Air Quality Consultants has updated a previous study that examined trends across the UK covering the period 2005 to 2016 (available here). The new report presents trends for 112 sites over the period 2005 to 2018 and for 183 sites over the more recent period 2010 to 2018. The findings were recently reported by Prof. Duncan Laxen to the Institute of Air Quality Management’s Routes to Clean Air Conference in London.
Overall downward trends have been identified across all sites, of -3.1% per year for NO2 and -3.0% per year for NOx, for the period 2010 to 2018. The downward trends are steeper at rural sites, at -3.4% per year for NO2 and -4.1% per year for NOx, than at road and urban sites, at -3.1 and -3.0% per year for NO2 and NOx respectively. These downward trends are greater than those previously identified for the period 2005 to 2016.
Air Quality Consultants previously found no significant trend over the period 2005 to 2016 for sites across central and northern England and north Wales, in contrast to the rest of the UK. This area of the UK has now caught up, with slightly steeper downward trends than the rest of the UK over the period 2010 to 2018.
The downward trends in NOx concentrations (2005 to 2018) are smaller than those for estimated UK NOx emissions (2005 to 2017), which suggests that emission reductions presented in the National Atmospheric Emission Inventory (NAEI) are overly optimistic. The report recommends that the validity of the NOx emissions estimates used in the NAEI, especially those for road vehicles, is reviewed in light of these observations.
Defra publishes projections of roadside and background concentrations. Recent projections of NO2 concentrations, from a base year of 2017, now match the overall trends in measured roadside and rural NO2 concentrations for the years 2017 and 2018.
The pattern of trends over time, which for NO2 is of little change between 2005 to 2010 and then a steeper downward trend from 2010 to 2018, is consistent with evidence of an increasing ratio of NO2:NOx in vehicle emissions in the period to 2010 and then a decline from 2010. There is now evidence of the NO2:NOx ratio increasing for Euro 6 light duty vehicles, which is likely to limit the current decline in NO2 concentrations to be less than that of NOx.
While the observed reductions in NOx and NO2 concentrations are encouraging, additional measures and actions will be required in order for the reducing trend to continue into the future. The report concludes that it is important to regularly monitor trends in ambient concentrations.
Overall Trend in Nitrogen Dioxide Concentrations Across All UK Sites, 2005-2018
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