UV climatology at McMurdo Station, Antarctica, based on version 2 data of the National Science Foundation's Ultraviolet Radiation Monitoring Network

Spectral ultraviolet (UV) and visible irradiance has been measured near McMurdo Station, Antarctica, between 1989 and 2004 with a SUV-100 spectroradiometer. The instrument is part of the U. S. National Science Foundation's UV Monitoring Network. Here we present a UV climatology for McMurdo based on the recently produced "version 2" data edition. Compared to the previously published "version 0" data set, version 2 data differ by -5 to 12% in the UV, depending on wavelength, solar zenith angle (SZA), and year. A comparison with results of a radiative transfer model confirmed that measurements of different years are consistent to within +/- 5%. Clear-sky spectra measured between October 1991 and March 1992 were significantly lower than spectra of other years because of the presence of volcanic aerosols. Total ozone column was calculated from UV spectra and was found in excellent agreement with collocated measurements of a Dobson spectrophotometer and satellite observations. Effective surface albedo was also estimated from clear-sky spectra. Monthly average albedo ranges between 0.69 for March and 0.84 for October. Biologically effective UV radiation is largest in November and December when low total ozone amounts coincide with relatively small SZAs. During these months, the noon-time UV Index typically varies between 2 and 5.5, but indices as high as 7.5 have been observed. The largest erythemal daily dose of 6.7 kJ/m(2) was measured on 28 November 1998. Linear regression analyses did not indicate statistically significant trends in UV nor visible radiation for the months September to January. For February and March, we found large, statistically significant positive trends in the UV and visible as well as for short-wave (0.3-3.0 mm) irradiance, ranging between 12 and 30% per decade. These trends are likely caused by changes in cloudiness and/or surface albedo, but the data do not allow unambiguous attribution of the increase to one of the two factors. On average, clouds reduce UV irradiance at 345 nm by 10% compared to clear-sky levels. Reductions vary substantially by month and year, can exceed 60% on rare occasions, and generally increase with wavelength. Between September and November, the variability in UV introduced by changes in total ozone is about twice as high as the UV variability due to clouds.