Atmos. Chem. Phys., 5, 3441-3450, 2005
www.atmos-chem-phys.net/5/3441/2005/
doi:10.5194/acp-5-3441-2005
© Author(s) 2005. This work is licensed under the
Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.
On the changing seasonal cycles and trends of ozone at Mace Head, Ireland
D. C. Carslaw
Institute for Transport Studies, University of Leeds, Leeds LS2 9JT, UK

Abstract. A seasonal-trend decomposition technique based on a locally-weighted regression smoothing (Loess) approach has been used to decompose monthly ozone concentrations at Mace Head (Ireland) into trend, seasonal and irregular components. The trend component shows a steady increase from 1990–2004, which is confirmed by statistical testing which shows that ozone concentrations at Mace Head have increased at the p=0.06 level by 0.18±0.04 ppb yr−1. By considering different air mass origins using a trajectory analysis, it has been possible to separate air masses into "polluted" and "unpolluted" origins. The seasonal-trend decomposition technique confirms the different seasonal cycles of these air mass origins with unpolluted air mass maxima in April and polluted air mass maxima in July/August. A detailed consideration of the seasonal component reveals different behaviour depending on the air mass origin. For baseline unpolluted air arriving at Mace Head there has been a gradual increase in the seasonal amplitude, driven by a declining summertime component. The amplitude of the seasonal component of baseline air is controlled by a maximum in April and a minimum in July. For polluted air mass trajectories, there was a substantial reduction in the amplitude of the seasonal component from 1990–1997. However, post-1997 results indicate that the seasonal amplitude in polluted air masses arriving at Mace Head is increasing. Furthermore, there has been a shift in the months controlling the size of the seasonal amplitude in polluted air from a maximum in May and minimum in January in 1990 to a maximum in April and a minimum in July by 2001. This finding suggests that there has been a steadily decreasing influence of polluted air masses arriving from Europe. These air masses have therefore increasingly taken on the attributes of baseline air.

Citation: Carslaw, D. C.: On the changing seasonal cycles and trends of ozone at Mace Head, Ireland, Atmos. Chem. Phys., 5, 3441-3450, doi:10.5194/acp-5-3441-2005, 2005.
 
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