Pollutant Information: Nitrogen Oxides 

About Nitrogen Oxides

Category: Air pollutants

Studies have shown that oxides of nitrogen (NOx) can cause lung irritation as well as lowering people's resistance to pneumonia and bronchitis and other respiratory infections. In the presence of sunlight, NOx can react to produce a photochemical smog. If hydrocarbons are also present, ozone can be produced, which has a similar health effect to NOx. Although higher concentrations of NOx are found in city areas, resulting ozone concentrations tend to be higher in rural areas, where crop yields can be reduced as a result.

Almost all NOx is emitted during the combustion of fuels: other sources contributed just under 5% of UK emissions in 2021. Emissions occur from all fuels, although the emission rate does vary from fuel to fuel and from sector to sector. Transport sources, and the energy and manufacturing industries are the most important sources. Contributions to UK emissions of NOx in 2021 were 27% for road transport, 20% for other forms of transport (including off-road vehicles and mobile machinery), 10% from power stations and other energy producers, and 13% from other industrial sites. Most of the road transport emissions in 2021 were from diesel vehicles. The estimation of the emissions from combustion sources is complex since the nitrogen can originate from either the fuel (in the case of solid fuels) or atmospheric nitrogen. The emission is dependent on the conditions of combustion, in particular temperature and excess air ratio, which can vary considerably. Thus, combustion conditions, load and even state of maintenance are important.

Since 1990, overall NOx emissions have decreased by 76%. This decrease occurs because of reductions in emissions from all of the broad sectors mentioned above. Emissions from road transport have fallen by 84% due to the introduction of catalytic converters and stricter regulations (i.e. Euro Standards). Emissions from the other transport sources have fallen 57% due to reductions in emissions from agricultural and other off-road vehicles, and reductions in fuel oil burnt by shipping. Lastly, emissions from energy and manufacturing industries have fallen by 84% and 56% respectively, due mainly to reduced use of coal in power stations, better control of emissions from those power stations, reduced use of fuel oil as an industrial fuel, and reduced emissions from cement kilns. The UK is currently on track to meet the Gothenburg Protocol target in 2030, but emissions will need to be reduced by a further 32% from 2021 to meet this target.

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Time series graph

Notable events

Start year End year Sector Information Impact
1970 1991 Road Transport Increase in road traffic without any abatement technology fitted. Increase in emissions
1992 2018 Heavy duty vehicles Stricter Euro I - VI emission regulations come in on trucks and buses offsets increasing vehicle km.  Impact takes time to have an impact as only new vehicles need to meet standards. Decrease in emissions
1970 1999 Public Electricity and Heat Production Increased electricity generated in nuclear plant. Decrease in emissions
1988 2011 Public Electricity and Heat Production The electricity generators programme of progressively fitting low NOx burners to their 500 MWe (megawatt electric) or larger coal fired units to comply with LCPD restrictions. Decrease in emissions
2007 2011 Public Electricity and Heat Production Programme of fitting over-fire-air burners to comply with LCPD restrictions. Decrease in emissions
1990 1999 Passenger Cars Introduction of petrol cars with early 3-way catalysts. Decrease in emissions
1990 2018 Public Electricity and Heat Production Deregulation in the use of gas for electricity production.  Increased use of Combined Cycle Gas Turbine (CCGT) stations and other gas fired plant rather than coal for electricity generation. Decrease in emissions
1993 1993 Agriculture Field burning of agricultural waste stopped after 1993. Decrease in emissions
1990 2010 Passenger Cars Switching from petrol to diesel cars Increase in emissions
1970 1985 Iron and Steel Decline in the energy intensive iron and steel industry and other heavy industries. Decrease in emissions
2020 2020 Road transport, aviation and railways Because of measures in place during the COVID-19 pandemic, NOx emissions from road transport, aviation and railways in 2020 were 25% and 60% and 31.4% lower than they had been in 2019 respectively. Decrease in emissions