Polar Regions UV Spectroradiometer Monitoring Program: Observations in UV Irradiance at the South Pole and Barrow, Alaska

J. Ehramjian, C.R. Booth, L.W. Cabasug, J.S. Robertson, and T. Mestechkina Biospherical Instruments Inc., San Diego, California 92110-2621

Introduction

The antarctic ultraviolet (UV) spectroradiometer monitoring network was established by the U.S. National Science Foundation (NSF) in 1988 in response to predictions of increased UV radiation in the polar regions. The network consists of several automated, high resolution spectroradiometers; five are placed in strategic locations in Antarctica and the arctic, one is established in San Diego to collect data and serve as a training and testing facility (Table 1), and a portable system is used at instrument intercomparisons [Sechmeyer et al., 1995; Thompson et al., 1997; Early et al., 1998]. Now in its 10th year of operation, the network continues to make measurements of UV spectral irradiance and provides a variety of biological dosage calculations of UV exposure. Biospherical Instruments Inc. (Sand Diego, California), under contract to Antarctic Support Associates (ASA), directed by NSF, is responsible for operating and maintaining the network and distributing data to the scientific community.

The spectroradiometers used in the system are Biospherical Instruments, Inc. Model SUV-100. Each instrument contains a diffusing irradiance collector, a double holographic grating monochromator, a photo-multiplier tube (PMT), and calibration lamps. A vacuum-formed Teflon diffuser serves as an all-weather irradiance cosine collector. This collector is heated by the system to deter ice and snow accumulation. Tungsten-halogen and mercury vapor calibration lamps are used for daily automatic internal calibrations of both responsivity and wavelength registration. All instrument functions, calibration activities, and solar data acquisition are computer controlled. Further details on the spectroradiometers can be found in Booth et al. [1998].

UV Radiation climate at SPO and BRW

Typical applications and observations that can be made from datasets produced from the network are presented. The South Pole Observatory, Antarctica (SPO) and the Barrow Observatory, Barrow, Alaska (BRW) network installations are in locations that also have CMDL facilities, and emphasis in this report is given to measurements made at these sites.

The SPO site is located away from the influence of mountains in a region of almost constant albedo. UV irradiance observations at SPO are well suited for atmospheric research because they are made in nearly constant high-albedo condition, further idealized by the air purity and high-altitude of the site. Cloud cover is also relatively infrequent and it is generally thin when it does occur. The very small hourly change in the solar zenith angle at SPO supports examination of changes in total column ozone (as estimated by UV irradiance) at 15-minute to 1-hour resolution [Booth and Madronich, 1993]. For example, a substantial decrease is seen in irradiance at 300 nm in late November 1993 (Figure 1(a)). Meanwhile, Total Ozone Mapping Spectrometer (TOMS Nimbus-7) data report a substantial increase in the 300 nm irradiance between November 15 and November 20, 1993.

BRW contrasts with SPO in that it is located where a significant change in surface albedo occurs due to both the springtime snowmelt [Dutton and Endres, 1991] and changes in sea ice coverage. Also, BRW experiences significant changes in incident irradiance during arctic storms. The contrast in irradiances between BRW and SPO is seen in Figure 1(b), which depicts the integrated local solar noontime irradiances over the UV-A spectrum (320-400 nm) from January 1993 through December 1997.

The integral of spectral irradiance from 298.507 to 303.03 nm is sensitive to changes in total ozone (and solar angle). A strong correlation between the ozone concentration and terrestrial UV irradiance is illustrated by the BRW 1997 example in Figure 2. Note the unusually low ozone over BRW in March 1997 [Savage et al., 1997] as measured by the TOMS satellite. During these low ozone occurrences, the measured integral of spectral irradiance from 298.507 to 303.03 nm is the highest recorded in BRW for those dates. In 1996 relatively high UV levels at SPO were observed at typical times (from late October into December) during the season, with an early termination of influence of the “ozone hole” in mid-December (Figure 3). Lower overall UV levels occurred through most of the 1997 season, with some higher than typical UV levels late in the year (December), suggesting that the effects of ozone depletion lingered that year.

TABLE 1. Installation Sites

*ARO: Atmospheric Research Observatory, system relocated to this new, joint CMDL facility in January 1997.

†CADIC: Centro Austral de Investigaciones Cientificas, Argentina.

‡Ukpeagvik Inupiat Corporation-National Arctic Research Laboratory, Alaska.

Fig. 1. Local solar noontime integrated spectral irradiance at Barrow, Alaska, and South Pole, Antarctica, from January 1993 through December 1997. The left panel shows the integrated irradiance around 300 nm (293.507-303.03 nm) and is contrasted with the right panel that illustrates the UF-A irradiance (320-400 nm). The higher irradiance values at Barrow are due to the higher sun elevation. Normally, irradiance at Barrow peaks in June, while irradiance at the South Pole peaks in December. Note that the 1997 South Pole and 1997 November-forward data are preliminary and subject to revision.

Fig. 2. Comparison of the 298.507-303.03 nm integrated spectral irradiance and TOMS total ozone measurements made at Barrow during Boreal spring 1997. Note that the left axis is a log scale to emphasize early “season” variation (lower solar zenith angles). The right axis is inverted to made the effect of decreasing ozone on increasing readily apparent.

In addition to the production of a long time series of data, the high spectral resolution data are calculated into several spectral integrals and biological doses for the convenience of the researcher. These include several spectral integrals (UV-A₁, UV-A₂, UV-B₁, UV-B₂, Visible, etc.) and a number of dose weightings (Erythemal, Setlow, Caldwell, Hunter, etc.). A simple use of this data shows the maximum UV-B irradiances (290-320 nm) recorded at each site for 1990-1997 (Table 2.).

Fig. 3. Integrated 298.507 - 303.03 nm spectral irradiance at South Pole. The thin lines represent historical (1991-1995) minimum and maximum observations. The 1996 data are represented by open squares, and 1997 by filled diamonds.

For the data of specific interest to the researcher, the effects of other environmental conditions can be verified using the other available integrals and doses. For instance, higher UV irradiances are sometimes observed in conditions of partial cloud cover than completely clear days, due to reflections off of cloud surfaces. This was San Diego that was verified using the Weighted Total Scene Irradiance integral [Booth et al., 1998]. Another environmental factor particular to polar UV measurements is high albedo due to snow and ice coverage in addition to well-defined ozone depletion occurrences. An example of the high albedo effect on UV measurements can be seen in the Palmer Station UV-B maximum occurrence (Table 2.). It was known that ice coverage was heavy at the time of this observation.

TABLE 2. UV-B (290-320 nm) Maxima (µW/cm²)

Summary

High spectral resolution scanning UV spectroradio-meters were established at six sites and are successfully providing multi-year data sets to the research community. Resulting data have been used to test radiative transfer models [Lubin and Frederick, 1992; Smith et al., 1992], investigate radiation amplification [Booth and Madronich, 1993; Madronich, 1994], derive ozone concentrations [Stamnes et al., 1992], investigate ozone depletion as a function of chlorofluorocarbons [Rowland, 1996], examine the biological impact of enhanced UV [Cullen et al., 1992; Anderson et al., 1993; Benavides et al., 1993; Holm-Hansen et al., 1993; Madronich, 1994; Diaz et al., 1994; Ladizesky et al., 1995; Karentz et al., 1995, Boucher et al., 1996] explore geographical differences in the UV [Booth et al., 1995; Diaz et al., 1994; McKenzie et al., 1994; Seckmeyer et al., 1995; Bojkov et al., 1995], and evaluate long term trends [Gurney, 1998].

Data, referenced to both beginning- and end-of-season calibration constants are distributed on CD-ROM and are available to interested researchers. For more information, please contact Biospherical Instruments Inc., 5340 Riley Street, San Diego, CA 92110-2621, at www.biospherical.com for online data ordering information.

Acknowledgments. This research and monitoring activity was funded by contracts SCK-M-18914-02, SCE-M34578-01, and SFJ-M42664-01 from Antarctic Support Associates under the direction of Polly Penhale at the National Science Foundation, Office of Polar Programs. R. McPeters of the National Aeronautics and Space Administration (Goddard Space Flight Center) provided TOMS Total Ozone data for comparison purposes. Barrow (CMDL) operators are D. Endres and M. Gaylord. The Ukpeagvik Inupiat Corporation of Barrow provided assistance in the original installation. Operators at Palmer, South Pole and McMurdo were provided by ASA.

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