Antarctica, ozone depletion

Heterogeneous chemistry occurring on polar stratospheric cloud particles of ice and nitric acid trihydrate has been estabUshed as a dorninant factor in the aggravated seasonal depletion of o2one observed to occur over Antarctica. Preliminary attempts have been made to parameterize this chemistry and incorporate it in models to study ozone depletion over the poles (91) as well as the potential role of sulfate particles throughout the stratosphere (92). 

Recent research suggests that ozone depletion has affected Antarctica s climate, cooling the interior and warming the extremities. 

A detailed analysis of the atmospheric measurements over Antarctica by Anderson et al. (19) indicates that the cycle comprising reactions 17 -19 (the chlorine peroxide cycle) accounts for about 75% of the observed ozone depletion, and reactions 21 - 23 account for the rest. While a clear overall picture of polar ozone depletion is emerging, much remains to be learned. For example, the physical chemistry of the acid ices that constitute polar stratospheric clouds needs to be better understood before reliable predictions can be made of future ozone depletion, particularly at northern latitudes, where the chemical changes are more subtle and occur over a larger geographical area. 

Consequences of Ozone Depletion. Ozone depletion over Antarctica is causing renewed concern about the consequences of increased levels of UV reaching the earth s biosphere. One area of concern involves the free-floating microscopic plants, known collectively as phytoplankton (the grass of the sea), which through the process of photosynthesis, fix carbon dioxide into living organic matter. Phytoplankton forms the basis of the marine food chain on which zooplankton (animal plankton) and all other components of the ecosystem depend for their sustenance. 

Effect of UV on Productivity of the Southern Ocean. Has ozone depletion over Antarctica affected the productivity of the Southern Ocean There is no easy answer. First, one has to take into account the fact that the drastic decrease of ozone over Antarctica has been reported as recently as 1976, a relatively short time in the evolution of the organisms to develop mechanisms to cope with elevated UV. One of the most vexing problems in studying the effects of UV radiation on productivity, is a dearth of historical data on the level of UV. Without these baselines, normal fluctuations could easily be interpreted as decline in productivity. Second, there is a host of biotic and abiotic factors that play significant roles in governing the productivity of the Southern Ocean (40). Ultraviolet radiation is but one more complicating factor to be considered in an already stressful environment. 

The development of the ozone hole over Antarctica is accelerated by heterogeneous catalysis on microciystals of ice. These microcrystals form in abundance in the Antarctic spring, which is when the ozone hole appears. Ice microciystals are less common in the Arctic atmosphere, so ozone depletion has not been as extensive in the Northern Hemisphere. 

Clearly, given the magnitude of ozone depletion in Antarctica in the spring, as well as its large geographical extent, an increase in UV might be expected to be most clearly observed there and indeed, this is the case, with UV increases as much as a factor of two over that... 

In the mid-1970s F. Sherwood Rowland and M. Molina showed evidence that Freons might drift freely and remain unaffected for up to 100 years, eventually move to the ozone layer, produce free chlorine atoms, and possibly destroy ozone (Molina and Rowland, 1974, pp. 810-812). They thus laid the foundation for a theory of ozone depletion and, together with Paul Crutzen of the Max Planck Institute, received the obel Prize for chemistry in 1995. In 1985 Joseph C. Farman, et al. published an article demonstrating the large volume of diminished ozone over Antarctica (Farman, Gardiner, and Shanklin, 1985, pp. 207-210). 

In the winter of 1984, massive losses of stratospheric ozone were detected in Antarctica over the South Pole (Halley Bay). This ozone depletion is known as the ozone hole. We know now that it also forms over the Arctic, although not as dramatically as in the Antarctic. Stratospheric ozone protects life on the surface of the Earth by screening harmful UV radiation coming from the sun through a photodissociation mechanism (see Chapter 4). 

Ozone depletion events have also been reported off the rim of Antarctica (Wessel, 1996 FrieB, 1997 Krecher et al., 1997 Wagner and Platt, 1998 Wessel et al., 1998 Roscoe et al., 2001). Therefore, as most impressively shown by satellite measurements of vertical column BrO (Wagner et al., 2001 Richter et al., 1998, 2002, see also Section 4.02.3.2), accompanying polar boundary layer springtime ozone depletion events are widespread. They are more pronounced in the lower parts of the boundary layer (Anlauf et al., 1994 Rasmussen et al., 1997). 

FIGURE 20.30 Worsening ozone depletion over Halley Bay, Antarctica. These measurements were all taken in the Antarctic spring (October) when depletion is at its worst. 

The possible implication of CFCs in the depletion of stratospheric ozone, postulated in 1974 and reinforced in the late 1980s by the discovery of a possible link to the thinning of the ozone layer over Antarctica during springtime, had profound effects on the fluorocarbon industry. The discovery of the ozone-depleting properties of CFCs led to the Montreal Protocol and the London and Copenhagen amendments (1990, 1992), which scheduled the end of production of CFCs by the end of 1995. 

In 1984, a remarkable and totally unpredicted phenomenon was discovered by the British Antarctic Survey, the so-called ozone hole. The discovery of ozone depletion over Antarctica during the spring period provided the first observational support for the possible effect of CFCs on the stratospheric ozone. However, the observation of ozone loss did not indicate its cause. From 1984 to 1988, several theories were postulated from CFC chemistry to atmospheric dynamics or even to cosmic electron fluxes. It was not until 1988, with the results of the 1987 Airborne Antarctic Ozone Expedition, that a probable link with CFCs was established. This prompted the Natural Resources Defense Council, an American pressure group, to sue the EPA to fulfill its 1980 promise to seek legislation to further control the manufacture and use of CFCs in the United States of America. 

Some of the first modeling efforts to quantify the effects of ozone depletion on aquatic ecosystems were with phytoplankton photosynthesis in Antarctica where the close proximity of regions of high and low ozone facilitated intercomparisons [97,98]. These studies indicated that inhibition of phytoplankton photosynthesis related to ozone depletion was <5%. In more recent years enough information has accumulated on BWFs for photosynthesis in phytoplankton that comparative summaries are possible. Work on the Rhode River, a sub-estuary of the Chesapeake Bay, has demonstrated short-term variation in the BWFs for phytoplankton, but no significant patterns of seasonal variation [99]. Comparisons among systems in this same study demonstrated striking similarities for the BWFs from the Rhode River and Antarctic phytoplankton. 

Anthropogenic emissions cause the depletion of the ozone. This is generally known through reports on the hole in the ozone layer. Although these reports referred only to the Antarctica area, ozone depletion is now also discernible to a lower degree on the mid-latitudes (e.g. Europe). 

De Zafra, R.L., M. Jaramillo, J. Barrett, L.K. Emmons, P.M. Solomon, and A. Parrish, New observations of large concentration of CIO in the springtime lower stratosphere over Antarctica and its implications for ozone-depleting chemistry. J Geophys Res 94, 11,423, 1989. 

Since about 1985, scientists have noted a precipitous drop in stratospheric ozone over Antarctica. This area of ozone depletion, known as the ozone hole, is unprecedented in the history of ozone observations. Scientists subsequently noted a similar decrease in ozone over Arctic regions, and in 1988 they detected a depletion of ozone over the United States for the first time. Three years later, scientists determined that the rate of ozone depletion was two to three times faster than originally anticipated. Many in the scientific community blame recently observed increases in cataracts and skin cancer as well as diminished plant growth on the ultraviolet radiation that has penetrated the reduced ozone layer. It has been predicted that erosion of the protective ozone layer will cause an additional 200,000 deaths from skin cancer over the next 50 years. 

---