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The U.S. National Science Foundation's (NSF's) Ultraviolet Spectral Irradiance Monitoring Network (UVSIMN) has been measuring global UV irradiance at seven locations in Antarctica, South America, Southern California, and the Arctic, starting in 1988. Data products include spectra of global (sun and sky) irradiance, sampled quarter-hourly between 280 and 600 nm; integrated irradiance (e.g., UV-B, UV-A); biologically effective dose-rates (e.g., the UV Index); total ozone; effective albedo; cloud optical depth; actinic flux; photoloysis rates; and complementing spectra calculated with a radiative transfer model. Data are disseminated via the project's website www.biospherical.com/NSF. During the last year, data have also been submitted to international data repositories, including (1) the World Ozone and UV Data Center (WOUDC), which is part of the World Meteorological Organization's Global Atmosphere Watch (GAW) program; (2) the Cooperative Arctic Data and Information Service (CADIS), which supports the Arctic Observing Network (AON), an NSF initiative for the International Polar Year (IPY); and (3) the SeaWiFS Bio-optical Archive and Storage System (SeaBASS), which serves NASA's calibration and validation activities for ocean-viewing satellites. We also plan to submit a subset of the dataset to (4) the Network for the Detection of Atmospheric Composition Change (NDACC). The main objective of NDACC is to further understanding of stratospheric changes to the troposphere. UVSIMN data have been adjusted to better serve the needs of these diverse research communities. This paper details the background, format, and volume of these new datasets.

Multiband filter radiometers (MBFRs) are extensively used in national networks for UV climate monitoring and information to the public about the potential risk of solar UV exposure. In order to provide an international, uniform expression of the Global UV index measurements, a harmonized calibration scale is needed. In this paper we present the results of the first international intercomparison of MBFRs held in Oslo in 2005. The purposes are to evaluate the UV-index scale of different radiometers and to provide a harmonized UV-index scale based on the radiometers individual directional and absolute spectral response functions. In total 43 MBFR radiometers and 4 high resolution spectroradiometers were assembled, representing UV-monitoring networks operated by institutions in US, Spain, Greece, Poland, Belgium, UK, Austria, Norway, Sweden and Finland. The radiometers are operating worldwide, with stations in the Antarctica and Arctic, North- and South-America, Africa, Europe, Middle-East and Nepal. All sky conditions were realized during the campaign period. The agreement between the users' own processed UVI and the reference is generally very good; within ±5% for 22 out of 26 data sets (75%) and ±10% for 23 out of 26 (88%). Solar zenith dependent discrepancies and drift in the users' UVI scales is seen, but the performance of most radiometers is generally very good. All the objectives planned for the intercomparison were fulfilled and the campaign considered a success.

Spectral ultraviolet (UV) and visible irradiance has been measured at Palmer Station, Antarctica, between 1988 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 Palmer Station based on the recently produced “Version 2” data edition. This data set will supersede the original release “V ersion 0”. Corrections applied to the new version increased biologically effective UV dose rates by 0-9%. Values of UV-A irradiance changed by –8% to +10%. A comparison with results of a radiative transfer model confirmed that meas urements of different years are consistent to within ±5%. Total ozone column was calculated from UV spectra and was found to agree with measurements of NASA’s Total Ozone Mapping Spectrometer (TOMS) installed on the Nimbus-7 satellite to within 1%. TOMS measurements on the Earth Probe satellite are 3% lower than SUV-100 data. Effect ive surface albedo was estimated from clear sky spectra. Between August and November, albedo typically ranges between 0.6 and 0.95. After melting of snow and sea ice, albedo varies between 0.3 and 0.5. Biologically effective UV radiation is largest in November and December when low total ozone amounts coincide with relatively small solar zenith angles (SZA). During these months, the noon-time UV Index typically varies between 4 and 7, but UV indices as high as 14.8 have been observed. The largest erythemal daily dose of 8.8 kJ/m² was measured on 11/10/97 and 12/7/98. Linear regression analyses did not indicate statistically significant trends in UV or visible radiation, with the exception of February when small downward trends with statistical significance were observed. On average, clouds reduce UV irradiance at 345 nm between 28% (October and November) and 42% (February) compared to clear sky levels. In extreme cases, reductions by clouds can be as high as 90%. Between September and November, the variability introduced by ozone is similar to that caused by clouds. Keywords: Solar ultraviolet radiation, Antarctica, climatology, total ozone, albedo.

Spectral ultraviolet (UV) and visible irradiance has been monitored at the South Pole with a SUV-100 spectroradiometer since 1991. A new data version labeled “Version 2” has recently been produced, which has a higher accuracy than the original released “Version 0”. We have established a UV climatology for the South Pole based on the new data set, focusing on the effects of cloudiness, total column ozone, and volcanic aerosols. In addition, total column ozone was calculated from the spectral UV measurements for solar zenith angles (SZA) up to 87° using a recently developed algorithm. This new total ozone data set can complement Dobson ozone observations to validate satellite total ozone data. At the South Pole, NASA/TOMS Version 7 overpass data exceed SUV-100 total ozone measurements by 5-8%. In contrast, TOMS Version 8 data agree on average to within 2% with SUV-100 measurements, and the ratio of the two data sets shows virtually no dependence on SZA and the ozone amount. The good agreement confirms that the well-documented bias of TOMS Version 7 data at high austral latitudes was successfully reduced with the release of Version 8. A similar comparison between SUV-100, Dobson, and TOMS ozone measurements was performed for the NSF/OPP network site at Arrival Heights (78°S), and results are discussed.

Multi-channel moderate-bandwidth GUV filter radiometers have recently been added to the suite of instruments deployed in the US National Science Foundation Office of Polar Programs' UV monitoring network. The GUV instruments complement the stations' SUV-100 high-resolution scanning spectroradiometers, which have been monitoring UV levels in Antarctica, South America, and Alaska for more than a decade. The GUV instruments are used to help quality control SUV measurements, and to calculate total column ozone and a variety of biologically relevant UV integrals and dose-rates in real time. The results are updated every minute on web pages, and can be accessed via the website www.biospherical.com/nsf, or the stations' intranets. Online data may guide researchers on station in planning experiments, or for "first-look" analysis. The instruments underwent a detailed characterization. Their spectral response functions were measured with an apparatus that was specifically designed for this purpose. The apparatus and the data analysis method are described in detail with special attention given to a deconvolution method to correct measured data for the finite spectral resolution of the apparatus. The impact of uncertainties in measuring the spectral response of GUV channels on solar measurements is discussed. The GUV instruments are calibrated by comparison with a SUV-150B spectroradiometer, and dose-rates for 15 different biological action spectra are calculated based on an algorithm suggested by Dahlback. A comparison of calibrated GUV and SUV data indicates that erythemal (CIE) irradiance can be derived from GUV measurements to within 3% relative to the SUV up to a solar zenith angle (SZA) of 80°. A similar level of agreement can also be reached for other action spectra. Ozone values derived from GUV measurements at San Diego agree to within 3 Dobson Units (DU) with SUV ozone data and within a few DU with Earth Probe TOMS satellite observations.

The U.S. National Science Foundation’s network for monitoring UV radiation in polar regions is now in its 15 th year of operation. During this period, the deployed SUV-100 spectroradiometers have repeatedly been modified, and data processing methods have been changed. These modifications have continuously improved the quality of published data, but have also introduced step-changes into the data set. For example, a change of the wavelength calibration method in 1997 has improved the wavelength accuracy to ±0.04 nm (±1σ), but also lead to a step of 2-4% in published biological dose rates. In order to best assess long-term changes in UV at network locations, it is desirable to remove these steps and to homogenize the data set. This publication discusses possible ways to accomplish these objectives, with special emphasis on absolute calibration, wavelength accuracy, and the cosine error. To date, published data are not corrected for the instruments’ cosine errors. Such corrections are not straightforward, as older data are affected by an azimuth asymmetry of the irradiance collector, which was not constant over the years. A new method to correct the errors for both clear and cloudy sky conditions was developed, and is described here. Results indicate that dose rates published prior to the year 2000 are low by 2-5%, and exhibit a variation with the Sun’s azimuth angle. By modifying the instruments’ irradiance collectors in 2000, the azimuth asymmetry was virtually eliminated, however, the modification also lead to a step-change of about 3% in published data. The ability of the new correction algorithm to remove this step is demonstrated. Uncertainties in biologically weighted dose rates caused by the cosine error can be reduced with the correction procedure to ±2%. We are planning to reprocess the entire NSF data set with the new algorithms to improve both accuracy and homogeneity.

Global solar UV measurements performed with high-resolution SUV-100 spectroradiometers in Antarctica and Alaska are compared with results of the radiative transfer model UVSPEC/libRadtran. The instruments are part of the National Science Foundation's Office of Polar Programs (NSF/OPP) UV monitoring network, and are located at the South Pole (90 degree(s)S), McMurdo (78 degree(s)S), Palmer Station (65 degree(s)S), and Barrow, Alaska (71 degree(s)N). A new algorithm to retrieve total column ozone from the ratio of measured and modeled UV spectra is presented, which is then used to uncover biases in column ozone data from different sources (Earth Probe TOMS Version 7, Dobson, GOME, TOVS) at the previously mentioned high-latitude locations. The analyses suggest that EP/TOMS overestimates total column ozone at all Antarctic sites by 4-10%, which is consistent with recent findings reported elsewhere. SUV-100 and Dobson total column ozone measurements at the South Pole, Barrow and McMurdo agree to within +/- 1.5%, +/- 2%, and +/- 1%, respectively. GOME measurements at Palmer and McMurdo Station are 2% and 6% lower than the SUV-100 data. TOVS ozone values show in general a larger deviation. The data further reveal that ozone and temperature profiles used in the model have an important influence, particularly at low sun elevations. This is quantified by comparing the UV measurements with model calculations using either standard profiles or actual profiles measured by balloon sondes. When using Dobson ozone measurements and actual ozone profiles, and correcting SUV- 100 UV measurements for the cosine error of the entrance optics, spectral clear-sky measurements typically agree with model results to within +/- 5% for solar elevations greater than 5 degrees.

Ideally measurements of spectral ultraviolet irradiances require a perfect adaption of the entrance optics to the cosine of the incidence angle. Other requirements of the entrance optics are: No ageing, high throughput at all wavelengths, weatherproofness, and no fluorescence of their material. In practice, however, available entrance optics differ by more than 10% from the ideal cosine response for incident angles greater than 60 degree(s). Without a correction this introduces a great uncertainty in the absolute measurement of irradiances, especially when the sun is low. A measurement of the angular dependence of the entrance optics and a knowledge of the ratio of the direct to the diffuse component of the global irradiance can be used to correct this cosine error. The correction is dependent on wavelength and sun elevation. For our cosine diffuser the corrections vary between 3% and 18%. The accuracy of the corrections is limited by the accuracy of the measurement of the angular dependence of the cosine diffuser, by the knowledge of the ratio of the direct to the diffuse radiation and by the knowledge of the angular dependence of the radiance of the diffuse component. We assume that our method reduces the overall cosine uncertainty from about +/- 10% to about +/- 3%.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.