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Calibrating Defra’s Background NOx and NO2 Maps against 2016 Measurements


Air Quality Consultants has carried out an analysis of the validity of Defra’s national pollution maps of ambient background concentrations in 2016. 

These maps cover the whole country on a 1x1 km grid and are published for each year from 2013 until 2030. Mapped concentrations in 2013 were based on existing emissions from a range of different sources (including road traffic) calibrated against monitoring data for that year.  Emission projections are then used to predict concentrations in all subsequent years to 2030.  There is evidence that the current ‘official’ emission factors published by Defra (as used in the Emission Factor Tookit v7.0) over-predicted the rate at which road traffic emissions of nitrogen oxides will fall in the future.  This can affect the mapped concentrations in 2016, as they are derived from a 2013 base year.

Air Quality Consultants has compared Defra’s background mapped nitrogen oxides (NOx) and nitrogen dioxide (NO2) values for 2016 with national background measurements made at Automatic Urban and Rural Network (AURN) sites during 2016.  Based on the 56 sites with more than 75% data capture for 2016, the Defra maps have been found to under-predict the background NOx and NO2 concentrations by 20.8% and 10.2% on average, respectively (May 2017 analysis available here).

This analysis also has implications for the suggested approach to treating background concentrations of nitrogen dioxide (NO2) and nitrogen oxides (NOx) when using the Calculator Using Realistic Emissions for Diesels (CURED) V2A, which was published in September 2016 (note available here).  The latest analysis provides the necessary uplift factors to be applied to 2016 base year background concentrations before projecting forwards using the CURED approach.

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