calculate the maps are tabulated along with an analysis of the reliability of the maps in section 3, and a discussion of the maps is presented in section 4. This discussion also highlights the changes in mapping methods for each pollutant, from those used in previous work.

The maps presented here are of annual mean concentration for all pollutants except ozone. The national air quality standards each pollutant are listed in Table 1 and, for some pollutants, are defined in terms of shorter averaging periods than annual mean. The corresponding national air quality objectives are that these standards should be achieved at a particular percentile by 2005 (DoE, 1997). In some cases reasonably robust empirical relationships between annual mean concentrations and these high percentiles have been derived. A comprehensive review of these relationships has been compiled by Willis et al (1998) and some relationships have also been discussed by Stedman et al (1997b).

Table 1. UK air quality standards and objectives.

Pollutant	Standard		Objective - to be
	concentration	measured as	achieved by 2005
Benzene	5 ppb	running annual mean	5 ppb
1,3-Butadiene	1 ppb	running annual mean	1 ppb
Carbon monoxide	10 ppm	running 8-hour mean	10 ppm
Lead	0.5µg/m3	annual mean	0.5 µg/m3
Nitrogen dioxide	150 ppb 21 ppb	1 hour mean annual mean	150 ppb, hourly mean* 21 ppb, annual mean*
Ozone	50 ppb	running 8-hour mean	50 ppb, measured as the 97th percentile*
Fine particles (PM10)	50 µg/m3	running 24-hour mean	50 µg/m3 measured as the 99th percentile*
Sulphur dioxide	100 ppb	15 minute mean	100 ppb measured as the 99.9th percentile*

ppm = parts per million; ppb = parts per billion; µg/m³ = micrograms per cubic metre
* = these objectives are to be regarded as provisional.

INDEX