Scientists report they have duplicated in the laboratory the same ice-cold reactions -- involving chemicals used in refrigeration and aerosol spray cans, ice clouds and the low antarctic sun -- that many of them say may cause the annual "ozone hole" over Antarctica.
Results of the experiments "fit very well with the observations of this last team of scientists to come back" from Antarctica, said Mario J. Molina, leader of a team from the Jet Propulsion Laboratory of the California Institute of Technology.
The Jet Propulsion team findings appear in an article in the latest issue of Science magazine, along with a companion article by a group from SRI International at Menlo Park, Calif., led by Margaret A. Tolbert.
Although it is a pollutant at low altitudes, ozone high in the stratosphere protects the Earth from the ultraviolet rays of the sun. Since the mid-1970s, ozone over Antarctica has fallen sharply every September. The "hole" has been getting deeper every year -- this year for the first time half the ozone disappeared before recovery to normal levels in October.
The laboratory work investigated the role of the high ice clouds over the antarctic continent. The ice clouds have always been there and exist nowhere else. Several scientists have theorized that the ice particles speed reactions that liberate chlorine and destroy the ozone.
The chlorine, most scientists believe, originates in the chlorofluorocarbon (CFC) compounds used since the 1930s for refrigeration fluids, foam blowing agents and, outside the United States, aerosol propellants. Major industrial nations agreed in September to cut emissions of these compounds in half by 1999.
Molina and Sherwood Rowland in 1974 were the first to suggest the danger to the ozone layer from CFC compounds. Their work led directly to the U.S. ban on CFC use as aerosol propellants in 1978.
The CFC compounds are broken down by ultraviolet light mostly over the tropics and temperate zones. The leftover chlorine is then carried to the Antarctic by such chemicals as chlorine nitrate and hydrochloric acid.
Molina's team showed that hydrochloric acid penetrates ice easily, unlike most substances, "over a distance of millimeters on a time scale of minutes."
According to the two articles, chlorine nitrate is attracted to the ice surface. There it can react in a few minutes with the ice to form nitric acid and hypochlorous acid. That, in turn, forms chlorine dioxide.
More important, the chlorine nitrate reacts with hydrochloric acid in the ice to form nitric acid and molecular chlorine, containing two chlorine atoms, "on a time scale of at most a few milliseconds," Molina's team wrote.
There is conflicting evidence as to whether ozone has been reduced over temperate zones and the tropics.
The Antarctic depletion alone has little effect on any humans there. Since the sun is so low in the sky there, the sun's rays travel slantwise through the ozone layer. Even passing through the depleted September ozone layer, those rays traverse more ozone than rays striking the United States.