Science: Cold is the key to ozone loss

 作者:季镫     |      日期:2019-03-02 05:03:05
By JOHN GRIBBIN Destruction of ozone in polar stratospheric clouds (PSCs) in winter does not depend on the type of particles in those clouds, but almost entirely on temperature. Studies have shown that the reactions that convert chlorine into an active form that destroys ozone will take place on the surface of particles in the clouds as the temperature drops below 200 K, regardless of the composition of the particles (Science, vol 161 p 1418). The chlorine in the destructive compounds comes from chlorofluorocarbons (CFCs) released by human activities. Over most of the stratosphere for most of the year, the chlorine is locked up in stable compounds. But the presence of cold, icy particles in polar stratospheric clouds acts as a catalyst for reactions that release chlorine in an active, ozone-destroying form. Active chlorine is released during the cold winter months, and builds up in the polar stratosphere. It reacts with ozone in spring, when sunlight becomes available to trigger photochemical reactions. But the returning sunlight also warms the stratosphere, evaporating the particles in the PSCs and preventing the release of more active chlorine. So the severe ozone depletion happens only in the spring. PSC particles come in different varieties. Some are made of nitric acid trihydrate, others are thought to be nitric acid dihydrate, or liquid solutions of nitric acid, or more exotic compounds. Researchers had thought that the exact composition of aerosol particles in the PSCs might be important in ozone depletion; but the study, carried out by Mario Molina and colleagues at the Massachusetts Institute of Technology, suggests this is not so. The MIT team finds that the key to chlorine activation is that temperatures should be low enough for hydrogen chloride to begin to condense on the particles. The hydrogen chloride forms an extremely thin semi-liquid layer in which further reactions take place, ultimately releasing chlorine gas. These reactions take place in the laboratory on a wide variety of particles, including the compounds thought to be present in PSCs. The particles simply provide a large surface area on which the reactions can take place and their identity is immaterial. One implication is that particles entering the stratosphere after large volcanic eruptions may trigger ozone depletion in the same way that PSCs do,