Introduction

When the ozone hole was discovered in 1984 (Chubachi, 1984; Farman et al., 1985), there was concern about increased levels of ultraviolet radiation in Antarctica. UV radiation was not measured in Antarctica at that time, prompting the U.S. National Science Foundation (NSF) to establish a UV monitoring program, which is now known as the "NSF Ultraviolet Spectral Irradiance Monitoring Network (UVSIMN)" (Booth et al., 1994). Similar monitoring activities also commenced in the 1980s and 1990s in Canada (Fioletov et al., 2001), Europe (e.g., Gröbner et al., 2006), New Zealand (McKenzie et al., 1999), the United States (Kaye et al., 1999), and other regions. The UVSIMN network currently consists of six sites at high latitudes and a system at San Diego, California. It has been operated by Biospherical Instruments Inc. (BSI) since 1988. An overview of network sites is provided in Table 3.1 and Fig. 3.1. The program employs SUV-100 and SUV-150B spectroradiometers, which measure global spectral irradiance between 280 nm and 600 nm with a bandpass of about 1.0 nm (SUV-100) or 0.63 nm (SUV-150B) full width at half maximum (FWHM). Results have been used in about 100 peer-reviewed publications as well as for scientific assessments of ozone depletion published by the World Meteorological Organization (e.g., WMO, 2007). A complete list of references and additional information about the network can be found in Operations Reports (e.g., Bernhard et al., 2006a) and at the project's website http://www.biospherical.com/NSF.

Table 3.1 Network sites

Site

Latitude Longitude Elevation Established Used in Study

McMurdo, Antarctica 77°50'S 166°40'E 183 m

Palmer, Antarctica 64°46'S 64°03'W 21 m

Ushuaia, Argentina 54°49'S 68°19'W 25 m

San Diego, California 32°46'N 117°12'W 22 m

Summit, Greenland 72°35'N 38°27'W 3200 m

Feb 1988 Jan 1991 - Jan 2007

Mar 1988 Dec 1989 - Jan 2007

May 1988 Mar 1990 - Apr 2006

Nov 1988 Nov 1988 - Jun 2005

Oct 1992 Oct 1992 - Aug 2006

Dec 1990 Jan 1991 - Apr 2007

Aug 2004 Aug 2004 - Nov 2006

Figure 3.1 Map of UVSIMN sites (created by Eric Gaba based on a Fuller map). Vertical lines indicate latitudes of 0°, ±20°, ±40°,and ±60°.(The Fuller Projection Map design is a trademark of the Buckminster Fuller Institute © 1938, 1967 and 1992. All rights reserved, http://www.bfi.org)

Figure 3.1 Map of UVSIMN sites (created by Eric Gaba based on a Fuller map). Vertical lines indicate latitudes of 0°, ±20°, ±40°,and ±60°.(The Fuller Projection Map design is a trademark of the Buckminster Fuller Institute © 1938, 1967 and 1992. All rights reserved, http://www.bfi.org)

In this chapter, a climatology of UV radiation at UVSIMN sites is presented. UV radiation at high latitude sites is distinct from conditions at lower latitudes due to small solar elevations; up to 24 hours of sunlight during spring and summer; extended periods of darkness during winter; the annually occurring ozone hole over Antarctica and recent episodes of severe ozone depletion over the Arctic; high surface albedo from seasonal or year-round snow and ice cover; small influence of clouds in the interior of Antarctica and Greenland due to low atmospheric water content; and small aerosol optical depth. Large changes in UV radiation due to the ozone hole can be expected, although other factors are also important. However, a confirmation of trends in UV based on measurements of the UVSIMN remains elusive. By analyzing measurements from South Pole, Palmer, and McMurdo, Bernhard et al. (2004; 2005; 2006b) concluded that linear trend estimates are by and large not significant at the 95.5% confidence level. Significant linear trends were observed only for the months of February and March at McMurdo (owing to changes in cloudiness and/or albedo), and for February at Palmer. Several factors contribute to this finding: (1) the network's operation started only in the late 1980s after the ozone hole had already been observed; (2) time-series of about 15 - 18 years are still considered short for reliable trend detection (Weatherhead et al., 1998); (3) there is a large year-to-year variability in total ozone, cloudiness, and albedo at most network sites, which obstructs the detection of possible long-term changes; (4) measurement uncertainties affect the detection of trends; and (5) the stratospheric chlorine loading (and the potential for ozone depletion) was highest at the turn of the century (WMO, 2007), approximately in the middle of UVSIMN time-series. The last factor suggests that changes in UV radiation over the period of UVSIMN operations should be described with a second-order rather than a linear function. In addition to the availability of ozone-depleting chemicals, the depth and extent of the ozone hole is also largely controlled by meteorology, planetary wave activity, and stratospheric temperatures (Rex et al., 2004; WMO, 2007). These factors show large changes from year to year. Despite these variabilities, the largest UV intensities at austral UVSIMN sites were observed in 1997 and 1998 when stratospheric chlorine concentrations were at their maximum (Bernhard et al., 2004, 2005, 2006b). While we do not attempt trend estimates in this study, we do estimate past UV levels at five network sites from historical measurements of total ozone, and contrast these estimates with the climatology established from recent measurements of the UVSIMN. This analysis documents large changes in the Antarctic UV climate that have occurred during the last 40 years.

Was this article helpful?

0 0

Post a comment