In terms of possible reduction in the ozone layer, erupting volcanoes are the biggest "spray cans" on earth. US Geological Survey vulcanologist David A. Johnson estimates that the amount of ozone-destroying chlorine shot into the stratosphere by volcanoes rivals that due to the breakdown of chlorofluorocarbon propellents from aerosol cans.
Reporting his research in the journal Science, he says that atmospheric chemists cannot understand the possible human impact on the ozone layer until the impact of volcanoes has been properly assessed.
Dr. Johnston has made general estimates of the likely chlorine release by volcanoes and a specific estimate for the 1976 series of eruptions of Augustine Volcano in Alaska. The amount of stratospheric chlorine from Augustine -- somewhere between 82 and 175 million kilograms -- ws "equivalent to 17 to 36 percent of the 1975 world industrial production of chlorine in fluorocarbons," Dr. Johnston says. He also estimates that chlorine injection by the Bishop Tuff eruption in California 700,000 years ago may have been equivalent to 570 times the 1975 fluorocarbon production.