A comparison of cloudless sky erythemal ultraviolet radiation at two sites in southwest Sweden

This case study examines the variability of ultraviolet erythemal radiation at two different but nearby locations in southwest Sweden. Data from two identical radiometers were analysed. One instrument sampled the urbanised environment of Goteborg (latitude: 57.70°N, longitude: 12.00°E), while the second one sampled a rural site, Koster Island (latitude: 58.83°N; longitude: 11.00°E), some 143 km to the north. Both radiometers were intercompared during the field season, and one radiometer had its calibratio traced to WMO/STUK intercomparison performed in 1995. Only cloudless data for the period 1 April 1996-30 September 1996 are examined in this study. Higher depletion was reported at Koster Island for the 4 days which were simultaneously cloudless at both sites. A cloudless model developed by Green et al. (Green, A.E.S., Cross, K.R. and Smith, L.A. 1980. Photochem. Photobiol., 31, 59-65.) was used to determine daily measured/model ratios for both sites. However, there is some uncertainty in these ratios due to the spatial variability in total column ozone. Excamining seasonal trends, much scatter is reported at both sites, although after day 226 (13 August 1996), distinctly lower ratios were reported at Koster Island. By examining pyranometer and relative humidity data at both sites, it is argued that coastal haze at Koster Island may explain the higher depletion there.