Scottish Air Quality Advisory Group advice note 1, June 2025

Summary of advice

The level of ambition proposed by the Scottish Government is broadly welcomed as a commitment to delivering further air quality improvements in Scotland. However the 2030 target date is only likely to be achievable based on current policies and does not in itself drive further change. More ambitious targets set further in the future, with interim targets, would be more effective in driving improvements.

There is potentially a considerable expansion in the number of air quality objectives (new daily means and new exposure indicators for both NO₂ and PM₂.₅), in addition to the tightening of existing objectives. Setting new objectives in legislation is a significant commitment; it is important that there is a strong rationale for the proposed amendments, the additions to objectives and the increased complexity. Whilst the health case is clear and strongly supported by evidence, there also needs to be evidence for the practical likelihood of Scotland meeting the proposed new objectives, including economic evidence, if relevant. This will come from analysis of current trends in measurements, but mainly from modelling. Also, demonstration of the achievement of the targets needs to be both measurable and verifiable. 

Part of the rationale is the balance between ambition and achievability, including knowledge of the extent to which actions/policy in Scotland can achieve the new targets versus the extent of reliance on actions in the rest of the UK, in Ireland and in rest of Europe, due to atmospheric transport across borders. A key aspect of this is knowledge on the specific emissions sources contributing to ambient concentrations in Scotland, as well as the policy commitments of the other UK nations and the EU. 

The UK Government has commissioned modelling studies to inform the development of its annual PM₂.₅ objective and the associated exposure reduction target for England. The emission reductions explored by the UK Government in these model scenarios were implemented across the whole of the UK and the data therefore are of value to understanding potential concentration changes in Scotland also [1] [2]. Assessment of the utility of other existing emission scenarios and models would also be beneficial for understanding the ranges of potential future air pollutant concentrations that will inform any new air quality objectives.

The inclusion of PM₁₀, NO₂ and daily targets would require far more complex modelling than undertaken at UK level to assess what emission reductions would be required to meet the proposed objectives. As a very local pollutant, NO₂ requires high resolution modelling, also necessary for the modelling of daily targets, with model performance validated against historical measurements. 

For NO₂, and to a lesser extent PM₂.₅ the move away from petrol and diesel vehicles will deliver improvements. The coarse aerosol fraction (PM₁₀-PM₂.₅) has a significant contribution from non-exhaust emissions (tyre, brake and road wear), which will likely decrease much less with the move towards EVs. Brake wear may go down due to regenerative braking, but road wear might go up due to vehicles becoming heavier. So non-exhaust emissions will continue to make an important contribution to PM₁₀, along with smaller PM size fractions. Sea salt will also likely continue to be a major component of PM₁₀ in coastal areas. Airports and shipping will likely remain significant sources of NOx during the move to net zero and could drive local exceedances of stricter PM and NO₂ targets. Improved source apportionment will be necessary in this context.

Reducing annual mean concentrations should lead to reductions in elevated daily means, so particular thought needs to be given to the values (and allowed exceedances, if relevant) of daily mean targets. Both annual and daily mean targets should be challenging but achievable. It may be that Scotland has greater influence on controlling elevated daily means (particularly for NO₂) than annual means, so this could be a way in which Scotland can take more meaningful action, especially given the extensive evidence base around the health impacts of short term air pollution episodes [3] [4] [5] [6]. Reducing daily means will automatically contribute to annual mean reductions.

A greater focus on emissions and source contributions could also help to drive further air quality improvements. Thought also needs to be given to how it is most appropriate to quantify exposure for any exposure reduction targets. As these will be set in legislation, they will be subject to scrutiny and potential challenge. Ideally, quantification needs to be measurement based.

To date, relatively little consideration has been given in the UK to the individual size fractions and components of PM (beyond the mass-based metric of PM₂.₅) when setting targets and objectives. As concentrations continue to decline, this will become increasingly important in developing and implementing effective policy interventions.

There is a need to consider how the increased uncertainty in PM measurements at lower concentrations impinges on the ability to be confident in whether lower value objectives are being met. In many parts of Scotland, concentrations are already at or close to levels which represent accurate limits of detection for much of current monitoring technology. There is also limited evidence of equivalence of current PM measurement approaches for Scottish PM chemical composition. 

Additional uncertainty is generated by the high proportion of PM₂.₅ which is non local. Relying on modelling alone to determine the local/regional split is probably insufficient; it has however been demonstrated that the local fraction can be separated from the regional background by measuring at higher time resolutions (one minute intervals).  

A similar argument applies to the measurement of NO₂ although the regional background contribution is much smaller than with PM₂.₅. It is also more challenging to operate some of the chemiluminescent NO₂ monitors at one minute resolution but it is possible. This is because they do not measure NO₂ directly but by difference (NOx - NO) which requires a cycle time often approaching one minute. There are ways to achieve this by careful setup of the monitors. Low cost NO₂ sensors can also achieve this with reasonable accuracy. At the same time, there is some evidence that chemiluminescent monitoring includes artefacts in the rural background in Scotland. These points should be considered when designing a monitoring strategy for assessing progress against any new objectives.

Caution should be exercised when considering whether to allow subtracting a background component from PM₂.₅ for compliance assessment. Firstly, there is no generally agreed definition of what constitutes natural background. Almost everything that might be considered natural has an anthropogenic influence, with the possible exceptions of sea salt and volcanic eruptions, but even then it could be argued that factors driving sea salt uplift are at least to some degree influenced by human-induced climate change. Also, subtraction of a natural component would require modelling, which introduces uncertainty.  

Further important issues are that natural components can act as carriers of toxic material attaching to the particles or resulting from particle-based chemistry, and that air quality guidelines are based on total PM. Expert reviews  continue to state that all components of PM must for the present continue to be treated as equally toxic to human health and be used for health impact quantifications. A further consideration is that subtraction could be perceived negatively by the general public. It should also be noted that, the more ambitious any targets are, the less headroom there is for external influences (both from other countries and natural sources).

Local authorities will continue to be responsible for much of the delivery of further air quality improvements. The current system of Local Air Quality Management (LAQM) has been in place throughout the UK for nearly 30 years, with relatively little significant change to processes and procedures over that period. A fundamental review of how LAQM operates in Scotland is likely to be necessary to ensure it remains fit for purpose.

Finally, the role of medium and long term climate change in influencing the nature and duration of short term air pollution episodes, and how this might change over time, will require some investigation. This is particularly important given the Scottish Government’s stated aim of closely linking the new air quality policy framework to 2045 net zero targets, and for maximising co-benefits for greenhouse gas and air pollutant reductions.

References

[1] Development of the Environment Act Targets - DEFRA UK Air - GOV.UK

[2] Analysis of abatement options to reduce PM2.5

[3] Respiratory Admissions Linked to Air Pollution in a Medium Sized City of the UK: A Case-crossover Study - Aerosol and Air Quality Research

[4] Short-Term Exposure to Ambient Air Pollution and Mortality From Myocardial Infarction - PubMed

[5] Public Health and Air Pollution in Asia (PAPA): a multicity study of short-term effects of air pollution on mortality - PubMed

[6] Spatio-temporal associations of air pollutant concentrations, GP respiratory consultations and respiratory inhaler prescriptions: a 5-year study of primary care in the borough of Lambeth, South London - PubMed