Beautiful mother-of-pearl clouds have been observed for centuries at high latitudes over Scandinavia (Figure 1) . The clouds are glimpsed perhaps once a year on those rare winter days when stratospheric temperatures are exceptionally cold. These clouds were thought to be little more than a colorful curiosity until the 1980s when the Antarctic ozone hole was discovered and satellites showed that polar stratospheric clouds (PSCs) formed much more frequently than observers on the ground had previously thought. Researchers at NASA Ames were the first to suggest that PSCs were composed of nitric acid particles instead of water ice crystals. A series of polar expeditions since 1987 has confirmed that the condensation of nitric acid and the adsorption of hydrogen chloride onto the PSC surfaces are critical ingredients in the formation of the ozone hole.
Significant polar ozone loss requires PSCs to be present while low temperatures (below -80 degrees C) are needed to form the clouds. Our research at NASA Ames has shown that there are many possible types of PSCs. Figure 2 illustrates the process by which various types of particles form in the stratosphere as the temperature cools. Generally, stratospheric sulfate aerosols serve as the nuclei, or foundation, for the condensation of PSCs. Sulfate aerosols in the stratosphere are either composed of aqueous sulfuric acid solutions or are frozen into crystalline sulfuric acid tetrahydrate (SAT). Our research shows that if the sulfate aerosols are liquid then nitric acid vapor dissolves in the aerosols forming ternary droplets of nitric acid-sulfuric acid-water. However, if the sulfate aerosols are frozen, then deposits of nitric acid vapor on SAT produces an amorphous (glassy) solid solution of nitric acid. Both ternary and amorphous solutions of nitric acid may then transform into a crystalline phase of nitric acid and water, such as nitric acid trihydrate (NAT). These nitric acid-containing particles can also serve as nuclei for the formation of ice crystals when temperatures dip below the ice frost point. As the stratosphere warms some of these transformations can be reversed.
Discovering the conditions controlling which of these compounds occurs in PSCs is essential for evaluating future changes in stratospheric ozone levels. During the next century the lowering of stratospheric temperatures due to the greenhouse effect and increased concentrations of nitrogen from fleets of high flying supersonic aircraft could modify the frequency of PSC formation, leading to enhanced ozone loss.
For more informtion about stratospheric ozone research at Ames Research Center, click here.
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