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Higher Atmosphere

Chlorofluorocarbons and HCFC's

The investigation of the ozone hole and the measures to ban chlorofluorocarbons (CFC's) world-wide is probably the most important example of a successful environmental protection policy. In this unit we look at the role played by CFC's, what role they are still playing and how they are being replaced.

1. Long term trend of CFC11 since 1977: mixing ratios in ppt at different measurement stations. Please note that the stations are at ground level. In the stratosphere the maximum is reached about 5 years later.
Source: NOAA/CMDL
Please click to enlarge! (15 K)

Trends in CFC concentrations

As seen in Basics Unit 2 the CFCs have been very useful compounds for diverse applications, since they are not toxic and inert gases. For the atmosphere they have a twofold importance. They are greenhouse gases with a very high global warming potential GWP and they have a very long lifetime. The latter allows them to stay in the atmosphere and to be transported slowly to the stratosphere. Although CFCs have been banned within a few years after the Montreal Protocol signed in 1987 it takes a rather long time until they level out.

2. The more recent trend for CFC11 and CFC12: mixing ratios in ppt
Source: NOAA/CMDL
Please click to enlarge! (50 K)

The maximum at the ground for CFC11 (lifetime 45 years) has been reached around 1994, the maximum for CFC12 (lifetime 100 years) may be reached now. The values for the northern hemisphere are slightly higher, because hemispherical mixing is always faster than the transport across the equator.

Comparison of CFCs and HCFCs

Global warming potentials (GWPs) are a measure of the relative radiative effect of a given substance compared to CO₂, integrated over a chosen time horizon. In easier words, the GWP tells us by which factor the greenhouse effect caused by this gas exceeds the one of CO₂.

The ozone depletion potential (ODP) is a relative number comparing how dangerous the different compounds are for the ozone layer.

CFCs have a twofold negative impact as ozone killers in the stratosphere and as greenhouse gases in the troposphere. The following table gives an overview of a few CFCs compared to the Hydrofluorocarbons HCFC which shall replace them.

Direct Global Warming Potentials (mass basis) relative to carbon dioxide (for gases for which the lifetimes have been adequately characterised). Data from IPCC TAR 2001

Gas		Lifetime (years)	Ozone depletion potential (ODP)	Global Warming Potential Time horizon


				20 years	100 years

Carbon dioxide	CO₂			1	1
Methane	CH₄	12.0*		62	23
Nitrous oxide	N₂O	114*		275	296
Chlorofluorocarbons
CFC-11	CCl₃F	45	1.0	6300	4600
CFC-12	CCl₂F₂	100	0.82	10200	10600
CFC-13	CClF₃	640		10000	14000
Hydrochlorofluorocarbons
HCFC-21	CHCl₂F	2.0		700	210
HCFC-22	CHClF₂	11.9	0.04	4800	1700
HCFC-123	CF₃CHCl₂	1.4	0.014	390	120
Hydrofluorocarbons
HFC-23	CHF₃	260	<0.0004	9400	12000
HFC-32	CH₂F₂	5.0		1800	550
HFC-41	CH₃F	2.6		330	97

* The values for CH₄ and N₂O are adjustment times including feedbacks of emission on lifetimes. Ozone depletion potentials are normalized and model derived, from WMO scientific assessment of ozone depletion 1994

For an overview of more species please click here.

It is obvious that the lifetime of HCFCs is much shorter. They are primarily decomposed in the troposphere and the probability that they reach the stratosphere and promote the ozone hole destruction is by far lower. However, the global warming potential of this gases is still high. They can be regarded as a compromise but not as the ideal solution.

About this page:

author: Dr. Elmar Uherek - Max Planck Institute, Mainz
scientific reviewer: Dr. Christoph Brühl - Max Planck Institute for Chemistry, Mainz
educational proofreading: Michael Seesing - Uni Duisburg - 2003-08-07
last published: 2004-05-11

Higher Atmosphere

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