Changes of the lower stratospheric ozone over Europe and Canada

Ozonesonde measurements from Europe and Canada for the period 1973-1994 stratified to the height of the tropopause, were analyzed to estimate regional changes in stratospheric ozone. The calculations were performed 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%) are clearly seen in 1976 (at 12-20 km), 1983 (at 17-22 km), 1985 and 1988 (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 pressure, takes place at 17-20 km over Europe in January-April and for Canadian stations from March through June. For the period 1973-1994 the decline 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 1979-1994. The estimation of trends as a function of altitude from the actual tropopause level reduces the relative error in the 7 to 13-km layer by more than one third, which permits better understanding of the ozone 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.