Ozone at Mace Head, Ireland from 1987 to 2021: Declining baselines, phase-out of European regional pollution, COVID-19 impacts

Just under three hundred thousand hourly O3 observations from the Mace Head, Ireland atmospheric monitoring station have been assembled into a complete dataset covering the thirty five-year period from April 1, 1987 through to May 31, 2022. Of these, seventy thousand hourly observations were assigned to baseline air masses over the 422 months in the study. Annual mean baseline mixing ratios rose from 34 ppb in 1988 to a peak of 42 ppb in 1999 before declining to 39 ppb in 2021. Monthly mean baseline mixing ratios reached a peak of 53 ppb in April 1999. Baseline O3 mixing ratios exhibit a marked seasonal cycle, with maxima in April and minima in August. A detailed examination of the differences between the baseline and complete monthly mean O3 mixing ratios revealed a set of monthly differences with some striking long-term changes. Wintertime differences were dominated by a series of O3 depletion events in which, during the early years of the study, reduced ozone to near zero in long-range transport events. European NOx emission reductions have reduced significantly the number and severity of these wintertime O3 depletion events. Summertime differences were dominated by a series of regional photochemical episodes. In the early years of the record, the regional photochemical episodes were intense enough to raise the monthly means in the complete record above the baseline means. European volatile organic compound (VOC) and NOx precursor emission reductions have dramatically reduced the number and intensity of these events. Episodic peak O3 levels have also fallen steadily as a result of concerted, European action on regional VOC and NOx emissions. Subsets of the baseline and complete ozone datasets between 2017 and 2022 were scrutinised for possible COVID-19 impacts. Baseline ozone levels were lower than expected during 2020 and 2021 but the statistical significance of these impacts is difficult to judge, and reduced stratosphere-troposphere exchange rather than COVID-19 emission reductions may be the cause.