Peak UV

For some biological processes, peak UV intensity may be more important than the mean value, and while we cannot be sure about evolutionary pressures, it is possible that UV intensities were much greater during the earlier history of the planet than at present (Björn and McKenzie, 2007). However, from our current perspective, changes in UV over the past, and future, few decades are more relevant. It is likely that peak UV over these time scales has already occurred, since it is expected that ozone will continue to recover throughout the 21st century (WMO, 2007).

2.4.1 Peak UV Index

Here we examine the geographical distribution of peak UV. There is considerable geographic variability, as illustrated in Fig. 2.3 (Liley and McKenzie, 2006), which shows calculated peak values of the UVI based on over 20 years of measurements from the NASA Total Ozone Monitoring Spectrometer (TOMS)

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Figure 2.2 Global distribution of the average cloud corrected erythemal daily dose (in kJ m 2 per day) for June 2005 (upper panel) and December 2005 (lower panel) derived from OMI measurements. These surface UV irradiance images were supplied by Aapo Tanskanen of the Meteorological Institute. OMI is a joint effort of KNMI, NASA, and FMI, and is managed by NIVR/Netherlands. The unshaded (white) areas are those with no UV data coverage

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Figure 2.2 Global distribution of the average cloud corrected erythemal daily dose (in kJ m 2 per day) for June 2005 (upper panel) and December 2005 (lower panel) derived from OMI measurements. These surface UV irradiance images were supplied by Aapo Tanskanen of the Meteorological Institute. OMI is a joint effort of KNMI, NASA, and FMI, and is managed by NIVR/Netherlands. The unshaded (white) areas are those with no UV data coverage satellite-borne instruments. Generally, the peak UVI values decrease from equator to pole. However, at mid-latitudes, and even at high latitudes, peak values can approach those in the tropics. UV doses can also be exceptionally high in the high-altitude Altiplano region of South America, where the UVI can reach values of 25 during the month of February when the noon sun is approximately

Figure 2.3 Global values of the peak UVI derived from 20 years of TOMS satellite data

overhead (Liley and McKenzie, 2006). Fittingly, the location of this peak is near Cuzco, Peru (13.5°S, 72°W, alt: ~3300 m a.s.l), which was the capital of the ancient sun-worshipping Inca civilization. In locations including the city of La Paz (population ~1 million), so called "extreme" UVI values (UVI > 10) are reached on two out of three days every year (F. Zaratti, Personal communication, 2006).

These global peak UVI values are nearly a factor of two greater than at unpolluted mid-latitude, lower-altitude sites, such as Lauder, New Zealand (45°S). Although the intensities there are not extreme in a global sense, they are approximately 40% higher than at corresponding latitudes in the Northern Hemisphere, and are more reminiscent of those at latitude 5° closer to the equator and 2000 m higher in altitude (Fig. 2.4). These differences are caused by: (1) the phasing of the Earth's orbit about the sun (closest in January and furthest in July), (2) lower ozone amounts in the Southern Hemisphere summer, and (3) the generally lower pollution in the Southern Hemisphere.

Overall differences are much smaller in the winter, so in the Southern Hemisphere the summer-winter contrast in UVEry is also more marked. These huge seasonal changes in UV radiation have important implications for human health. High UV irradiance in summer contributes to skin cancer, while low irradiance in winter results in ailments associated with vitamin D deficiency. In New Zealand, for example, the skin cancer rates are among the highest in the world, yet a significant fraction of the population has insufficient vitamin D in the winter (Livesey et al., 2007). Tanning of the skin, induced by the high summertime UV

Figure 2.4 Upper panel: Peak UV at 45°S (Lauder, NZ—labeled NZ01) compared with sites in North America of comparable latitude. The high-altitude sites in North America, which tend to be at lower latitudes, are labeled (CO11, CO01, UT01). Lower panel: The sites selected, which are a subset of the USDA UV network sites maintained by the University of Colorado. The Lauder site is shown on the inverted map of New Zealand, which is superimposed over its corresponding range of latitudes in North America

Figure 2.4 Upper panel: Peak UV at 45°S (Lauder, NZ—labeled NZ01) compared with sites in North America of comparable latitude. The high-altitude sites in North America, which tend to be at lower latitudes, are labeled (CO11, CO01, UT01). Lower panel: The sites selected, which are a subset of the USDA UV network sites maintained by the University of Colorado. The Lauder site is shown on the inverted map of New Zealand, which is superimposed over its corresponding range of latitudes in North America irradiance, may further exacerbate the winter problem since the increased pigmentation in response to sun exposure blocks UV penetration into the skin and so inhibits the production of vitamin D. Other factors, such as skin type, and lifestyle choices, which in turn are influenced by parameters, such as ambient temperature, also contribute to the high rates of skin cancer in New Zealand.

2.4.2 Peak UV Daily Dose

Unlike the peak UVI values, the peak daily UV doses derived from satellite data also depend on the length-of-day and on cloud patterns. The precise geographic location for these peak UV doses depends on the temporal averaging period and the spectral weighting function in question. Table 2.1 shows peak values of monthly mean daily UV doses and their locations for several different weighting functions. In all cases, the peak values occur in the Altiplano region, although for these monthly means, the peak occurs at a slightly higher latitude—where the summer day-length is longer than for the peak instantaneous UVI shown in Fig. 2.3. Whereas the peak UVI occurred in the February/March period (close to the period when the sun moves directly overhead), the peak monthly means occur closer to the summer solstice. The daily dose of erythemally weighted UV can exceed 12 kJ m (or 120 SED (standard erythemal dose)) on cloudless days (Lee-Taylor and Madronich, 2007), which corresponds to ~50 MED (minimum erythemal dose) for fair skinned individuals. For all of the weightings listed, the peak occurs in January, generally at latitude 22.5°S. In the case of UVA radiation, which is independent of ozone, the peak value occurs at higher latitude. The peak values for the month of December are only slightly less than those for January and all occur near latitude 5°S.

Table 2.1 Global peak in UV doses, calculated from monthly values, including the effects of clouds, which in each case reduce the clear-sky values by 10% ± 1%. All peak sites are in the Atacama Desert of the Andes (up to ~5,000 m) near the border of Chile, Bolivia, and Argentina. All are south of Cuzco, where the peak UVI occurred in February. For snow-covered surfaces, doses are about 35% greater. With the alternative definition of UVB (280 nm - 320 nm), the peak doses are approximately a factor of 2 greater (137 kJm 2 per day). Data extracted from a 20-year climatology (Lee-Taylor and Madronich, 2007)

Table 2.1 Global peak in UV doses, calculated from monthly values, including the effects of clouds, which in each case reduce the clear-sky values by 10% ± 1%. All peak sites are in the Atacama Desert of the Andes (up to ~5,000 m) near the border of Chile, Bolivia, and Argentina. All are south of Cuzco, where the peak UVI occurred in February. For snow-covered surfaces, doses are about 35% greater. With the alternative definition of UVB (280 nm - 320 nm), the peak doses are approximately a factor of 2 greater (137 kJm 2 per day). Data extracted from a 20-year climatology (Lee-Taylor and Madronich, 2007)

Weighting

Month

Max Value (kJm 2day ')

Latitude (°S)

Longitude (°W)

Erythema

01 12

11.8 10.5

22.5 25.5

66.9 68.1

Vitamin D

01 12

21.3 19.0

22.5 25.5

66.9 68.1

Non Melanoma Skin Cancer (NMSC)

01 12

23.4 21.0

22.5 25.5

66.9 68.1

UVA (315 nm - 400 nm)

01 12

2220 2060

26.5 25.5

68.1 69.4

UVB (280 nm - 315 nm)

01 12

74.0 67.2

22.5 25.5

66.9 68.1

During the latter period of the springtime Antarctic ozone hole, when solar elevations are higher and day length is longer, daily UV doses at high southern latitudes can reach values comparable with mid to low latitudes, such as San Diego (Kerr et al., 2003; Bernhard et al., 2008).

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