Defra recently published predicted background NOx, NO2, PM10 and PM2.5 concentrations which it verified against measurements made in 2021. AQC (part of Logika Group) has now compared Defra’s predictions against measurements made up to the end of 2024.
For NOx and NO2, we found a clear delineation between the performance of Defra’s predictions inside London and across the rest of the UK. We also found appreciable spatial variability in different parts of London. On average, Defra has over-predicted NOx and NO2 concentrations in central and inner London, and under-predicted outside London. Despite Defra’s modelling nominally including features such as the Ultra Low Emissions Zone (ULEZ), the overestimation in London has grown more pronounced over time, and spread outward coinciding with the expansion of the ULEZ. Not allowing for this bias could cause modelling studies to under-predict roadside concentrations and so we have provided correction factors which others can use in their models.
For PM10 and PM2.5, the greatest geographical pattern is a systematic underprediction at coastal sites, particularly those on the south coast of England. This suggests that Defra’s predictions may underestimate the local effects of sea salt. Despite this, it is not appropriate to apply any correction to the PM10 or PM2.5 mapped concentrations based on our national-scale analysis.
The report is available to download here.
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