The Ozone Problem, CFCs and Alternatives

Dimension of the Ozone Problem

Graph showing minimum levels of Antarctic ozone in Dobson Units (DU) from 1979 to 1996. (From Ministry for the Environment) A Dobson Unit is a basic measure used in ozone research. It is named after G.M.B. Dobson, who designed the 'Dobson Spectrometer' - the standard instrument used to measure ozone from the ground. The Dobson spectrometer measures the intensity of solar UV radiation at four wavelengths, two of which are absorbed by ozone and two of which are not. For more on the Dobson Unit go to: Total Ozone Mapping Spectrometer.
Scientific evidence accumulated over more than two decades of study by the international research community has shown that human-produced chlorofluorocarbons (CFCs) are responsible for observed depletions of the ozone layer over Antarctic (the ozone hole; recall Section 2.7). CFCs are a family of human-made compounds (chlorofluorocarbons), which are useful in many industrial applications (such as refrigeration, air conditioning, foam blowing, cleaning of electronic components, solvent cleaning, sterilization, fire extinguishing, coatings, paints, and aerosols). These compounds have done much damage to the ozone layer. By 1994, the average total Antarctic ozone level was less than half what it was in the 1970s. By 1998, Antarctic ozone levels reached record lows. In addition, CFCs are volatile and are extremely powerful greenhouse gases.

UV Radiation on the Rise
Ozone effectively shields the Earth against ultraviolet radiation by soaking-up UV(B) rays and preventing them from reaching the Earths surface. Not surprisingly, therefore, ground levels of harmful UV radiation have climbed as ozone levels have fallen. Most of the problems with UV(B) radiation are confined to the higher latitudes. In addition to harmful effects on humans, like an increases skin cancer, increased UV radiation in humans and stress on plants and land animals. Australia, for example, noted a dramatic rise in skin cancer rates linked to the rise in average daily levels of UV radiation.

The Montreal Protocol
In 1987, an international agreement known as the Montreal Protocol set a target of reducing the global production of CFCs by 50% by 1998. A further agreement was made in 1992 in to phase-out production of all CFCs in developed countries by 1996 and in developing countries by 2010. These measures have already made a difference: The total concentration of chlorine in the lower atmosphere that can be carried to the stratosphere has already peaked and is starting to steadily decrease.

The amount of ultraviolet radiation reaching a particular spot on Earth depends on several factors including the position of the Sun, amount of cloud cover and pollution and ozone levels. This table shows the relationship between ozone loss and UV(B) increase for different positions of the Sun (i.e. seasonal and hemisphere changes) assuming that cloud cover and pollution are the same. (From: WMO, UNEP, NOAA, NASA and EC (1998).)
CFCs are long-lived however (remaining in the atmosphere over time periods of 10-100s of years), and will continue to impact the ozone layer. Also, they will contribute to the enhanced greenhouse effect for some time.

Compounds that are analogous to CFCs in their industrial uses, but also ozone-friendly are continuously being invented and applied (for example, bromofluorocarbons). Among these there may be some with unfavorable properties for atmospheric chemistry. Hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) are categories of synthetic chemicals that are being used as alternatives to the ozone depleting substances. Because HFCs and PFCs do not directly deplete the stratospheric ozone layer, they are not controlled by the Montreal Protocol. These compounds, however, along with sulfur hexafluoride (SF6), are potent greenhouse gases. In addition to having high global warming potentials, SF6 and many HFCs and PFCs have extremely long atmospheric lifetimes, resulting in their essentially irreversible accumulation in the atmosphere. Sulfur hexafluoride, so far, is the most potent greenhouse gas the IPCC has evaluated. The story of the Montreal and Helsinki agreements illustrates a number of processes that are important in guessing future developments, as follows:
  1. The modern chemical industry is capable of producing a great number of highly useful substances, which have never before entered the environment and whose potential effects are entirely unknown and unsuspected. Such substances, as long as they seem without harm to people and other living things, will be tested in the real environment at the risk of the entire community.
  2. If substantial harm is detected, especially to people, the world community will in fact find the political will to respond, even at some economic cost. The danger must be serious and well defined, not too far into the future, and the remedy must not be too costly in terms of economic hardship.
  3. The remedy of ceasing emissions will not immediately alleviate the problem that has developed already. Only future additional damage is averted when it is applied. Recovery can be a long-term process (as it is in this case).

Measurements of atmospheric chlorofluorocarbon -11 (CFC-11) concentrations in the lower atmosphere versus time. Notice the slow but steady decrease in the growth rate of this substance after the year 1994. The data were first published in Nature 364, 783-786 (1993).