CHANGES OF THE LOWER STRATOSPHERIC OZONE OVER EUROPE AND CANADA

Ozonesonde measurements from Europe and Canada for the period 1973-199 4 stratified to the height of the tropopause, were analyzed to estimat e regional changes in stratospheric ozone. The calculations were perfo rmed for 1-km layers from the actual tropopause up to 30 km and within +/-4 km around the tropopause. The long-term changes in ozone partial pressure are very similar for both regions, and ozone anomalies show many common features. Negative deviations of 15-25 nbar (or 10-20%) ar e clearly seen in 1976 (at 12-20 km), 1983 (at 17-22 km), 1985 and 198 8 (at 18-25 km), 1989 and 1990 (at 12-20 km), and 1992 and 1993 (at 12 -23 km). In January-April 1995, negative deviations of 15-25 nbar (at 12-23 km) were observed again over the two regions. Trend estimations demonstrate that the strongest decline, in units of ozone partial pres sure, takes place at 17-20 km over Europe in January-April and for Can adian stations from March through June. For the period 1973-1994 the d ecline over both regions is 10-13 nbar per decade (or 5-10% per decade ) and is similar to 1.5 times larger for the shorter time interval 197 9-1994. The estimation of trends as a function of altitude from the ac tual tropopause level reduces the relative error in the 7 to 13-km lay er by more than one third, which permits better understanding of the o zone changes in these critical altitudes. Using stratified ozone data around the height of the tropopause, the negative stratospheric winter -spring trends became significant (10-15 nbar per decade or 12-17% her decade) at only 1-2 km above the tropopause.