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ScienceWeek
EARTH SCIENCE: ON THE OZONE HOLE
The following points are made by Susan Solomon (Nature 2004 427:289):
1) The discovery in 1985 of an unprecedented "hole" in Antarctica's ozone layer heralded the beginning of one of the most influential environmental stories of the late 20th century(1). The tale of the missing ozone at the bottom of the world was made all the more intriguing by its strong seasonal behaviour. Ozone levels decline rapidly each year in August and September (Antarctic spring), and the hole is typically at its deepest by late September to early October. It then largely fills in through mixing of surrounding ozone-rich air by late January, ready for the next year's cycle.
2) Each Antarctic spring in the late 1980s and early 1990s, enthusiastic scientists shared the news about the hole with a fascinated and equally enthusiastic public. Slowly, the mystery surrounding events in the Great White South evolved into understanding for both groups(2). The root cause of the hole was identified as a number of industrially produced chemicals. Policy-makers worldwide quickly agreed on the Montreal Protocol to phase out these chemicals, and by the late 1990s global production of these gases had dropped by more than 90%.
3) The ozone hole has been considered by many to be the success story of global environmental policy ever since the Montreal Protocol came into force. But in the past few years there has been concern about a public that seems to be more confused than intrigued by the news reports that greet the appearance of the hole each year. Despite the apparent success of the Montreal Protocol, the elimination of the ozone hole may seem to progress at a remarkably uneven rate. And as the hole changes in size and shape, communicating to the public what this means has become much harder.
4) The author suggests that this presents the scientific community with a new challenge. Given that global production of ozone-damaging compounds is now nearing zero, yet ozone depletion will continue for many years to come, how can scientists help to preserve the remarkable success in public understanding of the ozone hole? How can we best explain why this is so in language understandable to students, teachers, scientific colleagues in other fields -- indeed to everyone who shares our interest in a phenomenon that is not just scientific but also historic, not just technical but also sociological? How do we distinguish between what is news and what is not as the ozone hole moves into an expected period of very slow recovery? Are there any observable connections between the behaviour of the ozone hole and whether or not the Montreal Protocol is working? If not, how can we tell if the protocol is on or off track?
5) Within five years of the ozone hole's discovery, the cause had been established. Direct observations of chemicals in the atmosphere showed that an increase in chlorine concentrations in the stratosphere was the key agent responsible(3). This rise was mainly due to chlorofluorocarbons (CFCs) -- long-lived chemicals produced by the chemical industry and used variously as, for example, coolants in refrigerators and air conditioners, foam-blowing agents and solvents.
6) The chemical reactions that destroy ozone can occur with unparalleled efficiency in Antarctica because of the very cold conditions in the polar stratosphere during the winter and spring months. These allow special high-altitude clouds to form as soon as temperatures dip below -85 C (4). Cold surfaces within these clouds serve as the sites for very rapid reactions that convert inactive chlorine into its ozone-destroying forms. The hole appears during the Antarctic spring because the key ozone-destroying reactions are initiated by sunlight, a factor largely absent during winter months.
7) Why is there no Arctic hole? Ultracold temperatures are far more widespread and persistent in the Antarctic winter and spring atmosphere than in the Arctic, where the flow of air over the Himalayas and Rocky Mountains and land–sea temperature contrasts can generate very large "atmospheric waves". On the ground, we experience many of these waves as the passage of storms, and some travel upwards to the stratosphere, ultimately mixing warmer mid-latitude air with cold polar air. So the varied topography of the Northern Hemisphere gives it a greater number of atmospheric waves and a warmer polar stratosphere in the winter and spring on average than in the south(5). In brief, the ozone hole is driven by a blend of three factors: excess chlorine, which is why the ozone hole is a recent phenomenon; cold temperatures, which account for its occurrence in Antarctica; and sunlight, which explains why the hole opens up in spring, as light returns to the polar cap.
References (abridged):
1. Farman, J. C., Gardiner, B. G. & Shanklin, J. D. Nature 315, 207–210 (1985)
2. Cagin, S. & Dray, P. Between Earth and Sky (Pantheon Books, New York, 1993)
3. Scientific Assessment of Ozone Depletion: 1991 (Rep. 25, World Meteorological Organization, Geneva, 1991)
4. Solomon, S., Garcia, R. R., Rowland, F. S. & Wuebbles, D. J. Nature 321, 755–758 (1986)
5. Scientific Assessment of Ozone Depletion: 1998 (Rep. 44, World Meteorological Organization, Geneva, 1998)
Nature http://www.nature.com/nature
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ON STRATOSPHERIC OZONE
The following points are made by D.W. Fahey and A.R. Ravishankara (Science 1999 285:208):
1) Average ozone concentrations in the polar stratosphere show a pronounced cyclical variation over the course of the year. In winter and early spring, ozone builds up at the poles as ozone-rich air is transported from lower latitudes toward the polar regions. But when transport to high latitudes slows and solar illumination increases in late spring and summer, catalytic ozone destruction leads to a substantial decrease (approximately 30 percent).
2) Ozone is produced via solar ultraviolet photolysis of oxygen and destroyed through catalytic cycles involving reactive nitrogen, halogen (chlorine and bromine), and hydrogen species.
3) The balance between photolytic production, transport, and chemical destruction determines the abundance of ozone at any particular stratospheric location. This balance is also strongly season dependent. In addition, the relative contributions of the 3 types of catalytic destruction of ozone differ between the summer and winter-spring seasons.
4) During the summer, large regions of the polar stratosphere receive uninterrupted sunlight for many weeks. Photolysis reactions, several of which are complete ozone destruction cycles, occur continuously under these conditions. Total ozone concentrations therefore continuously decrease throughout high latitudes in late spring and early summer.
5) The authors suggest that we now understand in some detail how the combined effects of transport, chemical ozone production, and catalytic ozone loss control ozone during the annual cycle of stratospheric conditions. The summer ozone decreases at high latitudes will persist in the future because natural reactive nitrogen rather than human-induced reactive halogen species are primarily responsible for ozone destruction in those regions. In contrast, the winter-spring ozone destruction will gradually lessen in the next decades as halogen emissions steadily decrease -- barring other changes to the stratosphere such as major cooling of this region due to greenhouse gases.
Science http://www.sciencemag.org
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OZONE DEPLETION AND PLANT DNA DAMAGE
Ozone [O(sub3)] is a compound formed by several means, including a) exposure of oxygen gas to ultraviolet radiation; and b) the passage of electric sparks through air. Ozone is a blue gas and a blue-black solid and liquid, melting point -193 degrees centigrade, boiling point -112 degrees centigrade. Ozone gas is present in only trace quantities in the atmosphere of Earth: if all the ozone in the atmosphere were brought down to sea level, the layer of ozone would be only approximately 4 millimeters thick. Nevertheless, ozone in the outer atmosphere (lower stratosphere; 15 to 30 kilometers above the surface) acts to shield the Earth from excessive radiation, particularly ultraviolet radiation of 280 to 315 nanometers wavelength (UV-B), the UV band that is most dangerous to living systems.
2) UV-B radiation is lethal to simple unicellular organisms (algae, bacteria, protozoa), and to the surface cells of higher plants and animals. UV-B radiation damages DNA and is responsible for sunburn in human skin. In addition, the incidence of skin cancer in humans has been statistically correlated with the observed surface intensities of the UV wavelengths between 290 and 320 nanometers which are not totally absorbed by the ozone layer. In the lower atmosphere (*troposphere), ozone forms from combustion gases and is a major air pollutant contributing to *photochemical smog. Since the discovery in 1985 that an ozone hole develops over the Antarctic in late winter and early spring, intense research efforts have been devoted to clarifying the roles of atmospheric transport and chemistry in stratospheric ozone changes.
The following points are made by M.C. Rousseaux et al (Proc. Nat. Acad. Sci. 1999 96:15310):
1) The most important consequence of the depletion of stratospheric ozone is the increased transmission of solar UV-B radiation to the Earth's surface. Present levels of stratospheric ozone are at the lowest point since the measurement began in the 1970s. Ozone depletion is most pronounced over the Antarctic continent, where ozone levels commonly decline by more than 70 percent during late winter and early spring. Acute effects of ozone depletion on native organisms have been documented only for marine ecosystems of Antarctic waters. For example, it has been shown that increased UV-B can reduce *phytoplankton photosynthesis in the marginal ice zone when the ozone hole is overhead, reduce phytoplankton cell densities, and increase the DNA damage burden in *icefish eggs. Virtually nothing is known about the consequences of ozone depletion and increased solar UV-B on natural ecosystems located outside Antarctica.
2) The authors report that the temperate ecosystems of southern South America have been subjected to increasingly high levels of ozone depletion during the last decade. In the spring of 1997, despite frequent cloud cover, the passages of the ozone hole over Tierra del Fuego (latitude 55 degrees south) caused concomitant increases in solar UV, and the enhanced ground-level UV led to significant increases in DNA damage in the native plant Gunnera magellanica (a perennial herb). The fluctuations in solar UV explained a large proportion (up to 68 percent) of the variation in DNA damage, particularly when the solar UV was weighted for biological effectiveness according to *action spectra that assume a sharp decline in *quantum efficiency with increasing wavelength from the UV-B into the UV-A regions of the spectrum.
3) The authors conclude: "Our data indicate that the UV variations that take place during early spring, which to a large extent are caused by ozone depletion, result in corresponding changes in DNA damage density in naturally occurring individuals of G. magellanica... The high correlation in the present case is probably due to the fact that, as a photosynthetic organism, G. magellanica is obligatorily exposed to sunlight, and therefore to solar UV. Animals may afford to colonize habitats less exposed to radiation, and mobile forms may even actively seek shelter in response to high UV-B or to environmental variables that correlate with UV-B radiation levels."
Proc. Nat. Acad. Sci. http://www.pnas.org
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Notes:
troposphere: The term "troposphere" refers to the lowest 10 to 20 kilometers of the atmosphere (with the lower boundary the surface of the Earth).
photochemical smog: Air pollution in the form of a brown haze often seen over cities, occurring on sunny days in locations with large volumes of automobile traffic. Such smog is produced when sunlight acts on nitrogen oxides, ozone, and hydrocarbons. Photochemical smog is a respiratory irritant in man, and can kill or alter plant tissues.
phytoplankton photosynthesis: In general, the term "photosynthesis" refers to the series of chemical reactions by which plant cells transform light energy into chemical energy through the production of various compounds and oxygen from carbon dioxide and water. Phytoplankton (photoplankton) are small, usually microscopic, aquatic plants capable of photosynthesis; e.g., unicellular algae. Phytoplankton and plankton are not equivalent. The term "plankton" is a general designation for various drifting microscopic aquatic organisms in the upper regions of the oceans, both photosynthetic and non-photosynthetic.
icefish: A member of the family Salangidae; small teleost fishes. A "teleost fish" is one of a group of bony fish, with over 17,000 different species ranging from eels to trout.
action spectra: In general, an "action spectrum" is a graph showing the range of wavelengths over which a photochemical reaction occurs. The action spectrum indicates which wavelengths of light are most effective for driving the reaction.
quantum efficiency: In the context of a radiation-induced process, the term "quantum efficiency" refers to the actual number of species which are decomposed or reacted per quantum of energy absorbed.
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