DEFRA Technical Guidance

Part IV of the Environment Act 1995

Local Air Quality Management

Technical Guidance LAQM. TG(03)

www.defra.gov.uk

Part IV of the Environment Act 1995

Local Air Quality Management

Technical Guidance LAQM. TG(03)

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Published by the Department for Environment, Food and Rural Affairs. Printed in the UK, January 2003, on material containing 75% post-consumer waste and 25% ECF pulp

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Chapter 1: Introduction

1.01 Introduction 1-1

1.03 Role of this guidance 1-1

1.07 Statutory background 1-3

10. The role of review and assessment 1-4
11. The phased approach to review and assessment 1-5

1.16 Review and assessment Progress Reports 1-7

1.19 Public exposure 1-8

1.22 On what basis should an AQMA be declared? 1-10

1.25 On what basis should an AQMA be revoked or amended? 1-11

1.28 Background pollutant concentrations 1-12

1.31 Monitoring data 1-12

34. Exceedences and percentiles 1-13
35. Strategies for reviews and assessments 1-14
36. What is expected in the review and assessment report? 1-15

1.40 What lessons have been learnt from the first round of review

and assessment? 1-15

1.46 What happens after an AQMA is declared? 1-16

1.48 Other LAQM guidance documents 1-17

Chapter 2: Review and assessment of carbon monoxide

2.01 Introduction 2-1

2.03 What areas are at risk of exceeding the objectives? 2-1

2.03 The national perspective 2-1

2.06 The local perspective what conclusions have been drawn from  2-1 the first round of the review and assessment process?

2.09 The Updating and Screening Assessment for carbon monoxide 2-2

13. Background concentrations 2-3
14. Monitoring data 2-3

2.18 Screening assessment for road traffic sources 2-4

Chapter 3: Review and assessment of benzene

3.01 Introduction 3-1

3.04 What areas are at risk of exceeding the objectives? 3-1

3.04 The national perspective 3-1

3.08 Impact of petrol stations 3-2

3.11 The local perspective what conclusions have been drawn  3-2 from the first round of the review and assessment process?

3.13 The Updating and Screening Assessment for benzene 3-3

3.18 Background concentrations 3-4

3.20 Monitoring data 3-4

3.25 Screening assessment for industrial sources 3-6

3.39 Screening assessment for road traffic sources 3-10

3.44 Screening assessment for combined sources 3-11

3.46 The Detailed Assessment for benzene 3-17

3.50 Monitoring 3-17

3.52 Modelling 3-18

Chapter 4: Review and assessment of 1,3-butadiene

4.01 Introduction 4-1

4.03 What areas are at risk of exceeding the objectives? 4-1

4.03 The national perspective 4-1

4.06 The local perspective what conclusions have been drawn from  4-1 the first round of the review and assessment process?

4.08 The Updating and Screening Assessment for 1,3-butadiene 4-2

12. Background concentrations 4-2
13. Monitoring data 4-3

4.16 Screening assessment for industrial sources 4-3

4.30 The Detailed Assessment for 1,3-butadiene 4-9

Chapter 5: Review and assessment of lead

5.01 Introduction 5-1

5.03 What areas are at risk of exceeding the objectives? 5-1

5.03 The national perspective 5-1

6. The local perspective what conclusions have been drawn from  5-1 the first round of the review and assessment process?

8. The Updating and Screening Assessment for lead 5-2

5.11 Monitoring data 5-2

5.14 Screening assessment for industrial sources 5-3

5.28 The Detailed Assessment for lead 5-9

5.32 Monitoring 5-9

5.35 Modelling 5-10

Chapter 6: Review and assessment of nitrogen dioxide

6.01 Introduction 6-1

6.04 What areas are at risk of exceeding the objectives? 6-1

6.04 The national perspective 6-1

6.10 The local perspective what conclusions have been drawn from  6-2 the first round of the review and assessment process?

6.14 The Updating and Screening Assessment for nitrogen dioxide 6-3

6.22 Background concentrations 6-5

6.24 Monitoring data 6-5

6.28 Screening assessment for road traffic sources 6-9

6.34 Screening assessment for industrial sources 6-10

6.46 Screening assessment for other transport sources 6-14

6.53 The Detailed Assessment for nitrogen dioxide 6-28

6.58 Monitoring 6-29

6.60 Modelling 6-30

6.69 Relationships between NOx and NO2 6-32

Chapter 7: Review and assessment of sulphur dioxide

7.01 Introduction 7-1

7.03 What areas are at risk of exceeding the objectives? 7-1

7.03 The national perspective 7-1

6. The local perspective what conclusions have been drawn from  7-1 the first round of the review and assessment process?

8. The Updating and Screening Assessment for sulphur dioxide 7-1

7.11 Background concentrations 7-2

7.13 Monitoring data 7-2

7.17 Screening assessment for industrial sources 7-4

7.26 Screening assessment for domestic sources 7-5

7.29 Screening assessment for other transport sources 7-6

7.32 The Detailed Assessment for sulphur dioxide 7-14

7.37 Monitoring 7-14

7.39 Modelling 7-15

7.46 Domestic coal use 7-16

7.49 Industrial emissions 7-17

7.51 Emissions from shipping movements 7-17

Chapter 8: Review and assessment for PM10

8.01 Introduction 8-1

8.05 What areas are at risk of exceeding the objectives? 8-2

8.05 Sources of particles 8-2

8.08 Policy measures and current PM10 concentrations 8-4

8.13 The local perspective what conclusions have been drawn  8-6 from the first round of the review and assessment process?

8.16 The Updating and Screening Assessment for PM10 8-6

8.22 Background concentrations 8-8

8.24 Monitoring data 8-8

8.30 Screening assessment for road traffic sources 8-11

8.35 Screening assessment for industrial sources 8-12

8.58 Screening assessment for domestic solid fuel sources 8-18

8.64 Screening assessment for fugitive and uncontrolled sources 8-20

8.69 Screening assessment for other transport sources 8-21

8.73 The Detailed Assessment for PM10 8-35

8.78 Monitoring 8-35

8.80 Modelling 8-36

8.88 Road traffic sources 8-37

8.94 Domestic solid fuel use 8-38

8.97 Industrial emissions 8-39

8.99 Uncontrolled and fugitive emissions 8-40

Annex 1: Monitoring

A1.01 Introduction A1-1 A1.05 Air pollution monitoring methods A1-2 A1.12 Planning, setting up and operating a monitoring campaign A1-4

A1.13 Selecting monitoring equipment A1-4

A1.14  Other equipment A1-6

A1.15 Frequently Asked Questions on monitoring equipment A1-6

A1.22  Choosing a site A1-8

A1.25  Identifying relevant locations A1-10

A1.31  Local siting criteria A1-11

A1.33  Site numbers A1-12

A1.34  Frequently Asked Questions on monitoring locations A1-13

A1.39  Monitoring period A1-14

A1.44  Screening surveys A1-17

A1.48  Detailed monitoring A1-18

A1.51  Routine site operations A1-19

A1.53  Monitoring costs A1-19

A1.54  Frequently Asked Questions on monitoring procedures A1-20 A1.56  Monitoring for individual pollutants A1-20

A1.57  Monitoring for benzene A1-20

A1.58  Monitoring for 1,3-butadiene A1-21

A1.59  Monitoring for carbon monoxide A1-21

A1.60  Monitoring for lead A1-21

A1.62  Monitoring for nitrogen dioxide A1-22

A1.65  Monitoring for PM10 A1-22

A1.67  Monitoring for sulphur dioxide A1-23

A1.69 Examples of typical monitoring strategies A1-24 A1.70  Quality assurance and quality control QA/QC A1-26

A1.71  Collection, ratification and reporting of monitoring data A1-26

A1.72  Data quality objectives A1-26

A1.76  QA/QC of non-automatic data A1-27

A1.77  QA/QC of passive samplers A1-27

A1.86  QA/QC of automatic monitoring data A1-30

A1.95  Data processing A1-32

A1.99  Data ratification A1-33

A1.103  Reporting of monitoring data A1-37 A1.106  Frequently Asked Questions on QA/QC A1-39

Appendix A Definition of site classes A1-42 Appendix B Conversion factors A1-44

Annex 2 Estimating emissions

A2.01 Introduction A2-1 A2.06 Road transport A2-2

A2.06 DMRB Screening Model assessment data A2-2

A2.08 Detailed Assessment data A2-3

A2.12 Frequently Asked Questions A2-9

A2.39 Background road data A2-15 A2.56 Point sources A2-18

A2.56 Introduction A2-18

A2.64 Identifying priority sectors and pollutants A2-22

A2.65 Point source data gathering A2-22

A2.71 Getting data from the Public Registers A2-23

A2.74 Public register for Part A industrial processes A2-23

A2.77 Public register for Part B industrial processes A2-25

A2.84 Getting data from the regulator A2-27

A2.89 Getting data from the operator A2-28

A2.92 Getting data from Process Guidance Notes A2-29

A2.95 Emissions estimation A2-29

A2.97 Using monitoring or emission limit data A2-30

A2.110 Using emissions factors A2-33

A2.119 Material balance (mass balance) and engineering judgement A2-34

A2.121 Estimating future emissions A2-35

A2.122 Estimating fugitive emissions A2-35

A2.123 Operating profiles A2-35 A2.125 Other stationary sources A2-36

A2.127 Low-level domestic and commercial combustion A2-36

A2.136 Other area sources A2-38 A2.138 Other mobile sources A2-39

A2.138 Emissions from aircraft movements in the vicinity of airports A2-39

A2.141 Emissions from inshore and estuarine shipping A2-39

Appendix A Worked examples A2-40 Appendix B Sources of emissions factors A2-53 Appendix C Airport activity in 2000 A2-55 Appendix D The NAEI s data warehouse A2-57

Emissions data warehouse A2-57

Local authority data warehouse A2-57 Appendix E Significant point source processes lists A2-59 Appendix F Point source data pro-forma A2-64

Annex 3: Modelling

A3.01 Introduction A3-1 A3.05 LAOM tools A3-2 A3.12 Source data requirements A3-3 A3.15 Road traffic sources A3-3

A3.33 Detailed Assessment of road traffic emissions A3-6 A3.43 Emission Factors Toolkit A3-7

A3.48 How detailed should my traffic flows be at the detailed  A3-8 modelling stage?

A3.56 Street canyons and complex junctions A3-10 A3.67 Point sources A3-12 A3.77 Complex effects A3-14

A3.84 Atmospheric chemistry A3-15

A3.85 Other sources A3-15

A3.88 Fugitive emissions A3-15 A3.94 Meteorological data A3-17

A3.96 Meteorology used in screening models A3-18 A3.98 Inter-year variability of source contributions, as a result of  A3-18 differences in meteorological data?

A3.104 How many years of meteorological data? A3-19 A3.115 Meteorological data and background A3-21 A3.119 Sources of meteorological data A3-22

A3.120 Pollution episodes A3-22 A3.122 Lifetime of meteorological data sets A3-22 A3.124 Treatment of calm and missing meteorological data A3-23 A3.129 Calculation of percentiles and/or number of exceedences A3-24

A3.136 Urban meteorology A3-25 A3.142 Coastal effects A3-27

A3.144 Estimating background concentrations in modelling studies A3-27 A3.149 How detailed should my modelled area be? A3-28

A3.155 Interpretation of concentration contours A3-30 A3.160 Model validation, verification, adjustment and uncertainty A3-31 A3.170 What sites should be included in verification and adjustment? A3-33

A3.181 NOx/NO2 verification and adjustment A3-35 A3.188 Methods of model verification and adjustment A3-36 A3.193 What if hourly results need assessing? A3-42

A3.200 Uncertainty estimates A3-43 Appendix A Fugitive PM10 case study at grain handling facility A3-45 Appendix B List of key model users whose views on the First Round  A3-46

have been sought

Appendix C List of key model suppliers whose views on the First Round A3-47

have been sought

Annex 4: Abbreviations and glossary A4-1

Introduction

1. This technical guidance document replaces two earlier versions, issued as LAQM.TG4(98) and LAQM.TG4(00), and is designed to support local authorities in carrying out their duties under the Environment Act 1995 and subsequent Regulations. These duties require local authorities to review and assess air quality in their area from time to time. These reviews and assessments form the cornerstone of the system of local air quality management (LAQM). LAQM itself forms a key part in the Government s and the Devolved Administrations strategies to achieve the Air Quality Objectives[1]. These objectives are set out in Table 1.1. A general introduction to the system of local air quality management (LAQM) is provided in the Policy Guidance documents[2].
2. All aspects of the technical guidance are now brought together in this one document, with the revised guidance on monitoring, estimating emissions and the selection and use of dispersion models being provided as annexes. This document supersedes all previous technical guidance documents[3].

Role of this guidance

3. This document is designed to guide local authorities through the review and assessment process. It sets out the general approach to be used, together with detailed technical guidance, which is provided on a pollutant by pollutant basis.
4. In addition to the objectives set out in the Air Quality Regulations 2000, and the Air Quality (Amendment) Regulations 2002 ( the Regulations ), the EU has set limit values in respect of nitrogen dioxide and benzene, to be achieved by 1 January 2010, as well as indicative limit values for PM , also to be achieved by 2010[4]. Local

10

authorities currently have no statutory obligation to assess air quality against these limit values, but they may find it helpful to do so, in order to assist with longer-term planning, and the assessment of development proposals in their local areas. In circumstances where the base input data have already been collated (for example, for assessment of the 2010 objective for benzene, or the 2010 objectives for PM10 in Scotland) then the additional work required for inclusion of the other pollutants may be deemed beneficial and cost-effective. Therefore, this document provides informal guidance on how to assess against the time-frame of the limit values in 2010.

1-2

5. The new particles objectives for England, Wales, Northern Ireland and Greater London are not currently included in Regulations for the purpose of LAQM. The Government, the Welsh Assembly Government and the Department of the Environment in Northern Ireland will, however, consider whether the new particles objectives will be included in Regulations as soon as practicable after the review of the EU s first air quality daughter directive, which is due to be completed in 2004. The new particles objectives for England, Wales, Northern Ireland and Greater London are shown in Table

1.2. Whilst authorities have no obligation to review and assess against them, they may find it helpful to do so, in order to assist with longer-term planning, and the assessment of development proposals in their local areas. Suitable methods for assessment against these proposed objectives are provided in this document.

6. The methodologies described within this guidance are based upon the most up- to-date understanding of pollutant concentrations and sources, and of methods to predict future levels. They draw, as appropriate, on experience from the first round of review and assessment. The Government and the Devolved Administrations are continuing to sponsor research into all of these areas, and it is therefore inevitable that some of the assessment methodologies may need to be revised at some stage in the future. Supplementary or revised technical guidance will be issued periodically in light of any new information. New information is also made available, as it arises, through the Frequently Asked Questions sections of the websites operated by the Helpdesks (Box 1.1). Local authorities are encouraged to check these before contacting the Helpdesks.

Statutory background

7. This guidance is issued by the Department for Environment, Food and Rural Affairs

(Defra), the Scott ish Executive and the Welsh Assembly Government under section 88(1)

of the Environment Act 1995 ( the Act ). It replaces the guidance previously issued as

LAQM.TG1(00) to TG4(00). Under section 88(2) of the Act, local authorities are required

to take account of this guidance when carrying out any of their duties under or by virtue

of Part IV of the Act. Following suspension of the Northern Ireland Assembly, an 1-3 Environment (Northern Ireland) Order 2002 is being taken through parliament and is

scheduled to take legal effect in January 2003. Part III of the Order will require Northern Ireland relevant authorities to take account of this guidance when carrying out their duties under or by virtue of the Environment (Northern Ireland) Order 2002.

8. The Greater London Authority Act 1999 provides for the Mayor of London to publish an air quality strategy for the capital. The Mayor s Air Quality Strategy was published in September 2002 and sets out the steps the Mayor will take towards meeting the national Air Quality Objectives in London. It also contains further advice to London Boroughs in respect of LAQM.
9. The Mayor s Air Quality Strategy will not replace local authority duties under LAQM. However, London local authorities will have to take account of it when carrying out their LAQM duties. London local authorities must consult with the Mayor as well as the Secretary of State with regard to their reviews and assessments of air quality, and

on their air quality management area (AQMA) designations and action plans. The Mayor of London will have to take account of this guidance in exercising any powers of direction under section 85(2) to (4) of the Environment Act 1995.

The role of review and assessment

10. The Air Quality Strategy5 establishes the framework for air quality improvements. Measures agreed at the national and international level are the foundations on which

the strategy is based. It is recognised, however, that despite these measures, areas of

poor air quality will remain, and that these will best be dealt with using local measures implemented through the LAQM regime. The role of the local authority review and assessment process is to identify these areas, where it is considered likely that the Air Quality Objectives will be exceeded. Experience has shown that such areas may range from single residential properties to whole town centres.

1-4 5 The Air Quality Strategy for England, Scotland, Wales and Northern Ireland. Working Together for Clean Air.

January 2000. Cm 4548, SE 2000/3 and NIA 7.

The phased approach to review and assessment

11. This guidance document builds upon the phased approach to review and assessment established in previous technical guidance, LAQM.TG4(00). The intention is that local authorities should only undertake a level of assessment that is commensurate with the risk of an air quality objective being exceeded. Not every authority will, therefore, need to proceed beyond the first step in the second round of review and assessment. A description of the 2-step approach is set out in Box 1.2.

12. The first step of the review and assessment process is an Updating and Screening Assessment, which is to be undertaken by all authorities. This is based on

a checklist to identify those matters that have changed since the first round was completed, and which may now require further assessment. This Updating and Screening Assessment should cover: new monitoring data; new objectives; new sources or significant changes to existing sources, either locally or in neighbouring authorities; other local changes that might affect air quality, etc. If there is a risk that these changes may be significant, then a simple screening assessment should be carried out. Nomograms and similar tools are provided to help with this screening assessment.

13. Where the Updating and Screening Assessment has identified a risk that an air quality objective will be exceeded at a location with relevant public exposure, the authority will be required to undertake a Detailed Assessment following the guidance set out in this document. The aim of this Detailed Assessment should be to identify

with reasonable certainty whether or not a likely exceedence will occur. The assumptions within the Detailed Assessment will need to be considered in depth, and

the data that are collected or used, should be quality-assured to a high standard. This is

to ensure that authorities are confident in the decisions they reach. Where a likely exceedence is identified, then the assessment should be sufficiently detailed to

determine both its magnitude and geographical extent. Local authorities should not declare an Air Quality Management Area (AQMA) unless a Detailed Assessment has been completed.

14. The Updating and Screening Assessment report should be completed by all authorities (excluding those in Greater London that have declared AQMAs, and Northern Ireland) by the end of May 20036. This report should clearly identify any locations and pollutants for which it is considered necessary to carry out a Detailed Assessment. The evidence to support the conclusion to proceed, or not to proceed, to a Detailed Assessment should be provided in the report.
15. The Detailed Assessment report, if required, should be completed by the end of April 20047. It is expected that local authorities will then undertake reviews and assessments of air quality every three years. Updating and Screening Assessments will therefore be required to be submitted during the first four months of 2006 and 2009, and (if required) Detailed Assessments by the end of April 2007 and 2010 (see Table

1.3 and Box 1.3 for timescales).

6 London local authorities with AQMAs are expected to submit their Updating and Screening Assessments by the end of 2003 or earlier, where possible. Separate reporting timescales for authorities in Northern Ireland have been set.

7 In London, by the end of 2004. Separate reporting timescales for authorities in Northern Ireland have been set.

Review and assessment Progress Reports

16. The evaluation of the first round of reviews and assessments recommended that local authorities should prepare annual air quality Progress Reports between subsequent rounds of reviews and assessments. The concept is that this will ensure continuity in the LAQM process.
17. The precise format for the Progress Report has not yet been determined, but will essentially follow the checklist approach that is set out in subsequent chapters of this document. Further details on the Progress Reports will be provided via the Helpdesks by the middle of 2003. It is envisaged that these Progress Reports could be useful for the compilation of annual state of the environment reports that many authorities already prepare for Committees and members of the public.
18. In preparing these reports it is helpful if monitoring data are reported over the period of a calendar year. Progress Reports are therefore due by the end of April 2005 (which would cover the calendar year 2004) and April 2008. For those authorities that do not have to carry out Detailed Assessments following the Updating and Screening Assessments in April 2003, 2006 and 2009, they will be expected to also submit Progress Reports in April 2004, 2007 and 2010. This will set the future reporting timescales for LAQM reviews and assessments and Progress Reports (see Table 1.3 and Box 1.3).

Public exposure

19. The Regulations make clear that likely exceedences of the objectives should be assessed in relation to  the quality of the air at locations which are situated outside of buildings or other natural or man-made structures, above or below ground, and where members of the public are regularly present . Reviews and assessments should thus be focussed on those locations where members of the public are likely to be regularly present and are likely to be exposed over the averaging period of the objective. Authorities should not consider exceedences of the objectives at any location where relevant public exposure would not be realistic[1].

20. Several factors have been taken into account when developing the guidance on locations considered relevant:

¥ The Regulations refer to locations where members of the public are regularly present. This does not imply that it must be the same persons regularly present at that location. This is important for an understanding of relevant exposure where a short-term objective allows a number of exceedences of the standard. The standard is the basis for a potential risk to health, thus a single exposure of an individual above the standard is to be avoided. The objective allows a number of exceedences of the standard because of considerations of feasibility and practicability. Thus for sulphur dioxide, where there is a 15-minute standard, a relevant location would be anywhere where a member of the public might be exposed for a single 15-minute period, as long as members of the public are regularly present at that location. The allowance of up to 35 exceedences before the objective is breached determines the need to control concentrations at that location, not whether that location is relevant in terms of exposure.

¥ The long-term objectives apply where members of the public are likely to be exposed over the averaging period of the objective. As with the discussion of short- term objectives, this does not require the same individual to be present for a full year at a particular location, but the location must be one where people are likely to be regularly present for long periods. For instance, in the case of the 24-hour objectives, a relevant location would be one where members of the public may be exposed for 8 hours or more in a day, while for the annual mean objectives this might be where people are exposed for a cumulative period of 6 months in a year.

¥ There is a link between pollutant concentrations measured both inside and outside of a building. For this reason it is considered appropriate to measure at the building fa ade to represent relevant exposure. Thus, for exposure alongside a busy road, it is considered reasonable to select the fa ade of residential properties closest to the road as a representative location to assess exposure for pollutants with a 24-hour or annual mean objective.

21. For the purpose of assisting local authorities, some examples of where the objectives should, and should not apply, are summarised in Box 1.4. However it should be borne in mind that it is not possible to be prescriptive in this matter, and authorities should bear local circumstances in mind when considering the application of the objectives. The examples given in the table are not intended to be a comprehensive list, and it is expected that local judgement will often be required. In the case of doubt, further guidance may be obtained from the Review and Assessment Helpdesk.

On what basis should an AQMA be declared?

22. The local authority should aim at the end of the review and assessment process to be confident that it has identified all locations and pollutants for which it is likely that the air quality objective will be exceeded in the relevant future year and beyond10. This confidence will be determined by uncertainties in the various steps by which future concentrations are predicted. When carrying out a Detailed Assessment, the authority should be applying best estimates for all the components that go into producing the estimated future concentrations. The authority should be wary of adding one worst- case assumption to another, as this could lead to an unrealistic estimate of likely future concentrations.

9 Such locations should represent parts of the garden where relevant public exposure is likely, for example where there are seating or play areas. It is unlikely that relevant public exposure would occur at the extremities of the garden boundary, or in front gardens, although local judgement should always be applied.

10 The objectives are to be met in all future years beyond the target dates set out in Regulations. In the vast majority

of cases, it may be expected that air quality will improve in future years due to more stringent emission controls. However, in circumstances where authorities are concerned that emissions may rise in future years (for example in the vicinity of major airports, or where large scale developments are planned), then review and assessment for years beyond the target date may be appropriate.

23. Authorities should demonstrate that they are aware of the uncertainties involved in all of the data inputs, and show what steps have been taken to minimise these uncertainties. This will be particularly important where the final outcome of the review and assessment, to declare an AQMA or not, is finely balanced. By way of example, the level of confidence in the outcome will be greater if model validation is based on monitoring carried out to the standards applied in the national network. Similarly, the confidence in model results for a section of road will be greater if the traffic data are based on detailed traffic counts, rather than flows derived from a traffic model. Where uncertainties are potentially high and the outcome is marginal, then the authority should look to obtaining more reliable data to improve the confidence in its decision.
24. Whilst authorities are encouraged to be aware of the uncertainties in their reviews and assessments, it is not generally recommended that uncertainty estimates are applied to absolute monitoring or predicted data values as a means of correction, unless there is good reason to do so. Such estimates of uncertainty may however be useful in assisting the authority in its decision as to the geographical extent of the AQMA boundary. Further guidance on this approach is given in the document published by the National Society for Clean Air and Environmental Protection (NSCA) Air Quality Management Areas: Turning Reviews into Action . The document is available directly from the NSCA, or may be downloaded from the Internet site (www.stanger.co.uk/airqual/modelhlp).

On what basis should an AQMA be revoked or amended?

25. There is the potential that the review and assessment process may result in the need for an existing AQMA to be amended or revoked. The process for amending or revoking an AQMA is similar (from the technical point of view) to that for declaring an AQMA in the first instance. The authority will therefore need to be able to demonstrate the same degree of confidence in its decision to revoke or amend an AQMA, as was provided for the original declaration.
26. In the majority of cases, it is envisaged that a Detailed Assessment will be required to support any decision to amend or revoke an AQMA. The Updating and Screening Assessment may be sufficient in circumstances where:

¥ It can be demonstrated that the source(s) giving rise to the original AQMA declaration have been removed, for example, due to the closure of an industrial process or road;

¥ The pollutant emissions assumed for the original AQMA declaration have significantly changed, for example, due to the construction of a bypass around a town centre, or modification to an industrial process.

27. Government and the Devolved Administrations expect that all decisions to amend or revoke AQMAs should be subject to full consultation. Further guidance on the recommended approach to consultation is given in the Policy Guidance documents.

Background pollutant concentrations

28. Emissions from local pollutant sources (such as roads, chimney-stacks etc) will be added to local background concentrations. In many situations, the background contribution may represent a significant or dominant proportion of the total pollutant concentration, and it is thus important that authorities give careful consideration to background levels and how they are estimated for future years.
29. Background concentrations for all the regulated pollutants are expected to decline in future years, as a result of Government and EU policies and legislation to reduce pollutant emissions. Advice on how to treat background concentrations is given in the following chapters. In many instances it is recommended that use is made of the empirically-derived national background maps, which are provided for each 1x1 km grid square[1]. Where appropriate these data can be supplemented by local measurements of background, although care should be exercised to ensure that the monitoring site is representative. If the local background is derived from area-wide modelling, then the results should be validated against background monitoring sites and/or compared with the national maps.
30. When using these background maps, care may need to be taken in certain circumstances to avoid double-counting , for example where there is a very busy road passing through a rural or suburban area. A recommended approach to avoid double counting is given in Box 1.5.

Monitoring data

31. Monitoring data are likely to be available from a variety of sources, including national networks, regional networks, local data collected by the authority, and local data collected by other bodies or organisations. For the purpose of review and assessment, authorities will need to have confidence in the quality of the monitoring data, and further guidance is provided in Annex 1.
32. There are a number of different terminologies in use. The process of validation generally involves a first level screening of the data (by manual and/or automatic methods), to remove obvious erroneous values. These data will have been suitably calibrated against reference standards where appropriate. Within the national monitoring networks, these data are labelled provisional. The process of data ratification involves a more thorough checking of the data, for example data rescaling to allow for drift in the calibration standards, or data adjustments following site audits which have identified problems that could not have been identified remotely (for example, internal sampling leaks).

33. Authorities must always use validated data, and are advised to use ratified data from the national networks, wherever possible. If provisional data must be used, it should be noted that the process of ratification will be unlikely to affect the measured annual mean, but may change the number of shorter-term (for example, hourly or 15-minute) means.

Exceedences and percentiles

34. The short-term objectives are framed in terms of the number of occasions in a calendar year on which the objective concentration should not be exceeded. Wherever possible, authorities are encouraged to express the results of their monitoring and modelling in terms of the number of hours, days etc above the objective level. This is the clearest basis for strict comparison with the objectives set out in the 2000 and 2002 Regulations. However, for a strict comparison on this basis, there must be a minimum of 90% data capture throughout a calendar year12. In certain circumstances, where measured data capture is less than 90%, it may be appropriate to express short-term concentrations as percentile values that approximate the permitted number of exceedences. Where modelling predictions are carried out, the specific model used may not permit the number of exceedences to be calculated, or the meteorological dataset may contain less than 90% valid observations in the year. Once again, it may be more appropriate to express the results as a percentile. Further guidance is provided in Annexes 1 and 3. Relationships between the permitted number of exceedences of short-period concentrations and the equivalent percentiles are provided in Table 1.4 below to help express results in relevant terms.

12 An exceedence of the objectives may of course be demonstrated with a much lower data capture rate.

Strategies for review and assessment

35. Authorities are advised to give careful consideration to the monitoring and modelling strategies that they employ, particularly when they undertake work for a Detailed Assessment. Monitoring and modelling can prove to be both resource and cost-intensive activities, and authorities are recommended to make use of the guidance on monitoring, emissions and modelling as set out in Annexes 1 to 3. Authorities should also consult with the relevant Helpdesks to ensure that appropriate monitoring and modelling is carried out. Some general tips for monitoring and modelling strategies are provided in Box 1.6 below.

What is expected in the review and assessment report?

36. For the review and assessment process, all local authorities should complete an Updating and Screening Assessment report. Some authorities will also need to produce a Detailed Assessment report. These reports should describe all the information used to carry out the assessment in sufficient detail to justify the decision(s) to proceed from:

1. The Updating and Screening Assessment to a Detailed Assessment;
2. The Detailed Assessment to the decision to declare, alter or revoke an AQMA.

37. The precise format of these reports is the responsibility of the individual authorities. They may though find it useful to have regard to the checklists used when Defra and the Devolved Administrations carry out Appraisals of the review and assessment reports. These are made available on the Review and Assessment website (see Box 1.1).
38. Authorities may also find it helpful to look at the useful examples of reports produced during the first round of review and assessment. These are also held on the Review and Assessment Helpdesk website (see Box 1.1).
39. Given the importance of providing details of all the assumptions that have been made, particularly during a Detailed Assessment, authorities may wish to consider the publication of a summary report for wider distribution, together with a detailed technical report for distribution to a more limited audience.

What lessons have been learnt from the first round of review and assessment?

40. The initial stages of the first round of review and assessment are now practically complete (apart from Northern Ireland), and it is valuable to draw upon the experience gained during this exercise. To assist this process, Government and the Devolved Administrations commissioned an Evaluation Report, which has collated feedback from local authorities, and identified areas for improvement in the guidance. This report is available from the Review and Assessment Helpdesk website (see Box 1.1) and the Defra website[1]. Where appropriate, the conclusions of this report have been incorporated into this technical guidance.
41. To date, more than 100 AQMAs have been declared, which represents about one-third of the local authorities in England, Wales and Scotland. Of these, the vast majority of declarations are related to road traffic emissions, where attainment of the annual mean nitrogen dioxide objective is considered unlikely, sometimes in association with expected exceedences of the 24-hour PM10 objective.
42. A much smaller number of AQMA declarations associated with industrial processes have been reported. AQMAs associated with PM10 emissions have been declared for a steelworks, unregulated coal-fired boilers at a food factory, and a foundry. AQMAs associated with SO2 emissions have been declared at a cellophane

works, unregulated coal-fired boilers at a food factory, and a large hospital boiler. Fugitive emissions of PM10 have resulted in the declaration of AQMAs in two local authority areas. The declaration of an AQMA due to local coal-burning is also under consideration.

43. Other transport-related emissions have been an issue of concern in some areas, with AQMAs declared in the vicinity of Heathrow, Gatwick and East Midlands airports, and the ferry terminals at Dover.
44. The first round of reviews and assessments has highlighted the importance of considering public exposure, and the need for local authorities to focus upon those locations where they expect pollutant concentrations to be highest (sometimes referred to as hot spots ). It is likely to be more cost effective to start with an examination of worst-case locations and then work outward if exceedences are found, rather than take an unfocussed look at a large geographical area. If there is no exceedence at the most polluted location, there should be no exceedences elsewhere. This approach should also help ensure that potential areas of exceedence are not missed.
45. Examples of this approach are outlined in the subsequent chapters, on a pollutant-by-pollutant basis, together with indications as to where the most polluted locations are likely to be found. In the case of traffic this takes account of evidence from the first round that outside of major conurbations, exceedences are more likely where exposure occurs within 5m of the kerb. This can include roads with modest traffic flow, around 10,000-20,000 veh/day, in narrow congested town centre streets. It also takes account of the finding that concentrations are not as high alongside dual carriageways and motorways in open rural areas, as might be expected from their high traffic flows.

What happens after an AQMA has been declared?

46. Section 84(1) of the Environment Act 1995 requires authorities to carry out a further assessment of existing and likely future air quality in any area which has been designated as an AQMA[1]. This further assessment is intended to supplement information the authority has already produced during a Detailed Assessment. The intention is that this further assessment will be used to define the relative contribution of different sources within the areas of exceedence, so as to allow a focused action plan to be prepared.
47. Guidance on the preparation of these further assessments has already been provided. Defra and the Devolved Administrations will give consideration to the need to update this guidance for future rounds of review and assessment, if this is deemed necessary.

Other LAQM guidance documents

48. Authorities are reminded that this document is part of the LAQM series that has been published by Government and the Devolved Administrations. Authorities are strongly encouraged to read the relevant Policy Guidance documents15 which set out in detail the local air quality management process and the policy background.

15 Separate Policy Guidance documents have been published in England and Wales, Scotland and Northern Ireland.

Introduction

1. The Government and the Devolved Administrations have adopted an 8-hour running mean concentration of 11.6 mg/m3 as the air quality standard for carbon monoxide. The new objective has been set at a slightly tighter level of 10 mg/m3 as a maximum daily running 8-hour mean concentration[1], to be achieved by the end of 2003, bringing it into line with the second Air Quality Daughter Directive limit value.
2. This section of the guidance provides advice to local authorities on how to identify areas within their locality, at risk of exceeding the Air Quality Objectives for carbon monoxide.

What areas are at risk of exceeding the objectives?

The national perspective

3. The main source of carbon monoxide in the United Kingdom is road transport, which accounted for 67% of total releases in 2000 (the most recent year for which estimates are available). Annual emissions of carbon monoxide have been falling steadily since the 1970s, and are expected to continue to do so. Current projections indicate that road transport emissions will decline by a further 42% between 2000 and 2005.
4. A summary of measured maximum daily running 8-hour mean carbon monoxide concentrations at UK national network sites is shown in Table 2.1 for the period 1999-2001. There were no measured exceedences of the objective at any site during this period. In general, concentrations at kerbside or roadside sites were higher than at urban background or urban centre sites. However, under certain meteorological circumstances, pollutant emissions within urban areas may accumulate, resulting in higher concentrations at urban background or urban centre sites. This effect was noted during December 2001, when the objective was approached (within 20%) at 4 sites (Bolton, Glasgow Centre, Stoke-on-Trent Centre and Bradford Centre).
5. Carbon monoxide concentrations adjacent to major roads have also been modelled at a national level. The results of this assessment suggest that existing policies will be sufficient to reduce maximum daily running 8-hour mean concentrations of carbon monoxide below 10 mg/m3 by about 2003.

The local perspective what conclusions have been drawn from the first round of the review and assessment process?

6. There have been no AQMAs declared from the first round of reviews and assessments in respect of the previous 2003 air quality objective (11.6 mg/m3, as defined in the Regulations 2000). This conclusion supports the studies carried out at a national level.

7. Studies at a national level, based on both measured and modelling data, suggest that there is little likelihood of the new objective for carbon monoxide being exceeded by 2003. Whilst the maximum daily running 8-hour concentrations in December 2001 approached the objective at some urban background and urban centre sites, levels by 2003 are expected to continue to decline, and the likelihood of any exceedence is considered to be low. In addition, any consideration of these urban scale episodes would require a level of detail beyond that expected within the review and assessment process.
8. Whilst national-scale studies suggest that the objective will be achieved, it is important that local circumstances are fully taken into consideration. All authorities are therefore required to carry out a review and assessment for carbon monoxide. It is envisaged that this can be achieved without the need for further significant work, and it is highly unlikely that any authority will be required to proceed beyond the Updating and Screening Assessment.

The Updating and Screening Assessment for carbon monoxide

9. In completing the Updating and Screening Assessment, authorities are encouraged to maximise and build upon the data collation and assessments completed during the first round of review and assessment.
10. All local authorities should undertake the updating assessment for carbon monoxide as part of the review and assessment process.
11. In order to complete the updating assessment, authorities should draw upon the information compiled as part of the First Stage report, completed in the first round of review and assessment.
12. This part of the review and assessment is based upon a checklist approach, a summary of which is provided in Box 2.1. Authorities may wish to consider formatting their reports for the Updating and Screening Assessment with these section headings in mind. A detailed checklist for each source or location is set out in Box 2.2 (at the end of the Updating and Screening Assessment). This describes the information that authorities should collate for the review and assessment against the 2003 objective. The first column describes the source, location or data that need to be considered, and the subsequent columns describe the steps that need to be taken.

Background concentrations

13. Estimated annual mean background concentrations for 2001 have been mapped for the UK, and can be accessed from the internet at the following address (www.airquality.co.uk/archive/laqm/tools.php). Details of the mapping process can also be found at this site. In using these maps, authorities are advised to take care to avoid

 double counting . For example, in areas where there is a section of heavily-trafficked road it may be more appropriate to derive the background concentration from

adjacent grid squares. A recommended approach in such cases is set out in Box 1.5. Factors to adjust mapped or measured background concentrations to 2003 are

provided in Box 2.3.

Monitoring data

14. Data collected from national monitoring networks, or from local monitoring campaigns, are expected to give a more accurate indication of carbon monoxide concentrations than modelling studies. Authorities are recommended to prioritise the use of measured carbon monoxide concentrations wherever suitable data are available. It is emphasised that monitoring sites will need to be at locations relevant for public exposure, and where the maximum impact of the source (i.e. the highest concentrations) are expected to be measured. For the review and assessment of carbon monoxide, only monitoring data collected at roadside sites need be considered (see Box 1.4).
15. Guidance on monitoring methods and strategies for carbon monoxide is set out in more detail in Annex 1. Ideally, monitoring should have been carried out for a period of one year (with 90% data capture), although a shorter period (for example, 6 months) may be sufficient to demonstrate that the risk of exceedence of the objectives

is negligible, or has occurred. In circumstances where authorities have less than

12 months data, they should contact the relevant Helpdesk for assistance (see Box 1.1).

16. An evaluation of monitoring data from national network sites has indicated a poor relationship between the annual mean concentration and the maximum daily running 8-hour mean. It is therefore not practicable to adjust the measured maximum daily running 8-hour concentration forwards to 2003. For this step of the assessment, authorities should assume that the measured concentration in the year of monitoring is applicable to 2003.
17. Where the measured maximum daily running 8-hour concentration exceeds the

objective, the authority will need to proceed to a Detailed Assessment. However, it is

considered unlikely that any authority will need to proceed beyond the screening 2-3

assessment for carbon monoxide, and guidance on the Detailed Assessment has not been included in this document. In the event that any authority considers that there is a significant risk of exceeding the 2003 objective, they are advised to contact the relevant Helpdesk (see Box 1.1) for guidance.

Screening assessment for road traffic sources

18. As indicated in Box 2.2, authorities need only undertake a screening assessment for road traffic sources in respect of the 2003 objective, where daily average (AADT) flows exceed the stated threshold criteria.
19. Authorities are reminded that for the review and assessment of the maximum daily running 8-hour mean objective, predictions should be carried out at relevant roadside locations (see Box 1.4).
20. The screening assessment for road traffic sources in 2003 may be carried

out using the screening model which has been prepared for the Design Manual

for Roads and Bridges (DMRB) and has been published by the Highways Agency[1]. A suitable version of the DMRB Screening Model (v1.01) is available in Excel spreadsheet form and can be downloaded from the following internet address: www.airquality.co.uk/archive/laqm/tools.php. The DMRB model requires input data on annual average daily traffic flow (AADT), annual average speeds, the traffic composition (proportion of different vehicle types), the type of road, and the distance from the centre of the road to the receptor. The user is also required to input the 2003 annual mean background carbon monoxide concentration (see Para 2.13 above).

21. The DMRB model predicts the annual mean concentration for 2003. Authorities may assume that where the predicted annual mean concentration is below 2 mg/m3, there is little likelihood of the maximum daily running 8-hour mean concentration exceeding the objective.
22. The revised DMRB model is expected to provide a slightly conservative assessment of the impact in most cases. This is appropriate for a screening model and should prevent authorities unnecessarily proceeding to a Detailed Assessment. However, the validation work carried out by the Highways Agency has indicated that the model may significantly underpredict concentrations of carbon monoxide alongside urban city-centre roads classified as street canyons . In this context, a street canyon may be defined as a relatively narrow street with buildings on both sides, where the height of the buildings is generally greater than the width of the road. To avoid missing potential exceedences of the objective in such locations, the authority should multiply the predicted annual mean carbon monoxide road traffic component concentration, in the local output sheet in the DMRB, by a factor of 2, to take account of the model underprediction. This should then be added to the background concentration to give

the total concentration. Locations where this factor has been used should be clearly identified in the review and assessment report.

23. It is considered unlikely that any authority will need to proceed beyond the Updating and Screening Assessment for carbon monoxide. Guidance on the Detailed Assessment has therefore not been included in this document. In the event that any authority considers that there is a significant risk of exceeding the 2003 objective, they are advised to contact the relevant Helpdesk (see Box 1.1) for guidance.

1. Where 2 or more roads intersect, for example at a junction, the traffic flows from each road should be added to give a combined total. For example at a crossroads with 2 roads intersecting, where road [A] has an AADT flow of 38,000 vehicles per day and road [B] has an AADT flow of 44,000 vehicles per day, assume a combined flow of 82,000 vpd. If there are 3 links to the junction, add the flows and multiply by 2/3.
2. To avoid missing potential exceedences of the objective in street canyons, the predicted annual mean carbon monoxide road traffic component concentration, in the local output sheet in the DMRB, should be multiplied by a factor of 2. This should then be added to the background concentration to give the total concentration (see para 2.22).

Introduction

1. The Government and the Devolved Administrations have adopted a running annual mean concentration of 16.25 g/m3 as the air quality standard for benzene, with an objective for the standard to be achieved by the end of 2003. However, in light of the health advice from EPAQS and the Department of Health s Committee on Carcinogenicity of Chemicals in Food, Consumer Products and the Environment (COC) to reduce concentrations of benzene in air to as low a level as possible, additional tighter objectives have also been set. The additional objective is for an annual mean of 5 g/m3 to be achieved by the end of 2010 in England and Wales. In Scotland and Northern Ireland, a running annual mean of 3.25 g/m3 has been adopted as an additional objective, to be achieved by the end of 2010.
2. The second Air Quality Daughter Directive also sets a limit value for benzene, which has been transposed into UK legislation. The Directive includes an annual mean of 5 g/m3 to be achieved by 1 January 2010.
3. This section of the guidance provides advice to local authorities on how to identify areas within their locality, at risk of exceeding the Air Quality Objectives for benzene.

What areas are at risk of exceeding the objectives?

The national perspective

4. The main sources of benzene emissions in the UK are petrol-engined vehicles, petrol refining, and the distribution and uncontrolled emissions from petrol station forecourts without vapour recovery systems.
5. A number of policy measures already in place, or planned for future years, will continue to reduce emissions of benzene. Since January 2000, EU legislation has reduced the maximum benzene content of petrol to 1%, from a previous upper limit of 5%. The European Auto-Oil programme will further reduce emissions for cars and light- duty vehicles, and emissions of benzene from the storage and distribution of petrol are controlled by vapour recovery systems.
6. A summary of measured benzene concentrations at UK national network sites is shown in Table 3.1 for the period 1999 to 2001. Measured concentrations at all urban background and roadside sites were significantly below the 2003 running annual mean objective of 16.25 g/m3. In more recent years (2001) the concentrations measured at urban background locations were also below the tighter 2010 objectives, although

there is a reduced number of monitoring locations due to restructuring of the

network[1]. The elevated concentrations at London Marylebone Road (kerbside site) indicate the important contribution of traffic emissions, and current levels remain above the 2010 objectives.

7. Forecasts based on national mapping suggest that the policy measures currently in place will achieve the 2003 objective at all urban background and roadside/kerbside locations. Whilst the 2010 objectives are expected to be met at all urban background, and most roadside locations, there is the possibility for some remaining exceedences which will require additional measures at a local level.

Impact of petrol stations

8. The potential impact of emissions arising from petrol stations has been recently investigated by Defra and the Devolved Administrations. There are two possible major sources of benzene from evaporative emissions at petrol stations. The first when petrol vapour is displaced when filling underground storage tanks, termed Stage 1 emissions. The second when petrol vapour is displaced from vehicle petrol tanks during refuelling, termed Stage 2 emissions. All petrol stations with a petrol throughput of greater than 1000m3/annum were required to fit Stage 1 vapour recovery before 1 January 1999. Petrol stations with a throughput of less than 1000 m3/annum are very unlikely to have any significant effect on the local concentrations of benzene. Stage 1 emissions are therefore, unlikely to have any significant influence on concentrations of benzene in the vicinity of petrol stations. As yet there are no legal requirements to fit Stage 2 vapour recovery systems at petrol stations.
9. A 12 month study[1] of concentrations of benzene in the vicinity of petrol stations concluded that the presence of a petrol station is unlikely to have a significant influence on the concentrations of benzene close to residential properties, where:

¥ The petrol throughput is less than 2000m[2]/annum.

¥ The petrol distribution pumps are more than 10m from residential properties, either horizontally or vertically. Petrol stations located immediately below residential properties may result in elevated concentrations of benzene in the vicinity of the residential property.

¥ The petrol station is fitted with a canopy. The presence of a canopy seems to have little effect on the concentration of benzene in the immediate vicinity of petrol stations.

10. The study did not include any petrol stations fitted with Stage 2 vapour recovery. However, Stage 2 vapour recovery will further reduce emissions of benzene from petrol stations where it is fitted.

The local perspective what conclusions have been drawn from the first round of the review and assessment process?

11. There have been no AQMAs declared from the first round of reviews and assessments in respect of the 2003 air quality objective, which supports the studies carried out at a national level. There is therefore no requirement for authorities to

consider road traffic emissions in their review and assessment of the 2003 objective. Only those authorities with relevant locations in the vicinity of major industrial processes that store, handle or emit benzene, will need to progress beyond the Updating and Screening Assessment for the 2003 objective.

12. Data collected during the first round of reviews and assessments have indicated that there are current exceedences of the 2010 objectives at locations in close vicinity to industrial sites (petrochemical processes), and in close proximity to busy roads. Studies carried out in the vicinity of a major refinery have also measured current exceedences of the objectives, but not in an area of relevant public exposure. The potential impact of emissions from petrol stations has been highlighted in Paras 3.08 to

3.10 above. Whilst concentrations are expected to decline in the future (as background concentrations fall), authorities may need to consider these sources in their review and assessment of the 2010 objective.

The Updating and Screening Assessment for benzene

13. This updating review is intended to identify any significant changes that may have occurred since the first round of review and assessment was completed. This will, for example, include new monitoring data, new objectives, new or significantly changed emissions sources (either locally or in neighbouring authorities), or other local changes that might affect air quality, etc.
14. In completing the Updating and Screening Assessment, authorities are encouraged to maximise and build upon the data and assessments completed during the first round of review and assessment.
15. All local authorities should undertake the Updating and Screening Assessment for benzene as part of the review and assessment process.
16. In order to complete the updating assessment, authorities should draw upon the information compiled as part of the First Stage report, completed in the first round of review and assessment. However, as there is the potential for exceedences of the 2010 objectives at locations in close proximity to heavily-trafficked roads, information on road traffic may also be required.
17. This part of the review and assessment is based upon a checklist approach, a summary of which is provided in Box 3.1. Authorities may wish to consider formatting their reports for the Updating and Screening Assessment with these section headings in mind. A detailed checklist for each source or location is set out in Box 3.2 (at the end of the Updating and Screening Assessment section). This describes the information that authorities should collate for the review and assessment against the 2003 and 2010 objectives. The first column describes the source, location or data that need to be considered, and the subsequent columns describe the steps that need to be taken.

Background concentrations

18. Estimated annual mean background concentrations for 2001, 2003 and 2010 have been mapped for the UK, and can be accessed from the internet at the following address: (www.airquality.co.uk/archive/laqm/tools.php). Details of the mapping process can also be found at this site. Factors to adjust measured background concentrations to 2003 and 2010 are given in Box 3.3.
19. In using these maps, authorities are advised to take care to avoid double counting . For example, in areas where there is a section of heavily-trafficked road it may be more appropriate to derive the background concentration from an adjacent grid square. A recommended approach in such cases is set out in Box 1.5.

Monitoring data

20. Data collected from national monitoring networks, or from local monitoring campaigns, are expected to give a more accurate indication of benzene concentrations than modelling studies. Authorities are recommended to prioritise the use of measured benzene concentrations wherever suitable data are available. It is emphasised that monitoring sites will need to be at locations relevant for public exposure, and where the maximum impact of the source (i.e. the highest concentrations) are expected to be measured. In the case of roads, the highest relevant exposure is likely to be at the building facade. In the case of fugitive emissions from industrial or fuel storage/handling sources, then the highest relevant exposure will usually be at the closest residential property in the downwind direction. In the case of emissions from chimney stacks, the highest concentrations may be some hundreds of metres downwind, and may be determined by modelling.

21. Guidance on monitoring methods and strategies for benzene is set out in more detail in Annex 1. Monitoring using diffusion tubes is widely used in the UK to characterise ambient concentrations of benzene. Whilst this method can produce data that are comparable to reference methods (continuous or pumped samplers) care must be taken with the storage, handling and analysis of the tubes. It is imperative that suitable QA/QC procedures are employed, and that the diffusion tubes have been validated and bias corrected against a reference method (see Annex 1). The measured annual mean concentration may be assumed to be equivalent to the annual and running annual mean (where appropriate) and no correction is required.
22. Ideally, monitoring should have been carried out for a period of one year, although a shorter period (for example, 6 months) may be sufficient to demonstrate that the risk of exceedence of the objectives is negligible. Further guidance on how short-term periods of monitoring data may be adjusted for assessment against the annual and running annual mean objectives is given in Annex 1.
23. Where measured data have been collected in the vicinity of a road, then the concentrations will need to be adjusted for the relevant future year (2003 or 2010). Correction factors (using 2001 as the reference year ) are provided in Box 3.4 and have been derived from the estimated reduction in road traffic emissions in future years.
24. Where measured data have been collected in the vicinity of an industrial source, concentrations should not be corrected for future years (2003 and 2010), unless there is substantive evidence that emissions will be reduced as a result of abatement technologies. If there is sufficient evidence to guarantee that emissions will be reduced by a quantified amount in future years, then this can be used to adjust the measured values. A suggested approach is set out in Box 3.5 below.

Screening assessment for industrial sources

25. A list of industrial processes with the potential to emit significant quantities of benzene in 2003 and 2010 is set out in Annex 2, Appendix E. In the case of new industrial sources (since the last round of review and assessment), authorities are advised to check whether an air quality assessment has already been carried out as part of the planning or authorisation process. This should certainly be the case for all Part A (and Part B in Northern Ireland) regulated processes. If a suitably detailed assessment has been completed, then the authority may rely on these results, and should cite the conclusions in the review and assessment report.
26. To simplify the assessment of industrial benzene emissions, several nomograms have been prepared3. These estimate the emission rate (in tonnes per annum) that would produce a maximum running annual mean ground-level concentration of 1.625 g/m3 (for assessment of the 2003 objective) and a maximum annual mean concentration of 0.22 g/m3 (for assessment against 2010 objective). If the actual emission rate from the process exceeds the threshold, then it will be necessary to proceed to a Detailed Assessment.
27. The nomograms are shown at the end of this chapter. Authorities may also download various LAQM Tools from the internet at the following address: www.airquality.co.uk/archive/laqm/tools.php, which contain the calculations in an Excel spreadsheet form, and which requires the user to enter simple details on the emission rate and release conditions (stack height and diameter).

28. Where the process emits benzene from a stack of greater than 10 metres height, the nomograms in Figures 3.1 (for assessment against the 2003 objective) and 3.2 (2010 objective) should be used. Where the emissions arise from a low stack (less than 10 metres height) or from a fugitive source, then the nomograms in Figures 3.3 (2003 objective) and 3.4 (2010 objective) should be used.
29. To assist authorities in the compilation of data related to Part A (and Part B in Northern Ireland) processes[1], the Environment Agency (EA), the Scott ish Environment Protection Agency (SEPA), and the Northern Ireland Environment and Heritage Service (EHS), have committed to provide information on any changes that may affect emissions from existing processes, and any new processes that have been, or will

be, authorised. The information will be provided from the local office on request.

A pro-forma request sheet for this information is provided in Annex 2 (Appendix F).

30. Authorities may also wish to consider checking information derived from their first round of review and assessment if there were any doubts regarding their validity. If this information was obtained from the regulatory agency, or directly from the Public Register, then there should be no need to undertake this task.

Stack emissions

31. To use the nomograms (Figures 3.1 and 3.2) it is necessary to estimate:

¥ the rate of emission of benzene in tonnes per annum.

¥ the stack height.

¥ the stack diameter.

¥ the height of the tallest building within 5 stack heights of the chimney.

32. The emission rate for Part A (and Part B in Northern Ireland) processes can be obtained from the regulatory agencies (see Para 3.29) or the operator directly. Emissions for processes in England and Wales are also posted on the internet at the following address: http://www.environment-agency.gov.uk. Emission rates for Part B (Part C in Northern Ireland) processes may be obtainable from the authority s authorisation documents. Estimated emissions data and emissions factors are also available from the UK Emissions Factor Database (www.naei.org.uk). Details of stack heights, diameters and building heights can also be obtained from the regulator, the operator or the authorisation documents, or may be estimated from a visual inspection.
33. The nomograms use the effective stack height. This can be assumed to be equal to the actual (physical) stack height unless:

¥ The height of release is greater than 3m above the building on which it sits, but less than 2.5 times the height of the tallest adjacent building. In this case the effective stack height can be calculated from the following formula:

Ueff = 1.66(U act H)

where: H is the height (m) of the tallest adjacent building within 5 actual (physical) stack heights distance;

Ueff is the effective stack height; and Uact is the actual (physical) stack height.

34. To use the nomograms, identify the line that corresponds to the diameter of the stack under consideration, and locate the point on this line whose coordinates equal the effective stack height. Figure 3.1 should be used for assessment of the 2003 objective, and Figure 3.2 for the 2010 objective. Read off the corresponding emission rate on the horizontal axis, and compare this with the actual emission rate for the process. If the actual emission rate is greater than or equal to the emission rate derived from the chart, the authority should proceed to a Detailed Assessment.

NOTE:If the stack height is less than any adjacent building within 5 actual (physical) stack heights distance, the authority should, in the first instance, undertake the assessment assuming the release is from a low-level source (see Para 3.35). If this assessment indicates a problem, it will be necessary to proceed to a Detailed Assessment if the emission is greater than 0.25 tonne per annum.

Where there are multiple stacks at the same site, a precautionary approach may be taken by assuming the total emissions (from all stacks) are released from the smallest stack. Where there are complex sites, with many stacks, the above nomograms are unlikely to be applicable, and authorities are advised to proceed to the Detailed Assessment.

Short stack or fugitive emissions

35. To use the nomograms (Figures 3.3 and 3.4) it is necessary to estimate:

¥ the rate of emission of benzene in tonnes per annum

¥ the stack height (assumed to be zero for fugitive emissions)

36. The emission rate for Part A (and Part B in Northern Ireland) processes can be obtained from the regulatory agencies or the operator directly. Emissions for processes in England and Wales are also posted on the internet at the following address: www.environment-agency.gov.uk. Emission rates for Part B (Part C in Northern Ireland) processes may be obtainable from the authority s authorisation documents. Estimated emissions data and emissions factors are also available from the UK Emissions Factor Database (www.airquality.co.uk/archive/laqm/tools.php). Details of stack heights, diameters and building heights can also be obtained from the regulator, the operator or the authorisation documents, or may be estimated from a visual inspection.
37. To use the nomograms, identify the line which corresponds to the height of the stack under consideration (assume zero for a fugitive emission), and locate the point on this line whose coordinates equal the closest relevant receptor location. Figure 3.3 should be used for assessment of the 2003 objective, and Figure 3.4 for the 2010 objective. Read off the corresponding emission rate on the vertical axis, and compare this with the actual emission rate for the process. If the actual emission rate is greater than or equal to the emission rate derived from the chart, the authority should proceed to a Detailed Assessment.

NOTE:Where there are multiple stacks or fugitive releases at the same site, a precautionary approach may be taken by assuming the total emissions (from all stacks or releases) are released from the smallest stack (or from a zero height source). Where there are complex sites, with many stacks or releases, the above nomograms are unlikely to be applicable, and authorities are advised to proceed to a Detailed Assessment.

38. The authority will need to consider the impact of emissions from stacks within neighbouring areas, if there is a potential for these to be significant. As a guide, for medium size sources with chimneys between about 40 to 100 metres, impacts should be considered at a distance up to about 5 km. For small sources with chimneys between about 20 to 40 metres, impacts should be considered at a distance up to about 2 km. For fugitive emissions sources, impacts should be considered up to a distance of about 1 km. If there are several sources which line up, or are close to each other, or there are doubts regarding the potential impact, the authority is advised to take a conservative view and proceed to a Detailed Assessment.

Screening assessment for road traffic sources

39. As indicated in Box 3.2, authorities need only undertake a screening assessment for road traffic sources in respect of the 2010 objective, where the 2010 annual mean background concentration exceeds 2 g/m3 and the 2010 AADT flow exceeds 80,000 vehicles per day (single carriageway roads), 120,000 vehicles per day (dual carriageway roads) or 140,000 vehicles per day (motorways).
40. Authorities are reminded that for the review and assessment of the annual mean objectives, predictions should be carried out at relevant roadside locations (see Para 3.20).
41. The screening assessment for road traffic sources in 2010 may be carried out using the screening model which has been prepared for the Design Manual for Roads

and Bridges (DMRB) and has been published by the Highways Agency[1]. A suitable version of the DMRB Screening Model (v1.01) is available in Excel spreadsheet form and can be downloaded from the following internet address: www.airquality.co.uk/archive/laqm/tools.php. The DMRB model requires input data on annual average daily traffic flow (AADT), annual average speeds, the traffic composition (i.e. the proportion of different vehicle types), the type of road, and the distance from

the centre of the road to the receptor. The user is also required to input the 2010 background benzene concentration (see Section 3.18) which is available from the

internet site: www.airquality.co.uk/archive/laqm/tools.php.

42. The revised DMRB model is expected to provide a slightly conservative assessment of the impact in most cases. This is appropriate for a screening model and should prevent authorities unnecessarily proceeding to a Detailed Assessment. The validation work carried out by the Highways Agency has indicated that the model may significantly underpredict concentrations of nitrogen dioxide and carbon monoxide alongside urban city-centre roads classified as street canyons . There is no clear evidence, however, that this is the case for benzene, thus no adjustment is required for street canyons in the case of benzene.

43. The DMRB model predicts the annual mean concentration for direct comparison with the 2010 objective in England and Wales. Authorities in Scotland and Northern Ireland may assume that the running annual mean is equivalent to the predicted annual mean (i.e. the predicted annual mean may be directly compared with the running annual mean objective, with no adjustment or correction).

Screening assessment for combined sources

44. There are circumstances in which it may be necessary to take account of the potential combined impact of different sources, for example, an industrial or fugitive source, or petrol station close to a major road. The potential impact of combined sources need only be considered in respect of the 2010 objective, and a simplified (precautionary) approach is set out below.
45. The threshold emission rates for industrial and fugitive sources (as described in Sections 3.30 to 3.36 above) are based upon a concentration (0.22 g/m3) equivalent to approximately 10% of the 2010 objectives. This can be adjusted, depending upon the actual emission rate, using the procedure set out in Box 3.6 below.

1. Where 2 or more roads intersect, for example at a junction, the traffic flows from each road should be added to give a combined total. For example at a crossroads with 2 roads intersecting, where road [A] has an AADT flow of 38,000 vehicles per day and road [B] has an AADT flow of 44,000 vehicles per day, assume a combined flow of 82,000 vehicles per day. If there are 3 links to the junction, add the flows and multiply by 2/3.
2. No adjustment is required for street canyons (Para 3.42).

The Detailed Assessment for benzene

46. Where the screening assessment has indicated that there is a risk of the Air Quality Objectives for benzene not being achieved by 2003 or 2010, then the authority will need to carry out a Detailed Assessment. The aim of this Detailed Assessment is to determine with reasonable certainty whether or not there is a likelihood of the objectives not being achieved. The assumptions within this Detailed Assessment should be considered in depth, and the data that are used or collected, quality-assured to a high standard. This is to allow the authority to have confidence in the decision that it reaches to declare, not declare, or revoke an Air Quality Management Area. Where a likely exceedence of the objectives is identified, then the authority will also need to determine the magnitude and geographical extent, so that the AQMA boundary can be set.
47. Because of the wide variety of potential sources and local circumstances that may prevail, it is not possible to set prescriptive guidance for the Detailed Assessment. However, wherever possible, lessons learnt from the First Round of review and assessment have been drawn upon. It is also expected that authorities will make use of the various Helpdesks that have been established (see Box 1.1).
48. Specific guidance related to monitoring, emissions data and dispersion modelling, is provided in Annexes 1 to 3 of this document. Whilst important aspects are highlighted in this section, authorities who are undertaking a Detailed Assessment are strongly advised to read the relevant sections of the Annexes before commencing their work.
49. In undertaking the Detailed Assessment it is important to give consideration to the points of maximum relevant public exposure (i.e. those locations where the highest concentrations of benzene are expected). In the case of emissions from road traffic or fugitive sources (such as petroleum storage facilities) the highest concentrations are generally likely to occur at the closest point to the road or facility in the prevailing downwind direction. Where emissions are released from a stack or vent, then the point of highest concentration will occur some distance downwind. In the latter case, simple dispersion modelling may help identify this location.

Monitoring

50. As for the screening assessment, measured data are expected to give a more accurate indication of benzene concentrations than modelling studies. This is particularly the case where fugitive emissions (for example, from refineries or petroleum storage facilities) are of the greatest significance.
51. Guidance on monitoring methods, monitoring strategies and suitable QA/QC procedures are set out in greater detail in Annex 1 to this document. The following points are highlighted:

¥ Whilst passive diffusion tubes have a useful role to play for the Detailed Assessment, authorities should ensure that the tubes are adequately calibrated against a

reference sampler or equivalent, and that suitable correction factors are applied to

the data (see Annex 1). If possible, a European reference sampler (or equivalent) should be located at the point of expected maximum relevant exposure, with additional passive tubes set out to define the area of exceedence.

¥ All samplers should be located at points of relevant public exposure. There is unlikely to be any point in monitoring concentrations at locations where the objectives do

not apply.

¥ Ideally, monitoring should be carried out for a period of 12 months, with 90% data capture, and periods of missing data spread evenly throughout the year. In many circumstances this may not be possible, although data collected from shorter campaigns of 6, or even 3 months will be useful if concentrations are well below or above the objective.

¥ Where dispersion modelling studies are to be carried out (see 3.52 below) the results from the monitoring campaign will be used to verify the model at a local level. Specific consideration as to the type of sampling location that will fulfil this purpose needs to be given at the start of the monitoring. Samplers located at background locations are of little use in model verification. Ideally, a sampler located at the point of maximum public exposure will be chosen.

Modelling

52. Where emissions of benzene can be accurately quantified, and are released from a defined source such as road traffic, a chimney stack or vent, or storage tanks, then the likelihood of exceedence may be determined using a suitable air quality dispersion model. For the Detailed Assessment, authorities will need to have confidence in their results, which will be subsequently used to indicate the likelihood of whether the objectives will be exceeded or not. It is therefore important that the model is verified.
53. Issues regarding model validation and verification are discussed in further detail in Annex 3. Model validation generally refers to detailed, peer-reviewed studies that have been carried out by the model supplier, or a regulatory agency (for example, USEPA). All models used in the Detailed Assessment should have an appropriate pedigree, and have been subject to detailed and documented validation trials.
54. Model verification refers to checks that are carried out on model performance at a local level. This basically involves the comparison of predicted versus measured concentrations. Where there is a disparity between the predicted and measured concentrations, the first step should always be to check the input data and model parameters in order to minimise the errors. If required, the second step will be to determine an appropriate bias correction that can be applied. For the review and assessment of fugitive or road traffic sources it is essential that model verification is carried out. For the review and assessment of point sources, verification may be more difficult to carry out. In this case, the use of an appropriate validated model will be sufficient, provided that the source emissions can be accurately quantified. However, in cases where there are local factors contributing to uncertainty, such as certain types of batch processes, complex topography etc, then local verification studies may be required, and/or an additional consideration of uncertainties taken into account.

55. In all cases where model verification has been carried out, the approach should be fully documented in the review and assessment report, and any bias correction applied should be explicitly stated.

Meteorological data

56. The choice of meteorological data for use in the dispersion model can have an outcome on the result, depending on whether a worst case or typical year is selected. A detailed discussion on the selection of meteorological data is provided in Annex 3 and all authorities are advised to read this section prior to undertaking a modelling study. In summary, the guidance recommends that a single year of sequential meteorological data is used. If possible, meteorological, background and emissions data should all be derived from the same year.
57. In the case of emissions from point sources, the main effect of different meteorological years will be to affect the precise location of the maximum predicted concentration. Authorities are advised to take into consideration the potential effects of fluctuating wind directions in different years. A suggested means of dealing with this is to assume that the actual maximum concentration may occur within a 45 degree arc centred on the predicted maxima, and all properties lying within this arc should be considered. Where the authority decides to use multiple years (3 or more) of meteorological data, it is recommended that the results for all years are reported, but that any decision is based upon the worst-case result.

Receptor spacing

58. The importance of giving due consideration to potential hot spots has already been highlighted in Para 3.49. For the purpose of dispersion modelling, this requires the user to ensure that a suitable resolution for receptor grid spacing is used, or that specific receptors (representing the locations of maximum public exposure) are included6. If the grid spacing is set to a low resolution (for example, several hundred metres) and no specific receptors are included, then there is the potential that the model will not predict the highest concentrations relevant to public exposure. A more detailed discussion on receptor spacing is provided in Annex 3.
59. In selecting specific receptors along roads, authorities are reminded to pay careful attention to the alignment of the road in their model. Some models may describe curved sections of road as straight-line links. In some instances this may cause the distance from the road to the specific receptor to be significantly distorted (distances over the first 10 metres or so from the kerbside are critical). Authorities are advised to ensure that road alignments do not cause this type of problem.

6 Some dispersion models allow the user to incorporate specific roadside receptors automatically i.e. to place

receptors at a specified distance from the kerbside. Whilst this can be an acceptable approach, authorities should document that this option has been included, and ensure that the actual distances used in the model are consistent with the distances of the closest receptors to the road.

Road traffic sources

60. There are a number of commercial models available on the market that can be used to predict concentrations of benzene arising from road traffic. Whilst the accuracy of predictions may not improve substantially over those obtained from the DMRB model, authorities may find these models convenient as:

¥ They may be used for modelling of other road traffic emissions (for example, NOx and PM10).

¥ The output can be displayed in the form of concentration isopleths, and used to clearly indicate areas of exceedence.

61. Regardless of the model that is selected, improved accuracy is likely to be gained from improved input data. Careful consideration should be given to the assumptions

that have been made for traffic flows, speeds and vehicle mix. Specific consideration should be given to flows and speeds through junctions, and in particular to avoid

 double counting of vehicles where turning movement assessments are not available.

62. Road traffic models should always be verified at a local level. Ideally, verification should take place at a receptor location that represents the maximum point of relevant public exposure, and where a reference sampler or equivalent has been used.

An approach to this verification is suggested in Box 3.7.

Industrial sources

63. Where emissions of benzene can be accurately quantified from a stack or vent, then the impact can be determined using a suitable point source model.

¥ It is probably only necessary to determine the annual emission rate from the chimney even if there are significant variations. Additional input data required include the conditions of release (stack height, diameter, exit temperature).

¥ Whilst it is more difficult to verify point source models, this is a more straightforward process for annual mean concentrations, and is recommended for benzene. The process for verification is identical to that described above in Box 3.7. Ideally, the monitoring location will have been selected with the aid of a dispersion model.

64. Emissions arising from petroleum storage tanks etc can be quantified provided that details of the storage design, product throughput etc are known. The modelling process can then be treated as described in Para 3.52 above. Where emissions arise from a fugitive source, and are generally unquantifiable, then the Detailed Assessment will need to rely predominantly upon monitoring data.