National statistics

Nitrogen dioxide (NO2)

Updated 27 April 2023

National Statistics

Air quality statistics in the UK, 1987 to 2022 - Nitrogen dioxide (NO2)

Updated 27 April 2023

1. Why measure NO2?

Nitrogen dioxide (NO2) is a gas that is mainly produced during the combustion of fossil fuels, along with nitric oxide (NO). NO2 can also be formed when NO reacts with certain gases in the atmosphere. These reactions take place very quickly and are reversible, so the two gases are referred to together as nitrogen oxides (NOx).

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. NO2 can also 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.

In addition, NOx are 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 Air Quality Standards Regulations 2010 require that the annual mean concentration of NO2 must not exceed 40 µg/m3 and that there should be no more than 18 exceedances of the hourly mean limit value (concentrations above 200 µg/m3) in a single year.

2.1 Annual mean concentrations of NO2 in the UK, 1990 to 2022

The NO2 index shows the annual mean, averaged over all included sites that had annual data capture greater than or equal to 75%. The shaded areas represent the 95% confidence interval for the annual mean concentration for roadside sites, urban background sites and rural background sites. The intervals narrow over time because of an increase in the number of monitoring sites and a reduction in the variation between annual means at monitoring sites for NO2. Annual means for individual sites can be found in the NO2 statistical tables which accompany this release.

Figure 1: Annual mean concentrations of NO2 in the UK, 1990 to 2022

Figure 1: Annual mean concentrations of NO2 in the UK, 1990 to 2022

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Urban background NO2 pollution has reduced both in the long-term and in recent years

The annual mean concentration of NO2 at urban background sites has decreased over the time series to 15.6 µg/m3 in 2022. This is a 1% decrease since 2021.

Between 1992 and 2002 inclusive, the annual mean NO2 concentration at urban background sites reduced rapidly: by an average of 2.7 µg/m3 each year. This reduction was observed at most monitoring sites across the UK and could be a consequence of the large reduction in emissions of nitrogen oxides over this time period in the UK and in Europe.

Between 2002 and 2006 the annual mean concentration fluctuated with no clear trend, and this was observed at most monitoring sites across the UK. Emissions of nitrogen oxides in the UK and Europe were still decreasing over this period but these years also coincided with increased use of coal in power stations, and with the increasing popularity of new diesel cars (which produce greater nitrogen oxide emissions than petrol equivalents).

Between 2006 and 2019 inclusive, the annual mean NO2 concentration at urban background sites reduced by an average of 0.9 µg/m3 each year. Reductions in concentrations were observed at most monitoring sites across the UK. Emissions of nitrogen oxides in the UK and Europe have continued to decrease as newer road vehicles subject to stricter emission standards enter the fleet and power generation moves away from the use of coal, particularly in the UK.

In 2020, the annual mean NO2 concentration at urban background sites reduced further by 4.5 µg/m3 (23%). It is likely that a reduction in traffic as a result of COVID-19 restrictions was a contributing factor to this relatively large decrease.

In 2021, concentrations rose by 5% from the lowest point in the time series in 2020. It is likely that the lifting of some COVID-19 restrictions in 2021 was a contributing factor to this increase. Concentrations then decreased slightly (by 1%) from 2021 to 2022.

Roadside NO2 pollution has reduced in the long-term and in recent years, having been stable for most of the 2000s

The annual mean concentration of NO2 at the roadside has decreased over the time series to 23.6 µg/m3 in 2022. The annual mean NO2 concentration in 2022 is greater at roadside sites compared to urban background sites. This is most likely due to substantial emissions of nitrogen oxides from road transport sources, as the majority of concentrations at the roadside come from local transport sources.

For most of the 2000s, the annual mean NO2 concentration was stable, likely as a result of the increased ownership of diesel-fuelled vehicles which historically emitted far more nitrogen oxides compared to equivalent petrol-fuelled vehicles. This may have offset the impact of reduced emissions from other sources.

Between 2006 and 2019 inclusive, the annual mean NO2 concentration at roadside sites reduced by an average of 1.8 µg/m3 each year. This reduction was observed at most long-running monitoring sites across the UK and could be a consequence of the large reduction in road transport emissions of NO2 over the same period in the UK, as newer vehicles subject to stricter emissions standards enter the transport fleet.

In 2020, the annual mean NO2 concentration at the roadside reduced further by 8.21 µg/m3 (26%) to the lowest point in the time series. It is likely that a reduction in traffic as a result of COVID-19 restrictions was a contributing factor to this relatively large decrease.

In 2021, the annual mean NO2 concentration at the roadside increased by 1.8 µg/m3, rising by 8% from 2020. This is likely a result of increased road traffic following the removal of lockdown restrictions. In 2022, concentrations decreased slightly from 2021 and remain 24% lower than concentrations in 2019.

Rural background NO2 pollution has reduced at a gradual rate over time and is at low concentrations generally

The annual mean concentration of NO2 at rural background sites has decreased 66% over the time series to 6.0 µg/m3 in 2022. Since the start of the time series in 1997, the annual mean NO2 concentration at rural background sites has reduced by an average of 0.5 µg/m3 (or 3%) each year since 1997. This reduction was observed at most monitoring sites across the UK; which could be a consequence of the large reduction in emissions of nitrogen oxides over the same period in the UK and in Europe.

3. Average hours spent in ‘Moderate’ or higher NO2 pollution

This metric measures the annual trend in the number of hours on average that concentrations are recorded at levels that may have impacts on human health. For NO2, ‘Moderate’ air pollution (which requires action by citizens who are vulnerable to the health impacts of air pollution) is triggered when the latest hourly mean concentration is greater than 200 µg/m3. Urban background sites rarely record concentrations at these levels so we have not included a graph for this and NO2 pollution was low at all urban background sites throughout 2022. The coloured categories relate to the categories of the Daily Air Quality Index (see Table 20 in the statistical tables that accompany this release).

Figure 2: Mean hours when NO2 pollution was moderate or higher for roadside sites, 1997 to 2022

Figure 2: Mean hours when NO2 pollution was moderate or higher for roadside sites, 1997 to 2022

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Roadside monitoring sites have recorded a decreasing trend in hours of ‘Moderate’ air pollution due to NO2 since 2008. 2022 had the fewest hours of ‘Moderate’ air pollution per site due to NO2 since the beginning of the time series.

At roadside sites between 2021 and 2022 there was a decrease in the number of hours for which the mean NO2 concentration over the previous hour exceeded 200 µg/m3; from 0.1 hours to 0.0 hours per site.

For PM10 and PM2.5, ‘Moderate’ and higher air pollution has historically been experienced at many monitoring sites across the UK, but for NO2 ‘Moderate’ air pollution has been experienced by only a few particular sites. The overall trend follows the same trend as the London Marylebone Road site which recorded a peak of 853 hours in the ‘Moderate’ or higher categories in 2005 and has recorded 0 hours in this category in each year since 2019.

‘Moderate’ NO2 pollution at the roadside is usually the consequence of a build-up of emissions of nitrogen oxides from transport sources, and the presence of street canyons or wind conditions preventing dispersion of pollutants. For example, the London Marylebone Road site is on a six-lane congested road surrounded by high-rise buildings in central London.

4. Temporal variations in concentrations of NO2 in the UK, 2022

4.1 Weekday variations

The NO2 index shows the weekday mean, averaged over all included sites that had data capture greater than or equal to 75% for all instances of that weekday in a given year. The shaded areas represent the 95% confidence interval for the weekday mean concentration for roadside sites, urban background sites and rural background sites.

Figure 3: Weekday mean NO2 concentration at roadside, urban background and rural background sites, 2022

Figure 3: Weekday mean NO2 concentration at roadside, urban background and rural background sites, 2022

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NO2 concentrations tend to be lower at the weekend compared to during weekdays, particularly for roadside sites. This is likely to be primarily driven by less road traffic at the weekends. In 2022, the Monday-to-Friday mean concentration at roadside sites was 25.2 µg/m3, which was 28% greater than the mean concentration at the weekend of 19.7 µg/m3.

4.2 Hourly variations

The NO2 index shows the hourly mean, averaged over all included sites that had data capture greater than or equal to 75% for all instances of that hour in a given year. The shaded areas represent the 95% confidence interval for the hourly mean concentration for roadside sites, urban background sites and rural background sites.

Figure 4: Hourly mean NO2 concentration at roadside, urban background and rural background sites, 2022

Figure 4: Hourly mean NO2 concentration at roadside, urban background and rural background sites, 2022

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NO2 concentrations tend to be much greater during the morning and evening rush hour compared to other times of the day for roadside and urban background sites. This is most likely due to commuter traffic. This pattern of concentrations follows the distribution of road traffic by time of day [footnote 1].

5. Sections in this release

Summary

Background to concentrations of air pollutants

Concentrations of particulate matter (PM10 and PM2.5)

Concentrations of ozone

Days with ‘Moderate’ or higher air pollution (includes sulphur dioxide)

Compliance with the Code of Practice for Statistics and Defra group Statistics quality principles, and recent changes to the publication

Statistical tables (ENV02 – Air quality statistics)