Spectral UV Measurements at Sodankylä during 1990–2001

Abstract

A Brewer spectrophotometer MK II has measured UV irradiances since 1990 at Sodankylä (67°N,27°E), Finland. In this paper, the observed spectral features of the time series 1990–2001 are studied. In order to exclude the seasonal variation of irradiances due to the variation of solar zenith angle (SZA), only the narrow SZA interval from 63° to 65° was chosen. The data have been temperature, cosine, noise spike and wavelength‐shift corrected. In order to study long‐term changes in spectral UV irradiance, a linear regression line was fitted to the monthly mean irradiances from April to August. No statistically significant changes were observed for any months for the 12‐year period 1990–2001. One reason is that the period is rather short in comparison to the high natural inter‐annual variability of UV. The indistinct trend over this particular period could be also expected from the corresponding ozone changes: A decrease in the early 1990's with an increase toward more normal values in recent years, the smallest value occurring in the middle of the period. This makes the calculated UV changes highly dependent on the time period chosen. The biggest effect was seen in April, where the relative change at 305 nm varied from −5.0% to 12.1% per year for the periods 1993–2001 and 1990–1997, respectively. The analysis further shows that, although many other parameters, such as changes in cloudiness, aerosols, and albedo, contribute to UV changes, ozone has been a prominent factor at Sodankylä at the short wavelength end of the spectrum.