Long-term erythemal UV doses at Sodankyla estimated using total ozone, sunshine duration, and snow depth

A method for estimating daily erythemal UV doses using total ozone, sunshine duration, and snow depth has been developed. The method consists of three steps: (1) daily clear-sky UV doses were simulated using the UVSPEC radiative transfer program, with daily values of total ozone as input data, (2) an empirical relationship was sought between the simulated clear-sky UV doses, the measured UV doses, and the duration of bright sunshine, and (3) daily erythemal UV doses were estimated using this relationship. The method accounts for the varying surface albedo by dividing the period of interest into winter and summer days, depending on the snow depth. Using this method, the daily erythemal UV doses at Sodankyla were estimated for the period 1950-1999. This was done using Tromso's total ozone together with Sodankyla's own sunshine duration and snow depth as input data. Although the method is fairly simple, the results are in good agreement, even on the daily scale, with the UV radiation measured with the Brewer spectrophotometer at Sodankyla. Over the period 1950-1999 a statistically significant increasing trend of 3.9% per decade in erythemal UV doses was found for March. The fact that this trend is much more pronounced during the latter part of the period, which is also the case for April, suggests a connection to the stratospheric ozone depletion. For July, on the other hand, a significant decreasing trend of 3.3% per decade, supported by the changes in both total ozone and sunshine duration, was found.