Charles R. Booth

Geographical differences in the UV Measured by intercompared spectroradiometers

Five UV spectroradiometers representative of the types used in monitoring programs on several continents were intercompared at the Fraunhofer Institute for Atmospheric Environmental Research (IFU), Garmisch-Partenkirchen, Southern Germany, during a campaign in August 1994. Global spectral irradiances between 290 and l410 nm were measured over a range of solar zenith angles from 30{degrees} to 80{degrees}. Scans were synchronized to enable useful comparisons to be made under changing weather conditions, which included overcast, partly cloudy, and clear skies. No exchange of data was allowed between participating groups until after the campaign. At wavelengths longer than 310 nm, the spectra generally agreed to within {plus_minus}5%. At a wavelengths shorter than 3l10 nm, differences between instruments were larger, especially at larger solar zenith angles. Causes of differences are discussed. For all instruments, deviations in erythemally weighted irradiances were always less than 7% from the mean. The agreement between measurement systems is sufficient to allow an investigation of geographical differences in UV, under all observing conditions. UV doses measured at sites in the southern hemisphere are systematically larger than those measured at the corresponding northern latitudes. During the summer months the daily doses at the South Pole exceed those at mid-latitudes in the Northernmorexa0» Hemisphere. Further investigations must be performed to establish a global UV-climatology. 8 refs., 5 figs., 2 tabs.«xa0less

Version 2 data of the National Science Foundation's Ultraviolet Radiation Monitoring Network: South Pole

[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. INDEX TERMS: 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0394 Atmospheric Composition and Structure: Instruments and techniques; 3359 Meteorology and Atmospheric Dynamics: Radiative processes; KEYWORDS: ultraviolet radiation, UV, Antarctica, South Pole, uncertainty, total column ozone

Ultraviolet and visible radiation at Barrow, Alaska: Climatology and influencing factors on the basis of version 2 National Science Foundation network data

[1]xa0Spectral 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 Foundations 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.

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

[1]xa0Spectral 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 Foundations 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/m2 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 μm) 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.

Comparison of measured and modeled spectral ultraviolet irradiance at Antarctic stations used to determine biases in total ozone data from various sources

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 Foundations 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.

UV climatology at Palmer Station, Antarctica, based on version 2 NSF network data

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 Foundations 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 Version 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 measurements of different years are consistent to within ±5%. Total ozone column was calculated from UV spectra and was found to agree with measurements of NASAs 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. Effective 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.

Comparison of ultraviolet spectroradiometers in Antarctica

[1]xa0Solar ultraviolet irradiance has been monitored in Antarctica for almost two decades by a network of spectroradiometers established by the National Science Foundation. Data have been used for investigating increases in ultraviolet radiation in response to ozone depletion, validation of satellite observations, and the establishment of ultraviolet radiation climatologies and trends. To assess the quality of data collected, measurements of the monitoring spectroradiometer installed at Arrival Heights (78°S, 167°E) were compared with an independently calibrated, state-of-the art instrument, which was installed next to the monitoring system for a three-month campaign. Measurements of the two instruments differed by 5–7% on average. The discrepancy is quantitatively explained by the different irradiance scales used by the two systems, a bias in determining the reference plane of fore-optics, drifts of calibration standards, some temperature-dependence in the transmission of the entrance optics, and nonlinearity of one of the systems. The wavelength accuracy of data from both instruments was also tested with two commonly used correlation methods. Wavelength shifts determined with the two methods agreed to within 0.003–0.006 nm. Results of the campaign suggest that data collected by the monitoring instrument are of adequate quality for submission to the Network for the Detection of Atmospheric Composition Change.

The quality of data from the National Science Foundation's UV monitoring network for polar regions

The U.S. National Science Foundation’s network for monitoring UV radiation in polar regions is now in its 15th 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.

Dissemination of data from the National Science Foundation's UV monitoring network

The U.S. National Science Foundations (NSFs) 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 projects 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 Organizations 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 NASAs 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.

Comparison of UV climates at Summit, Greenland; Barrow, Alaska and South Pole, Antarctica

An SUV-150B spectroradiometer for measuring solar ultraviolet (UV) irradiance was installed at Summit, Greenland, in August 2004. Here we compare the initial data from this new location with similar measurements from Barrow, Alaska and South Pole. Measurements of irradiance at 345 nm performed at equivalent solar zenith angles (SZAs) are almost identical at Summit and South Pole. The good agreement can be explained with the similar location of the two sites on high-altitude ice caps with high surface albedo. Clouds have little impact at both sites, but can reduce irradiance at Barrow by more than 75%. Clear-sky measurements at Barrow are smaller than at Summit by 14% in spring and 36% in summer, mostly due to differences in surface albedo and altitude. Comparisons with model calculations indicate that aerosols can reduce clear-sky irradiance at 345 nm by 4?6%; aerosol influence is largest in April. Differences in total ozone at the three sites have a large influence on the UV Index. At South Pole, the UV Index is on average 20?80% larger during the ozone hole period than between January and March. At Summit, total ozone peaks in April and UV Indices in spring are on average 10?25% smaller than in the summer. Maximum UV Indices ever observed at Summit and South Pole are 6.7 and 4.0, respectively. The larger value at Summit is due to the sites lower latitude. For comparable SZAs, average UV Indices measured during October and November at South Pole are 1.9?2.4 times larger than measurements during March and April at Summit. Average UV Indices at Summit are over 50% greater than at Barrow because of the larger cloud influence at Barrow.