Overview Introduction       Sources of PM10      National perspective      General Approach      Internet Maps
Introduction

1. This document has been prepared by Stanger Science & Environment (SSE), in association with Air Quality Consultants (AQC) and the National Environmental Technology Centre (NETCEN), on behalf of the Department of the Environment, Transport and the Regions (DETR), the Welsh Office and the Scottish Office. It's aim is to provide informal assistance to those local authorities who may wish to assess the likelihood of areas within their locality exceeding the proposed EU Stage 1 Limit Values for PM₁₀.
   
2. The Government has recently published its proposals for review of the National Air Quality Strategy (NAQS)1, and in particular its proposals for revised and additional objectives for the pollutants. During the review process, as a result of the work of the Airborne Particles Expert Group (APEG), it became apparent that the original PM₁₀ objective was unrealistic. The APEG report2 also highlighted the importance of European sources and the limit to which local authority action could control levels of PM₁₀. The Government has therefore proposed replacing the objective in regulation with the proposed EC Air Quality Daughter Directive Stage 1 Limit Values (see Box 1).
   
   BOX 1: Summary of the existing and proposed objectives         
   

   Existing Objective	Proposed Objectives
   50 mg/m3, measured as the 99th percentile of the daily maximum running 24 hour mean (equivalent to 4 exceedances per year), to be achieved by 31.12.2005 Objective is based on measurements carried out using the TEOM analyser, or equivalent	40 mg/m3 as the annual mean, and 50 mg/m3 as a fixed 24 hour mean, maximum of 35 exceedances per year (approximately equivalent to the 90th percentile) to be achieved by 31.12.2004 Objectives are based on the European gravimetric transfer reference sampler or equivalent

   
   
3. Following consultation, the Government may conclude that the national objective for PM₁₀ should be replaced in regulation by the EC Stage 1 Limit Values. Such a change would clearly have significant implications for the scale of work local authorities undertake on the review and assessment of PM₁₀ and the conclusions they reach.
   
4. This note suggests a methodology which may be used by local authorities to assess PM₁₀ levels against the proposed new objectives. The methods described within this note follow the phased approach to review and assessment previously described within the LAQM Technical Guidance Note series (LAQM.TG1 to LAQM.TG4)^3,4,5,6. The intention of that guidance is that local authorities should only undertake as much work as necessary, commensurate with the extent of air quality problems in their area, and the likelihood of a prescribed objective being exceeded.
   
5. The First Stage review and assessment is intended to assist the local authority undertake an initial screening of industrial, transport and other PM₁₀ sources which may have a significant impact within, or close to their boundary. If this initial screening suggests that an exceedance of the proposed new objectives is likely, then the authority may proceed to the Second Stage review and assessment, which is likely to involve the use of simple screening models. In certain circumstances, the authority may choose to proceed directly to the Third Stage review and assessment, ignoring the suggested Second Stage. However, the simple methodologies suggested for the Second Stage may form a useful next step in preparing for the more detailed Third Stage work.
   
6. The aim of the Second Stage review and assessment is to provide a further screening of PM₁₀ concentrations within local authority areas. It is not intended that it should provide an accurate prediction of current or future air quality across the whole of the authority's area, nor does it require an authority to estimate every potential area of exceedance. Authorities are advised to focus upon locations where the highest PM₁₀ concentrations are likely to occur (highlighted by the First Stage), and to determine whether there is a significant risk of the proposed objectives being exceeded by the end of 2004.
   
7. Where the Second Stage review and assessment indicates that the proposed objectives will not be achieved by the end of 2004, or where there are significant uncharacterised PM₁₀ sources which cannot be adequately assessed by screening methods, then the authority may proceed to the Third Stage review and assessment. This may require the use of detailed emissions inventories, modelling and monitoring work, and should only be undertaken if necessary. It is also recognised that some authorities may be already undertaking a detailed Third Stage review and assessment of their area, and may not need to carry out the Second Stage review and assessment. However, it is strongly recommended that all authorities complete the information for the First Stage review and assessment, as set out in the guidance note 'Framework for review and assessment of air quality' LAQM.G1(97).
   
   

   BOX 2: Gravimetric vs TEOM measurement data

   
   
8. It should be noted that the methodology described within this note reflects the most up-to-date understanding of airborne particles in the UK, their concentrations, sources, and methods of predicting future levels. However, the science surrounding airborne particulates is rapidly evolving, and further research to improve our knowledge is in progress. It is inevitable that the assessment methodologies may need to be revised at some stage in the future, to better reflect our understanding of both sources and emissions. It is not the intention of this document to be prescriptive in all circumstances, and it is recognised that local authorities will need to use their professional and technical judgement in applying the detail of review and assessment, in the light of local circumstances.
   
   Sources of PM10
   
   
9. There is a wide range of emission sources which contribute to PM₁₀ concentrations in the UK. These can usefully be divided into 3 main source categories:
   
   • Primary Combustion Particles - particles emitted directly from combustion processes such as road traffic, power generation, industrial combustion processes etc. These particles are generally less than 2.5 µm and often well below 1 µm in diameter;
     
   • Secondary Particles - particles formed in the atmosphere following their release in the gaseous phase. These include sulphates and nitrates, formed from emissions of SO2 and NO_x; these particles are again generally less than 2.5 µm in diameter;
     
   • 'Coarse' or 'Other' Particles - the so-called 'coarse' or 'other' particles component comprises of emissions from a wide range of non-combustion sources. These include resuspended dust from road traffic, construction and mineral extraction processes, wind-blown dusts and soils, and sea salt. These particles are generally greater than 2.5 µm in diameter.
     
10. There are several reasons why it is important to bear in mind the different source categories, and their respective contribution to PM₁₀ concentrations, within the review and assessment process:
    
    • The expected reduction in particle emissions in future years is different for each type of source. For example, emissions from road transport will be governed by new legislation on vehicle emission standards; emissions of secondary particles will be largely governed by controls on power generation, industrial and transport SO2 and NO_x emissions, both in the UK and in Europe; emissions of coarse particles are largely uncontrolled, and in general are not expected to decline in future years. In forecasting future emissions it is therefore essential to treat each source category separately. It should be noted that it is not appropriate to extrapolate measured PM₁₀ concentrations forwards to 2004 using a simple trend analysis (eg plotting measured PM₁₀ concentrations between 1992 and 1998 and then extrapolating this line to 2004) - a suitable methodology for forecasting future concentrations is provided in later sections.
      
    • The principal focus of Local Air Quality Management should be towards the control of emissions at a local level. It is therefore important that the review and assessment process identifies the contribution of local emission sources, so that the effectiveness of control policies or action plans can be evaluated.
      
11. A description of the different source categories, and their approximate contribution to annual mean background concentrations is described in Box 3. A significant proportion of current annual mean PM₁₀ is derived from regional (including long distance transport from Europe) background sources. The exact regional background contribution at any site is variable, and will be dependent upon the precise geographic location. Typical regional annual mean background contributions are currently within the range of about 18-26 µg/m3, gravimetric and are outside of the control of local authorities. Where exceedances of the proposed objectives are predicted, local authorities are strongly advised to focus their efforts on the identification of the contribution of local sources to overall PM₁₀ concentrations.
    
    

    Box 3. Approximate contributions to PM10 concentrations 1998 - click here

    
    National perspective - what areas are at risk of exceeding the proposed objective?
    
    
12. For the purpose of review and assessment of PM₁₀, local authorities should focus their work upon locations where individuals are likely to be exposed over the averaging period of the objectives. It is recommended that the review and assessment should focus upon ground level outdoor locations where the public might reasonably be expected to be exposed over a period of 24 hours or longer. This will include background and roadside sites (i.e. sites close to the facade of a building) where there is housing, and other locations where potentially significant groups may be exposed such as schools or hospitals. It is unlikely to include kerbside sites (i.e. sites within 1 or 2 metres of the kerb) unless there are building facades or other sensitive locations within this distance.
    
13. PM₁₀ data from monitoring sites within the Automatic Urban and Rural Networks are shown summarised in Table 1 for the 1997 calendar year. All of these sites are equipped with TEOM analysers, and the data have been multiplied by 1.3 as previously described in Box 2. These data indicate that the proposed annual mean objective (40 µg/m3, gravimetric) is estimated to have been met at all sites in 1997, with the exception of a kerbside site in London (Camden Roadside), which exceeded by less than 2 mg/m³. The 24-hour objective (50 µg/m3, gravimetric, maximum of 35 exceedances per year) is estimated to have been exceeded at most of the roadside/kerbside sites, and exceeded or closely approached at about 30% of urban and suburban locations. Concentrations at rural/remote sites were well below the objective.
    
14. The data presented in Table 1 are for 1997, and it is confidently expected that PM₁₀ concentrations will have fallen by the end of 2004, when the proposed objectives are to be met. An analysis of PM₁₀ projections for 2004 is presented in the review of the National Air Quality Strategy. The analysis was carried out using both 1995 and 1996 monitoring data, because 1996 was characterised by a much higher frequency of easterly winds associated with the transport of polluted air from mainland Europe to the UK. The 1996 meteorology occurs about once every five to ten years and may therefore be described as 'atypical', and as such represents a worst-case, pessimistic scenario upon which to base predictions of future PM₁₀ concentrations. The analysis has indicated that with existing national policy measures and atypical meteorology, exceedances of the proposed objectives might be found in the following areas:
    
    • urban background sites in central London;
      
    • areas adjacent to busy roads, particularly within major urban areas;
      
    • areas which have significant emissions from the domestic burning of solid fuels;
      
    • areas in the vicinity of industrial plant, or which have significant uncontrolled or fugitive emissions (for example quarrying, materials handling facilities etc).
      
    Such areas are expected to form the focus of more detailed review and assessment for local authorities. It is considered unlikely that the proposed objectives will be exceeded at locations other than those listed above. It is not anticipated that local authorities will generally need to proceed beyond the First Stage review and assessment in areas other than those listed above.
    
    General approach to assessment
    
15. The monitoring data in Table 1 shows that the proposed 24-hour objective is more stringent than the annual mean objective. However, the 24-hour mean objective (expressed as 50 mg/m³, gravimetric, to be exceeded no more than 35 times per year) is potentially a difficult standard against which to carry out an assessment, due to the day-to-day variations in PM₁₀ concentration and composition. It is therefore recommended that the initial stages of review and assessment are carried out by calculating the annual mean PM₁₀ concentration and then estimating the 90th percentile concentration. The 90th percentile of daily means in a calendar year is approximately equivalent to 35 exceedance days.
    
16. An empirical relationship between the annual mean concentration and the 90th percentile of daily means has been derived from an analysis of monitoring data at UK automatic sites between 1992 and 1997, and is shown in Figure 1.
    
17. Any approach based upon an empirical relationship needs to be precautionary, and a 'best fit' line has been drawn which ensures that exceedances will not be underestimated, apart from very extreme cases. This gives the equation:
    PM₁₀ (90th percentile of daily means) = PM₁₀ (annual mean)*1.79
    The proposed 24-hour objective is therefore highly unlikely to be exceeded if the annual mean concentration is below 28 mg/m³, gravimetric.
    
18. Local authorities undertaking reviews and assessments of the proposed PM₁₀ objectives may find the various Technical Guidance Notes published by DETR, the Welsh Office and the Scottish Office (LAQM.TG1, LAQM.TG2 and LAQM.TG3) provide useful supplementary information to this document. Authorities are also reminded that DETR provides a number of telephone advice helplines which can be contacted to give specific advice on any areas of uncertainty. The helpline contact points are shown summarised in Box 4.
    
    
    

    BOX 4: Review and assessment helplines
    Monitoring Helpline: operated by NETCEN
    tel: 01235 463356	e-mail: aqm.helpline@aeat.co.uk
    Emissions Helpline: operated by the London Research Centre
    tel: 0171 793 1965	e-mail: emissions.factors@london-research.gov.uk
    Modelling Helpline: operated by Stanger Science & Environment
    tel: 0181 256 4972	e-mail: modelhelp@stanger.co.uk
    Industrial Sources Helpline: operated by RSK
    tel: 01306 743312	e-mail: industrial.emissions@rsk.co.uk
    Pollutant Specific Guidance Helpline: operated by Air Quality Consultants/University of West of England
    tel: 0117 976 3837	e-mail: aqm-review@uwe.ac.uk

    
    Access to Internet maps
    
19. Throughout this document, continued reference is made to the use of background PM₁₀ concentration maps which have been published on the Internet. The information in these maps will form an essential component of the review and assessment process, and authorities are advised to try to ensure that access to the Internet is available to them. The relevant maps can be found at the following address (/netcen/airqual)
    
20. Where authorities experience difficulties in accessing or obtaining the relevant information from the Internet maps, they are advised to contact the Pollutant Specific Guidance Helpline (see Box 4).
    

Contents          First Stage

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Report prepared by Stanger Science and Environment
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