Analysis and interpretation of 25 years of ozone observations at the Mace Head Atmospheric Research Station on the Atlantic Ocean coast of Ireland from 1987 to 2012

Observations of surface ozone have been made at the Mace Head Atmospheric Research Station on the Atlantic Ocean coastline of Ireland from April 1987 through to December 2012. Using meteorological analyses and a sophisticated Lagrangian dispersion model, the hourly observations have been sorted into baseline northern hemisphere mid-latitude, European regionally-polluted and southerly sub-tropical maritime air masses. Baseline monthly average levels showed a pronounced seasonal cycle with spring maxima and summer minima. Baseline levels have shown an average annual increase of +0.25 +/- 0.09 ppb year(-1) which has been stronger in the winter and spring and weaker in the summer months. The rate of this annual increase has slowed over the last decade to the extent that annual levels have been relatively constant through the 2000s. Annual mean O-3 levels in European air masses have shown much reduced upwards trends compared with baseline air masses. European levels show a seasonal cycle with a spring maxima and a winter minima. Policy actions to reduce European regional-scale NOx emissions have led to an increase in wintertime O-3 levels and a decrease in summertime peak levels. Levels in southerly sub-tropical maritime air masses have shown different seasonal cycles, different seasonal trends and systematically lower levels compared with baseline air masses. The baseline observations have been compared against a number of policy-relevant O-3 metrics and these have demonstrated the potential importance of the growing northern hemisphere O-3 baseline levels over the 25 year study period. (C) 2013 Elsevier Ltd. All rights reserved.