[1] Spectral ultraviolet (UV) and visible irradiance has been measured at the South Pole between 1991 and 2003 by a SUV-100 spectroradiometer, which is part of the U.S. National Science Foundation's UV Monitoring Network. Here we present a new data edition, labeled ''Version 2.'' The new version was corrected for wavelength shift errors and deviations of the spectroradiometer from the ideal cosine response. A comprehensive uncertainty budget of the new data set was established. Below 400 nm the expanded standard uncertainty (coverage factor 2) varies between 4.6 and 7.2%, depending on wavelength and sky condition. The uncertainty of biologically relevant UV irradiances is approximately 6%. Compared to the previously published data set, Version 2 UV data are higher by 5-14%, depending on wavelength, solar zenith angle (SZA), and year of observation. By comparing Version 2 data with results of a radiative transfer model, the good consistency and homogeneity of the new data set were confirmed. The data set is used to establish a UV climatology for the South Pole, focusing on the effects of aerosols, clouds, and total column ozone. Clouds are predominantly optically thin; 71% of all clouds have an optical depth between 0 and 1. The average attenuation of UV irradiance at 345 nm by clouds is less than 5% and no attenuations greater than 23% were observed. Attenuation by homogeneous clouds is generally larger in the visible than in the UV. The wavelength dependence of cloud attenuation is quantitatively explained with the wavelength-dependent radiance distribution on top of clouds and the incidence-angle dependence of cloud transmittance. Largest radiation levels occur in late November and early December when low stratospheric ozone amounts coincide with relatively small SZAs. Owing to the large effect of the ''ozone hole,'' short-and long-term variability of UV during the austral spring is very high. When the ozone hole disappears, DNA-damaging irradiance can decrease by more than a factor of two within 2 days. Typical summer UV index values range between 2 and 3.5 and vary by ±30% (±1s) between different years. Linear regression analyses did not indicate statistically significant UV trends owing to the large year-to-year variability and the fact that the network was established only after the first occurrence of the ozone hole. Current measurements therefore document variability on an elevated level.
[1] Spectral ultraviolet (UV) and visible irradiance has been measured near Barrow, Alaska (71°N, 157°W), between 1991 and 2005 with a SUV-100 spectroradiometer. The instrument is part of the U.S. National Science Foundation's UV Monitoring Network. Here we present results based on the recently produced ''version 2'' data release, which supersedes published ''version 0'' data. Cosine error and wavelength-shift corrections applied to the new version increased biologically effective UV dose rates by 0-10%. Corrected clear-sky measurements of different years are typically consistent to within ±3%. Measurements were complemented with radiative transfer model calculations to retrieve total ozone and surface albedo from measured spectra and for the separation of the different factors influencing UV and visible radiation. A climatology of UV and visible radiation was established, focusing on annual cycles, trends, and the effect of clouds. During several episodes in spring of abnormally low total ozone, the daily UV dose at 305 nm exceeded the climatological mean by up to a factor of 2.6. Typical noontime UV Indices during summer vary between 2 and 4; the highest UV Index measured was 5.0 and occurred when surface albedo was unusually high. Radiation levels in the UV-A and visible exhibit a strong spring-autumn asymmetry. Irradiance at 345 nm peaks on approximately 20 May, 1 month before the solstice. This asymmetry is caused by increased cloudiness in autumn and high albedo in spring, when the snow covered surface enhances downwelling UV irradiance by up to 57%. Clouds reduce UV radiation at 345 nm on average by 4% in March and by more than 40% in August. Aerosols reduce UV by typically 5%, but larger reductions were observed during Arctic haze events. Stratospheric aerosols from the Pinatubo eruption in 1991 enhanced spectral irradiance at 305 nm for large solar zenith angles. The year-to-year variations of spectral irradiance at 305 nm and of the UV Index are mostly caused by variations in total ozone and cloudiness. Changes in surface albedo that may occur in the future can have a marked impact on UV levels between May and July. No statistically significant trends in monthly mean noontime irradiance were found.
The investigation of the effect of solar ultraviolet (UV) and visible radiation on biological organisms and photochemical reactions requires spectral measurements of the desired radiation parameters of high accuracy. The Network for the Detection of Stratospheric Change (NDSC) and the World Meteorological Organization have set up stringent requirements for high-quality spectral measurements of ultraviolet radiation. It is shown that two new instruments comply with these standards. One is the newly developed spectroradiometer of the Institute of Meteorology and Climatology, University of Hannover, Hannover, Germany. It is capable of covering the spectral range from the UV to the near-infrared (290-1050 nm) in a comparably fine resolution. One major aim is to deploy this instrument as a traveling NDSC spectroradiometer. The other new instrument is built for the U.S. National Science Foundation's UV Monitoring Network. It is designed to monitor UV and visible irradiance at high latitudes and covers a wavelength range from 280 to 600 nm. Data of both instruments show deviations of less than 5% for a wide range of atmospheric conditions compared to a NDSC spectroradiometer owned by the Climate Monitoring and Diagnostics Laboratory during the fifth North American Interagency Intercomparison for UV Spectroradiometers. Such deviations represent state-of-the-art instrumentation for conducting long-term measurements of solar UV radiation capable of detecting trends and supporting long-term measurements by traveling standards. Furthermore, there is now an instrument capable of measuring solar irradiance in a wavelength range from 250 to 1050 nm.
[1] 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 À5 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.
Multichannel moderate-bandwidth ground-based ultraviolet (GUV) filter radiometers have recently been installed at several sites of the U.S. National Science Foundation's UV monitoring network where they complement high-resolution solar UV-100 (SUV-100) spectroradiometers. The five GUV channels are characterized for their spectral response and calibrated against SUV systems as well as irradiance standard lamps. Results indicate that accurate spectral characterization of GUV channels in the UV-B is crucial for obtaining high-quality UV measurements, in particular if instruments are calibrated with standard lamps. Using an inversion algorithm suggested by Dahlback (1996), total column ozone and approximately 30 different UV integrals and dose rates are routinely calculated from GUV measurements. For UV-A irradiance, GUV and SUV data agree to within Ϯ5% for solar zenith angles (SZAs) up to 90 deg. A similarly good agreement can be achieved for the UV index if solar measurements are restricted to SZAs smaller than 78 deg. The agreement for data products that are dominated by wavelengths in the UV-B is generally worse, but can be substantially improved if GUV instruments are equipped with an additional channel at 313 nm. GUV total column ozone values agree on average to within Ϯ5% with observations from the National Aeronautics and Space Administration's (NASA's) total ozone mapping spectrometer (TOMS) on board the earth probe satellite. GUV data products are disseminated via the website www.biospherical.com/NSF in near real time.
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