Following the submission of an Air Quality Assessment our Air Quality team received comments from an Environmental Health Officer (EHO) seeking further advice after a combination of factors led to the prediction of ‘minor’ to ‘major’ significant adverse effects and new exceedances of the annual mean air quality objective (AQO) for NO2, within an existing Air Quality Management Area (AQMA), as a result of a proposed development.
Following the submission of the Air Quality Assessment, data from a new strategic transport model (STM), developed for the purposes of the Local Authority’s emerging Local Plan, became available. With the modelled pollutant concentrations in future scenarios being unusually high compared to the Council’s monitoring data within the assessment extents, it became evident that a review of the model inputs and parameters was required. This would ensure that the assumptions used had been reasonable and therefore, that the results of the assessment were valid and reflective of realistic conditions. It was observed that the STM traffic flows for the verification year were significantly higher than those used for the Air Quality Assessment’s 2019 model verification scenario. Furthermore, the increase in flows from the verification year to the development opening year used in the Air Quality Assessment were viewed as unrealistically large, with a 25% increase in baseline traffic flows observed between 2019 (verification year) and 2026 (development opening year), on some road links.
Contract Challenges
In creating a robust assessment that utilises assumptions as part of the methodology, there is the potential for the model to predict pollutant concentrations that indicate a ‘moderate’ or ‘substantial’ impact, which would have a corresponding ‘significant’ effect at representative sensitive receptor locations. This would be due to the use of excessively conservative assumptions, with regard to emission factors, background concentrations and traffic data, rather than the utilisation of the most realistic datasets for the development opening year.
The challenge with the assumptions and limitations in determining the model input parameters is to achieve the correct balance between presenting a set of robust, yet realistic, assessment scenarios that demonstrate both compliance and assertiveness of the development’s potential impacts and their effects on the receiving environment.
Our Solution
It was agreed that an updated model should be developed for all model scenarios using updated traffic data and more realistic model input parameters, compared to the excessively conservative ones used previous, these are detailed below:
- Following submission of the original assessment by our Air Quality team, amendments and improvements were made to the STM from which the traffic data for the Air Quality Assessment were derived. This led to a better correlation between the verification and future baseline year scenarios, which allowed the assessment to focus on the actual impact of the development rather than apparent effects associated with disparity between the baseline traffic scenarios. Accordingly, updated verification and development year baseline traffic flows were used in the updated assessment.
- The modelled scenarios utilised emission factors that reflected a more realistic assessment approach and as such, development year (2026) emission factors were used for the assumed opening year scenarios.
- To further aid in the representation of a more realistic modelling scenario, development year background pollutant concentrations for the assumed opening year were utilised in the updated assessment.
- Additional updates to the model input parameters included speed corrections following a review of major junctions and reconsideration of electric vehicle uptake within the future scenarios’ emissions profiles.
Following the implementation of the above changes to the road traffic emissions model, the pollutant concentrations at sensitive receptor locations no longer indicated any potential for significant effects or exceedances of the AQO as a result of the proposed development. This was in part due to the updated verification year traffic data leading to a reduction in the calculated adjustment factor, resulting in the model being more representative of local pollutant monitoring, and the more realistic approach of using the assumed opening year (2026) background concentrations and emissions factors for the development scenarios.
Conclusion
The case study above highlights the requirement to question whether it is beneficial for Air Quality Assessments to be more representative of real-world conditions but with reduced robustness, or to continue to follow the most conservative approach at all times, with the risk of over-predicting impacts on the air quality of the local area. A sustainable compromise may be to employ the use of professional judgement as well as to carry out the assessment jointly between the Project Team and key regulatory stakeholders, to make sure that an assessment remains realistic and continues to follow accepted EPUK/IAQM modelling guidance, where appropriate.
