Comparison of Radiation Levels at Network Sites

There are substantial differences in the UV climatology between the various UVSIMN sites. A large portion of the differences can be traced to their geographical locations as lower latitudes experience higher sun elevations and more UV, all other factors being equal. However, the contention that low levels of UV will occur in Polar Regions because of the high latitude is shown here to not be true. Measurements from the seven network sites are presented below. The comparison is based on the average and maximum UV Index, as well as average and maximum erythemal daily dose. The results reveal that differences between the sites depend very much on the selection of the physical quantity used for the comparison.

High levels of UV radiation absorbed during short time-periods, ranging from minutes to hours, can be most detrimental for some organisms. This includes humans who may receive a "sunburn" after an exposure time of less than 20 minutes for UV Indices above 7 (Vanicek et al., 2000). The average noontime and daily maximum UV Index are therefore useful for quantifying the risk of getting sunburned. UV-B radiation is a risk factor for developing basal and squamous cell carcinoma (Moan and Dahlback, 1992), and the UV Index also provides guidance in avoiding overexposure with regard to cancer prevention. The "daily dose" is an appropriate quantity for investigating cumulative UV exposures over an extended time-period. This is relevant for plants and animals that cannot avoid the sun. This quantity, however, does not capture the impact of transient high levels of UV-B that may occur during episodic combinations of clear skies (or partly cloudy skies and high albedo) and severe ozone depletion. Such incidents may have biological significance in systems that do not obey reciprocity in terms of exposure intensity versus duration. This is particularly relevant for microorganisms, which have only a short life cycle, such as plankton (Moline et al., 1997). The maximum daily dose may be the best measure for quantifying the effect on these organisms.

Figure 3.11 shows a comparison of the noontime UV Index from the seven sites. The data are identical to those indicated by red lines in Figs. 3.4(a), 3.6, and 3.8. Measurements at San Diego exceed those at the other sites due to its lower latitude, and range between 2.5 during winter and 9.5 during summer.

Boreal month

- McMurdo

- Palmer

Usluiaia Sati Diego Barrow - Summit

UJ ASONDJ F M A M J Austral month

Figure 3.11 Comparison of average noontime UV Index from all sites

The average noontime UV Index at Palmer, Ushuaia, and Summit, observed close to the summer solstice, is about 5, or about 55% of the typical summer UV Index at San Diego. Average UV Indices for South Pole, Barrow and McMurdo extend up to 2, 3, and 4, respectively. The divergence relative to San Diego is even larger during autumn and winter when some sites experience extended periods of darkness.

The differences between sites show completely different patterns when maxima, rather than average values, are compared. Figure 3.12 shows the maximum daily UV Index, which was indicated by thin grey lines in Figs. 3.6 and 3.8. At Palmer Station, the maximum observed UV Index was 14.8. This value is 23% larger than the highest UV Index of 12.0 measured at San Diego. The maximum UV Index at

Boreal month

- McMurdo

- Palmer

Usluiaia Sati Diego Barrow - Summit

UJ ASONDJ F M A M J Austral month

Figure 3.11 Comparison of average noontime UV Index from all sites

Ushuaia was 11.5, which is comparable to summer-time values at San Diego. Maximum UV Indices at McMurdo, South Pole and Barrow are considerably smaller than at San Diego; however, the differences are considerably smaller when compared to average noontime values.

Boreal month

Boreal month

J ASOND J FMAMJ Austral month

Figure 3.12 Comparison of daily maximum UV Index from all sites, but Summit

J ASOND J FMAMJ Austral month

Figure 3.12 Comparison of daily maximum UV Index from all sites, but Summit

The picture changes again when comparing average daily erythemal doses (Fig. 3.13). The effect of 24 hours of sunlight during Arctic and Antarctic summers reduces the consequence of latitude differences. Although average summer doses at San Diego are still highest, average December UV doses at McMurdo, Palmer Station, South Pole, and Ushuaia, as well as June doses at Summit, amount to 65% - 95% of typical mid-summer San Diego conditions. Note that average daily doses at McMurdo and South Pole are very similar between mid-January and March but differ significantly in mid-November when doses at South Pole exceed those at McMurdo by up to 35%. The reasons are threefold: first, the influence of the ozone hole on UV levels is more pronounced at South Pole than at the Antarctic coast; second, the solar elevation at South Pole is constant for 24 hours;

Figure 3.13 Comparison of average daily erythemal dose from all sites and third, albedo at McMurdo is at its annual minimum during January and February. Average daily erythemal dose at Barrow is the lowest of all network sites mostly because ozone depletion in the northern hemisphere is much less severe than over Antarctica.

The importance of UV radiation for high latitudes becomes most obvious when comparing maximum daily doses. Figure 3.14 shows that the largest daily erythemal doses ever measured at South Pole and Palmer are 18% and 32%, respectively, higher than the San Diego record. Maximum doses at McMurdo and Ushuaia are comparable to San Diego levels. Note that the difference between McMurdo and South Pole is much smaller when maximum daily doses rather than maximum noontime UV Indices (Fig. 3.11) are compared. We attribute this to the difference in the diurnal cycle of the sun at these sites: at McMurdo, radiation levels peak at local solar noon whereas at the South Pole, there is virtually no change in solar elevation during a given 24-hour period.

Austral month

Figure 3.14 Comparison of maximum daily erythemal dose from all sites, but Summit

Austral month

Figure 3.14 Comparison of maximum daily erythemal dose from all sites, but Summit

Boreal month M J J A S

E

2500

2000

c

■r. ©

1500

"O

1000

a

500

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Boreal month M J J A S

Winter : Spring : Summer

Autumn

-

Figure 3.15 Comparison of average daily UV-A dose from all sites

McMurdo Palmer South Pole Ushuaia San Diego Barrow Summ it

Figure 3.15 Comparison of average daily UV-A dose from all sites

The effect of 24 hours of sunlight is best shown when comparing the average daily UV-A dose (Fig. 3.15). Since UV-A spectral irradiance is practically independent of atmospheric ozone concentrations, there are no ozone-related features in this figure. UV-A doses at the summer solstice at McMurdo, Summit, and South Pole exceed San Diego doses by 71% - 88%. In addition to 24 hours of sunlight, this difference can be explained by high surface albedo, and in the cases of South Pole and Summit, high altitude.

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