AQA Sensitivity Rationale

AIR QUALITY DISPERSION MODELLING FOR PLANNING

Sensitivity Analysis Rationale Note.

A disparity between the road transport emission projections and measured annual mean concentrations of nitrogen oxides and nitrogen dioxide has been recognised since 2011.¹ Whilst the emission projections suggested that both annual mean nitrogen oxides and nitrogen dioxide concentrations should have fallen by around 15-25% over the past decade or so, at many monitoring sites levels have remained relatively stable, or have even shown a slight increase.

This disparity led to a detailed review of the emission factors and fleet mix for UK conditions, and from 2012 Defra has issued several revisions of the Emissions Factors Toolkit (EFT). The reason for the disparity is thought to relate to the on-road performance of modern diesel vehicles. New vehicles registered in the UK have to meet progressively tighter European type approval emissions categories, referred to as "Euro" standards. While the nitrogen oxides emissions from newer vehicles should be lower than those from equivalent older vehicles, the on-road performance of some modern diesel vehicles is often no better than that of earlier models.

The latest versions of the EFT go some way to addressing the disparity between air quality measurements and emissions, but do not fully address it, and it is recognised that the forecast reductions may still be optimistic in the near-term². Defra’s EFT uses emission factors from COPERT (v5.0). COPERT is updated every few years and version 5 incorporates different emissions functions from Euro 6 vehicles to broadly coincide with the phased introduction of (RDE) tests for vehicle type-approval.

The Institute of Air Quality Management (IAQM) note there is uncertainty in the impact on emissions of the introduction of the World-harmonised Light-duty Test Procedure (WLTP), which replaced the current driving cycle and the RDE test procedure in September 2017 for new model types, and which will become the standard test for all new cars from 2019. Many diesel vehicles satisfying the requirements of the early stages of the Euro-6 emissions standards (Euro-6a and 6b) are on the road and have been shown in emissions tests to have substantially lower NOx emissions than Euro-5 vehicles however, these vehicles were not required to meet the WLTP. Therefore, the new forecast reductions in nitrogen oxides
emissions may still be optimistic in the near-term.

To account for this uncertainty, where a planning application is in an area within or adjacent to an AQMA or areas where measured pollutant concentrations are within 10% of the Air Quality Objective, WRS require a sensitivity test should be conducted assuming that the future road traffic emissions per vehicle are unchanged from baseline year values. The
predictions within this sensitivity test are likely to be over-pessimistic, as new, lower-emission Euro VI and Euro 6 vehicles will be on the road and are expected to deliver real on-road reductions in nitrogen oxides emissions. However, it is considered this will provide a more appropriate conservative model of air quality impacts from developments that
the Local Authority can be confident.

¹ Carslaw, D.C., Beevers, S.D., Westmoreland, E., Williams, M.L., Tate, J.E., Murrells, T., Stedman, J., Li, Y., Grice, S., Kent, A. and I. Tsagatakis (Jul 2011). Trends in NOx and NO2 emissions and ambient measurements in the UK.
² IAQM (Jul 2018) ‘Dealing with Uncertainty in Vehicle NOx Emissions within Air Quality Assessments’ Version 1.1.

V4 – 29.01.2019