1 Introduction

1.1 Background 1.2 Gravimetric Measurement Data 1.3 Relationship between 90 th percentile 24-hour concentrations and the annual mean

1.1 BACKGROUND
The Department of the Environment, Transport and the Regions (DETR) in conjunction with The Scottish Office and The Welsh Office has issued guidance under the 1995 Environment Act on the approaches that can be adopted to help identify areas within their locality at risk of exceeding the objectives prescribed in the Air Quality Regulations, 1997. The Government has recently published proposals to amend the National Air Quality Strategy (NAQS) (DETR et al, 1999): these proposals include an amendment of the air quality objective for PM₁₀. The proposed new objectives for PM₁₀ (measured by gravimetric or equivalent methods) for 31 December 2004 are:

• an annual objective of 40 mg m-3;
  
• a 24 hour objective of 50 mg m-3 with a maximum of 35 exceedences (approximately 90 th percentile of 50 mg m-3).
  

The Government commissioned AEA Technology, Stanger Science and Environment and Air Quality Consultants to prepare informal technical information for local authorities on the approaches that they might take to help identify areas within their locality at risk of exceeding these new objectives for PM₁₀ (Moorcroft et al, 1999).

This report summarises some research carried out using dispersion models and the development of new maps of estimated background PM₁₀ concentrations in support of the development of this technical information for Local Authorities on the review and assessment of PM₁₀. The research covered:

• the development of a simple screening tool for the assessment of emissions of particles from domestic coal burning;
  
• the development of screening tools for the assessment of industrial stack emissions;
  
• maps of estimated secondary PM₁₀ for 1996 and total PM₁₀ for 2004
  

and is described in this report.


1.2 GRAVIMETRIC MEASUREMENT DATA
The amended air quality objectives for PM₁₀proposed within the review of the NAQS (DETR et al, 1999) are derived from the stage 1 limit values of the European Union 'Daughter Directive'. The reference monitoring method for these limit values is gravimetric but equivalent methods can also be used to implement the Directive. Monitoring of PM₁₀levels in the UK has, to date, been largely based upon the use of TEOM analysers. A principal concern with the TEOM instrument is that the filter is held at an elevated temperature (50°C) in order to minimise errors associated with the evaporation and condensation of water vapour. This can lead to the loss of the more volatile species (some hydrocarbons, nitrates etc.) and has led to the identification of differences between TEOM and gravimetric measurements at co-located sites. The recently published APEG report (APEG, 1999) concluded that at concentrations around 50mg/m³ the TEOM tends to under-read by between 15 and 30% compared with a gravimetric sampler. However, this effect is not constant, and varies depending upon the mass concentration, the distance from a specific source, and the environmental conditions.

There is therefore a potential inconsistency between measurements of PM₁₀concentrations made using a TEOM or equivalent analyser and the proposed objectives - for example, a daily mean concentration of 45 mg/m3 measured using a TEOM or equivalent analyser could be underestimating the 'gravimetric' or equivalent concentration by 15 mg/m3 or more. It is therefore necessary to apply a 'correction factor' when assessing TEOM or equivalent measured concentrations against the proposed objectives. For the purpose of the review and assessment procedure Moorcroft et al, (1999), a constant factor of 1.3 has been used (consistent with the APEG report), and is applied to TEOM measured concentrations.

Concentrations of PM₁₀throughout both this report and the published informal technical information are therefore referred to as either (mgm^-3, gravimetric) meaning gravimetric or equivalent measurements or (mgm^-3, TEOM) meaning TEOM or equivalent measurements.


1.3 RELATIONSHIP BETWEEN 90 TH PERCENTILE 24-HOUR CONCENTRATIONS AND THE ANNUAL MEAN

Analysis of data from urban monitoring stations in the UK national monitoring networks indicates that the following relationship between the 90 th percentile fixed 24-hour PM₁₀average concentration (i.e. 35 exceedences per annum) and the annual mean is reasonably robust:


90 th_percentile = 1.79 x Annual_mean

There is therefore a risk that the proposed 24-hour objective of 50 mgm^-3, gravimetric will be exceeded if the annual average PM₁₀concentration is more than 28 mgm^-3, gravimetric.


Executive Summary          Section 2

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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