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Concentration of ozone-depleting substances, 1980-2002

The concentration of ozone-depleting substances in the atmosphere continues to fall. Recovery of the ozone layer is only expected to become apparent after 2010.

Fall in concentration of ozone-depleting substances continues


The total amount of ozone-depleting substances in the atmosphere is gradually decreasing. This is the result of the reduction in emissions of substances containing chlorine. The potential bromine concentration is still rising because of continuing emissions of substances containing bromine (WMO, 2002).
The potential chlorine concentration in the lower atmosphere increased in the past from a natural background level of 0.6 ppb to more than 3.5 ppb in 1993 (see figure 1). Since then, the concentration in the lower atmosphere has gradually decreased (Montzka et al., 1999; Prinn et al., 2000).

The effect of the Montreal Protocol


The reduction of the chlorine concentration is a result of international measures set out in the Montreal Protocol (UNEP, 1987-2000). In particular, the fall in concentrations of methyl chloroform (CH3CCl3) (see figure 2) resulted in a relatively rapid drop in potential chlorine concentrations during the nineties. The rate at which potential chlorine continues to decrease is falling because the other substances containing chlorine remain in the atmosphere much longer than methyl chloroform.

Increase in HCFC concentration


The concentration of HCFCs has risen sharply in recent years (see figure 2) but their contribution to ozone depletion is still small. HCFCs reach the stratosphere less easily than CFCs and therefore have a less destructive effect on stratospheric ozone. These HCFCs can therefore be used temporarily as substitutes for CFCs; in industrialised countries until 2030 at the latest. Both CFCs and HCFCs are covered by the Montreal Protocol.

What is potential chlorine concentration and potential bromine concentration?


The potential chlorine concentration and potential bromine concentration in the lower atmosphere are indicators of the amount of ozone-depleting substances which have already been released but which have not yet reached the stratosphere. Ozone-depleting substances reach the stratosphere a few years after they are released. Once they reach the stratosphere, they break up into smaller molecules that can attack the ozone molecules. Halons and CFCs are released after a delay of many years from stocks of existing products.

Initial recovery of ozone layer expected after 2010


It is probable that the peak concentration of ozone-depleting substances in the stratosphere has now been almost reached. As a result, the ozone layer may recover in time. However, this recovery is not expected to become apparent before 2010. Complete recovery will take more than 50 years.

The increase in greenhouse gases in the atmosphere cools down the stratosphere and may have an effect on the recovery of the ozone layer. It is expected that, as a result, ozone will be broken down less quickly in the higher stratosphere. But as a result of uncertainty about the effect of reduced temperatures on ozone in the lower stratosphere, the effect on the ozone layer as a whole is not yet properly understood. The effect may be either positive or negative (WMO, 2002).

The Montreal Protocol for substances that deplete the ozone layer


International policy aims to restrict or stop the production and use of ozone-depleting substances. This objective was set out in 1987 in the Montreal Protocol (UNEP, 1987-2000). The countries that signed the protocol have undertaken to use no more chlorinated fluorocarbons (CFCs) from 1996 onwards. For developing countries, this obligation will apply from 2010 onwards. The protocol permits the use of HCFCs for a number of years to come.
As a result of the protocol, global production and use of ozone-depleting substances have fallen sharply in the last ten years. The Montreal Protocol can therefore be described as a major success. The measures taken are intended to result in the recovery of the ozone layer (so that it becomes thicker), restoring the situation in 1980

Concentration measurement methods


Concentrations of ozone-depleting substances are measured in a limited number of locations on earth. This sites are selected in such a way that they are far away from the sources. This makes them representative for large areas. The global average is calculated as the average for these sites.

References


  • Fraser, P. J. et al. (1999). Southern Hemisphere halon trends (1978-1998) and global halon emissions. Journal of Geophysical Research, vol. 104, 15985-15999.
  • Montzka, S.A., J.H. Butler, et al.(1999). Present and future trends in the atmospheric burden of ozone-depleting halogens. Nature, vol. 398, pp 690-694.
  • Oram, D. E. et al. (1995). Measurements of HCFC-142b and HCFC-141b in the Cape Grim air archive: 1978-1993. Geophys. Res. Lett. vol. 22, pp. 2741-2744.
  • Prinn, R.G., R.F. Weiss, et al. (1997). The ALE/GAGE/AGAGE database, DOE-CDIAC World Data Center, Dataset No. DB-1001, 1998.
  • UNEP The Montreal Protocol on substances that deplete the ozone layer (with amendments).
  • WMO (2003) (World Meteorological Organization): Scientific Assessment of Ozone Depletion: 2002. Global ozone research and monitoring project - report no. 47, Geneva, 2003.

Relevant sections and indicators in the Environmental Data Compendium


Relevant information outside of the Environmental Data Compendium
This page was last changed on November 22, 2005  (version 01).