Spring ozone column values over Thule, Greenland in the period 1991-1998

Eight years of continuous measurements of the ozone column at Thule, Greenl and are presented. Monthly average values of the ozone column have been obs erved to decrease significantly in spring during this decade. Lower ozone c olumns with large interannual variations have been observed when the polar vortex was above the site. In 1995 and 1996 March monthly mean values were very low with an average deficit in the vertical column of more than 30 % o r 150 DU, when considering only days where the site was inside the vortex. There are two contributions to the changes in the total column ozone one is transport and the other is chemical depletion. From the variation in the g eopotential height of 250 hPa lever and the temperature at the 475 K isentr opic level an estimate of the part of the decrease in the vortex March mont hly mean ozone column which could be explained by vertical transport was ob tained. It was found that vertical transport could explain a 10 % decrease in 1995 and a 5 % decrease in 1996; leaving 25 % to be explained by chemica l ozone destruction.