YEARLY CYCLE OF LOWER TROPOSPHERIC OZONE AT THE ARCTIC-CIRCLE

Measurements of tropospheric ozone at three sites at the Arctic Circle in the Finnish Lapland are presented. The variability of ground-level ozone over the diurnal and seasonal cycles in 1992-93 is discussed fo r the sites of Oulanka and Pallas. The variability with height and ove r the annual cycle in 1989-94 is discussed for the Sodankyla aerologic al Observatory, which has the longest record on the vertical distribut ion of ozone in the Nordic region. Seasonally resolved ozone statistic s and the differences between the sites are accounted for. At the surf ace, ozone levels peak in the spring, but they decline rapidly in the early summer (remote area feature) with a 15-30% seasonal difference. The seasonal difference between spring and summer decreases with heigh t in the lower troposphere and at 850 and 700 hPa, the spring maximum continues as high ozone levels in the summer (an anthropogenic feature ). At these two levels, the relative differences in ozone between spri ng and summer were 4% and -1.3%, respectively. The summertime high ozo ne levels in the lower troposphere highlight the importance of transpo rt of anthropogenic precursors of ozone for the regional lower troposp here. A three-dimensional trajectory climatology is used for assessing tropospheric transport patterns. Air mass transport occurs from both remote and polluted source regions. The Arctic is the most important s ource region at the 950 hPa level. With increasing altitude, the contr ibutions of the European and the Atlantic regions become comparable. T he evolution of snow cover and surface-based inversions affect the var iability of ozone, through variations in the deposition sink strength and the boundary layer stability.