Analysis and assessment of the observed long-term changes over three decades in ground-level ozone across north-west Europe from 1989-2018

Annual maximum 8-hourly (AM8) mean ozone mixing ratios have been studied at 56 long-running European rural ozone monitoring stations over the 30-year period from 1989 to 2018. AM8s at almost all stations and in almost all years have exceeded the World Health Organization (WHO) air quality guideline set at 50 ppb for the protection of human health. The long-term changes in AM8s have been quantified using a transported baseline (TB) contribution and a regional photochemical enhancement (RPE). The transported baseline contribution was defined using the observations for baseline air masses arriving at Mace Head, Ireland having traversed the North Atlantic Ocean. Regional photochemical enhancements were found by subtracting the transported baseline contributions from the AM8 ozone mixing ratios at each of the EMEP stations. Regional photochemical enhancements have declined strongly at all 56 long-running rural monitoring stations due to actions taken to control ground-level ozone formation through the reduction of European regional ozone precursor emissions. The relative magnitudes of the two contributions, explain why the WHO air quality guideline for ozone is widely exceeded. Progress towards meeting the WHO air quality guideline has been slow because the transported baseline contributions alone have approached or exceeded the WHO guideline throughout much of the 1989-2018 period. This exceedance arises from intercontinental transport of photochemically-produced ozone from North America, Asia and ultimately from Europe itself, by transport around the globe, imposed upon the stratospheric contribution. Without policy actions to further reduce European regional ozone precursor emissions and without new policy actions to reduce the transported baseline contributions, exceedances of the WHO air quality guideline will continue across Europe into the foreseeable future. At present, there is no leading international agency addressing the policy problems associated with hemisphere-wide ozone transport and coordinating policy actions in Europe, North America and Asia.

Automated summary

The study analyzed the annual maximum 8-hourly mean ozone mixing ratios at 56 European rural ozone monitoring stations from 1989 to 2018. It found that almost all stations and years exceeded the World Health Organization's air quality guideline for human health protection. The long-term changes in ozone mixing ratios were measured using a transported baseline contribution and a regional photochemical enhancement. The study revealed that regional photochemical enhancements have significantly decreased at all rural monitoring stations due to actions taken to control ozone precursor emissions. However, the transported baseline contributions, mainly from intercontinental transport of ozone, have remained high and explain why the WHO guideline is frequently exceeded.