On the changing seasonal cycles and trends of ozone at Mace Head, Ireland

[摘要] A seasonal-trend decomposition technique based on a locally-weightedregression smoothing (Loess) approach has been used to decompose monthlyozone concentrations at Mace Head (Ireland) into trend, seasonal andirregular components. The trend component shows a steady increase from1990–2004, which is confirmed by statistical testing which shows that ozoneconcentrations at Mace Head have increased at the p=0.06 level by0.18±0.04 ppb yr⁻¹. By considering different air mass origins usinga trajectory analysis, it has been possible to separate air masses into"polluted" and "unpolluted" origins. The seasonal-trend decompositiontechnique confirms the different seasonal cycles of these air mass originswith unpolluted air mass maxima in April and polluted air mass maxima inJuly/August. A detailed consideration of the seasonal component revealsdifferent behaviour depending on the air mass origin. For baselineunpolluted air arriving at Mace Head there has been a gradual increase inthe seasonal amplitude, driven by a declining summertime component. Theamplitude of the seasonal component of baseline air is controlled by amaximum in April and a minimum in July. For polluted air mass trajectories,there was a substantial reduction in the amplitude of the seasonal componentfrom 1990–1997. However, post-1997 results indicate that the seasonalamplitude in polluted air masses arriving at Mace Head is increasing.Furthermore, there has been a shift in the months controlling the size ofthe seasonal amplitude in polluted air from a maximum in May and minimum inJanuary in 1990 to a maximum in April and a minimum in July by 2001. Thisfinding suggests that there has been a steadily decreasing influence ofpolluted air masses arriving from Europe. These air masses have thereforeincreasingly taken on the attributes of baseline air.

[学科分类] 大气科学