Basildon Council (BC) and Essex County Council (ECC) received a Legal Directive to resolve perceived exceedances of the EU Limit Value, following the publishing of the UK plan for tackling roadside nitrogen dioxide concentrations by Department for Environment Food and Rural Affairs (Defra) in 2017.
Introduction
Following the publishing of the UK plan for tackling roadside nitrogen dioxide concentrations by Department for Environment Food and Rural Affairs (Defra) in 2017, Basildon Council (BC) and Essex County Council (ECC) received a Legal Directive to resolve perceived exceedances of the EU Limit Value (40 μg/m3) adjacent to the A127 east of the Fortune of War junction, as indicated by Defra’s Pollution Climate Mapping (PCM) model.
A monitoring survey was implemented in February 2018 using diffusion tubes at over 80 locations, with the aim of verifying the PCM model results and identifying the extent of exceedances if they existed, and also to identify other exceeding locations with relevant exposure. Monitoring indicated exceedances of the Limit Value at multiple locations along the A127 within Basildon, as well as other locations like East Mayne and Upper Mayne.
As a result a number of mitigation measures were developed and implemented to achieve compliance in the shortest possible time. The works undertaken have been split into three stages, which are discussed further below:
- A127 Speed Management Scheme (2017 - 2020)
- East Mayne Walkway Removal (2020 - 2023)
- Monitoring and Evaluation (2023 - present).
Method
A127 Speed Management Scheme
Traffic and air quality modelling was undertaken to establish when ‘natural compliance’ may be achieved (i.e. when all concentrations across Basildon would reduce to below 40 μg/m3 in the absence of any other actions). The results indicated that intervention would be required to bring forward compliance, so a 50 mph speed limit on the A127 was tested in the models. This indicated that the lower, more efficient speed would reduce NOx emissions, and therefore NO2 concentrations, thereby bringing the expected compliance year forward. This measure was implemented in 2020 and resulted in an approximately 13 to 15 per centreduction in NOx emissions, although total NO2 concentrations remained above the Limit Value.
East Mayne Walkway Removal
Whilst the A127 speed management scheme had some success on the A127, it had minimal impact on monitored NO2 concentrations on East Mayne, where modelling indicated particularly high NO2 concentrations adjacent to the pedestrian walkway on the central reservation. Whilst this would not be considered a ‘sensitive receptor’ for Local Air Quality Management (LAQM) purposes, the Limit Value applies where there is evidence of exposure on sections of walkway representative of 100 meter, of which East Mayne is an example. Further traffic and air quality modelling was undertaken to assess the impact of different measures on future NO2 concentrations and the compliance year. The measure which achieved concentrations below the Limit Value in the quickest possible time was the removal of the walkway, and the realignment of the signalised crossings. This was implemented in early 2022, with compliance achieved at this location upon removal of the central reservation walkway, owing to the removal of the receptor.
Monitoring and evaluation
Following the successful implementation of the two measures, the project entered the monitoring and evaluation stage, which involved continued monitoring at key and supporting locations, and approximation of when compliance would potentially be achieved across Basildon. The diffusion tube monitoring survey was expanded in 2019 to over 170 sites, which covered ‘gaps’ in the survey identified by the modelling exercises undertaken. Since then the survey has been reduced year on year, with 69 locations currently being monitored. For two years (2022 and 2023) air quality sensors (AQS1 units) were installed on East Mayne alongside VivaCity traffic sensors, which allowed detail analysis as to the causes of elevated NO2 concentrations at key diffusion tube monitoring locations.
Lessons learned
Joint Air Quality Unit (JAQU) recently released a document to guide local authorities still in the Plan (i.e. still monitoring concentrations above the Limit Value) through the ‘Exit Process’. This involves additional analysis to understand when compliance is likely to be achieved, if additional measures are required to bring it forward, and an accelerated process to assess and implement these measures if required.
The outcomes of this work and previous work are being used to develop bespoke mitigation measures at key locations with persistent exceedances of the Limit Value, which will be assessed and potentially implemented, if deemed likely to bring compliance forward.