Ozone (O3)
Updated 27 April 2023
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National Statistics
Air quality statistics in the UK, 1987 to 2022 - Ozone (O3)
Updated 27 April 2023
1. Why measure O3?
Ozone (O3) 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. In addition, ozone can have adverse effects on the environment through oxidative damage to vegetation including crops. Ozone can also react with other chemicals in the air to form smog. Therefore, it is important to measure ozone concentrations to identify whether they pose risks to public health and the environment.
In contrast to many air pollutants measured in the UK, there are no major emission sources of ozone itself. The vast majority of ozone is instead formed in the air from reactions between other pollutants. For example, non-methane organic volatile compounds (NMVOCs) can photochemically react with other air pollutants outdoors in the presence of sunlight (via ultraviolet radiation) to produce ground-level ozone. Similar reactions can occur with nitrogen oxides (NOx).
The interdependence between concentrations of ozone and its precursor pollutants makes both assessing and tackling exposure to ozone and its precursor pollutants difficult. For example, modelling suggests that, as NOx concentrations decrease in urban areas, there may be an associated increase in ozone concentrations until there is a significant reduction of both ozone and its precursor pollutants like NOx. Another layer of complexity in assessing exposure to ozone is that once formed, ozone can travel long distances and can reach high concentrations far from the original sources of pollution. These characteristics mean that estimating emissions or measuring concentrations of ozone precursor pollutants alone is insufficient to assess potential exposure to ozone in the UK. Therefore, not only is measuring ozone concentrations one of the only methods of effectively assessing ozone pollution generally in the UK, but it is also important to help quantify any perverse outcomes of tackling exposure to other pollutants like NOx.
The Air Quality Standards Regulations 2010 set the target for ozone to a maximum daily eight hour mean concentrations of 120mg. This target value is not to be exceeded on more than 25 days per calendar year averaged over three years.
2. Trends in concentrations of O3 in the UK
2.1 Annual mean concentrations of O3 in the UK, 1987 to 2022
The O3 index shows the annual mean of the daily maximum 8-hour mean, averaged over all 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 urban background sites and rural background sites. Annual means for individual sites can be found in the O3 statistical tables which accompany this report.
Figure 13: Annual mean concentrations of O3 in the UK, 1987 to 2022
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Urban background O3 pollution has shown a long-term increase.
The annual average daily maximum eight hour mean concentration of ozone at urban background sites has shown an increasing trend since 1992. In 2022, the annual average daily maximum eight hour mean concentration of ozone reached 65.1 µg/m3 which is the highest value in the time series. This was an increase of 4% compared to 2021.
Ozone concentrations can vary substantially between years because of inter-annual variability in meteorology. Some variation from year-to-year is expected due to fluctuations in the occurrence of hot summer weather conditions which are associated with high ozone concentrations.
From the start of the time series in 1992 to the mid-2000s, urban background ozone concentrations were increasing. This may have been due to the reduction in emissions of nitrogen oxides in the UK and Europe, which can inhibit the formation of ozone in urban areas.
Concentrations fluctuated from the mid-2000s to 2016 with no obvious trend.
Since 2017, urban background ozone concentrations have followed an upward trend, with 2022 being highest in the time series. Particularly hot weather summer occurred in 2022, which possibly led to more ozone formation than usual. The highest ozone concentrations at urban background sites have all been observed in the last 5 years (from 2018 to 2022).
Rural background O3 pollution has shown no clear long-term trend.
The average daily maximum eight hour mean concentration has fluctuated since the start of the time series in 1987 and was 73.8 µg/m3 in 2022. Since 2017, rural background concentrations of ozone have followed an upward trend, with 2022 also being highest in the time series. There was a 5% increase in concentrations from 2021 to 2022. Some variation from year-to-year is expected due to fluctuations in the occurrence of hot summer weather conditions which are associated with high ozone concentrations. The future trend in concentrations may be dependent on global emissions of ozone precursor substances.
Ozone is formed through a complex chemical reaction involving nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. Ozone concentrations are typically higher in rural areas partly due to there being more pollutants in the air in urban areas, due to traffic, industrial activities and other sources, which can react with each other and can degrade ozone or inhibit its formation (for example nitric oxide (NO)). Overall, air pollutant concentrations in urban areas are on a long-term declining trend, leading to less degradation and increased formation of ozone, which contributes towards ozone concentrations increasing in these areas.
3. Average hours spent in ‘Moderate’ or higher O3 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 O3, ‘Moderate’ air pollution (which requires action by citizens who are vulnerable to the health impacts of air pollution) is triggered when the latest 8-hour running mean concentration is greater than 100 µg/m3. 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 14: Mean hours when O3 pollution was ‘Moderate’ or higher for rural background sites, 1987 to 2022
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Figure 15: Mean hours when O3 pollution was ‘Moderate’ or higher for urban background sites, 1992 to 2022
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Between 2017 and 2022, urban and rural background monitoring sites recorded an increased number of hours of ‘Moderate’ or higher O3 air pollution, after a decade of a relatively low number of hours.
At urban background sites between 2017 and 2020 there was a large increase in the number of hours for which the mean O3 concentration over the previous 8 hours exceeded 100 µg/m3 to the highest value in over a decade (130 hours in 2020). Overall, the time series remains volatile, with a considerably lower mean number of hours in 2021 (38 hours) followed by an increase to 111 hours in 2022.
There was a similar rise in rural background concentrations between 2017 and 2019. This was followed by a modest decrease in the number of hours of ‘Moderate’ or higher O3 pollution between 2019 and 2020 at rural background sites, though values remaining relatively high (161 hours). In 2021 this fell to 70 hours per site and in 2022 increased to 176 hours per site.
The overall trend in the rural indicator is a long-term decrease interrupted by several years where moderate pollution was more common. The long-term decrease is likely driven by reductions in global emissions of substances that lead to the formation of ozone such as nitrogen oxides and volatile organic compounds.
The overall trend in the urban indicator is less clear. Ozone concentrations are strongly influenced by weather, which likely contributes to the high variability over time and peaks such as in the hot, sunny summers of 2003, 2006, 2018, 2019, 2020 and 2022. This means that long time series are required to distinguish between weather effects and the effect of changes in pollutant emissions. In 2020, reductions in NOx concentrations in urban environments as a result of COVID-19 restrictions were probably also a large contributing factor to increased ozone pollution, since less NO will have been available to react with ozone and inhibit its formation.
4. Temporal variations in concentrations of O3 in the UK, 2022
4.1 Monthly variations
The O3 index shows the monthly mean, averaged over all included sites that had monthly data capture greater than or equal to 75% in a given year. The shaded areas represent the 95% confidence interval for the monthly mean concentration for and urban background sites and rural background sites.
Figure 16: Monthly mean daily maximum 8-hour mean concentrations of O3 in the UK, 2022
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For O3 the greatest average concentrations tend to occur during the spring and summer months. Ozone is a secondary pollutant, meaning it is created in the atmosphere through a cycle of reactions of its precursors nitrogen oxides and volatile organic compounds (VOC), sunlight is also a key part of the ozone production cycle. In the summer stagnant air leads to the build-up of these precursor compounds, while more sunlight and higher temperatures increase the rates of reactions that generate ozone.
In 2022, the month that had the greatest ozone concentrations was April for both rural and urban monitoring sites (with monthly mean concentrations of 86.7 µg/m3 and 81.0 µg/m3 respectively). It was notably dry in April 2022 with above average temperatures - Met Office. This sunny spell would have increased the rate of chemical reactions between ozone precursor pollutants that would have built up in the atmosphere.
Ozone exhibits strong seasonal and diurnal cycles. Monthly average ozone concentrations in the UK are usually at their maximum in April and May, in contrast to most of continental Europe where monthly ozone has a maximum in June and July. The amplitude of the annual cycle, that is the maximum and minimum extent, is around 25-30 µg/m3, being slightly greater at urban sites, which likely reflects increased rates of chemical removal in winter from increased NO emissions in winter. There is some evidence that the annual amplitude in ozone has been decreasing slightly - see AQEG report Ozone in the UK.
5. Sections in this release
Background to concentrations of air pollutants
Concentrations of nitrogen dioxide
Concentrations of particulate matter (PM10 and PM2.5)
Days with ‘Moderate’ or higher air pollution (includes sulphur dioxide)