Atmospheric Chlorine

The transition from CFCs to alternatives, including HCFCs, reduces atmospheric chlorine loading. Thus, the use of alternatives can reduce the risk of stratospheric ozone depletion.

Significant progress has been made to phase out CFCs and reverse the trend of increasing chlorine in the atmosphere.

HCFCs and HFCs are necessary to allow the rapid elimination of CFCs.

A rapid phase out of production - and hence of consumption and release - of CFCs is necessary to reverse the trends of increasing concentrations of atmospheric chlorine and increasing ozone depletion. The international scientific and technical experts conducting assessments for the United Nations Environment Programme (UNEP) have stated that availability of HCFCs and HFCs is necessary to allow a rapid, global CFC phaseout.

"It is technically feasible to virtually phase out controlled substances [CFCs, carbon tetrachloride, halons, and methyl chloroform] in developed countries by 1995-1997. This, however, depends on the extent of recycling and technical feasibility of equipment retrofit, on the availability of HCFC and HFC replacements and on their toxicological and environmental acceptability, on a regulatory regime which allows profitable investment in their production....."⁽¹⁾

"HCFCs are technically and economically necessary for the transition in:
        - the majority of refrigeration and air conditioning applications;
        - manufacture of insulating foams;
        - selected and limited solvent applications; and
        - certain fire protection applications where space constraints exist."⁽²⁾

The HCFCs and HFCs are considered environmentally superior to CFCs because they are largely destroyed in the lowest region of the atmosphere. The HFCs do not contain chlorine and have no potential to deplete ozone.⁽³⁾ HCFCs, however, do contain chlorine, but only a small percentage of that chlorine can affect the ozone layer; this is because most of the HCFCs released at ground level are destroyed in the lower atmosphere before they reach the stratospheric ozone layer.
An index called the Ozone Depletion Potential (ODP) has been adopted for regulatory purposes under the Montreal Protocol. The ODP of a compound is an estimate of the total ozone depletion due to 1 Kg of the compound divided by the total ozone depletion due to 1 Kg of CFC-11. Thus, the ODP shows relative effects of comparable emissions of the various compounds. Calculated values for ODPs of individual compounds change as the basic understanding of their interaction with the ozone layer changes. The standard values fixed in the Montreal Protocol and other regulations are shown in the figure below.⁽¹⁾

Ozone Depletion Potentials

Note: The HCFCs and HFCs listed above are the substances studied by AFEAS. All HFCs have zero ozone depletion potential.

Atmospheric chlorine concentrations should decrease with the phase out of CFCs and other controlled substances. This can only be accomplished by using alternative materials and technologies, including HCFCs; a conclusion confirmed by results presented in the UNEP Assessments.⁽¹⁾⁽²⁾ For a variety of technical and economic reasons, HCFCs will be used to replace only about 15-30% of CFCs;⁽¹⁾ additionally, the Montreal Protocol was amended in 1992 to limit both the total quantity and period of use of HCFCs. With these restrictions, HCFCs will contribute only small amounts of chlorine to the total reaching the ozone layer - much less than the CFCs they replace. This fact is demonstrated in the chart opposite, which shows stratospheric chlorine and bromine loadings as the "equivalent chlorine."⁽⁴⁾
As illustrated, HCFCs contribute only about 1% to the peak in atmospheric chlorine, when the risk of ozone depletion is highest, and their whole contribution to atmospheric chlorine, over the next 50 years, is only 5% of the total. Virtually all of that contribution happens when stratospheric chlorine, and the risk of stratospheric ozone depletion, is declining.⁽⁵⁾

Notes:

1.	Based on Reference 4 after adjustment for the stratospheric effectiveness of the halogen content of each compound, as described in Reference 7.

2.	The natural part of methyl bromide includes the large unknown source that may be man-made but is beyond the scope of the Montreal Protocol.

3.	Bromine from all sources (Halons 1211, 1301 and 2402, and methyl bromide) is shown as its equivalent in chlorine (see Reference 4).

4.	Equivalent halogen loading is calculated for the troposphere and is not adjusted for stratospheric effectiveness of any of the compounds.

Production of CFCs has dropped significantly, and recent evidence demonstrates that substantial progress has been made to reverse the trend of increasing chlorine concentrations.

"There is now clear evidence that the growth rates of the CFCs have slowed significantly in recent years, presumably in response to reduced emissions."⁽⁵⁾⁽⁶⁾

This evidence is in the form of measurements of the atmospheric concentrations of CFCs. In all cases, it is clear that emissions have slowed dramatically and, for some of the CFCs, atmospheric concentrations have stabilized or begun to fall.⁽⁷⁾⁽⁸⁾

References

(1)

Synthesis of the Reports of the Ozone Scientific Assessment Panel, Environmental Effects Assessment Panel, Technology and Economic Assessment Panel, prepared by the Assessment Chairs for the Parties to the Montreal Protocol, November 1991. This report provided the basis for the Copenhagen Amendments and Adjustments to the Montreal Protocol.

(2)

Report of the Technology and Economic Assessment Panel to the Parties to the Montreal Protocol, March 1994.

(3)

Ravishankara, A.R., A.A. Turnipseed, N.R. Jensen, S. Barone, M. Mills, C.J. Howard and S. Solomon, Do hydrofluorocarbons destroy stratospheric ozone? Science, 263, 71-75, 1994.

(4)

Stratospheric Ozone: 1996, the fifth report of the UK Stratospheric Ozone Review Group. HMSO, London, UK, 1996.

(5)

Scientific Assessment of Ozone Depletion: 1994; part 2.2.1, pp2.3-2.8. World Meteorological Organisation, Global Ozone Research and Monitoring Project - Report No. 37, WMO, P.O. Box 2300, Geneva, Switzerland, 1994.

(6)

Climate Change 1994: Radiative Forcing of Climate Change and an Evaluation of the IPCC IS92 Emission Scenarios. J.T. Houghton et al., (eds.), part 2.5.1.1, pp92-93, Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK, 1995.

(7)

Montzka S.A., J.H. Butler, R.C. Myers, T.M. Thompson, T.H. Swanson, A.D. Clarke, L.T. Lock, and J.W. Elkins. Decline in the tropospheric abundance of halogen from halocarbons: Implications for stratospheric ozone depletion, Science, 272, 1318-1322, 1996.

(8)

Simmonds P.G., R.G. Derwent, A. McCulloch, S. O'Dougherty and A. Gaudry. Long-term trends in concentrations of halocarbons and radiatively active trace gases in Atlantic and European air masses monitored at Mace Head, Ireland from 1987-1994, Atmos. Environ., 30 (23), 4041-4063, 1996.