Usenet 1994 January/usenetsourcesnewsgroupsinfomagicjanuary1994.iso/answers/ozone-depletion/stratcl

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From: [e-mail] (Robert Parson)
Subject: Ozone Depletion FAQ Part II: Stratospheric Chlorine and Bromine
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Summary: This is the second of four files dealing with stratospheric
         ozone depletion. It is concerned with sources of chlorine
         and bromine in the earth's stratosphere.
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Keywords: ozone layer cfc stratosphere chlorine bromine volcanoes
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* Copyright 1993 Robert Parson                                        *
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 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.

| 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, including any errors.

*** Corrections and comments are welcomed.

- Robert Parson
  Associate Professor
  Department of Chemistry and Biochemistry,
  University of Colorado  (for which I do not speak)

  [e-mail]
  [e-mail]


CONTENTS

1.  THE STRATOSPHERE

1.1) What is the stratosphere?
1.2) How is the composition of air described?
1.3) How does the composition of air change with height?
     (Or, "CFC's are heavier than air - so how can they get into
      the stratosphere?")

2.  CHLORINE IN THE STRATOSPHERE

2.1) Where does the Chlorine in the stratosphere come from?
2.2) What is the evidence for anthropogenic sources?
2.3) What is the trend of stratospheric chlorine concentration?
2.4) In what molecules is stratospheric chlorine found?
2.5) What happens to organic chlorine compounds in stratosphere?
2.6) How is chlorine removed from the atmosphere?
2.7) What are the possible sources of the HCl in the
     _stratosphere_?
2.8) What is the source of HCl in the troposphere?
2.9) How is chlorine distributed in the stratosphere?
2.10) Which source of stratospheric chlorine is supported by this
      evidence?
2.11) How do we know that the CFC's in the stratosphere are being
      photolyzed?
2.12) How do the CFCs produced in the Northern Hemisphere get to
      the  Antarctic?
2.13) Isn't it true that volcanoes put much more chlorine into the
      stratosphere than CFCs?
2.14) How much chlorine comes from rockets and Space Shuttle
      launches?

3.  BROMINE IN THE STRATOSPHERE

3.1) Is bromine important to the ozone destruction process?
3.2) How does bromine affect ozone concentrations?
3.3) Where does the bromine come from?

4.  REFERENCES
=================================================================

1.  THE STRATOSPHERE

1.1) What is the stratosphere?

The stratosphere extends from about 15 km to 50 km (the precise
altitude of the lower boundary, known as the tropopause, varies
between ~10 and ~17 km, depending upon latitude and season.) In the
stratosphere temperature _increases_ with altitude, due to the
absorption of UV light by oxygen and ozone.  This creates a global
"inversion layer" which impedes vertical motion into and within
the stratosphere. The word "stratosphere" has the same root as the
word "stratification" or layering.

 The stratosphere is often compared to the "troposphere", which is
the atmosphere below about 15 km.  The prefix "tropo" refers to
change:  the troposphere is the part of the atmosphere in which
weather occurs. This results in relatively rapid mixing of
tropospheric air. [Wayne] [Wallace and Hobbs]

1.2) How is the composition of air described?
     (Or, What is a 'mixing ratio'?)

 The density of the air in the atmosphere depends upon altitude,
and in a complicated way because the temperature also varies with
altitude.  It is therefore awkward to report concentrations of
atmospheric species in units like g/cc or molecules/cc.  Instead,
it is convenient to report the relative number of molecules - the
number of molecules of a given component in a small volume,
relative to the total number of molecules in that volume. Chemists
usually call this a mole fraction, but atmospheric scientists have
taken to calling it a "mixing ratio".  Typical units for trace
species are parts-per-billion by volume, or "ppbv". The phrase
"by volume" reflects "Avogadro's Law": for an ideal gas mixture,
equal volumes contain equal numbers of molecules.


1.3) How does the composition of air change with height?
(Or, "CFC's are heavier than air - so how can they get into the
stratosphere?")

In the earth's troposphere and stratosphere, most _stable_ chemical
species are well-mixed - their mixing ratios are independent of
altitude.  If a species' mixing ratio changes with altitude, some
kind of physical or chemical transformation is taking place.
That last statement may seem surprising - one might expect the
heavier molecules to dominate at lower altitudes.  The mixing ratio
of Krypton (mass 84), then, would decrease with altitude, while
that of Helium (mass 4) would increase.  In reality, however,
molecules do not segregate by weight in the troposphere or
stratosphere.  The relative proportions of Helium, Nitrogen, and
Krypton are unchanged up to about 100 km.

Why is this? Vertical transport in the troposphere takes place by
convection and turbulent mixing.  In the stratosphere and in the
next layer up, the "mesosphere", it takes place by "eddy diffusion"
- the gradual mechanical mixing of gas by small scale motions.
These mechanisms do not distinguish molecular masses.  Only at much
higher altitudes do mean free paths become large enough that
_molecular_ diffusion dominates and gravity is able to separate the
different species. [Wayne] [Wallace and Hobbs]

Experimental measurements of the fluorocarbon CF4 verify this
homogeneous mixing.  CF4 has an extremely long lifetime in the
stratosphere - probably many thousands of years.  The mixing ratio
of CF4 in the strat.  was found to be 0.056-0.060 ppbv from 10-50
km, with no overall trend.  [Zander et al. 1992]

An important trace gas that is *not* well-mixed is water vapor. The
lower troposphere contains a great deal of water - as much as 30,000
ppmv in humid tropical latitudes. High in the troposphere, however,
the water condenses and falls to the earth as rain or snow, so that
the stratosphere is extremely dry, typical mixing ratios being about
4 ppmv. Indeed, the transport of water vapor from troposphere to
stratosphere is even more inefficient than this would suggest, since
much of the small amount of water in the stratosphere is actually
produced _in situ_ by the oxidation of methane.

Sometimes that part of the atmosphere in which the chemical
composition of stable species does not change with altitude is
called the "homosphere". The homosphere includes the troposphere,
stratosphere, and the next layer up, the "mesosphere". The upper
regions of the atmosphere are then referred to as the
"heterosphere". [Wayne] [Wallace and Hobbs]


2.  CHLORINE IN THE STRATOSPHERE

2.1) Where does the Chlorine in the stratosphere come from?

~80% from CFC's and related manmade organic chlorine compounds (eg.
CCl4) ~15-20% from methyl chloride (CH3Cl), most of which (~3/4) is
natural.  A few % from inorganic sources, including volcanic
eruptions.  [WMO 1991] [Solomon] [AASE] [Rowland 1989,1991] [Wayne]

2.2) What is the evidence for anthropogenic sources?

 The numbers above come from measurements of the altitude and time
dependence of the natural and manmade chlorine- and
fluorine-containing compounds in the troposphere and stratosphere.
The mixing ratios of the manmade compounds are almost independent
of altitude in the troposphere and drop off rapidly in the
stratosphere. The mixing ratios of the inorganic chlorine compounds
drop off rapidly in the troposphere, then _increase_ rapidly in the
stratosphere, suggesting that they are being produced there by
photolysis of the organic chlorine compounds.  At the bottom of the
stratosphere nearly all of the chlorine is organic, at the top it
is all inorganic, suggesting a quantitative conversion from one to
the other. At the same time, the total amount of  fluorine in the
stratosphere has been increasing, suggesting that the additional
chlorine is coming from compounds that also contain Fluorine.
The details are presented in the next few sections.


2.3) What is the trend of stratospheric chlorine concentration?

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 such increases.  On the other
hand, emissions of CFC's and related manmade compounds have
increased enormously. The major sink for the CFC's has been firmly
established to be UV photolysis in the stratosphere. In 1989, the
concentrations of the major CFC's in the troposphere were
increasing at about 4% per year. [WMO 1991].

2.4) In what molecules is stratospheric chlorine found?

Let us divide up the chlorine compounds in the stratosphere into
"organic" (i.e.  carbon-containing) and "inorganic".  The major
inorganic chlorine compound in the troposphere is Hydrogen Chloride
(HCl); in the stratosphere the major compounds are HCl and
Chlorine Nitrate (ClONO2). These are called "chlorine reservoirs" -
they do not themselves react with ozone, but they generate a small
proportion of chlorine-containing radicals which do.  The various
chlorine-containing compounds are chemically active to varying extents,
and a complex chemical equilibrium involving the concentrations of the
various species and the local pressure and temperature results.

The major _organic_ chlorine compounds that reach the stratosphere
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.

There are many other organic chlorine compounds, natural and
manmade, but they are either produced in small quantities or have
short tropospheric lifetimes, and they have no relevance for
stratospheric ozone depletion. Vinyl chloride, for example, is
produced in much larger quantities than any of the CFC's, but it
is quickly destroyed in the troposphere [Brasseur and Solomon].
The chlorine atoms that are released eventually find their way
into HCl or other water-soluble compounds and are washed out.


2.5) What happens to organic chlorine compounds 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 and nitrogen oxides:

 Cl + O3 -> ClO + O2
 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 lead to
HCl and ClONO2. Most of the inorganic chlorine in the stratosphere
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]


2.6) How is chlorine removed from the atmosphere?

The major chlorine reservoir, HCl, is very soluble in water,
and is quickly washed out of the troposphere; its lifetime there
is estimated to be 1-7 days.  On the other hand, its stratospheric
lifetime is about 2 years, with the principal sink being transport
back down to the troposphere.


2.7) What are the possible sources of the HCl in the
_stratosphere_?

There are three possibilities:

 i.    It can _drift_ up from the troposphere.
 ii.   It can be _produced_ in the stratosphere, as the end product
       of photolysis of the organic chlorine compounds, as described
       above.
 iii.  It can be _injected_ into the stratosphere by a large volcanic
       eruption.

These can be distinguished by measuring how the HCl mixing ratio
varies with altitude.  In case (1), we expect to see a more-or-less
uniform distribution through the stratosphere.  In case (2) we
expect to see the HCl mixing ratio _increase_ strongly with
altitude in the stratosphere, since there is more short-wavelength
UV, and thus faster photolysis of the organic compounds, the higher
you go.  In particular we ought to be able to correlate the
concentrations of organic and inorganic chlorine compounds.  We can
develop a simple model for this if we assume that photolysis
instantaneously converts an organochlorine molecule into its final
inorganic products. Such a picture would be correct if the chemical
reactions following photolysis were very fast compared to vertical
transport. (As we have  seen above this is only partially true:
photolysis pops off a single Cl atom which reaches its final
destination quickly, but the remaining Cl atoms are removed by a
sequence of slower reactions. This turns out not to be such a
serious problem, however, and we can compensate for it in part by
measuring the reaction intermediates and including them in the
chlorine budget.)  If we do assume that the chemistry is fast
compared to vertical transport, then we ought to find that at any
altitude the mixing ratios of Cl from all species should add up to
a constant, with the relative proportion of inorganic Cl increasing
with altitude until eventually there is no organic chlorine left.
In case (3) we expect to see irregular behavior: the HCl should be
concentrated in the region of the volcanic plume immediately after
the eruption, and then diffuse out to a more uniform distribution.
We will deal with case (3) in more detail in another section, as a
number of complications arise in connection with it.


2.8) What is the source of HCl in the troposphere?

The principal source of HCl in the troposphere 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.) As mentioned above, however,
condensation of water vapor efficiently removes HCl from the upper
troposphere; in-situ and spectroscopic measurements that the HCl
mixing ratio is less than 0.1 ppbv at elevations above 3 km,  and
less than 0.04 ppbv at 13.7 km.  [Vierkorn-Rudolf et al. ]
[Harris et al.] Even the largest, low-altitude concentrations are
much smaller than the total chlorine from organic sources which
amounted to ~3.8 ppbv in 1989 [WMO 1991]. In the upper troposphere,
organic chlorine dominates overwhelmingly.


2.9) 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.  The basic trends have been clear since the late 1970's,
and they confirm that photolysis of CFC's and related compounds is
indeed the major source (scenario (2) above).

 The HCl distribution in the stratosphere differs markedly from that
in the troposphere. The HCl mixing ratio _increases_ rapidly with
altitude up to about 35 km, above which it increases more slowly,
up to 55 km and beyond. This was noticed as early as 1976
[Farmer et al.] [Eyre and Roscoe] and has been confirmed repeatedly
since.  The other important inorganic chlorine compound in the
stratosphere, Chlorine Nitrate, ClONO2, also increases rapidly in the
lower strat., then falls off at higher altitudes.  These results
strongly suggest that the HCl in the stratosphere is being _produced_
there, not drifting up from below.

Let us now look at the organic compounds.  Again, there is an
enormous body of data, all of which shows 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 drop-off in organic chlorine correlates
nicely to 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.  And,
_at the bottom of the stratosphere almost all of the chlorine is
organic_.  (This fact by itself is strong evidence that HCl
diffusing up from  the troposphere is _not_ the major source of
chlorine in the stratosphere.) [Fabian et al. ] [Zander et al. 1987]
[Zander et al. 1992] [Penkett et al.]

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, Mixing ratios in ppbv

Alt.,  CH3Cl CCl4 CCl2F2 CCl3F CHClF2 CH3CCl3 C3F3Cl3  COFCl
km
12.5  .580  .100  .310  .205  .066    0.096    0.021   0.004
15    .515  .085  .313  .190  .066    0.084    0.019   0.010
20    .350  .035  .300  .137  .061    0.047    0.013   0.035
30      -    -    .030   -    .042      -        -     0.029
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
30   1.452 1.016    .107  .077       2.652    0.131       2.78
40   2.213 0.010    .234  .142       2.607      -         2.61


(The complete tables give results every 2.5 km from 12.5 to 55km,
and also contain a similar inventory of the fluorine compounds.
Standard errors on total Cl were estimated to be 0.02-0.04 ppbv.)

2.10) Which source of stratospheric chlorine is supported by this
      evidence?

We see that the _total_ Cl concentration is roughly constant at
2.5-2.7 ppbv, suggesting that all but about 0.2 ppbv has been
accounted for.  There is nearly quantitative conversion of organic
chlorine in the lower stratosphere to inorganic chlorine in the
upper stratosphere.

Of course this approach - adding up mixing ratios at fixed altitude
- is simplistic.  Making it truly quantitative requires a lot
of work, accounting for vertical and horizontal transport time
scales and complex chemistry.  One finds, for example, that the
altitude profiles are "compressed" in the polar stratosphere in
winter, as the cold air descends.  When all of these factors are built
into atmospheric models, reasonably good agreement is achieved for
the altitude dependence of the major chlorine compounds.  [McElroy
and Salawich].

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.


2.11) How do we know that the CFC's in the stratosphere are being
photolyzed?

The previous argument - CFC mixing ratios decrease with altitude,
inorganic chlorine mixing ratios increase with altitude - certainly
suggests that one is being transformed into the other. But there is
direct evidence as well:

i.  Increasing concentrations of HF in the stratosphere - a factor
of 4 between 1978 and 1989 [Zander et al. 1990] HF is the major
reservoir for Fluorine in the stratosphere, just as HCl is the
major chlorine reservoir.  The Fluorine budget, as a function of
altitude, adds up in much the same way as the Chlorine budget.
There are some discrepancies in the lower stratosphere; model
calculations predict _less_ HF than is actually observed.  [Zander
et al. 1992].

ii.  Observation of reaction intermediates such as COF2 and COFCl.
 These are formed when the photolysis products react with oxygen.
 Looking back at the table, notice that COFCl appears in precisely
 the altitude range in which the CFC's are disappearing.


2.12) How do the CFCs produced in the Northern Hemisphere get 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 [Singh et al.]. [Elkins et al.]


2.13) Isn't it true that volcanoes put much more chlorine into the
stratosphere than CFCs?

Short Reply: No. They 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. Remember,
the mixing ratio of HCl _decreases_ with altitude in the troposphere,
reaching vanishingly small values at the tropopause, and then _increases_
with altitude in the stratosphere.  This rules out all processes in
which HCl slowly drifts upward from the troposphere. It does not,
however, rule out a _major_ volcanic eruption, which could 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,
                                   Megatons (Mt)

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            20 - 30
Krakatau, 1883      6              50 (est.)
Tambora, 1815       7              80-200 (est.)

[Smithsonian] [Symonds et al.] [Sigurdsson] [Pinatubo] [WMO 1988]

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 - you may remember that 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.  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 >1.2 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
ofvolcanic 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 - found in
Dixy Lee Ray's _Trashing the Planet_  - 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 arisen
from a disastrous editorial mistake in a scientific encyclopedia).

 It is very difficult to reconcile these upper limits 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 the HCl has an inorganic origin,
where is all that _organic_ 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], - 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.

------------------------------------------------------------------
 In summary:

 * 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
   whatsoever.

---------------------------------------------------------------

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. This claim was repeated in Dixy Lee Ray's books.
"21st Century" is published by Lyndon LaRouche's political
associates, although LaRouche himself usually keeps a low profile
in the magazine.  Mt. Erebus has in fact been simmering quietly for
over a century but the estimate of 1000 tons/day of HCl only applied
to an especially active period between 1976 and 1983.  Moreover that
estimate [Kyle et al.] 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. The only places where I have
ever seen Erebus described as a source of stratospheric chlorine is
in LaRouchian publications and in articles and books that, incredibly,
consider such documents to be reliable sources.


2.14) How much chlorine comes from rockets and Space Shuttle
launches?

 Very little. 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  68 metric tons of chlorine as
HCl; a full year's worth of shuttle and solid-  rocket launches
produces about 725 tons. This is negligible compared to chlorine
emissions in the form of CFC's and related compounds (1.2 million
tons/yr in the 1980's, of which ~0.3 Mt reach the stratosphere each
year). It is also negligible in comparison to natural sources, which
produce about 75,000 tons per year. [Prather et al.] [WMO 1991].

See also the sci.space FAQ, Part 10, "Controversial Questions".


3.  BROMINE

3.1) Is bromine important to the ozone destruction process?

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.

3.2) How does bromine affect ozone concentrations?

 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, the principal
pathway by which Br destroys ozone also involves Cl:

     BrO + ClO -> Br + Cl + O2
     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.


3.3) Where does the bromine come from?

The largest source of stratospheric Bromine is methyl bromide,
CH3Br.  Much of this is natural (as with CH3Cl), but 30 - 60% is
manmade. [Khalil et al.] It is widely used as a fumigant.
Another important source is the family of "halons".  Like
CFC's these compounds have long atmospheric lifetimes (72 years for
CF3Br) and very little is lost in the troposphere.  [Wayne p. 167].
At the bottom of the stratosphere the total Br mixing ratio is ~20
parts-per-trillion (pptv), of which ~8 pptv is manmade.  [AASE]
Uncertainties in these numbers are relatively larger than for Cl,
because the absolute quantities are so much smaller, and we should
expect to see them change.


4.  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.


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.

--------------------------------
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
Composition of the Global Stratosphere", _Science_ _243, 763, 1989.

[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",
 _Nature_ _347_, 347, 1990.

[Wallace and Hobbs] J. M. Wallace and P. V. Hobbs,
_Atmospheric Science: an Introductory Survey_, Academic Press, 1977.

[Wayne] R. P. Wayne, _Chemistry of Atmospheres_,
        2nd.  Ed., Oxford, 1991.

[WMO 1988] World Meteorological Organization,
_Report of the International Ozone Trends Panel_, Report # 18

[WMO 1991] World Meteorological Organization,
_Scientific Assessment of Ozone Depletion: 1991_, Report # 25
-----------------------------

More specialized articles:

[AASE] End of Mission Statement, second airborne arctic
stratospheric expedition, NASA 30 April 1992.

[Cadle] R. Cadle, "Volcanic emissions of halides and sulfur
compounds to the troposphere and stratosphere", J. Geophys. Res.
_80_, 1651, 1975]

[Delmas] R. J. Delmas, "Environmental Information from Ice Cores",
_Reviews of Geophysics_ _30_, 1, 1992.

[Elkins et al.] J. W. Elkins, T. M. Thompson, T. H. Swanson,
J. H. Butler, B. D. Hall, S. O. Cummings, D. A. Fisher, and
A. G. Raffo, "Decrease in Growth Rates of Atmospheric
Chlorofluorocarbons 11 and 12", _Nature_ _364_, 780, 1993.

[Eyre and Roscoe] J. Eyre and H. Roscoe, "Radiometric measurement
of stratospheric HCl", _Nature_ _266_, 243, 1977.

[Fabian et al. 1979] P. Fabian, R. Borchers, K.H. Weiler, U.
Schmidt, A. Volz, D.H. Erhalt, W. Seiler, and F. Mueller,
"Simultaneously measured vertical profile of H2, CH4, CO, N2O,
CFCl3, and CF2Cl2 in the mid-latitude stratosphere and
troposphere", J. Geophys. Res.  _84_, 3149, 1979.

[Fabian et al. 1981] P. Fabian, R. Borchers, S.A. Penkett, and
N.J.D. Prosser, "Halocarbons in the Stratosphere", _Nature_ _294_,
733, 1981.

[Farmer et al.] C.B. Farmer, O.F. Raper, and R.H. Norton,
"Spectroscopic detection and vertical distribution of HCl in the
troposphere and stratosphere", Geophys. Res. Lett. _3_, 13, 1975.

[Harris et al.] G.W. Harris, D. Klemp, and T. Zenker,
"An Upper Limit on the HCl near-surface mixing ratio over the
Atlantic", J. Atmos. Chem. _15_, 327, 1992.

[Johnston] D. Johnston, "Volcanic contribution of chlorine to the
 stratosphere: more significant to ozone than previously
estimated?" _Science_ _209_, 491, 1980.

[Khalil et al.] M.A.K. Khalil, R. Rasmussen, and R. Gunawardena,
"Atmospheric Methyl Bromide: Trends and Global Mass Balance"
 J. Geophys. Res. _98_, 2887, 1993.

[Kyle et al.] P.R. Kyle, K. Meeker, and D. Finnegan,
"Emission rates of sulfur dioxide, trace gases, and metals from
 Mount Erebus, Antarctica", _Geophys. Res. Lett._ _17_, 2125, 1990.

[Mankin and Coffey] W. Mankin and M. Coffey, "Increased
stratospheric hydrogen chloride in the El Chichon cloud",
_Science_ _226_, 170, 1983.

[Mankin, Coffey and Goldman] W. Mankin, M. Coffey and A. Goldman,
"Airborne observations of SO2, HCL, and O3 in the stratospheric
plume of the Pinatubo volcano in July 1991", Geophys. Res. Lett.
_19_, 179, 1992.

[Penkett et al.] S.A. Penkett, R.G. Derwent, P. Fabian, R.
Borchers, and U. Schmidt, "Methyl Chloride in the Stratosphere",
_Nature_ _283_, 58, 1980.

[Pinatubo] Special Mt. Pinatubo issue, Geophys. Res. Lett. _19_,
#2, 1992.

[Pinto et al.] J. Pinto, R. Turco, and O. Toon, "Self-limiting
physical and chemical effects in volcanic eruption clouds",
 J. Geophys. Res. _94_, 11165, 1989.

[Prather et al. ] M. J. Prather, M.M. Garcia, A.R. Douglass, C.H.
Jackman, M.K.W. Ko, and N.D. Sze, "The Space Shuttle's impact on
the stratosphere", J. Geophys. Res. _95_, 18583, 1990.

[Singh et al.] H. Singh, L. Salas, H. Shigeishi, and E. Scribner,
"Atmospheric Halocarbons, hydrocarbons, and sulfur hexafluoride
global distributions, sources, and sinks", _Science_ _203_, 899,
1974.

[Sigurdsson] H. Sigurdsson, "Evidence of volcanic loading of the
 atmosphere and climate response",  _Palaeogeography,
Palaeoclimatology, Palaeoecology_ _89_, 277 (1989).

[Smithsonian] Smithsonian Report, _Global Volcanism:1975-85_, p 14.

[Symonds et al.] R. B. Symonds, W. I. Rose, and M. H. Reed,
"Contribution of Cl and F-bearing gases to the atmosphere by
volcanoes", _Nature_ _334_, 415 1988.

[Tabazadeh and Turco] A. Tabazadeh and R. P. Turco, _Stratospheric
Chlorine Injection by Volcanic Eruptions: HCl Scavenging and
Implications for Ozone_, _Science_ _260_, 1082, 1993.

[Vierkorn-Rudolf et al.] B. Vierkorn-Rudolf. K. Bachmann, B.
Schwartz, and F.X. Meixner, "Vertical Profile of Hydrogen Chloride
in the Troposphere", J. Atmos. Chem.  _2_, 47, 1984.

[Zander et al. 1987] R. Zander, C. P. Rinsland, C. B. Farmer, and
R. H. Norton, "Infrared Spectroscopic measurements of halogenated
source gases in the stratosphere with the ATMOS instrument", J.
Geophys. Res. _92_, 9836, 1987.

[Zander et al. 1990] R. Zander, M.R. Gunson, J.C. Foster, C.P.
Rinsland, and J. Namkung, "Stratospheric ClONO2, HCl, and HF
concentration profiles derived from ATMOS/Spacelab 3 observations
- an update", J. Geophys. Res.  _95_, 20519, 1990.

[Zander et al. 1992] R. Zander, M. R. Gunson, C. B. Farmer, C. P.
Rinsland, F. W. Irion, and E. Mahieu, "The 1985 chlorine and
fluorine inventories in the stratosphere based on ATMOS
observations at 30 degrees North latitude",  J. Atmos. Chem. _15_,
171, 1992.

[Zreda-Gostynska et al.] G. Zreda-Gostynska, P. R. Kyle, and
D. L. Finnegan, "Chlorine, Fluorine and Sulfur Emissions from
Mt. Erebus, Antarctica and estimated contribution to the antarctic
atmosphere", _Geophys. Res. Lett._ _20_, 1959, 1993.