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Ozone Depletion FAQ Part II: Stratospheric Chlorine and Bromine |
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Archive-name: ozone-depletion/stratcl
Last-modified: 16 Dec 1997
Version: 5.9
-----------------------------
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Subject: Copyright Statement
***********************************************************************
* Copyright 1997 Robert Parson *
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* This file may be distributed, copied, and archived. All such *
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* "Caveat". Reproduction and distribution for personal profit is *
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* a reference for school projects; it should instead be used as a *
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-----------------------------
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
Associate Professor
Department of Chemistry and Biochemistry,
University of Colorado (for which I do not speak)
rparson@spot.colorado.edu
Robert.Parson@colorado.edu
-----------------------------
Subject: TABLE OF CONTENTS
How to get this FAQ
Copyright Notice
General Information
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
terrestrial mechanisms.
5. REFERENCES FOR PART II
Introductory Reading
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
radical:
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
chlorine nitrate:
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
km
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
km ||
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
chlorine compounds.
-----------------------------
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.
b.) Halons
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
together.
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
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