Trends in the vertical distribution of ozone: A comparison of two analysesof ozonesonde data

We present the results of two independent analyses of trends in the vertica l distribution of ozone. For most of the ozonesonde stations we use data th at were recently reevaluated and reprocessed to improve their quality and i nternal consistency. The two analyses give similar results for trends in oz one. We attribute differences in results primarily to differences in data s election criteria, rather than in statistical trend models. We find signifi cant decreases in stratospheric ozone at all stations in middle and high la titudes of the Northern Hemisphere from 1970 to 1996, with the largest decr eases located between 12 and 2i km, and trends of -3 to -10 %/decade near 1 7 km. The decreases are largest at the Canadian and the most northerly Japa nese station and are smallest at the European stations and at Wallops Islan d. The mean midlatitude trend is largest, -7 %/decade, from 12 to 18 km for 1970-1996, and the decrease in ozone is significant from 10.5 to 25 km. Fo r 1980-1996, the decrease is more negative by 1-2 %/decade, with a maximum trend of -9%/decade in the lowermost stratosphere. The trends vary seasonal ly from about 10 to 17 km, with largest ozone decreases in winter and sprin g. Trends in tropospheric ozone are highly variable and depend on the regio n. There are decreases or zero trends at the Canadian stations for 1970-199 6, and decreases of -2 to -8 %/decade for the mid-troposphere for 1980-1996 ; the three European stations show increases for 1970-1996, but trends are close to zero for two stations for 1980-1996 and positive for one; there ar e increases in ozone for the three Japanese stations for 1970-1996, but tre nds are either positive or zero for 1980-1996. The United States stations s how zero or slightly negative trends in tropospheric ozone after 1980. It i s not possible to define reliably a mean tropospheric ozone trend for north ern midlatitudes, given the small number of stations and the large variabil ity in trends. The integrated column trends derived from the sonde data are consistent with trends derived from both surface-based and satellite measu rements of the ozone column.