Variations of erythemal ultraviolet irradiance and dose at Tartu/Toravere, Estonia

Abstract

The variations of erythemally weighted ultraviolet (UV) irradiance and its relationships with atmospheric characteristics during 3 annual cycles (1998-2000) at 1 site (Toravere, Estonia; 58.3° N, 26.5° E; 70 m above sea level) were studied, with the further aim to estimate the erythemal doses for past years. In clear sky conditions the daily erythemal dose and noon irradiance exhibit total-ozone-related asymmetry in the annual cycle, with 27% lower values at spring equinox than at autumnal equinox. The clear sky ratio of erythemally weighted to broadband (300 to 3000 nm) irradiance increases from (0.5 ± 0.25) × 10-4 to (2 ± 0.1) × 10-4 with an increase in solar elevation from 10° to 55°. In overcast low cloudiness conditions this ratio is higher. A simple empirical method to estimate the summer half-year erythemal dose for past years is proposed, and the doses were calculated for 1967-2000. A sharp drop in summer half-year sunshine duration and erythemal dose was found around 1975, with a following quasi-linear recovery to the values met in 1967-1975. A comparison with the measured and Total Ozone Mapping Spectrometer (TOMS)-derived data is provided for 1998-2000. In the fine summer of 1999 and the 'normal' one of 2000, the measured, estimated and TOMS-derived half-year doses agreed to within 0.7%. In the extremely cloudy summer of 1998, the difference between the measured and estimated values was 1.7% and between the measured and TOMS-derived values 6%.