After two decades of probing the atmosphere with a welter of instruments carried on spacecraft, weather balloons and highflying U-2 aircraft, researchers thought the case was closed: man-made chemicals known as chlorofluorocarbons (CFCs) drift into the stratosphere, break down and release chlorine molecules, which in turn attack the ozone molecules that naturally shield Earth from cancer-causing ultraviolet radiation.
             Now, a furious counterattack has caught many researchers by surprise. Former Atomic Energy Commission Chairman Dixy Lee Ray, in her book Trashing the Planet, claims that any ozone depletion that has occurred is completely natural, the result of volcanic eruptions and sea spray, not man-made chemicals.
             Most of these counterarguments are easily dismissed. Measurements in the atmosphere confirm what basic chemistry suggested all along: chlorine from most natural sources never researchers the stratosphere. Natural chlorine-bearing chemicals, such as hydrochloric acid from volcanoes and sodium chloride from sea spray, are soluble in water and are washed out of the air by rain. Measurements taken after the 1991 Mount Pinatubo volcanic eruption in the Philippines showed no increase of atmospheric chlorine.
             The exception is when volcanoes erupt with such force that they inject material directly into the stratosphere, which begins at an altitude of about 60,000 feet. The 1976 eruption of Mount St. Augustine was one such exception; it deposited 175,000 tons of chlorine, but that is still much less than the 750,000 tons released each year from man-made chemicals.
             The only natural source of chlorine that routinely survives the journey to the stratosphere is methyl chloride, which is given off by ocean plankton; measurements show it accounts for only one sixth of the chlorine in the stratosphere. All the rest comes from man-made CFCs, and stratospheric chlorine has been increasing steadily since the 1950s, from
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