Re-Evaluation Of The 1950-1962 Total Ozone Record From Longyearbyen, Svalbard

The historical total ozone measurements taken with Dobson Spectrophotometer # 8 at Longyearbyen (78.2 degrees N, 15.6 degrees E), Svalbard, Norway, in the period 1950 1962 have been re-analyzed and homogenized based on the original measurement logs, using present-day procedures. In lack of sufficient calibration information, an empirical quality assessment was performed, based on a climatological comparison with ozone measurements in Tromso, using TOMS data at both sites in the period 1979 - 2001, and ground-based Dobson data in the period 1950 - 1962. The assessment revealed that the C wavelength pair direct-sun (DS) measurements are most trustworthy ( and most frequent), while the WMO standard reference mode AD direct-sun has a systematic bias. Zenith-blue (ZB) measurements at solar zenith angles (SZA) < 78 degrees were adjusted to DS data using different empirical functions before and after 1957 ( the start of the International Geophysical Year). ZB measurements at larger SZAs were homogenized by means of a normalization function derived from days with measurements over a wide range of SZAs. Zenith-cloudy measurements, which are particularly frequent during the summer months, were homogenized by applying correction factors depending on the cloud type ( high thin clouds and medium to low thick clouds). The combination of all measurements yields a total of 4685 single values, covering 1637 days from September 1950 to September 1962; moon measurements during the polar night add another 137 daily means. The re-evaluated data show a convincing consistence with measurements since 1979 ( TOMS, SAOZ, Dobson) as well as with the 1957 - 1962 data stored at the World Ozone and UV Data Centre (WOUDC).