Change of atmosphere
F. SHERWOOD ROWLAND
describes the discovery of ozone depletion and looks at prospects for the future
Chlorofluorocarbon (CFC) compounds were first synthesized about 70 years ago, entering commercial use soon afterwards, first in a refrigerant gas and then in aerosol propellants. Yearly production rose exponentially from the 1950s, reaching 100,000 tonnes per year in the mid-1960s. Concentrations in the atmosphere rose steadily, but stayed well below the levels at which they could be detected using the instruments of the time. So their increase went unnoticed until British scientist Jim Lovelock first found CFC-11 in Irish air at a level of approximately 50 parts per trillion by volume with an electron capture detector - an extremely sensitive instrument of his own invention. He then made extensive measurements on board ship from England to Antarctica, and determined that it was clearly detectable everywhere in the surface atmosphere.
In late 1973, intrigued by the presence of a new component of the Earth's atmosphere, Mario Molina and I began investigating the eventual atmospheric fate of CFCs. Most molecules released to the atmosphere are removed within hours to weeks by one of three general processes: by absorption of sunlight (photolysis), by dissolution in water (rainout), or by reaction with hydroxyl radical, HO, or ozone, O3 (oxidation). However, CFCs are transparent, insoluble and unreactive to atmospheric oxidizing agents; so none of these processes affects them and they can last a long time in the lower atmosphere.
Molina and I realized that these wandering CFC molecules would eventually be carried into the stratosphere by great storms in the equatorial regions. There, 25 to 30 kilometres up, they are destroyed by intense, very energetic, solar ultraviolet radiation. This UV-C radiation is not present near the Earth's surface because it is strongly absorbed by ozone in the stratosphere, and the CFC molecules have to rise above most of this before they encounter it. Ozone, the triatomic form of oxygen (O3), mostly resides where it is created, high overhead in the stratosphere, when the ordinary diatomic form of oxygen, O2, is broken into two O atoms by absorbing solar UV-C radiation. Most of these O atoms normally combine with another O2 molecule to make a molecule of ozone. Natural processes (such as reactions with other O atoms) convert the average atmospheric ozone molecule back into O2 in less than two weeks on average (the extremes range from a few minutes to many months).
Molina and I estimated in 1974 the average lifetime of a molecule of CFC-11 in the atmosphere at 40 to 80 years, and at 75 to 150 years for CFC-12. These predictions have been confirmed by measurements over the last 23 years which have shown their actual atmospheric lifetimes to be about 50 years for CFC-11 and 100 years for CFC-12.
Even the million tonnes of CFCs released per year during the 1970s and 1980s would be negligible globally if it were not for the special reactions of the chlorine atoms that are given off when they are broken down. After release in the upper part of the stratospheric ozone layer, each chlorine atom immediately attacks an ozone molecule, only to be set free again. In this way, the average chlorine atom removes about 100,000 molecules of ozone before it eventually migrates randomly down into the lower atmosphere and is itself removed.
The combination of a catalytic chain length of 100,000 with the release of about 1 million tonnes of CFCs per year puts the loss of ozone from this newly introduced anthropogenic pathway onto the same scale as the natural processes which have always regulated it. Adding CFCs reduced the average global lifetime of ozone from a normal 11 days to 10 or 9 days. The ozone layer will only return to normal when the CFCs have disappeared from the atmosphere - one or two centuries from now.
After the first burst of interest in the CFC/ozone problem during 1974 to 1977 - and the banning of CFCs in aerosol propellants for almost all uses in the United States, Canada, Norway and Sweden in the late 1970s - the issue receded from newspaper front pages back into the atmospheric science community. Most people, at least in North America, felt that the problem had been solved. The yearly world production of CFC-11 and
CFC-12 fell somewhat because of the aerosol propellant ban in those four countries, but then began to rise slowly in the early 1980s as other uses expanded. Meanwhile, the rapidly burgeoning microelectronics industry began using more and more of the cleaning agent, CFC-113, which also began accumulating in the atmosphere (with a lifetime between those of CFC-11 and CFC-12).
Both the scientific world and the public in general were taken by surprise, in 1985, by the discovery that very large losses of ozone were occurring every springtime over Antarctica. This Antarctic 'ozone hole', first noticed in about 1980, became progressively worse and now leaves only about 30 per cent as much ozone over the southern continent as during the 1950s and 1960s. It spreads over an area of about 14 million square kilometres, occurring within a few weeks after the reappearance of the sun following the darkness of the polar winter, and reaching its maximum early each October. It then remains until late November, when the ozone-depleted air is broken up and pushed northward by the more intense mid-spring sunlight.
Two scientific expeditions were sent to Antarctica in 1986 and 1987, and a third flew over the area from South America in 1987. Their measurements indicated that this remarkable loss of ozone was directly caused by the chlorine atoms released from the CFCs and other man-made organochlorine compounds, such as methyl chloroform and carbon tetrachloride. In March 1988 the World Meteorological Organization/NASA Ozone Trends Panel announced that ozone losses were now also being observed over North America, Europe and Japan - and manufacturers rapidly agreed that further CFC production should be limited. The United Nations Montreal Protocol on Substances that Deplete the Ozone Layer was agreed in September 1987. It originally provided for CFC manufacture to be reduced to half the 1986 amount by the end of the century. This was then raised to a complete phase-out at a meeting of the Protocol in London in 1990, and two years later, in Copenhagen, the deadline was brought forward to 1 January 1996. (A ten-year delay in implementing the ban has been permitted for developing countries.) Bans on further release were also imposed on methyl chloroform, carbon tetrachloride and halons (brominated molecules which also destroy ozone).
Precision measurements of the global total of the three important CFCs (-11, -12 and -113) have shown a rapid slowdown in amounts released to the atmosphere each year. The global cumulative amount of CFC-11 has already levelled off, and the total for all three of them is now barely increasing. The maximum amount of chlorine and bromine at ozone-layer altitudes will peak at about the year 2000, and then slowly decline.
The rapid loss of ozone over Antarctica has been traced to the special effects of polar stratospheric clouds (PSCs), which form during the polar winter. Chemical changes on the surfaces of the tiny particles which make up the clouds allow runaway ozone-removing reactions to take place in the early spring sunlight. The clouds form much less readily in the Arctic, but such reactions are also possible on them and substantial ozone losses have been periodically observed here too. The massive eruption of the Philippine volcano Mount Pinatubo in 1991 put large quantities of sulphuric acid particles into the stratosphere: these can also change the chlorine chemistry and result in additional ozone depletion.
Remaining concerns about depletion of the stratospheric ozone layer centre on black-market production of CFCs in Russia, the complicated chemistry of the fumigant methyl bromide (which also has natural sources and several processes that remove it in the lower atmosphere), and the changes in the structure of atmosphere temperature caused by the accumulation of greenhouse gases such as carbon dioxide, methane and nitrous oxide. Besides warming the Earth's surface, these gases cause a cooling of the lower stratosphere which has already been observed. And when methane reaches the stratosphere, as some of it does, its oxidization forms more water vapour. The combination of more gaseous water and lower temperatures in the stratosphere should make the formation of PSCs easier in the future than now, and this may increase the effectiveness of the remaining stratospheric chlorine in removing ozone.
If all goes well (i.e. there is no huge volcanic explosion, and not too much of an increase in PSCs), the ozone depletion observed in the past 20 years should be slowly reversed over many decades. During this period, we can expect greater exposures to UV-B radiation, with its risk to all species, and probably much more widespread measurement both of the amounts of UV-B and of its specific effects on individual species.
At the tenth anniversary of the Montreal Protocol, an appropriate assessment could be that of the Swedish Academy of Sciences, when announcing the Nobel Prize for Chemistry on 11 October 1995, that: 'by explaining the chemical mechanisms that affect the thickness of the ozone layer, the three researchers have contributed to our salvation from a global environmental problem that could have catastrophic consequences'
Professor F. Sherwood Rowland is currently Bren Research Professor at the University of California.