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Subject: Ozone Depletion FAQ Part II: Stratospheric Chlorine and Bromine
This article was archived around: 24 Dec 1997 20:49:55 GMT
Last-modified: 16 Dec 1997
Subject: How to get this FAQ
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Subject: Copyright Statement
* Copyright 1997 Robert Parson *
* This file may be distributed, copied, and archived. All such *
* copies must include this notice and the paragraph below entitled *
* "Caveat". Reproduction and distribution for personal profit is *
* not permitted. If this document is transmitted to other networks or *
* stored on an electronic archive, I ask that you inform me. I also *
* ask you to keep your archive up to date; in the case of world-wide *
* web pages, this is most easily done by linking to the master at the *
* ohio-state http URL instead of storing local copies. Finally, I *
* request that you inform me before including any of this information *
* in any publications of your own. Students should note that this *
* is _not_ a peer-reviewed publication and may not be acceptable as *
* a reference for school projects; it should instead be used as a *
* pointer to the published literature. In particular, all scientific *
* data, numerical estimates, etc. should be accompanied by a citation *
* to the original published source, not to this document. *
Subject: General Information
This part deals not with ozone depletion per se (that is covered
in Part I) but rather with the sources and sinks of chlorine and
bromine in the stratosphere. Special attention is devoted to the
evidence that most of the chlorine comes from the photolysis of
CFC's and related compounds. Instead of relying upon qualitative
statements about relative lifetimes, solubilities, and so forth, I
have tried to give a sense of the actual magnitudes involved.
Fundamentally, this Part of the FAQ is about measurements, and I
have therefore included some tables to illustrate trends; the
data that I reproduce is in every case a small fraction of what
has actually been published. In the first section I state the
present assessment of stratospheric chlorine sources and trends,
and then in the next section I discuss the evidence that leads to
those conclusions. After a brief discussion of Bromine and Iodine in
section 3, I answer the most familiar challenges that have been
raised in section 4. Only these last are actually "Frequently Asked
Questions"; however I have found the Question/Answer format to be
useful in clarifying the issues in my mind even when the questions
are rhetorical, so I have kept to it.
Subject: Caveats, Disclaimers, and Contact Information
| Caveat: I am not a specialist. In fact, I am not an atmospheric
| chemist at all - I am a physical chemist studying gas-phase
| processes who talks to atmospheric chemists. These files are an
| outgrowth of my own efforts to educate myself about this subject.
| I have discussed some of these issues with specialists but I am
| solely responsible for everything written here, especially errors.
| On the other hand, if you find this document in an online archive
| somewhere, I am not responsible for any *other* information that may
| happen to reside in that archive. This file should not be cited as
| a reference in publications off the net; rather, it should be used as
| a pointer to the published literature.
*** Corrections and comments are welcomed.
- Robert Parson
Department of Chemistry and Biochemistry,
University of Colorado (for which I do not speak)
Subject: TABLE OF CONTENTS
How to get this FAQ
Caveats, Disclaimers, and Contact Information
TABLE OF CONTENTS
1. CHLORINE IN THE STRATOSPHERE - OVERVIEW
1.1) Where does the Chlorine in the stratosphere come from?
1.2) How has stratospheric chlorine changed with time?
1.3) How will stratospheric chlorine change in the future?
2. THE CHLORINE CYCLE
2.1) What are the sources of chlorine in the troposphere?
2.2) In what molecules is _stratospheric_ chlorine found?
2.3) What happens to organic chlorine in the stratosphere?
2.4) How do we know that CFC's are photolyzed in the stratosphere?
2.5) How is chlorine removed from the stratosphere?
2.6) How is chlorine distributed in the stratosphere?
2.7) What happens to the Fluorine from the CFC's?
2.8) Summary of the Evidence
3. BROMINE AND IODINE
3.1) Does Bromine contribute to ozone depletion?
3.2) How does bromine affect ozone?
3.3) Where does the bromine come from?
3.4) How about Iodine?
4. COMMONLY ENCOUNTERED OBJECTIONS
4.1) CFC's are 4-8 times heavier than air, so how can they
4.2) CFCs are produced in the Northern Hemisphere, so how do
they get down to the Antarctic?
4.3) Sea salt puts more chlorine into the atmosphere than CFC's.
4.4) Volcanoes put more chlorine into the stratosphere than CFC's.
4.5) Space shuttles put a lot of chlorine into the stratosphere.
4.6) Most CFC's are decomposed by soil bacteria and other
5. REFERENCES FOR PART II
Books and Review Articles
More specialized references
Subject: 1. CHLORINE IN THE STRATOSPHERE - OVERVIEW
Subject: 1.1) Where does the Chlorine in the stratosphere come from?
~80% from CFC's and related manmade organic chlorine compounds,
such as carbon tetrachloride and methyl chloroform
~15-20% from methyl chloride (CH3Cl), most of which is natural.
A few % from inorganic sources, such as volcanic eruptions.
[Russell et al. 1996] [WMO 1991, 1994] [Solomon] [AASE]
[Rowland 1989,1991] [Wayne]
These estimates are based upon >20 years' worth of measurements of
organic and inorganic chlorine-containing compounds in the earth's
troposphere and stratosphere. Particularly informative is the
dependence of these compounds' concentrations on altitude and
their increase with time. The evidence is summarized in section 2
of this FAQ.
Subject: 1.2) How has stratospheric chlorine changed with time?
The total amount of chlorine in the stratosphere has increased by
a factor of 2.5 since 1975 [Solomon] During this time period the
known natural sources have shown no major increases. On the other
hand, emissions of CFC's and related manmade compounds have
increased dramatically, reaching a peak in 1987. Extrapolating
back, one infers that total stratospheric chlorine has increased
by a factor of 4 since 1950.
Subject: 1.3) How will stratospheric chlorine change in the future?
Since the 1987 Montreal Protocol (see Part I) production of
CFC's and related compounds has been decreasing rapidly, and
in consequence their rate of growth in the atmosphere has
fallen dramatically [Elkins et al. 1993] [Prinn et al. 1995]
[Montzka et al. 1996] The data below show that CFC-12 concentrations
have nearly stabilized while CFC-11 has actually begun to decrease.
Growth Rate, pptv/yr
Year CFC-12 CFC-11
1977-84 17 9 [Elkins et al. 1993]
1985-88 19.5 11 "
1993 10.5 2.7 "
1995 5.9 -0.6 [Montzka et al. 1996]
Methyl chloroform and carbon tetrachloride are also decreasing, while
CFC-113 has stabilized. Overall, tropospheric chlorine from halocarbons
peaked in 1995 and has begun to decline. The time scale for mixing
tropospheric and lower stratospheric air is about 3-5 years, so
_stratospheric_ chlorine is expected to peak in about 1998 and
then to decline slowly, on a time scale of about 50 years.
[WMO 1994] [Montzka et al. 1996]
Subject: 2. THE CHLORINE CYCLE
Subject: 2.1) What are the sources of chlorine in the troposphere?
Let us divide the chlorine-containing compounds found in the
atmosphere into two groups, "organic chlorine" and "inorganic
chlorine". The most important inorganic chlorine compound in the
troposphere is hydrogen chloride, HCl. Its principal source is
acidification of salt spray - reaction of atmospheric sulfuric and
nitric acids with chloride ions in aerosols. At sea level, this
leads to an HCl mixing ratio of 0.05 - 0.45 ppbv, depending strongly
upon location (e.g. smaller values over land.) However, HCl dissolves
very readily in water (giving hydrochloric acid), and condensation of
water vapor efficiently removes HCl from the _upper_ troposphere.
Measurements show that the HCl mixing ratio is less than 0.1 ppbv at
elevations above 7 km, and less than 0.04 ppbv at 13.7 km.
[Vierkorn-Rudolf et al.] [Harris et al.]
There are many volatile organic compounds containing chlorine, but
most of them are quickly decomposed by the natural oxidants in the
troposphere, and the chlorine atoms that were in these compounds
eventually find their way into HCl or other soluble species and are
rained out. The most important exceptions are:
ChloroFluoroCarbons, of which the most important are
CF2Cl2 (CFC-12), CFCl3 (CFC-11), and CF2ClCFCl2 (CFC-113);
HydroChloroFluoroCarbons such as CHClF2 (HCFC-22);
Carbon Tetrachloride, CCl4;
Methyl Chloroform, CH3CCl3;
and Methyl Chloride, CH3Cl (also called Chloromethane).
Only the last has a large natural source; it is produced
biologically in the oceans and chemically from biomass burning.
The CFC's and CCl4 are nearly inert in the troposphere, and have
lifetimes of 50-200+ years. Their major "sink" is photolysis by UV
radiation. [Rowland 1989, 1991] The hydrogen-containing halocarbons
are more reactive, and are removed in the troposphere by reactions
with OH radicals. This process is slow, however, and they live long
enough (1-20 years) for a large fraction to reach the stratosphere.
As a result of this enormous difference in atmospheric lifetimes,
there is more chlorine present in the lower atmosphere in
halocarbons than in HCl, even though HCl is produced in much larger
quantities. Total tropospheric organic chlorine amounted to
~3.8 ppbv in 1989 [WMO 1991], and this mixing ratio is very nearly
independent of altitude throughout the troposphere. Methyl Chloride,
the only ozone-depleting chlorocarbon with a major natural source,
makes up 0.6 ppbv of this total. Compare this to the tropospheric HCl
mixing ratios given above: < 0.5 ppbv at sea level, < 0.1 ppbv at 3 km,
and < 0.04 ppbv at 10 km.
Subject: 2.2) In what molecules is _stratospheric_ chlorine found?
The halocarbons described above are all found in the stratosphere,
and in the lower stratosphere they are the dominant form of chlorine.
At higher altitudes inorganic chlorine is abundant, most of it in
the form of HCl or of _chlorine nitrate_, ClONO2. These are called
"chlorine reservoirs"; they do not themselves react with ozone, but
they generate a small amount of chlorine-containing radicals - Cl,
ClO, ClO2, and related species, referred to collecively as the
"ClOx family" - which do. An increase in the concentration of
chlorine reservoirs leads to an increase in the concentration of
the ozone-destroying radicals.
Subject: 2.3) What happens to organic chlorine in the stratosphere?
The organic chlorine compounds are dissociated by UV radiation
having wavelengths near 230 nm. Since these wavelengths are also
absorbed by oxygen and ozone, the organic compounds have to rise
high in the stratosphere in order for this photolysis to take
place. The initial (or, as chemists say, "nascent") products are
a free chlorine atom and an organic radical, for example:
CFCl3 + hv -> CFCl2 + Cl
The chlorine atom can react with methane to give HCl and a methyl
Cl + CH4 -> HCl + CH3
Alternatively, it can react with ozone to give ClO:
Cl + O3 -> ClO + O2
which can go on to react with O to release Cl again, closing
a catalytic cycle:
ClO + O -> Cl + O2
or can react with nitrogen dioxide to form the metastable compound
ClO + NO2 -> ClONO2.
(There are other pathways, but these are the most important.)
The other nascent product (CFCl2 in the above example) undergoes
a complicated sequence of reactions that also eventually leads to
HCl and ClONO2. Most of the inorganic chlorine in the stratosphere
therefore resides in one of these two "reservoirs". The immediate
cause of the Antarctic ozone hole is an unusual sequence of
reactions, catalyzed by polar stratospheric clouds, that "empty"
these reservoirs and produce high concentrations of ozone-destroying
Cl and ClO radicals. [Wayne] [Rowland 1989, 1991]
Subject: 2.4) How do we know that CFC's are photolyzed in the stratosphere?
The UV photodissociation cross-sections for the halocarbons have been
measured in the laboratory; these tell us how rapidly they will
dissociate when exposed to light of a given wavelength and intensity.
We can combine this with the measured intensity of radiation in the
stratosphere and deduce the way in which the mixing ratio of a
given halocarbon should depend upon altitude. Since there is almost
no <230 nm radiation in the troposphere or in the lowest parts of
the stratosphere, the mixing ratio should be independent of altitude
there. In the middle stratosphere the mixing ratio should drop off
quickly, at a rate which is determined by the photodissociation
cross-section. Thus each halocarbon has a characteristic signature
in its mixing ratio profile, which can be calculated. Such calculations
(first carried out in the mid 1970's) agree well with the distributions
presented in the next section.
There is direct evidence as well. Photolysis removes a chlorine
atom, leaving behind a reactive halocarbon radical. The most likely
fate of this radical is reaction with oxygen, which starts a long
chain of reactions that eventually remove all the chlorine and
fluorine. Most of the intermediates are reactive free radicals, but
two of them, COF2 and COFCl, are fairly stable and live long enough
to be detected - and have been. [Zander et al. 1992, 1994].
Subject: 2.5) How is chlorine removed from the stratosphere?
Since the stratosphere is very dry, water-soluble compounds are
not quickly washed out as they are in the troposphere. The
stratospheric lifetime of HCl is about 2 years; the principal
sink is transport back down to the troposphere.
Subject: 2.6) How is chlorine distributed in the stratosphere?
Over the past 20 years an enormous effort has been devoted to
identifying sources and sinks of stratospheric chlorine. The
concentrations of the major species have been measured as a
function of altitude, by "in-situ" methods ( e.g. collection
filters carried on planes and balloons) and by spectroscopic
observations from aircraft, balloons, satellites, and the Space
Shuttle. From all this work we now have a clear and consistent
picture of the processes that carry chlorine through the stratosphere.
Let us begin by asking where inorganic chlorine is found. In the
troposphere, the HCl mixing ratio decreased markedly with increasing
altitude. In the stratosphere, on the other hand, it _increases_ with
altitude, rapidly up to about 35 km, and then more slowly up to 55km
and beyond. This was noticed as early as 1976 [Farmer et al.]
[Eyre and Roscoe] and has been confirmed repeatedly since. Chlorine
Nitrate (ClONO2), the other important inorganic chlorine compound in
the stratosphere, also increases rapidly in the lower stratosphere, and
then falls off at higher altitudes. These results strongly suggest
that HCl in the stratosphere is being _produced_ there, not drifting
up from below.
Let us now look at the organic source gases. Here, the data show
that the mixing ratios of the CFC's and CCl4 are _nearly independent
of altitude_ in the troposphere, and _decrease rapidly with altitude_
in the stratosphere. The mixing ratios of the more reactive
hydrogenated compounds such as CH3CCl3 and CH3Cl drop off somewhat
in the troposphere, but also show a much more rapid decrease in
the stratosphere. The turnover in organic chlorine correlates
nicely with the increase in inorganic chlorine, confirming the
hypothesis that CFC's are being photolyzed as they rise high enough
in the stratosphere to experience enough short-wavelength UV. At
the bottom of the stratosphere almost all of the chlorine is
organic, and at the top it is all inorganic. [Fabian et al. ]
[Zander et al. 1987, 1992, 1996] [Penkett et al.]
Finally, there are the stable reaction intermediates, COF2 and
COFCl. These have been found in the lower and middle stratosphere,
exactly where one expects to find them if they are produced from
organic source gases and eventually react to give inorganic chlorine.
For example, the following is extracted from Tables II and III of
[Zander et al. 1992]; they refer to 30 degrees N Latitude in 1985.
I have rearranged the tables and rounded some of the numbers, and
the arithmetic in the second table is my own.
Organic Chlorine and Intermediates, Mixing ratios in ppbv
Alt., CH3Cl CCl4 CCl2F2 CCl3F CHClF2 CH3CCl3 C2F3Cl3 || COFCl
12.5 .580 .100 .310 .205 .066 .096 .021 || .004
15 .515 .085 .313 .190 .066 .084 .019 || .010
20 .350 .035 .300 .137 .061 .047 .013 || .035
25 .120 - .175 .028 .053 .002 .004 || .077
30 - - .030 - .042 - - || .029
40 - - - - - - - || -
Inorganic Chlorine and Totals, Mixing ratios in ppbv
Alt., HCl ClONO2 ClO HOCl || Total Cl, Total Cl, Total Cl
|| Inorganic Organic
12.5 - - - - || - 2.63 2.63
15 .065 - - - || 0.065 2.50 2.56
20 .566 .212 - - || 0.778 1.78 2.56
25 1.027 .849 .028 .032 || 1.936 0.702 2.64
30 1.452 1.016 .107 .077 || 2.652 0.131 2.78
40 2.213 0.010 .234 .142 || 2.607 - 2.61
I have included the intermediate COFCl in the Total Organic column.
It should be noted that COFCl was not measured directly in this
experiment, although the related intermediate COF2 was.
This is just an excerpt. The original tables give results every 2.5km
from 12.5 to 55km, together with a similar inventory for Fluorine.
Standard errors on total Cl were estimated to be 0.02-0.04 ppbv.
[Zander et al. 1996] provide a similar inventory for the year 1994;
once again the total chlorine at any altitude is approximately
constant, but at ~3.5 ppbv instead of ~2.6 ppbv, indicative of
the increase in anthropogenic halocarbons between 1985 and 1994.
Notice that the _total_ chlorine at any altitude is nearly constant
at ~2.5-2.8 ppbv. This is what we would expect if the sequence of
reactions that leads from organic sources to inorganic reservoirs
was fast compared to vertical transport. Our picture, then, would be
of a swarm of organic chlorine molecules slowly spreading upwards
through the stratosphere, being converted into inorganic reservoir
molecules as they climb. In fact this oversimplifies things -
photolysis pops off a single Cl atom which does reach its final
destination quickly, but the remaining Cl atoms are removed by a
sequence of slower reactions. Some of these reactions involve
compounds, such as NOx, which are not well-mixed; moreover,
"horizontal" transport does not really take place along surfaces of
constant altitude, so chemistry and atmospheric dynamics are in fact
coupled together in a complicated way. These are the sorts of issues
that are addressed in atmospheric models. Nevertheless, this simple
picture helps us to understand the qualitative trends, and quantitative
treatments confirm the conclusions [McElroy and Salawich]
[Russell et al. 1996].
We conclude that most of the inorganic chlorine in the stratosphere
is _produced_ there, as the end product of photolysis of the organic
Subject: 2.7) What happens to the Fluorine from the CFC's?
Most of it ends up as Hydrogen Fluoride, HF. The total amount of HF
in the stratosphere increased by a factor of 3-4 between 1978 and
1989 [Zander et al., 1990] [Rinsland et al.]; the relative increase
is larger for HF than for HCl (a factor of 2.2 over the same period)
because the natural source, and hence the baseline concentration,
is much smaller. For the same reason, the _ratio_ of HF to HCl has
increased, from 0.14 in 1977 to 0.23 in 1990. As discussed above, the
decomposition of CFC's in the stratosphere produces reaction
intermediates such as COF2 and COFCl which have been detected in the
stratosphere. COF2 in particular is relatively stable and makes a
significant contribution to the total fluorine; the total amount
of COF2 in the stratosphere increased by 60% between 1985 and 1992
[Zander et al. 1994] The total Fluorine budget,
as a function of altitude, adds up in much the same way as the
chlorine budget. [Zander et al. 1992, 1994] [Luo et al.]
The most comprehensive measurements of stratospheric HF are those made
by the Halogen Occultation Experiment (HALOE) on the UARS satellite
[Luo et al.] [Russell et al. 1996] Information about HALOE is available
on the World-Wide-Web at http://haloedata.larc.nasa.gov/home.html .
Subject: 2.8) Summary of the Evidence
a. Inorganic chlorine, primarily of natural origin, is efficiently
removed from the troposphere; organic chlorine, primarily
anthropogenic, is not, and in the upper troposphere organic
chlorine dominates overwhelmingly.
b. In the stratosphere, organic chlorine decreases with altitude,
since at higher altitudes there is more short-wave UV available to
photolyze it. Inorganic chlorine _increases_ with altitude.
At the bottom of the stratosphere essentially all of the chlorine
is organic, at the top it is all inorganic, and reaction
intermediates such as COF2 are found at intermediate altitudes.
c. Both HCl and HF in the stratosphere have been increasing steadily,
in a correlated fashion, since they were first measured in the 1970's.
Reaction intermediates such as COF2 are also increasing.
Subject: 3. BROMINE
Subject: 3.1) Does Bromine contribute to ozone depletion?
Br is present in much smaller quantities than Cl, but it is
much more destructive on a per-atom basis. There is a large
natural source; manmade compounds contribute about 40% of the total.
In the antarctic chlorine is more important than Bromine, but at
middle latitudes their effects are comparable.
Subject: 3.2) How does bromine affect ozone?
Bromine concentrations in the stratosphere are ~150 times smaller
than chlorine concentrations. However, atom-for-atom Br is 10-100
times as effective as Cl in destroying ozone. (The reason for this
is that there is no stable 'reservoir' for Br in the stratosphere
- HBr and BrONO2 are very easily photolyzed so that nearly all of
the Br is in a form that can react with ozone. Contrariwise, F is
innocuous in the stratosphere because its reservoir, HF, is
extremely stable.) So, while Br is less important than Cl, it must
still be taken into account. Interestingly, one principal
pathway by which Br destroys ozone also involves Cl:
BrO + ClO -> BrCl + O2
BrCl + hv -> Br + Cl
Br + O3 -> BrO + O2
Cl + O3 -> ClO + O2
Net: 2 O3 -> 3 O2
[Wayne p. 164] [Solomon]
so reducing stratospheric chlorine concentrations will, as a
side-effect, slow down the bromine pathways as well.
Another important mechanism combines Br with HOx radicals:
BrO + HO2 -> HOBr
HOBr + hv -> Br + OH
Br + O3 -> BrO + O2
OH + O3 -> HO2 + O2
Net: 2 O3 -> 3 O2
Subject: 3.3) Where does the bromine come from?
a.) Methyl Bromide
The largest source of stratospheric Bromine is methyl bromide,
CH3Br. It is also the most poorly characterized source. Much of it is
naturally produced in the oceans, but a significant portion (30-60%,
according to [Khalil et al.) is manmade; it is widely used as a
fumigant. Methyl bromide is also produced during biomass burning,
which can be either natural or anthropogenic [Mano and Andreae]. The
1994 assessment from the World Meteorological Organization [WMO 1994]
estimates the major sources as:
Oceans: 60-160 ktons/yr
Fumigation: 20-60 ktons/yr
Biomass burning: 10-50 ktons/yr .
This assessment estimates the atmospheric lifetime of methyl bromide
to be 0.8-1.7 years (best estimate 1.3 years) and its ozone depletion
potential to be about 0.6 . However, recent laboratory and field
experiments [Shorter et al.] indicate that large amounts of methyl bromide
are consumed by soil bacteria. This would push the atmospheric lifetime
down to the lower limit of 0.8 years, and reduce the ozone depletion
potential to 0.4; it may also require adjustments in the estimated sources.
Methyl bromide is also produced in the combustion of leaded gasolines,
which use ethylene dibromide as a scavenger. One estimate for the methyl
bromide emissions from this source gave 9-22 ktons/yr, but another
estimate gave only 0.5-1.5 ktons/yr.
Another important Bromine source is the family of "halons", widely
used in fire extinguishers. Like CFC's these compounds have long
atmospheric lifetimes (65 years for CF3Br) and very little is lost in
the troposphere. [WMO 1994]. Halons are scheduled for phase-out
under the Montreal Protocol, and their rate of increase in the
atmosphere has slowed by a factor of three since 1989. (Before then
halon concentrations were increasing by 15-20% _per year_.)
Subject: 3.4) And how about about Iodine?
Since Chlorine and Bromine radicals both enter into ozone-destroying
catalytic cycles, it comes as no surprise that Iodine can do so as well.
One possible mechanism is:
ClO + IO -> Cl + I + O2
Cl + O3 -> ClO + O2
I + O3 -> IO + O2
Net: 2 O3 -> 3 O2
Note that this is precisely analogous to the Bromine/Chlorine cycle
given in section 3.2; the Iodine acts in concert with Chlorine. There
are also cycles in which Iodine and Bromine, and Iodine and OH, act
At present it is not known whether there is enough Iodine in the
stratosphere to make these reactions important for the overall ozone
balance. The principle source of atmospheric iodine is methyl iodide,
produced in large quantities by marine biota. Methyl iodide, like methyl
chloride and bromide, is insoluble in water and is thus not "frozen out"
at the tropopause; however it has a much shorter atmospheric lifetime
so only a small fraction survives long enough to reach the stratosphere.
It has recently been suggested [Solomon et al. 1994a,b] that this small
fraction may nevertheless be large enough to influence ozone depletion
in the lowest part of the stratosphere. (Current models using only
chlorine and bromine chemistry predict significantly less ozone loss in
these regions than has been observed.) More measurements will be needed
to resolve this issue.
Anthropogenic sources of stratospheric iodine are negligible.
Trifluoromethyliodide, CF3I, has been suggested as a substitute for
halons, since unlike halons, CF3I has a short atmospheric lifetime.
[Solomon et al. 1994b] estimate its ozone depletion potential (ODP) to
be less than 0.008 and probably less than 0.0001; CF3Br, in contrast,
has an ODP of 7.8. Iodine may be accelerating the rate at which
(mostly) anthropogenic chlorine and (partly) anthropogenic bromine
destroy ozone, but iodine in itself is not an anthropogenic influence.
Subject: 4. COMMONLY ENCOUNTERED OBJECTIONS
Subject: 4.1) CFC's are 4-8 times heavier than air, so how can they
reach the stratosphere?
This is answered in Part I of this FAQ, section 1.3. Briefly,
atmospheric gases do not segragate by weight in the troposphere
and the stratosphere, because the mixing mechanisms (convection,
"eddy diffusion") do not distinguish molecular masses.
Subject: 4.2) CFCs are produced in the Northern Hemisphere, so how do
they get down to the Antarctic?
Vertical transport into and within the stratosphere is slow. It
takes more than 5 years for a CFC molecule released at sea level to
rise high enough in the stratosphere to be photolyzed. North-South
transport, in both troposphere and stratosphere, is faster - there is
a bottleneck in the tropics (it can take a year or two to get across
the equator) but there is still plenty of time. CFC's are distributed
almost uniformly as a function of latitude, with a gradient of ~10%
from Northern to Southern Hemispheres.
[Singh et al. 1979] [Elkins et al. 1993]
Subject: 4.3) Sea salt puts more chlorine into the atmosphere than CFC's.
True, but not relevant because this chlorine is in a form (HCl) that
is rapidly removed from the troposphere. Even at sea level there is
more chlorine present in organic compounds than in HCl, and in the
upper troposphere and lower stratosphere organic chlorine dominates
overwhelmingly. See section 2.1 above.
Subject: 4.4) Volcanoes put more chlorine into the stratosphere than CFC's.
Short Reply: False. Volcanoes account for at most a few percent
of the chlorine in the stratosphere.
Long reply: This is one of the most persistent myths in this
area. As is so often the case, there is a seed of truth at the
root of the myth. Volcanic gases are rich in Hydrogen Chloride, HCl.
As we have discussed, this gas is very soluble in water and is
removed from the troposphere on a time scale of 1-7 days, so we can
dismiss quietly simmering volcanoes as a stratospheric source, just
as we can neglect sea salt and other natural sources of HCl. (In fact
tropospheric HCl from volcanoes is neglible compared to HCl from
sea salt.) However, we cannot use this argument to dismiss MAJOR
volcanic eruptions, which can in principle inject HCl directly into
the middle stratosphere.
What is a "major" eruption? There is a sort of "Richter scale" for
volcanic eruptions, the so-called "Volcanic explosivity index" or
VEI. Like the Richter scale it is logarithmic; an eruption with a
VEI of 5 is ten times "bigger" than one with a VEI of 4. To give a
sense of magnitude, I list below the VEI for some familiar recent
and historic eruptions:
Eruption VEI Stratospheric Aerosol,
Kilauea 0-1 -
Erebus, 1976-84 1-2 -
Augustine, 1976 4 0.6
St Helen's, 1980 5 (barely) 0.55
El Chichon, 1982 5 12
Pinatubo, 1991 5-6 30
Krakatau, 1883 6 50 (estimated)
Tambora, 1815 7 80-200 (estimated)
[Smithsonian] [Symonds et al.] [Sigurdsson] [Pinatubo] [WMO 1988]
[Bluth et al.] [McCormick et al. 1995]
Roughly speaking, an eruption with VEI>3 can penetrate the
stratosphere. An eruption with VEI>5 can send a plume up to 25km, in the
middle of the ozone layer. Such eruptions occur about once a decade.
Since the VEI is not designed specifically to measure a volcano's impact
on the stratosphere, I have also listed the total mass of stratospheric
aerosols (mostly sulfates) produced by the eruption. (Note that St.
Helens produced much less aerosol than El Chichon - St. Helens blew out
sideways, dumping a large ash cloud over eastern Washington, rather than
ejecting its gases into the stratosphere.) Passively degassing volcanoes
such as Kilauea and Erebus are far too weak to penetrate the
stratosphere, but explosive eruptions like El Chichon and Pinatubo need
to be considered in detail.
Before 1982, there were no direct measurements of the amount of HCl
that an explosive eruption put into the stratosphere. There were,
however, estimates of the _total_ chlorine production from an
eruption, based upon such geophysical techniques as analysis of
glass inclusions trapped in volcanic rocks. [Cadle] [Johnston]
[Sigurdsson] [Symonds et al.] There was much debate
about how much of the emitted chlorine reached the stratosphere;
estimates ranged from < 0.03 Mt/year [Cadle] to 0.1-1.0 Mt/year
[Symonds et al.]. During the 1980's emissions of CFC's and related
compounds contributed ~1 Mt of chlorine per year to the
atmosphere. [Prather et al.] This results in an annual flux of >0.3
Mt/yr of chlorine into the stratosphere. The _highest_ estimates
of volcanic emissions - upper limits calculated by assuming that
_all_ of the HCl from a major eruption reached and stayed in the
stratosphere - were thus of the same order of magnitude as human
sources. (There is no support whatsoever for the claim that a
_single_ recent eruption produced ~500 times as much chlorine as a
year's worth of CFC production. This wildly inaccurate number appears
to have originated as an editorial mistake in a scientific encyclopedia.)
It is very difficult to reconcile the higher estimates with the
altitude and time-dependence of stratospheric HCl. The volcanic
contribution to the upper stratosphere should come in sudden bursts
following major eruptions, and it should initially be largest in
the vicinity of the volcanic plume. Since vertical transport in the
stratosphere is slow, one would expect to see the altitude profile
change abruptly after a major eruption, whereas it has maintained
more-or-less the same shape since it was first measured in 1975.
One would also not expect a strong correlation between HCl and
organochlorine compounds if volcanic injection were contributing
~50% of the total HCl. If half of the HCl has an inorganic origin,
where is all that _organic_ stratospheric chlorine going?
The issue has now been largely resolved by _direct_ measurements of the
stratospheric HCl produced by El Chichon, the most important eruption of
the 1980's, and Pinatubo, the largest since 1912. It was found that El
Chichon injected *0.04* Mt of HCl [Mankin and Coffey]. The much bigger
eruption of Pinatubo produced less [Mankin, Coffey and Goldman] [Wallace
and Livingston 1992], - in fact the authors were not sure that they had
measured _any_ significant increase. Analysis of ice cores leads to
similar conclusions for historic eruptions [Delmas]. The ice cores show
significantly enhanced levels of sulfur following major historic
eruptions, but no enhancement in chlorine, showing that the chlorine
produced in the eruption did not survive long enough to be transported
to polar regions. It is clear, then, that even though major eruptions
produce large amounts of chlorine in the form of HCl, most of that HCl
either never enters the stratosphere, or is very rapidly removed from it.
Recent model calculations [Pinto et al.] [Tabazadeh and Turco]
have clarified the physics involved. A volcanic plume contains
approximately 1000 times as much water vapor as HCl. As the plume
rises and cools the water condenses, capturing the HCl as it does
so and returning it to the earth in the extensive rain showers that
typically follow major eruptions. HCl can also be removed if it
is adsorbed on ice or ash particles. Model calculations show that
more than 99% of the HCl is removed by these processes, in good
agreement with observations.
* Older indirect _estimates_ of the contribution of volcanic
eruptions to stratospheric chlorine gave results that ranged
from much less than anthropogenic to somewhat larger than
anthropogenic. It is difficult to reconcile the larger estimates
with the altitude distribution of inorganic chlorine in the
stratosphere, or its steady increase over the past 20 years.
Nevertheless, these estimates raised an important scientific
question that needed to be resolved by _direct_ measurements
in the stratosphere.
* Direct measurements on El Chichon, the largest eruption of
the 1980's, and on Pinatubo, the largest since 1912, show
that the volcanic contribution is small.
* Claims that volcanoes produce more stratospheric chlorine than
human activity arise from the careless use of old scientific
estimates that have since been refuted by observation.
* Claims that a single recent eruption injected ~500 times a year's
CFC production into the stratosphere have no scientific basis
To conclude, we need to say something about Mt. Erebus. In an
article in _21st Century_ (July/August 1989), Rogelio Maduro
claimed that this Antarctic volcano has been erupting constantly
for the last 100 years, emitting more than 1000 tons of chlorine
per day. Mt. Erebus has in fact been simmering quietly for over a
century [ARS] but the estimate of 1000 tons/day of HCl only applied
to an especially active period between 1976 and 1983 [Kyle et al. 1990].
Moreover, that estimate has been since been reduced to 167 tons/day
(0.0609 Mt/year). By late 1984 emissions had dropped by an order of
magnitude, and have remained at low levels since; HCl emissions
_at the crater rim_ were 19 tons/day (0.007 Mt/year) in 1986,
and 36 tons/day (0.013 Mt/year) in 1991. [Zreda-Gostynska et al.]
Since this is a passively degassing volcano (VEI=1-2 in the active
period), very little of this HCl reaches the stratosphere. The
Erebus plume never rises more than 0.5 km above the volcano,
and in fact the gas usually just oozes over the crater rim. Indeed,
one purpose of the measurements of Kyle et al. was to explain high
Cl concentrations in Antarctic snow.
Subject: 4.5) Space shuttles put a lot of chlorine into the stratosphere.
Simply false. In the early 1970's, when very little was known about
the role of chlorine radicals in ozone depletion, it was suggested
that HCl from solid rocket motors might have a significant effect
upon the ozone layer - if not globally, perhaps in the immediate
vicinity of the launch. It was immediately shown that the effect
was negligible, and this has been repeatedly demonstrated since.
Each shuttle launch produces about 200 metric tons of chlorine as
HCl, of which about one-third, or 68 tons, is injected into the
stratosphere. Its residence time there is about three years. A
full year's US schedule of shuttle and solid rocket launches injects
725 tons of chlorine into the stratosphere. The European Space Agency's
Ariane rocket makes a similar contribution, with 57 tons of HCl deposited
in the stratosphere for each launch. These inputs are negligible
compared to chlorine emissions in the form of CFC's and related
compounds (~ 1.0 million tons/yr in the 1980's, of which ~0.3 Mt reach
the stratosphere each year). It is also small in comparison to natural
sources of stratospheric chlorine, which amount to about 75,000 tons
per year. [Prather et al.] [WMO 1991] [Ko et al.]
See also the sci.space FAQ, Part 10, "Controversial Questions",
available by anonymous ftp from rtfm.mit.edu in the directory
pub/usenet/news.answers/space/controversy, or on the world-wide web at:
Subject: 4.6) Most CFC's are decomposed by soil bacteria and other
This argument is based upon a misinterpretation of measurements made by
Khalil and Rasmussen. These scientists did show that some CFC's such
as CFC-11 and CFC-12 (but not CFC-113) were taken up by soils in
Australia [Khalil and Rasmussen 1989] and by rice paddies in China
[Khalil et al. 1990]. However, the amounts that are disposed of in
this way are small compared to the amounts that end up in the
stratosphere. A recent summary [Khalil and Rasmussen 1993] concludes
that out of a total of 9152 Gigagrams (Gg) of CFC-11 produced, only 1
Gg has been removed by soils and 33 Gg reside in the oceans; in
contrast, 1709 Gg have been photolyzed in the stratosphere, 741 Gg are
presently in the stratosphere, and 5360 Gg are in the troposphere.
Most of the remainder is still trapped in foams, aerosols, etc. and
has not yet been released to the atmosphere.
(In contrast, soil bacteria do appear to consume large quantities of
methyl bromide, CH3Br. [Shorter et al.])
Subject: 5. REFERENCES FOR PART II
A remark on references: they are neither representative nor
comprehensive. There are _hundreds_ of people working on these
problems. For the most part I have limited myself to papers that
are (1) widely available (if possible, _Science_ or _Nature_ rather
than archival sources such as _J. Geophys. Res._) and (2) directly
related to the "frequently asked questions". (In this part, I have
had to refer to archival journals more often than I would have
liked, since in many cases that is the only place where the
question is addressed in satisfactory detail.) Readers who want to
see "who did what" should consult the review articles listed below,
or, if they can get them, the extensively documented WMO reports.
Subject: Introductory Reading
[Graedel and Crutzen] T. E. Graedel and P. J. Crutzen,
_Atmospheric Change: an Earth System Perspective_, Freeman, 1993.
[Rowland 1989] F. S. Rowland, "Chlorofluorocarbons and the
depletion of stratospheric ozone", _Am. Sci._ _77_, 36, 1989.
Subject: Books and Review Articles
[Brasseur and Solomon] G. Brasseur and S. Solomon, _Aeronomy of
the Middle Atmosphere_, 2nd Edition, D. Reidel, 1986.
[McElroy and Salawich] M. McElroy and R. Salawich, "Changing
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[Rowland 1991] F. S. Rowland, "Stratospheric Ozone Depletion",
_Ann. Rev. Phys. Chem._ _42_, 731, 1991.
[Solomon] S. Solomon, "Progress towards a quantitative
understanding of Antarctic ozone depletion",
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[Wallace and Hobbs] J. M. Wallace and P. V. Hobbs,
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[Wayne] R. P. Wayne, _Chemistry of Atmospheres_, 2nd Ed., Oxford, 1991.
[WMO 1988] World Meteorological Organization,
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[WMO 1991] World Meteorological Organization,
_Scientific Assessment of Ozone Depletion: 1991_, Report # 25
[WMO 1994] World Meteorological Organization,
_Scientific Assessment of Ozone Depletion: 1994_
Global Ozone Research and Monitoring Project - Report #37.
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