On shifts in the long-term Umkehr radiance records and their influence on retrieved ozone profiles.

The Umkehr technique has been used routinely for more than 40 years to asse ss features in the vertical distribution of ozone. The measurements represe nt the longest ozone profile data record in existence. A careful examinatio n of the long-term Umkehr N-value (radiance) measurement record, archived a t the WMO World Ozone and UV Data Center in Toronto, Canada, revealed shift s in the data. These shifts appear to be mainly related to Dobson instrumen t calibrations and/or to instrument changes at the observing stations. In m ost cases, the detected shifts exhibit solar zenith angle dependence. The s tandard procedure of subtracting the Umkehr measurement at 60-degree solar zenith angle (or the next smallest SZA available) from the measurement at t he other angles to eliminate the instrumental and extraterrestrial constant s from measurements does not necessarily remove the shifts. The shift error s would appear as shifts in the retrieved ozone profiles and could affect a ny ozone profile analysis (including ozone trends) as well as comparisons w ith other profiles derived from satellite and lidar measurements. A prelimi nary approach for detecting the shifts and allowing for corrections is deve loped and evaluated for its potential to improve the quality of the of the Umkehr records.