Publication - Advice and guidance

National low emission framework

Published: 14 Jan 2019
Environment and Forestry Directorate
Part of:
Environment and climate change

Methodology for local authorities to undertake air quality assessment through the National Low Emission Framework (NLEF) to inform decisions on transport-related actions.

31 page PDF

787.7 kB

31 page PDF

787.7 kB

National low emission framework
2. NLEF appraisal

31 page PDF

787.7 kB

2. NLEF appraisal

2.1 NLEF appraisal process – key stages

2.1.1 The NLEF appraisal process provides a consistent approach that can be applied across Scotland to inform decisions on transport-related actions to improve local air quality. It will support local authorities in considering transport-related issues in the context of local air quality management and help develop evidence to support consideration of the introduction of an LEZ as an appropriate option to improve air quality.

2.1.2 It is intended to be a two stage process consisting of screening and assessment. It is not the intention that all local authorities will have to conduct the full NLEF appraisal.

2.1.3 The initial screening stage should be completed by local authorities that:

  • have identified air quality problems (where transport is the primary cause) and declared an AQMA
  • have identified potential air quality problems (where transport is the primary cause) which may lead to an AQMA declaration in the future

2.1.4 Where the screening exercise indicates that further assessment should be carried out, the local authority should proceed to stage two. At stage two, the NMF will support the identification of the scope and key contributors to air quality issues and provide the evidence to help assess potential benefits of transport-related actions to address those issues, with a focus on considering the introduction of an LEZ.

2.1.5 The two key stages in conducting a NLEF appraisal are summarised in Table 1 below.

Table 1




Actions required



  • decision on whether to proceed to stage two assessment
  • screening process to identify actions that will benefit air quality within the AQMA
  • screening evidence should form part of the Annual Progress Report, with the decision agreed by Scottish Government and SEPA



  • decision to proceed with introduction of LEZ or identification of alternative transport-related measures required to improve air quality
  • Stage two assessment report agreed by Scottish Government and SEPA
  • NMF approach to support assessment of sources of pollution and options
  • quantitative impact assessment (based on predicted change in pollutant concentrations)
  • consideration of consequential impacts (e.g. congestion, export of pollution)

2.2 NLEF stage one - screening


2.2.1 Through the LAQM system, local authorities are required to review and assess air quality within their area on an annual basis and report the outcome via an Annual Progress Report (APR). Where an exceedance (or potential for exceedance) of the relevant air quality objectives is identified, it will result in the declaration of an Air Quality Management Area (AQMA) for the affected area or the situation will be kept under review as to whether an AQMA declaration is required in the future.

2.2.2 Where an AQMA is declared, local authorities are required to develop an air quality action plan (AQAP) which identifies the measures proposed to achieve compliance with the air quality objectives. Progress on AQAPs is also reported via the APR.

2.2.3 NLEF Screening will form part of the review and assessment process for

APRs and AQAPs, where relevant. It is intended that the screening process will encompass a review of proposed actions across the range of local authority functions, with a particular focus on transport. This will allow local authorities to assess whether these actions are likely to lead to significant improvements in air quality within the AQMA, in line with the requirement to meet the air quality objectives in the shortest possible timescale.

2.2.4 Where the proposed actions do not support delivery of this requirement, local authorities should move to stage two of the NLEF appraisal, to assess the scope for introducing an LEZ in support of meeting air quality objectives. The outcome of the screening exercise should be submitted as part of the APR and the AQAP should be updated.

2.2.5 The stage one screening exercise should use existing information and evidence from across the local authority's functions to identify and consider actions that could lead to improved air quality and contribute to the revocation of the AQMA.

2.2.6 Through the screening process, some AQMAs will be excluded from the need for further consideration through stage two assessment due to the nature of the AQMA (e.g. size, scale, contributing sources of pollution) or where other committed actions from the AQAP or other programme of work being undertaken by the local authority is likely to remove air quality exceedances in the shortest possible timescale.

2.2.7 In carrying out the screening, local authorities should consider all of their functions to ensure that any actions that could have an impact on air quality are taken account of.

2.2.8 It is intended that all local authorities with AQMAs will carry out the screening process for each AQMA where transport is a key contributor to exceedances. Local authorities will not be required to complete the NLEF screening where this is not the case.

2.2.9 Initially the results of the screening process, including the outcome of the analysis of the likelihood of removing air quality exceedances, will be reported by local authorities to Scottish Government and SEPA in a standalone report. In future years, it is intended that the screening will be reported through APRs and assessed by the Scottish Government and SEPA as part of the wider APR review process.

2.2.10 In those cases where the outcome of the screening exercise determines that an LEZ is a potentially appropriate option, the affected AQMA will move to the stage two assessment process.

2.2.11 It is anticipated that an LEZ is likely to be appropriate in circumstances where it cannot be demonstrated that committed measures are likely to remove air quality exceedances in the shortest possible timescale.

2.2.12 Should an AQMA be screened out on the basis of such committed action plan measures being in place, then it is expected that these measures will be delivered within the timescale defined in the AQAP.

2.2.13 An LEZ alone will not always be sufficient to improve air quality to a level which removes any exceedances and additional measures may need to be identified and implemented through the LAQM action planning process.

Screening – key stages

2.2.14 Existing air quality information, including data produced as part of the annual review and assessment process and air quality action plans, can be used in the screening assessment. It is not anticipated that there will be a requirement for local authorities to collect new data or information during the screening stage of the appraisal process.

2.2.15 The following key steps should be undertaken as part of the screening exercise:

  • Review of information on the main sources of poor air quality and other contributing factors within each AQMA. If transport is not a significant contributor within a particular AQMA, this AQMA should not be considered under the NLEF appraisal process.
  • Analysis of existing data including air quality, traffic and environmental data as well as information on existing and future action planning measures across all local authority functions which seek to address or are likely to contribute to improving air quality
  • Conduct the NLEF stage one screening process
  • Record the results of the screening process and the decision as to whether proposed measures are sufficient or whether any AQMA requires to progress to a stage two assessment

Information to support the screening exercise is attached at Appendix 2.

2.2.16 Local authorities should submit the completed screening assessment and findings to the Scottish Government and SEPA for review and agreement. Where the findings demonstrate that committed measures are likely to remove air quality exceedances in the shortest possible timescale, and the Scottish Government and SEPA are in agreement with this conclusion, then no further work will be required under the NLEF appraisal process at this time. Scottish Government and SEPA may seek the opinion of Transport Scotland and other appropriate authorities before agreeing the screening outcome.

2.2.17 It is anticipated that existing measures identified as part of the LAQM action planning process will require to be implemented in accordance with stated timescales, irrespective of the need to progress to a stage two assessment.

Data requirements and analysis

2.2.18 Potential data sources will include:

  • LAQM Review and Assessment reports
  • outcomes of local modelling (where available)
  • appraisals of major transport schemes
  • land use proposals and/or assessments
  • Annual Progress Reports and AQAP measures

2.2.19 For each AQMA, the main issues should be defined for any exceedances of the air quality objectives (AQO) for NO2 and / or PM10 and/or PM2.5 with a focus on:

  • the locations where Scottish AQOs for NO2and / or PM10 and / or PM2.5 concentrations are not being met (or may not be met)
  • the sources and percentage split of sources contributing to exceedances of Scottish AQOs for NO2, PM10 and / or PM2.5 (e.g. road transport, rail transport, industry, etc.)
  • where emissions from road transport are the significant contributor, the relative breakdown of the vehicle classes contributing to exceedances of Scottish AQOs for NO2and / or PM10 and / or PM2.5; e.g. Heavy Goods Vehicles (HGVs), buses, taxis, cars, etc.

Defining the issues

2.2.20 Existing data, largely produced through the LAQM review and assessment process, including in APRs, should be summarised to:

  • identify the main sources of emissions
  • describe the main AQAP measures already being implemented by the local authority and their expected impacts on the levels of AQO exceedance
  • identify whether there are any other committed actions that could help to in address the exceedance and the likely contribution to reducing exceedances

Screening outcome

2.2.21 The screening outcome should consider whether committed measures identified through the screening exercise will be likely to remove air quality exceedances in the shortest possible timescale.

2.2.22 It is anticipated that where the screening outcome finds that the committed measures are not likely to achieve this, then the affected AQMA should move to the stage two assessment.

2.2.23 If the air quality exceedance is very localised it may be more appropriate to address the issue by identifying additional location specific measures to be implemented through the AQAP, potentially through consideration of local transport measures. In this situation, the additional measures should be identified and the timescale for implementing them recorded in the screening outcome along with a description of the likely contribution to removing exceedances.

2.2.24 In other circumstances, it is likely that the AQMA could benefit from the introduction of an LEZ and should be progressed to stage two assessment.

Reporting requirements

2.2.25 It is anticipated that local authorities will report the outcome of their first stage one screening exercise by 30 June 2019. Screening should then be undertaken on an annual basis as part of the annual assessment and review process and reported through the Annual Progress Report (APR).

2.2.26 In the event that SEPA and Scottish Government cannot reach an agreement with the local authority on the outcome of the NLEF screening process, further discussions will be required. In some circumstances, further insight into the planned actions or additional environmental information may add clarity and help in reaching agreement.

2.3. NLEF stage two – assessment


2.3.1. The National Modelling Framework aims to standardise data collection requirements, analysis process and presentation of outputs to provide local authorities with information required to appraise measures for improving urban air quality. Local NMF models will represent a standardised approach to modelling air quality for local authorities undertaking a stage two assessment.

2.3.2 The stage two assessment will focus on:

  • assessing the impact of potential LEZ options with regard to air quality
  • identifying the preferred option, including consideration of geographical extent and scope of vehicles to be included
  • considering the wider impacts of the preferred option
  • supporting identification of the costs associated with implementing the preferred option

Objective setting

2.3.3 The starting point for the stage two assessment process will be to define the objectives for the potential LEZ, taking account of the pollutant(s) of concern and with regard to any available information on source apportionment that identifies particular vehicle types that are a significant contributor to any air quality exceedances.

Defining size and boundaries of options for consideration

2.3.4 The indicative boundary of potential options for consideration should be defined at the outset, taking account of local circumstances. Potentially, more than one boundary may be considered. For example, the AQMA boundary or one which covers just a few streets with the highest concentrations of air pollutants.

2.3.5 The area for consideration will be informed by:

  • the area of exceedance of air quality objectives and the main sources of pollutants
  • geographically discrete areas, such as a town centre, or other areas which are well defined (e.g. within an inner ring road)
  • features that may influence enforcement (e.g. an outer ring-road with junctions leading into exceedance areas, key access points such as bridges)
  • the potential need to allow vehicles to divert onto alternative routes to avoid the area of the LEZ
  • air quality along any such alternative routes to determine if they could be at risk of new exceedances as a result of displaced traffic
  • mapped emissions by vehicle type in order to identify areas where options are likely to be most effective. Mapping bus routes, taxi ranks and/or residential and commercial land-uses will be useful.

2.3.6 The size and extent of areas should be designed to meet the objectives that have been set for the LEZ but there is likely to be a range of other issues that will require to be considered such as access and traffic management and the effect on surrounding roads.

National Modelling Framework approach

2.3.7 The NMF provides a standardised approach to modelling air quality to support consideration of LEZs. New tools have been developed to ensure that the data is presented in a standardised way that supports the decisions-making process to help improve air quality. The NMF ensures that the analysis and generation of evidence to support decision-making is consistent across those local authorities undertaking the stage two assessment. SEPA will support local authorities throughout stage two assessment and the decision-making process, through the development of the NMF local model.

Figure 2 - National Modelling Framework – generating evidence

Figure 2 - National Modelling Framework – generating evidence

2.3.8 The NMF local models will utilise ADMS-Urban, a recognised system that is used around the world for modelling all aspects of air pollution across large urban areas. The model:

  • can simulate details on a range of scales, from street-to-street to city-wide
  • take account of the full range of pollution sources, including traffic
  • include an advanced description of meteorology, taking account of vertical changes in wind speed to allow for detailed characterisation of atmospheric turbulence and stability
  • include the effect of buildings on dispersion and the mixing of pollution induced by vehicles
  • the effect of chemical reactions on pollutant concentrations

2.3.9 An ADMS model for each option being considered will be built by quantifying emissions from a range of sources. Emissions from major roads will be defined explicitly in the model based on detailed traffic-count data and operator information. Good quality detailed traffic data is essential for good model performance and SEPA will work with the local authorities to ensure that the data is collected and presented in a format that aids decision making.

2.3.10 Other sources of pollution in the model include residential and industrial combustion, industry, waste, minor roads, shipping and railways. Emissions from these sources are not defined explicitly. Instead, emissions are derived from published UK National Atmospheric Emissions Inventory 1km2 emission grids. Where other significant sources are identified, such as largescale industrial emissions, shipping or major road networks (such as motorways), these will be assessed appropriately to ensure they are defined explicitly in the model. Finally, some pollution in the area will have been transported from outside of the region. These sources are represented in the model by 'background' measurements of pollution taken from an appropriate rural monitoring station. For comparison, Urban Background monitoring data will also be used to represent the effect of emissions from sources other than the main roads.

2.3.11 The model includes accurate dimensions of city streets which will be derived using Geographical Information Systems (GIS). These dimensions include the width and height of each road, as well as the width and height of the street 'canyon', as created by the surrounding buildings. This means the model can reproduce the 'trapping' of pollutants within some narrow streets.

2.3.12 The model uses hourly meteorological data from the most appropriate Met Office station for each proposed LEZ area. Meteorological data is used to calculate how well all sources of pollution are mixed and transported within, and across, the urban area.

2.3.13 Detailed traffic data will be collected for each proposed LEZ area to ensure an accurate representation across the road network. Data collection is processed to give Annual Average Daily Flow (AADF) (i.e. the typical number of vehicles driving along each road section during a 24 hour period). The traffic data collection process will aim to provide detailed traffic composition, so that vehicles can be classified into 11 different vehicle categories – motorcycle, car, taxi, light-goods vehicle (LGV), bus/coach (PSV) and six different types of heavy-goods vehicle (HGV) and pedal cycles. In addition, a smaller amount of very detailed data, collected through the recording of vehicle number-plates, will be analysed using information from the DVLA database. SEPA will work with the local authority to ensure the traffic data is collected in a manner that can be utilised within the NMF and future traffic models.

2.3.14 Local authorities will be responsible for the development of any traffic model that is deemed necessary for the assessment process. SEPA will work with the traffic modellers to ensure that the outputs are transferrable to assess local air quality issues in relation to modelled traffic changes.

Figure 3: Example of detailed traffic collection points, including turn counts, 12/24 traffic count locations and ANPR (data collected in Glasgow during winter 2017/18)

Figure 3: Example of detailed traffic collection points, including turn counts, 12/24 traffic count locations and ANPR (data collected in Glasgow during winter 2017/18)

2.3.15 Local bus operator data, collected directly for individual routes that operate within the modelled area, will ensure the correct proportions of Euro engine classifications are used within the model.

2.3.16 Compiled traffic data and other information detailed above are fed into the air quality model and the performance of the model can be checked against observed data.

2.3.17 The traffic information provides a rich source of data which forms part of the evidence for the development of appropriate interventions. Additionally, the data will be collected in such a way as to ensure that it can be used within the development of detailed traffic models in order to run traffic scenarios to help understand altered traffic flows. These outputs can then be used within ADMS to assess changes in air pollution concentrations. SEPA will work with the local authorities to provide appropriate interactive tools for using the data.

2.3.18 Model outputs are visualised using the interactive TIBCO Spotfire® software. This provides an interactive platform for sharing data outputs and building/presenting scenarios. Modelled data is presented as predicted annual mean concentrations at kerbside points along every road in the modelled area. This makes it possible for exceedances at other locations to be identified. Some of these points correspond to monitoring stations, where the model concentration can be compared against observed values to help validate the model outputs.

2.3.19 The sensitivity of the model predictions are explored under varying individual modelled parameters and varying metrological conditions. Computational Fluid Dynamic (CFD) models are used to identify complex air flow issues around specific monitoring stations or locations identified within the modelling domain. Although the CFD model does not represent the mass concentration of a pollutant, it allows the illustration of potential dead zones or areas of high flush, which would perhaps be missed elsewhere. This is useful where modelled predictions cannot be reproduced due to localised air flow issues.

2.3.20 The NMF local model will be used to estimate the contribution of each source to total pollution levels in the defined area. At kerbside locations, the contribution from road traffic is calculated against background pollution data. Estimates of contributions from vehicle types are calculated to provide the greatest contributors to the road-traffic component of air pollution in the area.

2.3.21 Detailed traffic count data is linked to an emission database to highlight the relative importance of different vehicles types.

2.3.22 The NMF local model will be used to estimate the potential improvement in air quality by removing, or reducing emissions from, all or part of the vehicle fleet. These traffic scenarios will be applied within the boundaries of the options identified. Scenarios could include removing diesel vehicles, restricting buses to EURO VI emission standards (including retrofit) or quantifying the effect of newer vehicles entering the fleet due to natural turnover.

2.4 Stakeholder input and consultation

2.4.1 The introduction of an LEZ has the effect of restricting access to the designated zone for non-compliant vehicles. This will impact on all those who wish to access the area in a vehicle, with the potential to affect bus service operators, businesses and individuals. It will be critical to engage with key stakeholders from the outset of the stage two assessment process to obtain data to support the assessment and to gather information on the potential issues that will inform potential LEZ options and will be critical to the design and implementation of any future LEZ.

2.5 Wider considerations

2.5.1 As part of the stage two assessment, local authorities should consider the potential for environmental impacts beyond air quality that could be associated with the introduction of an LEZ or, in some cases, other transport-related measures.

2.5.2 These considerations will be at a high-level, reflecting the early stage of development of any proposals. Broadly, the stage two assessment process should:

  • identify the range of likely impacts on the environment
  • identify whether these impacts will need to be considered further in later stages of the assessment process

2.5.3 As many AQMAs are located in urban locations and at a relatively small scale, the consideration of other environmental impacts may not necessarily be extensive. It will, however, be beneficial to consider as wide a range of impacts as possible.

2.5.4 Whilst reflecting the early stage of development of any plan, it is recommended that local authorities consider whether there is likely to be any requirement for a statutory assessment under the Environmental Assessment (Scotland) Act 2005 as part of the stage two assessment process. Guidance on Strategic Environmental Assessment is available online.

2.5.5 Whilst improved air quality will be beneficial for all, the potential for unintended impacts exists. It will be important for local authorities to consider the potential for impacts on equality, to identify issues which will require to be considered through an equalities impact assessment should the introduction of an LEZ be progressed.

2.5.6 The exclusion of certain vehicle classes from an area will have an impact on the overall transport offering. It will be important when considering impacts to ensure options for public transport, active travel and integrated transport are maximised and unintended consequences, such as impacts on equality and connectivity, are taken into account. It will also be important to ensure that other policy measures such as parking, planning and climate change are also considered.

2.5.7 Whilst costs will be dependent upon the scope and design of any LEZ or other transport-related measure being introduced, the stage two assessment work will allow a high-level estimate of costs to be made. The NMF process will support the consideration of costs, by helping to define the boundary and scope of any LEZ, allowing outline costs to be identified as part of the decision-making process.

2.6 Assessment outcome

2.6.1 The overall outcome of the stage two assessment process will be to identify whether the introduction of an LEZ should be progressed to support the delivery of improved air quality and, in particular, to reduce any exceedances of air quality standards.

2.6.2 The NMF approach will support this decision by providing evidence to:

  • show air quality impacts of potential LEZ options
  • help identification of the preferred option
  • aid consideration of wider impacts of the preferred option
  • help establish the likely costs of the preferred option

2.6.3 Alternatively, the stage two assessment may conclude that the introduction of an LEZ is not the most appropriate measure to improve air quality. In those cases, the information produced through the NMF will support the identification of alternative transport-related measures to reduce exceedances of air quality.

2.6.4 In such cases, the identification of these alternative transport measures should be considered as part of the appraisal process, with the outcomes reported through the stage two Assessment Report.

2.6.5 Any measures identified through the stage two assessment should be included within the AQAP with the date by which they will be delivered.

2.7 Reporting requirements

2.7.1 Local authorities should prepare a report on the outcome of the stage two assessment, documenting the work undertaken and the recommendation on whether an LEZ should be introduced or other identified measures implemented. SEPA will provide the local authorities with an interim report and a detailed NMF report for use in the preparation of the stage two assessment report.

2.7.2 The stage assessment two report will be reviewed and agreed by Scottish Government and SEPA.

2.8 Next steps

2.8.1 Where the outcome of the stage two assessment concludes that an LEZ is required then it is anticipated that this will be introduced by 2023 in line with Scottish Government commitments included within the Programme for Government 2017-18.


Email: Andrew Taylor