Emissions of air pollutants in the UK – Nitrogen oxides (NOx)

Question 1

1. What are nitrogen oxides and why are their emissions estimated?

Accepted Answer

Nitrogen oxides (NOx) refers to nitric oxide (NO) and nitrogen dioxide (NO2), both of which are mainly formed during the combustion of fossil fuels. The dominant portion of these gases is nitric oxide (NO). 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 2022), the UK was non-compliant with the annual mean concentration limit value for NO2 (as set in the Air Quality Standards Regulations (2010)) at a number of roadside locations in urban areas. It is estimated that, on average in 2022, 65 per cent of the NOx concentrations at the roadside originate as NOx emissions from road transport. Therefore, it is important we estimate these emissions to understand how they are changing and how we can reduce them to help reduce harmful roadside concentrations of NO2.

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 have environmental impacts. 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.

NOx can react with other air pollutants (e.g. NMVOCs) to form ground-level ozone. Ozone is a gas which is damaging to human health and can trigger inflammation of the respiratory tract, eyes, nose and throat, as well as asthma attacks. Ozone can also have adverse effects on the environment through oxidative damage to vegetation including crops.

NOx can be emitted both by natural processes, such as wildfires, and as a result of human activities, such as the combustion of fuels. The National Atmospheric Emissions Inventory, and the statistical tables published as part of this release, mostly covers NOx emissions from human activities, but there are a few exceptions included as memo items, such as forest fires. The information presented in this document only covers NOx emissions from human activities within the UK.

The Convention on Long Range Transboundary Air Pollution (CLRTAP)’s amended Gothenburg Protocol and National Emission Ceilings Regulations (2018) (NECR) requires the UK to reduce emissions of NOx by 55 per cent compared to emissions in 2005 by 2020 and in each subsequent year, up to and including 2029. The NECR also requires the UK to reduce emissions of NOx by 73 per cent compared to emissions in 2005 by 2030.

Question 2

2. Trends in total annual emissions of nitrogen oxides in the UK, 1970 to 2022

Accepted Answer

Figure 6: Annual emissions of nitrogen oxides in the UK: 1970 – 2022

Notes:

‘ERC’ refers to the 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’.
‘CLRTAP’ refers to the 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’.

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Emissions of NOx have decreased by 78 per cent since 1970, to 643 thousand tonnes in 2022. This trend was driven by a decline in coal use in power stations and by the modernisation of the road transport fleet. Emissions of NOx decreased by 4 per cent between 2021 and 2022. This is similar to the change since 1990 as total emissions have decreased by an average of 4 per cent per year between 1990 and 2022.

The latest data shows that the UK did meet the 55 per cent emission reduction commitment for NOx in 2022. Excluding agricultural sources (which are not required for the NECR assessment of reductions, see section 4 of the Background), there were 619 thousand tonnes of emissions of NOx in 2022, having fallen by 1076 thousand tonnes since 2005 (i.e. emissions decreased by 63 per cent).

Question 3

3. Major emission sources for nitrogen oxides in the UK

Accepted Answer

Figure 7: UK annual emissions of nitrogen oxides by 2022 major emission source: 1990, 2005, 2021 and 2022

Note:

‘Industrial combustion’ refers to emissions from the burning of fuels on manufacturing and construction sites to generate energy for industrial use, such as to drive mobile machinery or to create heat or electricity for industrial processes. This does not include emissions from combustion in the agricultural, forestry and fishing sectors, from the public sector, from the commercial sector, or emissions from energy industries.
‘Energy’ refers to fuel combustion in energy industries. This includes combustion in public electricity and heat production, petroleum refining and the production of other fuels.

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Road transport contributed 30 per cent of emissions of NOx in 2022. Increases in road traffic accounted for the steep climb in emissions of NOx between 1984 and 1989. The introduction of catalytic converters and stricter emissions regulations resulted in a downward trend in emissions after 1990. This is due to the replacement of older vehicles in the vehicle fleet with newer vehicles that meet stricter emissions standards, and more recently, the uptake of electric vehicles. Annual emissions from road transport decreased by 68 per cent between 2005 and 2022.

Emissions of NOx from non-road transport (aviation, rail, and shipping) decreased by 47 per cent between 2005 and 2022 and contributed 15 per cent of total NOx emissions in 2022. There was a very slight increase in emissions (4 per cent increase) between 2021 and 2022, partly due to a gradual increase in aviation activity levels following the COVID-19 pandemic. Emissions from all modes of non-road transport remain lower than before 2020.

Emissions from power stations (part of the energy sector) 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. As a result, annual emissions of NOx from fuel combustion in energy industries have decreased by 75 per cent between 2005 and 2022, while emissions from industrial combustion decreased by 54 per cent over the same period, largely due to the closure or conversion to biomass fuel of coal-fired power stations and plants.

Levels and trends in emissions from specific sources are available for the period 1990 to 2022 through the statistical tables that accompany this release.