Exploring the Ozone Hole: Mechanisms of Stratospheric Denitrification
The ozone layer is a blanket of ozone that exists six to 30 miles above the earth's surface in the stratosphere (EPA 2001). Crucial to life on earth, it protects against the sun's harmful ultraviolet radiation. However, chemicals used in refrigerants, pesticides, and industrial solvents pose a grave threat to the earth's ozone layer. Chlorine radicals that form from chlorofluorocarbons (CFCs) react with ozone to convert it into simple diatomic oxygen, rendering it incapable of absorbing harmful UV light. High levels of CFCs, which are used in numerous industrial and household contexts, have combined with blustery weather to create a gaping hole in the layer of ozone over Antarctica. This ozone hole' is a reduction of ozone of up to 70% that occurs seasonally in the stratosphere over Antarctica. This hole reached its record size in September 2000, when it claimed more than 15 million square miles, an area as large as the continent of North America.
CFC concentrations are normally held in check by nitrogen oxides, also called NOx. Molecules of NOx deactivate the chlorine radicals that destroy ozone. Any process that depletes the stratosphere of NOx reservoirs contributes to higher levels of chlorine radicals and therefore enhances ozone depletion. While the chemistry driving ozone depletion itself is understood, the process of denitrification still remains a mystery, with two prominent theories competing for viability.
Possible Mechanisms of Denitrification
The current denitrification theories rely on Polar Stratospheric Clouds (PSCs) as the denitrification agents. It is thought that PSC particles containing nitric acid settle out of the stratosphere, thus forming a permanent sink for nitrogen-containing species. However, there are two types of PSCs, each forming under different conditions and composed of different particles-which gives rise to two theories, each one dependent upon a single type of PSC.
Type I PSCs form at a few degrees below 196 K (-77ºC) and are composed of nitric acid hydrate droplets. They are relatively small, typically having a radius less than 10-10 m (Dessler 1999), and therefore fall very slowly from the stratosphere. Because of their small size, they also are at a higher risk of evaporation and therefore have a relatively short lifetime. Type 2 PSCs are composed of ice particles. They are less abundant than Type 1 PSCs because they form only at or below the frost point, 188K (-85ºC). These particles tend to be larger, with average radii of five to 20 micrometers (10-6 m) (Dessler 1999), and therefore have more rapid sedimentation velocities and longer lifetimes (Table 1).
Any denitrification mechanism must take into account two empirical facts: First, denitrification has been observed to occur without accompanying dehydration of the atmosphere (Fahey 1990). However, if appreciable sedimentation of either type of PSC occurred, dehydration would also be observed. A viable mechanism therefore must allow for nitrogen oxides to descend from the atmosphere without causing water to leave as well. Second, denitrification has been observed during middle to late June, a relatively warm period in the Antarctic (Tabazadeh 2001). However, Type 2 PSCs do not normally exist above the frost point. A denitrification mechanism must then either rely only on Type 1 PSCs or must somehow account for the existence of Type 2 PSCs above the frost point.
Type 1 PSC-Mediated Denitrification
The small size of Type 1 PSC particles causes them to fall very slowly through the atmosphere (Dessler 1999). They are therefore at high risk of evaporating before having fallen a significant distance. Because of this probability of evaporation, it is unlikely that these particles will be able, in their natural state, to carry NOx down from the atmosphere. Therefore, if Type 1 PSCs were to denitrify the atmosphere, a few particles must somehow grow into larger particles. The process by which the small Type 1 PSC particles might attain the size necessary for stratospheric denitrification has invited attention from the scientific community.
One group of scientists, headed by R.J. Salawitch, described the composition of these larger particles as well as the conditions necessary to create them. They proposed that in areas where the temperature drops slowly with descent through the stratosphere, ice could selectively deposit on PSC particles, allowing them to grow to sizes reaching up to a micrometer. In the presence of slow cooling rates, the first ice/PSC particles to form are energetically favored to have more ice deposited on them, while the others do not grow appreciably. Eventually, the ice/PSC particles reach a size that is favorable for sedimentation out of the atmosphere. When this occurs, denitrification results. Because ice deposition occurs on only a fraction of the PSC particles, little dehydration accompanies the denitrification.
The important feature of the Salawitch mechanism is the presence of slow cooling rates. If the cooling rates through the stratospheric column were rapid, the atmosphere would quickly become supersaturated with respect to HNOx, causing condensate to be deposited indiscriminately on all PSC particles. None of the particles would reach the size necessary for sedimentation. If slow cooling rates are empirically observed, this theory is a plausible explanation for denitrification without accompanying dehydration. Furthermore, because Type 1 PSCs form above the frost point, the observation that denitrification occurs at relatively warm temperatures is explained. Other studies of Type 1 PSCs employ a similar line of reasoning (Fahey 1990; Dessler 1999).
Type 2 PSC-Mediated Denitrification
While the studies discussed above explain denitrification through Type 1 PSC sedimentation, it has been argued that the slow cooling rates necessary for selective nucleation are not present in the atmosphere (Gary 1989). Thus, the questions of how denitrification can occur above the frost point and with little dehydration still stand. A mechanism that addresses these questions using Type 2 PSCs has been presented by S.E. Wofsy and his colleagues (1990).
The fundamental concept involving this second mechanism is the idea of a coating around the Type 2 PSC particles. Wofsy and his colleagues reasoned that a coating would protect Type 2 PSC particles from relatively warm temperatures, allowing them to denitrify the atmosphere even at temperatures above the frost point. These scientists observed that a Type 2 PSC loses 80% of its initial mass upon descending 100m through the stratosphere. However, after this initial descent, its size remains roughly constant.
Wofsy hypothesized that the coating involved in this process is composed of a hydrate of nitric acid. As the surrounding atmosphere cools, the partial pressure of nitric acid exceeds its equilibrium value and it begins to condense on ice particles. In doing so, it lowers the vapor pressure of the ice, preventing its evaporation even at temperatures several degrees above the frost point.
The Wofsy mechanism meets another criterion as well: it allows for denitrification to occur without dehydration. Because only a select few the ice particles are preserved long enough to attain a sufficient size for sedimentation, the vast majority of particles remain in the stratosphere. Thus, water is not removed from the stratosphere in any significant amount. It seems possible that coated Type 2 PSC particles could denitrify the atmosphere at temperatures above the frost point and without dehydration, in accordance with observations.
More recent experiments have supported Wofsy's hypothesis. Goodman et. al. (1997) proved that nitrate exists on ice crystals found in the stratosphere. Furthermore, Tolbert and Middlebrook showed that ice particles coated with an artificially induced thin layer of nitric acid hydrate survived to temperatures four degrees higher than particles lacking the coat (1990). However, questions remain about the exact method by which the nitrate coating forms. Some have criticized Wofsy's mechanism, arguing that it results in the creation of a highly porous nitric acid coating, one that does not succeed in prolonging the particle's lifetime (Biermann et al. 1998).
While the last decade has yielded important theoretical insights into atmospheric denitrification, questions still remain. The slow stratospheric cooling rates necessary for Type 1 PSC denitrification are not always observed, and there are still questions about how a coating that effectively prolongs the lifetime of Type 2 PSCs can form (Table 2).
The danger is that a rise in CFC concentrations would cool the stratosphere such that PSCs would routinely persist for this time period, not only over the Antarctic, but over the Arctic as well. Denitrification would consequently occur extensively over both poles, allowing for the existence of high concentrations of chlorine radicals and subsequent ozone depletion. The growing hole over the Antarctic would then be joined by an Arctic ozone hole.
Fortunately, CFC emissions have decreased in recent years. Because the issue of ozone depletion is such a pressing one for public health and safety, we must strive toward heightened public awareness of the dangers of ozone depletion and continue in the trend of decreased CFC emissions. On an individual level, this means repairing leaky air conditioners and reducing use of aerosol sprays. Industry must also take on the challenge of reducing CFC emissions by using alternatives to ozone-depleting substances. The Environmental Protection Agency has listed alternatives to commonly used industrial CFCs (EPA 2001) as a means of encouraging the public to use alternative products. More broadly, international cooperation and heightened awareness of environmental health hazards are necessary to address this global problem.
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