<P> In 1970 Paul Crutzen pointed out that emissions of nitrous oxide (N O), a stable, long - lived gas produced by soil bacteria, from the Earth's surface could affect the amount of nitric oxide (NO) in the stratosphere . Crutzen showed that nitrous oxide lives long enough to reach the stratosphere, where it is converted into NO . Crutzen then noted that increasing use of fertilizers might have led to an increase in nitrous oxide emissions over the natural background, which would in turn result in an increase in the amount of NO in the stratosphere . Thus human activity could affect the stratospheric ozone layer . In the following year, Crutzen and (independently) Harold Johnston suggested that NO emissions from supersonic passenger aircraft, which would fly in the lower stratosphere, could also deplete the ozone layer . However, more recent analysis in 1995 by David W. Fahey, an atmospheric scientist at the National Oceanic and Atmospheric Administration, found that the drop in ozone would be from 1--2 percent if a fleet of 500 supersonic passenger aircraft were operated . This, Fahey expressed, would not be a showstopper for advanced supersonic passenger aircraft development . </P> <P> In 1974 Frank Sherwood Rowland, Chemistry Professor at the University of California at Irvine, and his postdoctoral associate Mario J. Molina suggested that long - lived organic halogen compounds, such as CFCs, might behave in a similar fashion as Crutzen had proposed for nitrous oxide . James Lovelock had recently discovered, during a cruise in the South Atlantic in 1971, that almost all of the CFC compounds manufactured since their invention in 1930 were still present in the atmosphere . Molina and Rowland concluded that, like N O, the CFCs would reach the stratosphere where they would be dissociated by UV light, releasing chlorine atoms . A year earlier, Richard Stolarski and Ralph Cicerone at the University of Michigan had shown that Cl is even more efficient than NO at catalyzing the destruction of ozone . Similar conclusions were reached by Michael McElroy and Steven Wofsy at Harvard University . Neither group, however, had realized that CFCs were a potentially large source of stratospheric chlorine--instead, they had been investigating the possible effects of HCl emissions from the Space Shuttle, which are very much smaller . </P> <P> The Rowland--Molina hypothesis was strongly disputed by representatives of the aerosol and halocarbon industries . The Chair of the Board of DuPont was quoted as saying that ozone depletion theory is "a science fiction tale...a load of rubbish...utter nonsense". Robert Abplanalp, the President of Precision Valve Corporation (and inventor of the first practical aerosol spray can valve), wrote to the Chancellor of UC Irvine to complain about Rowland's public statements . Nevertheless, within three years most of the basic assumptions made by Rowland and Molina were confirmed by laboratory measurements and by direct observation in the stratosphere . The concentrations of the source gases (CFCs and related compounds) and the chlorine reservoir species (HCl and ClONO) were measured throughout the stratosphere, and demonstrated that CFCs were indeed the major source of stratospheric chlorine, and that nearly all of the CFCs emitted would eventually reach the stratosphere . Even more convincing was the measurement, by James G. Anderson and collaborators, of chlorine monoxide (ClO) in the stratosphere . ClO is produced by the reaction of Cl with ozone--its observation thus demonstrated that Cl radicals not only were present in the stratosphere but also were actually involved in destroying ozone . McElroy and Wofsy extended the work of Rowland and Molina by showing that bromine atoms were even more effective catalysts for ozone loss than chlorine atoms and argued that the brominated organic compounds known as halons, widely used in fire extinguishers, were a potentially large source of stratospheric bromine . In 1976 the United States National Academy of Sciences released a report concluding that the ozone depletion hypothesis was strongly supported by the scientific evidence . In response the United States, Canada and Norway banned the use of CFCs in aerosol spray cans in 1978 . Early estimates were that, if CFC production continued at 1977 levels, the total atmospheric ozone would after a century or so reach a steady state, 15 to 18 percent below normal levels . By 1984, when better evidence on the speed of critical reactions was available, this estimate was changed to 5 to 9 percent steady - state depletion . </P> <P> Crutzen, Molina, and Rowland were awarded the 1995 Nobel Prize in Chemistry for their work on stratospheric ozone . </P>

Rate of ozone depletion is greater in antarctica