1. Why are emissions of nitrogen oxides estimated?
Nitrogen oxides (NOx) are a group of gases that are mainly formed during the combustion of fossil fuels. The dominant portion of these gases is nitric oxide (NO). However, NO can react with other gases in the atmosphere to form nitrogen dioxide (NO2) which is harmful to health. These reactions take place very quickly and are reversible, so the two gases are referred to together as NOx.
In the most recent annual air quality assessment (for 2021), the UK was non-compliant with the limit value placed on the annual mean NO2 concentration at a number of roadside locations in urban areas. It has been estimated that on average 70 per cent of the NOx concentrations at the roadside originate as NOx emissions from road transport.
Short-term exposure to concentrations of NO2 can cause inflammation of the airways and increase susceptibility to respiratory infections and to allergens. NO2 can exacerbate the symptoms of those already suffering from lung or heart conditions. In addition, NOx can cause changes to the environment. Deposition of Nitrogen to the environment both directly as a gas (dry deposition) and in precipitation (wet deposition) can change soil chemistry and affect biodiversity in sensitive habitats.
Nitrogen oxides are also precursors for the formation of ozone. Ozone is a gas which is also damaging to human health and can trigger inflammation of the respiratory tract, eyes, nose and throat as well as asthma attacks. Moreover, ozone can have adverse effects on the environment through oxidative damage to vegetation including crops.
The revised Convention on Long Range Transboundary Air Pollution (CLRTAP) and National Emission Ceilings Regulations (NECR) requires the UK to reduce emissions of nitrogen oxides by 55 per cent compared to emissions in 2005 by 2020 and in each subsequent year, up to and including 2029 (and by 73 per cent compared to emissions in 2005 by 2030).
2. Trends in total annual emissions of nitrogen oxides in the UK, 1970 to 2021
Figure 6: Annual emissions of nitrogen oxides in the UK: 1970 – 2021
‘Domestic ERC’ refers to our emission reduction commitment applicable between 2020 and 2029, as set out in the National Emission Ceilings Regulations 2018. This applies to the series ‘Nitrogen oxides minus emissions from agriculture’.
‘International ERC’ refers to our emission reduction commitment applicable from 2020 onwards, as set out in the Convention on Long Range Transboundary Air Pollution. This applies to the series ‘Nitrogen oxides’.
Emissions of nitrogen oxides have fallen by 77 per cent since 1970, to 677 thousand tonnes in 2021. This trend was driven by a decline in coal use in power stations and modernisation of the road transport fleet. There was an increase of less than 1 per cent between 2020 and 2021. Total emissions have not changed by much since 2020, in part attributed to the Covid-19 restrictions which continued to reduce traffic on many roads in early 2021. This is in contrast to the long-term trend, since UK total emissions have fallen by an average of 4 per cent per year between 1990 and 2021.
The latest data shows that the UK achieved the emission reduction commitments for nitrogen oxides in 2021. Excluding agricultural sources, there were 649.6 thousand tonnes of emissions of nitrogen oxides in 2021, having fallen by 1060 thousand tonnes since 2005 (a 62 per cent reduction).
3. Major emission sources for nitrogen oxides in the UK
Figure 7: UK annual emissions of nitrogen oxides by 2021 major emission source: 1990, 2005, 2020 and 2021
- Industrial combustion refers to emissions from industrial burning of fuels, either to generate energy, or to drive mobile machinery. This does not include emissions from combustion in the agricultural, forestry and fishing sectors, or emissions from institutional/commercial combustion.
Increases in road traffic accounted for the steep climb in emissions of nitrogen oxides (NOx) between 1984 and 1989. The introduction of catalytic converters and stricter emissions regulations resulted in a downward trend in NOx emissions after 1990.
Road transport accounted for 27 per cent of emissions of nitrogen oxides in the UK in 2021, and other forms of transport (aviation, rail, and shipping) accounted for 14 per cent. There is a downward trend in emissions from road transport due to the replacement of older vehicles in the vehicle fleet with newer vehicles that meet stricter emissions standards. Annual emissions from road transport have fallen by 69 per cent between 2005 and 2021, and other forms of transport have reduced annual emissions by 45 per cent over the same period.
Emissions from power stations and industrial combustion plants have reduced substantially, reflecting a long-term trend away from the use of coal and oil in favour of natural gas and renewable energy sources. Annual nitrogen oxide emissions from energy industries have reduced by 74 per cent between 2005 and 2021, largely due to the closure or conversion to biomass fuel of coal-fired power stations.
Levels and trends in emissions from specific sources are available for the period 1990 to 2021 through the statistical tables that accompany this release.