This indicator tracks changes in rural background concentration of ozone (O3). Chemical reactions in the air involving nitrogen oxides (NOx) and volatile organic compounds (VOCs) produce the toxic gas O3 which can harm health, damage wild plants, crops, forests, and some materials, and is a greenhouse gas contributing to global warming.
This indicator is an assessment of clean air (reporting the condition of the atmosphere as an asset). Exposure to O3 can also be considered as a pressure on human health and thriving plants and wildlife. The indicator is determined by calculating the annual average of the maximum daily 8-hour mean concentrations of O3 measured at all rural measurement sites on Defra’s Automatic Urban and Rural Network (AURN).
Readiness and links to data
Data on the measured concentrations of O3 from the ARUN are published annually as National Statistics; Air Quality Statistics.
Figure A4: Rural background concentrations of ozone (O3) in England, 1987 to 2021
Table A4: Rural background concentrations of ozone (O3) in England, 1987 to 2021
|Year||95% confidence interval lower bound||95% confidence interval upper bound||Annual average maximum daily 8-hour mean o3 concentration (µg/m3)|
Trend description for A4
The average daily maximum 8 hour mean concentration of O3 has fluctuated considerably since the time series began, but overall it has increased by 25%, from 56.2 μg per m3 in 1987 to 70.3 μg per m3 in 2021. Some variance from year to year is expected due to fluctuations in the occurrence of hot summer weather conditions which are associated with high O3 concentrations.
Assessment of change
Rural background concentrations of O3 showed little or no change over the short and medium-term assessment periods. However, there has been an increase (deterioration) in rural background concentrations of O3 over the long term.
Change since 2018 has also been assessed. Little or no change was observed for rural background concentrations of O3 since 2018. However, this is based on only 4 data points so should be considered as indicative and not evidence of a clear trend.
Further information on this assessment, along with details on the methodology, is provided in the Assessment background page. Summaries by 25 Year Environment Plan goal and information on indicator links are presented in the Assessment results pages.
Table A4: Assessment of change
|Component||Subcomponent||Period||Date range||Percentage change||Smoothing function||Assessment of change|
|A4||Annual average maximum daily 8-hour mean O3 concentration (µg/m3)||Short term||2015 to 2020||4.47||Loess||Deterioration|
|A4||Annual average maximum daily 8-hour mean O3 concentration (µg/m3)||Medium term||2010 to 2020||5.31||Loess||Deterioration|
|A4||Annual average maximum daily 8-hour mean O3 concentration (µg/m3)||Long term||1987 to 2020||14.98||Loess||Deterioration|
Note that assessment categories for short, medium and long term were assigned based on smoothed data, so percentage change figures in Table A4 may differ from unsmoothed values quoted elsewhere. Percentage change refers to the difference seen from the first to last year in the specified date range.