In January 2020, Air Quality Consultants Ltd reported significant reductions in ambient nitrogen oxides (NOx) concentrations at roadside sites across the UK over the period 2013 to 2019. Although in detail, reductions prior to 2016 were limited, while significant reductions were evident from 2016.
Air Quality Consultants Ltd has now evaluated how Defra’s Emission Factor Toolkit (EFT) matches the measurements over the same period, 2013 to 2019; the new report is available here. The study has separated out the road component of the roadside concentrations by subtracting the local background, to show the trends in the road component of NOx. This has allowed a direct comparison with the trends in the road traffic component of NOx derived from the EFT.
The new analysis has found that over the 2013-2019 period as a whole, the scale of reduction in vehicle emissions of NOx predicted by the EFT is very similar to that measured at a national level. In more detail, it has been shown that the EFT has under-predicted measured reductions in London and slightly over-predicted them elsewhere during the period 2013-2016, but that since 2016 the EFT has tended to under-predict the reduction in Road-NOx in all geographical areas, especially in London.
The report concludes that the EFT is now unlikely to over-state the rate at which NOx emissions decline into the future at an ‘average’ site in the UK. Indeed, the balance of evidence suggests that, on average, NOx concentrations are likely to decline more quickly in the future than predicted by the EFT.
Prior to this new analysis, the evidence from around 2000 to 2015 had pointed to the EFT exaggerating reductions in NOx emissions, because expected reductions in emissions from diesel vehicles were not being seen in practice. To allow for this, Air Quality Consultants Ltd developed the CURED model to provide a more pessimistic projection of future concentrations, which has been widely adopted for the assessment of future year concentrations.
The analysis of recent NOx measurements now provides evidence that vehicle controls are working and as a result the EFT is now reflecting the rate of observed reductions. On this basis, there seems to be little value in continuing to use, or updating, the CURED model. In other words, the EFT may now be relied upon to predict the ‘most likely’ future emissions reductions, and it may even under-predict the rate of reduction. The only proviso is that the modelling must be verified against measurements made in the year 2016 or later.
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