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

We present the results of two independent analyses of trends in the vertical distribution of ozone. For most of the ozonesonde stations we use data that were recently reevaluated and reprocessed to improve their quality and internal consistency. The two analyses give similar results for trends in ozone. We attribute differences in results primarily to differences in data selection criteria, rather than in statistical trend models. We find significant decreases in stratospheric ozone at all stations in middle and high latitudes of the Northern Hemisphere from 1970 to 1996, with the largest decreases located between 12 and 21 km, and trends of -3 to -10 %/decade near 17 km. The decreases are largest at the Canadian and the most northerly Japanese station and are smallest at the European stations and at Wallops Island. 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. For 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 seasonally from about 10 to 17 km, with largest ozone decreases in winter and spring. Trends in tropospheric ozone are highly variable and depend on the region. There are decreases or zero trends at the Canadian stations for 1970-1996, 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 are increases in ozone for the three Japanese stations for 1970-1996, but trends are either positive or zero for 1980-1996. The United States stations show zero or slightly negative trends in tropospheric ozone after 1980. It is not possible to define reliably a mean tropospheric ozone trend for northern midlatitudes, given the small number of stations and the large variability in trends. The integrated column trends derived from the sonde data are consistent with trends derived from both surface-based and satellite measurements of the ozone column.