Impact of Solar UV on the Environment

The most important effect of solar UV on the environment is its photochemical interaction with oxygen, producing ozone, a GHG (greenhouse gas). The photochemistry of ozone behaves in different ways on the biosphere, depending on its height from the ground. The behavior of ozone at ground level demonstrates itself by its damaging effects on both human health (through inhaled air), and on vegetation. Because it is a major source of atmospheric oxidants, it regulates atmospheric composition and maintains a habitable atmosphere (IPCC, 2007). At ground level, ozone is formed when nitrogen oxides (NOx) and volatile organic compounds (VOC) react with sunlight. Nitrogen oxide is a by-product of high-temperature combustion created by fire, automobile emissions, and power plants. Volatile organic compounds include organic chemicals that vaporize easily, such as gasoline. Therefore, ozone is found in higher concentrations in urban areas resulting in dangerous effects on health.

The second behavior we discuss here is in the stratosphere (Chapman, 1930). About 90% of the ozone in the earth's atmosphere resides in the stratosphere, forming the "ozone layer" which shields life on earth from harmful UV radiation. In the mid-1970s, it was recognized that anthropogenic chlorofluorocarbons (CFCs) could deplete the ozone layer. Observation of the ozone layer indicated that depletion was occurring due to the buildup of CFCs and Halons in the stratosphere.

The discovery in the mid 1980s of dramatic stratospheric ozone depletion in Antarctica, which is commonly referred to as the ozone hole, and the consequent rising of UV levels in the troposphere, resulted in increased studies into the possibility of damaging effects on human health and biosphere.

The O3 concentration, as Total Ozone Content (TOC) in Dobson Units (DU; 1 DU = 2.7 • 1016 O3 molecules • cm-2) has fallen from about 350 DU in 1978, to less than 100 during the ozone hole season (approximately from the end of August to the end of November). The historical pictures elaborated by NASA for the ozone depletion in polar regions, October 1980 through 2004 and March 1979 through 1998, respectively, are shown in Figs. 4.5 and 4.6. The more recent conditions of Antarctic are shown in Fig. 4.6(b).

TOMS Total Ozone Monthly Averages March 1079 March 1 980 March 1981 March 1082 March I1)S3

TOMS Total Ozone Monthly Averages March 1079 March 1 980 March 1981 March 1082 March I1)S3

March 1994 March 1997 March 1998

Figure 4.5 Historical picture of NASA with Arctic ozone depletion trend in March (monthly averages)

Figure 4.5 Historical picture of NASA with Arctic ozone depletion trend in March (monthly averages)

Figure 4.6 (a) Historical picture of NASA with Antarctic ozone depletion trend in October (monthly averages); (b) Antarctic ozone depletion trend, since 1980 to 2004 (October averages)

The O3 trends registered between 1975 and 2005 are shown in Fig. 4.7(a), for Belgrano, and Fig. 4.7(b) for Ushuaia. In Fig. 4.7(a), Belgrano, the O3 is displayed as October mean values measured at ground level by the Brewer spectrophotometer and the Total Ozone Mapping Spectrometer (TOMS), the sensors installed on several satellites during their flying period (1979 - 2005). Comparisons are made with the absolute minimum values registered inside the ozone hole. A negative trend is clearly evident since the 1970s, with a sharp depletion since 1980. However, a stable tendency seems to have begun since the end of the 1990s (Knudsen et al., 2004; Velders, 2008). The day of the year during 1980 to 2005 when the ozone hole opens and closes (internationally defined as ozone concentration less or greater than 220 DU) is shown in Fig. 4.8(a). The opening period increases from approximately 270 DU to 210 DU. The period of closure moves from 300 DU to 350 DU. During the final part of the period, stabilization appears in the trend. Therefore, as shown in Fig. 4.8(b), the length of the ozone hole period increases from 20 days to 120 days, showing a constant trend after 1997. Is this an effect of the Montreal Protocol enforcement or a consequence of a constraining of the vortex? The answer to this question will be a future goal of research (Egorova et al., 2001; Tabazadeh and Cordero, 2004).

The Arctic polar vortex is not as stable as the Antarctic vortex, and abrupt ingress of warm air phenomena can occur (Salby and Garcia, 1990). These sudden warmings are due to streams of air from the southern latitudes aided by very active planetary waves (Stowasser et al., 2002; Latysheva et al., 2007; Manney et al., 2008). Antarctica had higher O3 values during 1998 and 2002 when the vortex

1990 Years (b)

Figure 4.7 The O3 trends registered between the years of 1975 - 2005: (a) in Belgrano-mean values during October by Brewer (dots) and TOMS (solid line) compared with the absolute minima (dotted line) registered inside the ozone hole; and (b) over Ushuaia-monthly mean values by Total Ozone Mapping Spectrometer (TOMS); the line is a generic fit to highlight the trend

1990 Years (b)

Figure 4.8 (a) Ozone hole in Antarctica, days of the year of opening and closing. Satellite data from TOMS; and (b) ozone hole in Antarctica, length ozone hole season. Satellite data from TOMS

area was not as wide and the temperatures were warmer. In particular, a major warming in September 2002 produced an early closing of the hole as a consequence of the splitting event (GrooB et al., 2005).

These studies are necessary to understand the dynamic inside the vortex and to distinguish natural trends from anthropogenic ones (Roscoe et al., 2005b). In fact, ozone concentration is not only influenced by man-made CFCs, but also by natural phenomena connected to volcanic eruptions (Tie and Brasseur, 1995; Grainger and Highwood, 2003), Quasi Biennial Oscillation (QBO) (Garcia and Solomon, 1987; Baldwin and Dunkerton, 1998; Han et al., 2000; Sitnov, 2004), and solar activity (Labitzke and van Loon, 1997; Shindell et al., 1999; Todorovich and Vujovic, 2008). This is well depicted in Fig. 4.7(b), where the ozone trend over Ushuaia is shown. Even when dealing with a short record, the 11-year solar cycle trend appears in the data superimposed on the depletion due to CFC presence in the atmosphere. This is due to major increases of extreme UV radiation, responsible for ozone creation, compared with the larger UV wavelengths during the maximum amount of solar activity.

Moreover, the sun also influences the terrestrial atmosphere by sporadic activity (e.g., flares). After large explosions on the sun's surface, Solar Energetic Particles (SEPs) are able to enter the terrestrial magnetosphere and produce additional ionization at polar latitudes (Storini et al., 2005). During and after these events, the chemistry in the mesosphere and stratosphere changes, and the ozone concentration undergoes a large degree of variability (Damiani et al., 2006; Damiani et al., 2008) since the HOx and NOx that is produced trigger catalytic cycles of O3 destruction. Depending on the seasonal conditions, these effects can be long lasting and could alter the background values. The absence of solar radiation during the winter does not allow NO production from the chemical reactions between N2O and atomic oxygen. Moreover, the transport from the low latitudes of air masses that have a high concentration of NO is blocked by the vortex. In these conditions, caused by the descending air masses from the mesosphere to the stratosphere (Engel et al., 2005; Müller et al., 2007), the NOX produced at elevated altitudes by SEPs assumes high relevance (Randall et al., 2006; Seppala et al., 2007). However, a low percentage of TOC variability can be attributed to these phenomena since the effects are concentrated on the middle atmosphere.

These phenomena obviously happen throughout the year, but due to the dark winters in the polar regions, the absence of solar radiation limits continuous sampling by ground-based measurements or by satellite instruments. For this reason, the stratospheric ozone is occasionally sampled at ground level by spectrophotometers using the solar UV reflected by the moon disk. Of course this possibility is only viable during limited periods of the polar night, depending on both the phase of the moon disk and the air mass factor (the optical path length in the atmosphere). In other periods, the UV fluxes are too dim for the sensitivity of the instruments. The winter surface air temperatures are not as cold as in the Arctic. The minimum air temperature at ground level is higher than - 26°C; therefore, it is possible to leave instruments outdoors and to carry out ozone and sulfur dioxide measurements (Rafanelli et al., 2008). Unfortunately, the very low winter surface temperatures and the frequent presence of katabatic winds in the Antarctic make open-air measurement in the polar night a sporadic event, which can only be accomplished with ozone-sondes on balloons.

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