Xref: info.physics.utoronto.ca news.answers:30603 sci.answers:1672 sci.environment:48626
From: email@example.com (Robert Parson)
Subject: Ozone Depletion FAQ Part I: Introduction to the Ozone Layer
Date: 10 Oct 1994 03:15:06 GMT
Organization: University of Colorado, Boulder
Summary: This is the first of four files dealing with stratospheric
ozone depletion. It provides scientific background for
the more detailed questions in the other three parts.
Keywords: ozone layer cfc stratosphere depletion
Last-modified: 9 October 1994
Subject: How to get this FAQ
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Subject: Copyright Statement
* Copyright 1994 Robert Parson *
* This file may be distributed, copied, and archived. All *
* copies must include this notice and the paragraph below entitled *
* "Caveat". Reproduction and distribution for personal profit is *
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Subject: General remarks
This is the first of four FAQ files dealing with stratospheric ozone
depletion. This part deals with basic scientific questions about the
ozone layer, and serves as an introduction to the remaining parts which
are more specialized. Part II deals with sources of stratospheric
chlorine and bromine, part III with the Antarctic Ozone Hole, and Part
IV with the properties and effects of ultraviolet radiation. The later
parts are mostly independent of each other, but they all refer back.
to Part I. I emphasize physical and chemical mechanisms
rather than biological effects, although I make a few remarks about
the latter in part IV. I have little to say about legal and policy
issues other than a very brief summary at the end of part I.
The overall approach I take is conservative. I concentrate on what
is known and on most probable, rather than worst-case, scenarios.
For example, I have relatively little to say about the effects
of UV radiation on terrestrial plants - this does not mean that the
effects are small, it means that they are as yet not well
quantified (and moreover, I am not well qualified to interpret the
literature.) Policy decisions must take into account not only the
most probable scenario, but also a range of less probable ones.
There have been surprises, mostly unpleasant, in this field in the
past, and there are sure to be more in the future.
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
| reactions 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 all 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 document should not be
| cited 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. THE STRATOSPHERE
1.1) What is the stratosphere?
1.2) How is the composition of air described?
1.3) How does the composition of the atmosphere change with
2. THE OZONE LAYER
2.1) How is ozone created?
2.2) How much ozone is in the layer, and what is a
2.3) How is ozone distributed in the stratosphere?
2.4) How does the ozone layer work?
2.5) What sorts of natural variations does the ozone layer show?
2.5.a) Regional and Seasonal Variation
2.5.b) Year-to-year variations.
2.6) What are CFC's?
2.7) How do CFC's destroy ozone?
2.8) What is an "Ozone Depletion Potential?"
2.9) What about HCFC's and HFC's? Do they destroy ozone?
2.10) *IS* the ozone layer getting thinner?
2.11) Is the middle-latitude ozone loss due to CFC emissions?
2.12) If the ozone is lost, won't the UV light just penetrate
2.13) Do Space Shuttle launches damage the ozone layer?
2.14) Will commercial supersonic aircraft damage the ozone layer?
2.15) What is being done about ozone depletion?
3. REFERENCES FOR PART I
Books and Review Articles
More Specialized References
Subject: 1. THE STRATOSPHERE
Subject: 1.1) What is the stratosphere?
The stratosphere extends from about 15 km to 50 km. 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 - since warmer air lies above colder air, convection
is inhibited. The word "stratosphere" is related to the word
"stratification" or layering.
The stratosphere is often compared to the "troposphere", which is
the atmosphere below about 15 km. The boundary - called the
"tropopause" - between these regions is quite sharp, but its
precise location varies between ~10 and ~17 km, depending upon
latitude and season. 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]
Above the stratosphere lie the "mesosphere", ranging from ~50 to
~100 km, in which temperature decreases with altitude; the
"thermosphere", ~100-400 km, in which temperature increases
with altitude again, and the "exosphere", beyond ~400 km, which
fades into the background of interplanetary space. In the upper
mesosphere and thermosphere electrons and ions are abundant, so
these regions are also referred to as the "ionosphere". In technical
literature the term "lower atmosphere" is synonymous with the
troposphere, "middle atmosphere" refers to the stratosphere
and mesosphere, while "upper atmosphere" is usually reserved for the
thermosphere and exosphere. This usage is not universal, however,
and one occasionally sees the term "upper atmosphere" used to
describe everything above the troposphere (for example, in NASA's
Upper Atmosphere Research Satellite, UARS.)
Subject: 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 "mole fraction", the relative
number of molecules of a given type in an air sample. Atmospheric
scientists usually call a mole fraction a "mixing ratio". Typical
units for mixing ratios are parts-per-million, billion, or
trillion by volume, designated as "ppmv", "ppbv", and "pptv"
respectively. (The expression "by volume" reflects Avogadro's Law -
for an ideal gas mixture, equal volumes contain equal numbers of
molecules - and serves to distinguish mixing ratios from "mass
fractions" which are given as parts-per-million by weight.) Thus
when it is said that the mixing ratio of hydrogen chloride at 3 km
is 0.1 ppbv, it means that 1 out of every 10 billion molecules in
an air sample collected at that altitude will be an HCl molecule.
[Wayne] [Graedel and Crutzen]
Subject: 1.3) How does the composition of the atmosphere change with
altitude? (Or, how can CFC's get up to the stratosphere
when they are heavier than air?)
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
mesosphere, it takes place by "eddy diffusion" - the gradual mechanical
mixing of gas by motions on small scales. These mechanisms do not
distinguish molecular masses. Only at much higher altitudes do mean
free paths become so large that _molecular_ diffusion dominates and
gravity is able to separate the different species, bringing hydrogen
and helium atoms to the top.
[Chamberlain and Hunten] [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 stratosphere 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
5 ppmv. Indeed, the transport of water vapor from troposphere to
stratosphere is even less efficient 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. [SAGE II]
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 mesosphere. The upper regions of the atmosphere
- the "thermosphere" and the "exosphere" - are then referred to as
the "heterosphere". [Wayne] [Wallace and Hobbs]
Subject: 2. THE OZONE LAYER
Subject: 2.1) How is ozone created?
Ozone is formed naturally in the upper stratosphere by short
wavelength ultraviolet radiation. Wavelengths less than ~240
nanometers are absorbed by oxygen molecules (O2), which dissociate to
give O atoms. The O atoms combine with other oxygen molecules to
O2 + hv -> O + O (wavelength < 240 nm)
O + O2 -> O3
Subject: 2.2) How much ozone is in the layer, and what is a
"Dobson Unit" ?
A Dobson Unit (DU) is a convenient scale for measuring the total
amount of ozone occupying a column overhead. If the ozone layer
over the US were compressed to 0 degrees Celsius and 1 atmosphere
pressure, it would be about 3 mm thick. So, 0.01 mm thickness at
0 C and 1 at is defined to be 1 DU; this makes the ozone layer over
the US come out to ~300 DU. In absolute terms, 1 DU is about
2.7 x 10^16 molecules/cm^2.
In all, there are about 3 billion metric tons, or 3x10^15 grams,
of ozone in the earth's atmosphere; about 90% of this is in the
The unit is named after G.M.B. Dobson, who carried out pioneering
studies of atmospheric ozone between ~1920-1960. Dobson designed
the standard instrument used to measure ozone from the ground. The
Dobson spectrometer measures the intensity solar UV radiation at
four wavelengths, two of which are absorbed by ozone and two of
which are not. These instruments are still in use in many places,
although they are gradually being replaced by the more elaborate
Brewer spectrometers. Today ozone is measured in many ways, from
aircraft, balloons, satellites, and space shuttle missions, but the
worldwide Dobson network is the only source of long-term data. A
station at Arosa in Switzerland has been measuring ozone since the
1920's, and some other stations have records that go back nearly as
long (although many were interrupted during World War II). The
present worldwide network went into operation in 1956-57.
Subject: 2.3) How is ozone distributed in the stratosphere?
In absolute terms: about 10^12 molecules/cm^3 at 15 km, rising to
nearly 10^13 at 25 km, then falling to 10^11 at 45 km.
In relative terms: ~0.5 parts per million by volume (ppmv) at 15 km,
rising to ~8 ppmv at ~35 km, falling to ~3 ppmv at 45 km.
Even in the thickest part of the layer, ozone is a trace gas.
Subject: 2.4) How does the ozone layer work?
UV light with wavelengths between 240 and 320 nm is absorbed by
ozone, which then falls apart to give an O atom and an O2 molecule.
The O atom soon encounters another O2 molecule, however (at all times,
the concentration of O2 far exceeds that of O3), and recreates O3:
O3 + hv -> O2 + O
O + O2 -> O3
Thus _ozone absorbs UV radiation without itself being consumed_;
the net result is to convert UV light into heat. Indeed, this is
what causes the temperature of the stratosphere to increase with
altitude, giving rise to the inversion layer that traps molecules in
the troposphere. The ozone layer isn't just _in_ the stratosphere; the
ozone layer is responsible for the _existence_ of the stratosphere.
Ozone _is_ destroyed if an O atom and an O3 molecule meet:
O + O3 -> 2 O2 ("recombination").
This reaction is slow, however, and if it were the only mechanism
for ozone loss, the ozone layer would be about twice as thick
as it is. Certain trace species, such as the oxides of Nitrogen (NO
and NO2), Hydrogen (H, OH, and HO2) and chlorine (Cl, ClO and ClO2)
can catalyze the recombination. The present ozone layer is a
result of a competition between photolysis and recombination;
increasing the recombination rate, by increasing the
concentration of catalysts, results in a thinner ozone layer.
Putting the pieces together, we have the set of reactions proposed
in the 1930's by Sidney Chapman:
O2 + hv -> O + O (wavelength < 240 nm) : creation of oxygen atoms
O + O2 -> O3 : formation of ozone
O3 + hv -> O2 + O (wavelength < 320 nm) : absorption of UV by ozone
O + O3 -> 2 O2 : recombination .
Since the photolysis of O2 requires UV radiation while
recombination does not, one might guess that ozone should increase
during the day and decrease at night. This has led some people to
suggest that the "antarctic ozone hole" is merely a result of the
long antarctic winter nights. This inference is incorrect, because
the recombination reaction requires oxygen atoms which are also
produced by photolysis. Throughout the stratosphere the concentration
of O atoms is orders of magnitude smaller than the concentration of
O3 molecules, so both the production and the destruction of ozone by
the above mechanisms shut down at night. In fact, the thickness of the
ozone layer varies very little from day to night, and above 70 km
ozone concentrations actually _increase_ at night.
(The unusual catalytic cycles that operate in the antarctic ozone
hole do not require O atoms; however, they still require light to
operate because they also include photolytic steps. See Part III.)
Subject: 2.5) What sorts of natural variations does the ozone layer show?
There are substantial variations from place to place, and from
season to season. There are smaller variations on time scales of
years and more. [Wayne] [Rowland 1991] We discuss these in turn.
Subject: 2.5.a) Regional and Seasonal Variation
Since solar radiation makes ozone, one expects to see the
thickness of the ozone layer vary during the year. This is so,
although the details do not depend simply upon the amount of solar
radiation received at a given latitude and season - one must also
take atmospheric motions into account. (Remember that
both production and destruction of ozone require solar radiation.)
The ozone layer is thinnest in the tropics, about 260 DU, almost
independent of season. Away from the tropics seasonal variations
become important, but in no case (outside the Antarctic ozone hole)
does the layer become appreciably thinner than in the tropics. For
Location Column thickness, Dobson Units
Jan Apr Jul Oct
Huancayo, Peru (12 degrees S) : 255 255 260 260
Aspendale, Australia (38 deg. S): 300 280 335 360
Arosa, Switzerland (47 deg. N): 335 375 320 280
St. Petersburg, Russia (60 deg. N): 360 425 345 300
These are monthly averages. Interannual standard deviations amount
to ~5 DU for Huancayo, 25 DU for St. Petersburg. [Rowland 1991].
Notice that the highest ozone levels are found in the _spring_,
not, as one might guess, in summer, and the lowest in the fall,
not winter. Indeed, at high latitudes in the Northern Hemisphere
there is more ozone in January than in July! Most of the ozone is
created over the tropics, and then is carried to higher latitudes
by prevailing winds (the general circulation of the stratosphere.)
[Dobson] [Garcia] [Salby and Garcia] [Brasseur and Solomon]
The antarctic ozone hole, discussed in detail in Part III, falls
*far outside* this range of natural variation. Mean October ozone
at Halley Bay on the Antarctic coast was 117 DU in 1993, down
from 321 DU in 1956.
Subject: 2.5.b) Year-to-year variations.
Since ozone is created by solar UV radiation, one expects to see
some correlation with the 11-year solar sunspot cycle. Higher
sunspot activity corresponds to more solar UV and hence more rapid
ozone production. This correlation has been verified, although
its effect is small, about 2% from peak to trough averaged over the
earth, about 4% in polar regions. [Stolarski et al.]
Another natural cycle is connected with the "quasibiennial
oscillation", in which tropical winds in the lower stratosphere
switch from easterly to westerly every 26 months. This leads to
variations of the order of 3% at a given latitude, although the
effect tends to cancel when one averages over the entire globe.
Episodes of unusual solar activity ("solar proton events") can
also affect ozone levels, as can major volcanic eruptions such as
Agung in 1963, El Chichon in 1982, and Pinatubo in 1991. (The
principal mechanism for this is _not_ injection of chlorine into
the stratosphere, as discussed in Part II, but rather the
injection of sulfate aerosols which change the radiation balance
in the stratosphere by scattering light, and which convert
inactive chlorine compounds to active, ozone-destroying forms.)
These are all small effects, however, (a few % at most in a global
average), and persist for short periods, 3 years or less.
Subject: 2.6) What are CFC's?
CFC's - ChloroFluoroCarbons - are a class of volatile organic compounds
that have been used as refrigerants, aerosol propellants, foam blowing
agents, and as solvents in the electronic industry. They are chemically
very unreactive, and hence safe to work with. In fact, they are so inert
that the natural reagents that remove most atmospheric pollutants do not
react with them, so after many years they drift up to the stratosphere
where short-wave UV light dissociates them. CFC's were invented in 1928,
but only came into large-scale production after ~1950. Since that year,
the total amount of chlorine in the stratosphere has increased by
a factor of 4. [Solomon]
The most important CFC's for ozone depletion are:
CFCl3 (CFC-11), and
In discussing ozone depletion, "CFC" is occasionally used to
refer to a somewhat broader class of chlorine-containing organic
compounds that have similar properties - unreactive in the
troposphere, but readily photolyzed in the stratosphere. These
HydroChloroFluoroCarbons such as CHClF2 (HCFC-22),
Carbon Tetrachloride, CCl4,
Methyl Chloroform, CH3CCl3,
and Methyl Chloride, CH3Cl.
(The more careful publications always use phrases like "CFC's and
related compounds", but this gets tedious.)
Only methyl chloride 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.
Most of Part II is devoted to stratospheric chlorine chemistry;
look there for more detail.
Subject: 2.7) How do CFC's destroy ozone?
CFC's themselves do not destroy ozone; certain of their decay products
do. After CFC's are photolyzed, most of the chlorine eventually ends
up as Hydrogen Chloride, HCl, or Chlorine Nitrate, ClONO2. These are
called "reservoir species" - they do not themselves react with ozone.
However, they do decompose to some extent, giving, among other things,
a small amount of atomic chlorine, Cl, and Chlorine Monoxide, ClO,
which can catalyze the destruction of ozone by a number of mechanisms.
The simplest is:
Cl + O3 -> ClO + O2
ClO + O -> Cl + O2
Net effect: O3 + O -> 2 O2
Note that the Cl atom is a _catalyst_ - it is not consumed by the
reaction. Each Cl atom introduced into the stratosphere can
destroy thousands of ozone molecules before it is removed.
The process is even more dramatic for Bromine - it has no stable
"reservoirs", so the Br atom is always available to destroy ozone.
On a per-atom basis, Br is 10-100 times as destructive as Cl.
On the other hand, chlorine and bromine concentrations in
the stratosphere are very small in absolute terms. The mixing ratio
of chlorine from all sources in the stratosphere is about 3 parts
per billion, (most of which is in the form of CFC's that have not
yet fully decomposed) whereas ozone mixing ratios are measured in
parts per million. Bromine concentrations are about 100 times
smaller still. (See Part II.)
The complete chemistry is very complicated - more than 100
distinct species are involved. The rate of ozone destruction at any
given time and place depends strongly upon how much Cl is present
as Cl or ClO, and thus upon the rate at which Cl is released from
its reservoirs. This makes quantitative _predictions_ of future
ozone depletion difficult. [Rowland 1989, 1991] [Wayne]
Subject: 2.8) What is an "Ozone Depletion Potential?"
The ozone depletion potential (ODP) of a compound is a simple measure of
its ability to destroy stratospheric ozone. It is a relative measure:
the ODP of CFC-11 is defined to be 1.0, and the ODP's of other compounds
are calculated with respect to this reference point. Thus a compound with
an ODP of 0.2 is, roughly speaking, one-fifth as "bad" as CFC-11.
More precisely, the ODP of a compound "x" is defined as the ratio of
the total amount of ozone destroyed by a fixed amount of compound x to
the amount of ozone destroyed by the same mass of CFC-11:
Global loss of Ozone due to x
ODP(x) == ---------------------------------
Global loss of ozone due to CFC-11.
Thus the ODP of CFC-11 =1.0 by definition. The right-hand side of the
equation is calculated by combining information from laboratory
and field measurements with atmospheric chemistry and tranport models.
Since the ODP is a relative measure, it is fairly "robust", not overly
sensitive to changes in the input data or to the details of the model
calculations. That is, there are many uncertainties in calculating the
numerator or the denominator of the expression, but most of these cancel
out when the ratio is calculated.
The ODP of a compound will be affected by:
The nature of the halogen (bromine-containing halocarbons usually
have much higher ODPs than chlorocarbons, because atom for atom Br
is a more effective ozone-destruction catalyst than Cl.)
The number of chlorine or bromine atoms in a molecule.
Molecular Mass (since ODP is defined by comparing equal masses
rather than equal numbers of moles.)
Atmospheric lifetime (CH3CCl3 has a lower ODP than CFC-11, because
much of the CH3CCl3 is destroyed in the troposphere.)
The ODP as defined above is a steady-state or long-term property. As
such it can be misleading when one considers the possible effects of CFC
replacements. Many of the proposed replacements have short atmospheric
lifetimes, which in general is good; however, if a compound has a short
_stratospheric_ lifetime, it will release its chlorine or bromine atoms
more quickly than a compound with a longer stratospheric lifetime. Thus
the short term effect of such a compound on the ozone layer is larger
than would be predicted from the ODP alone (and the long-term effect
correspondingly smaller.)(The ideal combination would be a short
tropospheric lifetime, since those molecules which are destroyed in the
troposphere don't get a chance to destroy any stratospheric ozone,
combined with a long stratospheric lifetime.) To get around this, the
concept of a Time-Dependent Ozone Depletion Potential has been
introduced [Solomon and Albritton] [WMO 1991]:
Loss of ozone due to X over time period T
ODP(x,T) == ----------------------------------------------
Loss of ozone due to CFC-11 over time period T
As T->infinity, this converges to the steady-state ODP defined previously.
The following table lists time-dependent and steady-state ODP's for
several halocarbons [Solomon and Albritton] [WMO 1991]
Compound Formula Ozone Depletion Potential
10 yr 30 yr 100 yr Steady State
CFC-113 CF2ClCFCl2 0.56 0.62 0.78 1.10
carbon tetrachloride CCl4 1.25 1.22 1.14 1.08
methyl chloroform CH3CCl3 0.75 0.32 0.15 0.12
HCFC-22 CHF2Cl 0.17 0.12 0.07 0.05
Halon - 1301 CF3Br 10.4 10.7 11.5 12.5
Subject: 2.9) What about HCFC's and HFC's? Do they destroy ozone?
HCFC's (hydrochlorofluorocarbons) differ from CFC's in that only
some, rather than all, of the hydrogen in the parent hydrocarbon
has been replaced by chlorine or fluorine. The most familiar
example is CHClF2, known as "HCFC-22", used as a refrigerant and
in many home air conditioners (auto air conditioners use CFC-12).
The hydrogen atom makes the molecule susceptible to attack by the
hydroxyl (OH) radical, so a large fraction of the HCFC's are
destroyed before they reach the stratosphere. Molecule for
molecule, then, HCFC's destroy much less ozone than CFC's, and
they were suggested as CFC substitutes as long ago as 1976.
Mos HCFC's have ozone depletion potentials around 0.01-0.1, so that
during its lifetime a typical HCFC will have destroyed 1-10% as
much ozone as the same amount of CFC-12. Since the HCFC's are more
reactive in the troposphere, fewer of them reach the stratosphere.
However, they are also more reactive in the stratosphere, so they
release chlorine more quickly. The short-term effects are therefore
larger than one would predict from the steady-state ozone depletion
potential. In deciding upon substitutes for CFC's, the "time-dependent
ozone depletion potential", discussed in the preceding section,
is more informative than the steady-state ODP. [Solomon and Albritton]
HFC's, hydrofluorocarbons, contain no chlorine at all, and hence
have an ozone depletion potential of zero. (In 1993 there were
tentative reports that the fluorocarbon radicals produced by
photolysis of HFC's could catalyze ozone loss, but this has now
been shown to be negligible [Ravishankara et al.]) A familiar
example is CF3CH2F, known as HFC-134a, which is being used in some
automobile air conditioners and refrigerators. HFC-134a is more
expensive and more difficult to work with than CFC's, and while it
has no effect on stratospheric ozone it is a greenhouse gas (though
somewhat less potent than the CFC's). Some engineers have argued
that non-CFC fluids, such as propane-isobutane mixtures, are better
substitutes for CFC-12 in auto air conditioners than HFC-134a.
Subject: 2.10) *IS* the ozone layer getting thinner?
There is no question that the ozone layer over antarctica has thinned
dramatically over the past 15 years (see part III). However, most of
us are more interested in whether this is also taking place at
middle latitudes. The answer seems to be yes, although so far the
effect are small.
After carefully accounting for all of the known natural variations,
a net decrease of about 3% per decade for the period 1978-1991
was found. This is a global average over latitudes from 66 degrees
S to 66 degrees N (i.e. the arctic and antarctic are excluded in
calculating the average). The depletion increases with latitude,
and is somewhat larger in the Southern Hemisphere. Over the US, Europe
and Australia 4% per decade is typical; on the other hand there is
no significant ozone loss in the tropics. The depletion is larger in
the winter months, smaller in the summer. [Stolarski et al.]
The following table, extracted from a much more detailed one in
[Herman et al.], illustrates the seasonal and regional trends in
_percent per decade_ for the period 1979-1990:
Latitude Jan Apr Jul Oct Example
65 N -3.0 -6.6 -3.8 -5.6 Iceland
55 N -4.6 -6.7 -3.1 -4.4 Moscow, Russia
45 N -7.0 -6.8 -2.4 -3.1 Minneapolis, USA
35 N -7.3 -4.7 -1.9 -1.6 Tokyo
25 N -4.2 -2.9 -1.0 -0.8 Miami, FL, USA
5 N -0.1 +1.0 -0.1 +1.3 Somalia
5 S +0.2 +1.0 -0.2 +1.3 New Guinea
25 S -2.1 -1.6 -1.6 -1.1 Pretoria, S. Africa
35 S -3.6 -3.2 -4.5 -2.6 Buenos Aires
45 S -4.8 -4.2 -7.7 -4.4 New Zealand
55 S -6.1 -5.6 -9.8 -9.7 Tierra del Fuego
65 S -6.0 -8.6 -13.1 -19.5 Palmer Peninsula
(These are longitudinally averaged satellite data, not individual
measurements at the places listed in the right-hand column. There
are longitudinal trends as well.)
Between 1991 and 1993 these trends accelerated. Satellite and
ground-based measurements showed a remarkable decline for 1992
and early 1993, a full 4% below the average value for the
preceding twelve years and 2-3% below the _lowest_ values observed
in the earlier period. In Canada the spring ozone levels were 11-17%
below normal [Kerr et al.]. However, by February 1994 ozone over
the United States had recovered to levels similar to 1991.
[Hofmann et al. 1994b] Sulfate aerosols from the July 1991 eruption
of Mt. Pinatubo are the most likely cause of this transient; these
aerosols can convert inactive "reservoir" chlorine into active
ozone-destroying forms, and can also interfere with the production
and transport of ozone by changing the solar radiation balance in
the stratosphere. [Brasseur and Granier] [Hofmann and Solomon]
[Hofmann et al. 1994a] Another cause may be the unusually strong
arctic polar vortex in 1992-93, which made the arctic stratosphere
more like the antarctic than is usually the case. [Gleason et al.]
[Waters et al.] In any event, the rapid ozone loss in 1992 and 1993
appears to have been a transient phenomenon, superimposed upon the
much slower downward trend identified for 1970-1991.
Subject: 2.11) Is the middle-latitude ozone loss due to CFC emissions?
That's the majority opinion, although not everyone agrees. The
present trends are too small to allow a watertight case to be made
(as _has_ been made for the far larger, but localized, depletion
in the Antarctic Ozone hole; see Part III.). Other possible causes
are being investigated. To quote from the 1991 Scientific Assessment
published by the World Meteorological Organization, p. 4.1 [WMO 1991]:
"The primary cause of the Antarctic ozone hole is firmly
established to be halogen chemistry....There is not a full
accounting of the observed downward trend in _global ozone_.
Plausible mechanisms include heterogeneous chemistry on sulfate
aerosols [which convert reservoir chlorine to active chlorine -
R.P.] and the transport of chemically perturbed polar air to middle
latitudes. Although other mechanisms cannot be ruled out, those
involving the catalytic destruction of ozone by chlorine and
bromine appear to be largely responsible for the ozone loss and
_are the only ones for which direct evidence exists_."
(emphases mine - RP)
The Executive Summary of the subsequent 1994 scientific assessment states:
"Direct in-situ meaurements of radical species in the lower
stratosphere, coupled with model calculations, have quantitatively shown
that the in-situ photochemical loss of ozone due to (largely natural)
reactive nitrogen (NOx) compounds is smaller than that predicted from
gas-phase chemistry, while that due to (largely natural) HOx compounds
and (largely anthropogenic) chlorine and bromine compounds is larger
than that predicted by gas-phase chemistry. This confirms the key role
of chemical reactions on sulfate aerosols in controlling the chemical
balance of the lower stratosphere. These and other recent scientific
findings strengthen the conclusion of the previous assessment that the
weight of scientific evidence suggests that the observed middle- and
high-latitude ozone losses are largely due to anthropogenic chlorine and
bromine compounds." [WMO 1994]
A legal analogy might be useful here - the connection between
_antarctic_ ozone depletion and CFC emissions has been proved beyond
a reasonable doubt, while at _middle latitudes_ there is only
probable cause for such a connection.
One must remember that there is a natural 10-20 year time lag
between CFC emissions and ozone depletion. Ozone depletion today is
(probably) due to CFC emissions in the 1970's. Present
controls on CFC emissions are designed to avoid possibly large
amounts of ozone depletion 30 years from now, not to repair the
depletion that has taken place up to now.
Subject: 2.12) If the ozone is lost, won't the UV light just penetrate
deeper into the atmosphere and make more ozone?
This does happen to some extent - it's called "self-healing" - and
has the effect of moving ozone from the upper to the lower
stratosphere. Recall that ozone is _created_ by UV with wavelengths
less than 240 nm, but functions by _absorbing_ UV with wavelengths
greater than 240 nm. The peak of the ozone absorption band is at
~250 nm, and the cross-section falls off at shorter wavelengths.
The O2 and O3 absorption bands do overlap, though, and UV radiation
between 200 and 240 nm has a good chance of being absorbed by
_either_ O2 or O3. (Below 200 nm the O2 absorption cross-section
increases dramatically, and O3 absorption is insignificant in
comparison.) Since there is some overlap, a decrease in ozone does
lead to a small increase in absorption by O2. This is a weak feedback,
however, and it does not compensate for the ozone destroyed. Negative
feedback need not imply stability, just as positive feedback need not
Numerical calculations of ozone depletion take the "self-healing"
phenomenon into account, by letting the perturbed ozone layer come
into equilibrium with the exciting radiation.
Subject: 2.13) Do Space Shuttle launches damage the ozone layer?
Very little. In the early 1970's, when 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 schedule of shuttle and solid rocket launches injects
725 tons of chlorine into the stratosphere. 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 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, and on the world-wide web at:
Subject: 2.14) Will commercial supersonic aircraft damage the ozone layer?
Short answer: Probably not. This problem is very complicated,
and a definite answer will not be available for several years,
but present model calculations indicate that a fleet of high-speed
civil transports would deplete the ozone layer by < 2%. [WMO 1991, 1994]
Long answer (this is a tough one):
Supersonic aircraft fly in the stratosphere. Since vertical transport
in the stratosphere is slow, the exhaust gases from a supersonic jet
can stay there for two years or more. The most important exhaust gases
are the nitrogen oxides, NO and NO2, collectively referred to as "NOx".
NOx is produced from ordinary nitrogen and oxygen by electrical
discharges (e.g. lightning) and by high-temperature combustion (e.g. in
automobile and aircraft engines).
The relationship between NOx and ozone is complicated. In the
troposphere, NOx _makes_ ozone, a phenomenon well known to residents
of Los Angeles and other cities beset by photochemical smog. At high
altitudes in the troposphere, similar chemical reactions produce ozone
as a byproduct of the oxidation of methane; for this reason ordinary
subsonic aircraft actually increase the thickness of the ozone layer
by a very small amount.
Things are very different in the stratosphere. Here the principal
source of NOx is nitrous oxide, N2O ("laughing gas"). Most of the
N2O in the atmosphere comes from bacteriological decomposition of
organic matter - reduction of nitrate ions or oxidation of ammonium
ions. (It was once assumed that anthropogenic sources were negligible
in comparison, but this is now known to be false. The total
anthropogenic contribution is now estimated at 8 Tg (teragrams)/yr,
compared to a natural source of 18 Tg/yr. [Khalil and Rasmussen].)
N2O, unlike NOx, is very unreactive - it has an atmospheric lifetime
of more than 150 years - so it reaches the stratosphere, where most of
it is converted to nitrogen and oxygen by UV photolysis. However, a
small fraction of the N2O that reaches the stratosphere reacts instead
with oxygen atoms (to be precise, with the very rare electronically
excited singlet-D oxygen atoms), and this is the major natural source
of NOx in the stratosphere. About 1.2 million tons are produced each
year in this way. This source strength would be matched by 500 of the
SST's designed by Boeing in the late 1960's, each spending 5 hours per
day in the stratosphere. (Boeing was intending to sell 800 of these
aircraft.) The Concorde, a slower plane, produces less than half as
much NOx and flies at a lower altitude; since the Concorde fleet is
small, its contribution to stratospheric NOx is not significant. Before
sending large fleets of high-speed aircraft into the stratosphere,
however, one should certainly consider the possible effects of
increasing the rate of production of an important stratospheric trace
gas by as much as a factor of two. [CIC 1975]
In 1969, Paul Crutzen discovered that NOx could be an efficient
catalyst for the destruction of stratospheric ozone:
NO + O3 -> NO2 + O2
NO2 + O -> NO + O2
net: O3 + O -> 2 O2
This sequence was rediscovered two years later by H. S. Johnston, who
made the connection to SST emissions. Until then it had been thought
that the radicals H, OH, and HO2 (referred to collectively as "HOx")
were the principal catalysts for ozone loss; thus, investigations of
the impact of aircraft exhaust on stratospheric ozone had focussed on
emissions of water vapor, a possible source for these radicals. (The
importance of chlorine radicals, Cl, ClO, and ClO2, referred to as -
you guessed it - "ClOx", was not discovered until 1973.) It had been
argued - correctly, as it turns out - that water vapor injection was
unimportant for determining the ozone balance. The discovery of
the NOx cycle threw the question open again.
Beginning in 1972, the U.S. National Academies of Science and
Engineering and the Department of Transportation sponsored an
intensive program of stratospheric research. [CIC 1975] It soon
became clear that the relationship between NOx emissions and the
ozone layer was very complicated. The stratospheric lifetime of
NOx is comparable to the timescale for transport from North to
South, so its concentration depends strongly upon latitude. Much
of the NOx is injected near the tropopause, a region where
quantitative modelling is very difficult, and the results of
calculations depend sensitively upon how troposphere-stratosphere
exchange is treated. Stratospheric NOx chemistry is _extremely_
complicated, much worse than chlorine chemistry. Among other
things, NO2 reacts rapidly with ClO, forming the inactive chlorine
reservoir ClONO2 - so while on the one hand increasing NOx leads
directly to ozone loss, on the other it suppresses the action
of the more potent chlorine catalyst. And on top of all of this, the
SST's always spend part of their time in the troposphere, where NOx
emissions cause ozone increases. Estimates of long-term ozone
changes due to large-scale NOx emissions varied markedly from year
to year, going from -10% in 1974, to +2% (i.e. a net ozone _gain_)
in 1979, to -8% in 1982. (In contrast, while the estimates of the
effects of CFC emissions on ozone also varied a great deal in these
early years, they always gave a net loss of ozone.) [Wayne]
The discovery of the Antarctic ozone hole added a new piece to the
puzzle. As described in Part III, the ozone hole is caused by
heterogeneous chemistry on the surfaces of stratospheric cloud
particles. While these clouds are only found in polar regions,
similar chemical reactions take place on sulfate aerosols which are
found throughout the lower stratosphere. The most important of the
aerosol reactions is the conversion of N2O5 to nitric acid:
N2O5 + H2O -> 2 HNO3 (catalyzed by aerosol surfaces)
N2O5 is in equilibrium with NOx, so removal of N2O5 by this
reaction lowers the NOx concentration. The result is that in the
lower stratosphere the NOx catalytic cycle contributes much less to
overall ozone loss than the HOx and ClOx cycles. Ironically, the
same processes that makes chlorine-catalyzed ozone depletion so
much more important than was believed 10 years ago, also make
NOx-catalyzed ozone loss less important.
In the meantime, there has been a great deal of progress in
developing jet engines that will produce much less NOx - up to a
factor of 10 - than the old Boeing SST. The most recent model
calculations indicate that a fleet of the new "high-speed civil
transports" would deplete the ozone layer by 0.3-1.8%. Caution
is still required, since the experiment has not been done - we have
not yet tried adding large amounts of NOx to the stratosphere. The
forecasts, however, are good. [WMO 1991, Ch. 10] [WMO 1994]
_Aside_: One sometimes hears that the US government killed the SST
project in 1971 because of concerns raised by H. S. Johnston's work
on NOx. This is not true. The US House of Representatives had already
voted to cut off Federal funding for the SST when Johnston began
his calculations. The House debate had centered around economics and
the effects of noise, especially sonic booms, although there were
some vague remarks about "pollution" and one physicist had testified
about the possible effects of water vapor on ozone. About 6 weeks
after both houses had voted to cancel the SST, its supporters
succeeded in reviving the project in the House. In the meantime,
Johnston had sent a preliminary report to several professional
colleagues and submitted a paper to _Science_. A preprint of
Johnston's report leaked to a small California newspaper which
published a highly sensationalized account. The story hit the press
a few days before the Senate voted, 58-37, not to revive the SST.
(The previous Senate vote had been 51-46 to cancel the project. The
reason for the larger majority in the second vote was probably the
statement by Boeing's chairman that at least $500 million more would
be needed to revive the program.)
Subject: 2.15) What is being done about ozone depletion?
The 1987 Montreal Protocol specified that CFC emissions should be
reduced by 50% by the year 2000 (they had been _increasing_ by 3%
per year.) This agreement was amended in London in 1990, to state
that production of CFC's, CCl4, and halons should cease entirely by
the year 2000. Restrictions have also been applied to other Cl
sources such as methylchloroform. (The details of the protocols are
complicated, involving different schedules for different compounds,
delays for developing nations, etc. See the book by [Benedick].)
The phase-out schedule was accelerated by four years by the 1992
Copenhagen agreements. A great deal of effort has been devoted to
recovering and recycling CFC's that are currently being used in
Recent NOAA measurements [Elkins et al.] show that the _rate of
increase_ of halocarbon concentrations in the atmosphere has decreased
markedly since 1987, by a factor of 4 for CFC-11 and a factor of 2
for CFC-12. It appears that the Protocols are being observed. Under
these conditions total stratospheric chlorine is predicted to peak
at 3.8 ppbv in the year 1998, 0.2 ppbv above current levels, and to
slowly decline thereafter. [WMO 1994] Extrapolation of current trends
suggests that the maximum ozone losses will be [WMO 1994]:
Northern Mid-latitudes in winter/Spring: 12-13% below late 1960's levels,
~2.5% below current levels.
Northern mid-latitudes in summer/fall: 6-7% below late 1960's levels,
~1.5% below current levels.
Southern mid-latitudes, year-round: ~ 11% below late 1960's levels,
~2.5% below current levels.
Very little depletion has been seen in the tropics and very little is
expected there. After the year 2000, the ozone layer will slowly
recover over a period of 50 years or so. The antarctic ozone hole
is expected to last until about 2045. [WMO 1991,1994]
Some scientists are investigating ways to replenish stratospheric
ozone, either by removing CFC's from the troposphere or by tying up
the chlorine in inactive compounds. This is discussed in Part III.
Subject: 3. REFERENCES FOR PART I
A remark on references: they are neither representative nor
comprehensive. There are _hundreds_ of people working on these
problems. Where possible I have limited myself to papers that
are (1) available outside of University libraries (e.g. _Science_
or _Nature_ rather than archival journals such as _J. Geophys. Res._)
and (2) directly related to the "frequently asked questions".
I have not listed papers whose importance is primarily historical.
Readers who want to see "who did what" should consult the review
articles listed below, or, if they can get them, the WMO reports
which are extensively documented.
Subject: Introductory Reading
[Garcia] R. R. Garcia, _Causes of Ozone Depletion_, _Physics World_
April 1994 pp 49-55.
[Graedel and Crutzen] T. E. Graedel and P. J. Crutzen,
_Atmospheric Change: an Earth System Perspective_, Freeman, NY 1993.
[Rowland 1989] F.S. Rowland, "Chlorofluorocarbons and the depletion
of stratospheric ozone", _American Scientist_ _77_, 36, 1989.
[Zurer] P. S. Zurer, "Ozone Depletion's Recurring Surprises
Challenge Atmospheric Scientists", _Chemical and Engineering News_,
24 May 1993, pp. 9-18.
Subject: Books and Review Articles
[Benedick] R. Benedick, _Ozone Diplomacy_, Harvard, 1991.
[Brasseur and Solomon] G. Brasseur and S. Solomon, _Aeronomy of
the Middle Atmosphere_, 2nd. Edition, D. Reidel, 1986
[Chamberlain and Hunten] J. W. Chamberlain and D. M. Hunten,
_Theory of Planetary Atmospheres_, 2nd Edition, Academic Press, 1987
[Dobson] G.M.B. Dobson, _Exploring the Atmosphere_, 2nd Edition,
[CIC 1975] Climate Impact Committee, National Research Council,
_Environmental Impact of Stratospheric Flight_,
National Academy of Sciences, 1975.
[Johnston 1992] H. S. Johnston, "Atmospheric Ozone",
_Annu. Rev. Phys. Chem._ _43_, 1, 1992.
[Ko et al.] M. K. W. Ko, N.-D. Sze, and M. J. Prather, "Better
Protection of the Ozone Layer", _Nature_ _367_, 505, 1994.
[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.
[Salby and Garcia] M. L. Salby and R. R. Garcia, "Dynamical Perturbations
to the Ozone Layer", _Physics Today_ _43_, 38, March 1990.
[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_,
Global Ozone Research and Monitoring Project - Report #18.
[WMO 1989] World Meteorological Organization,
_Scientific Assessment of Stratospheric Ozone: 1991_
Global Ozone Research and Monitoring Project - Report #20.
[WMO 1991] World Meteorological Organization,
_Scientific Assessment of Ozone Depletion: 1991_
Global Ozone Research and Monitoring Project - Report #25.
[WMO 1994] World Meteorological Organization,
_Scientific Assessment of Ozone Depletion: 1994_
Global Ozone Research and Monitoring Project - Report #25.
(Executive Summary; the full report is to be published in November 1994).
Subject: More Specialized References
[Brasseur and Granier] G. Brasseur and C. Granier, "Mt. Pinatubo
aerosols, chlorofluorocarbons, and ozone depletion", _Science_
[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.
[Gleason et al.] J. Gleason, P. Bhatia, J. Herman, R. McPeters, P.
Newman, R. Stolarski, L. Flynn, G. Labow, D. Larko, C. Seftor, C.
Wellemeyer, W. Komhyr, A. Miller, and W. Planet, "Record Low Global
Ozone in 1992", _Science_ _260_, 523, 1993.
[Herman et al.] J. R. Herman, R. McPeters, and D. Larko,
"Ozone depletion at northern and southern latitudes derived
from January 1979 to December 1991 TOMS data",
J. Geophys. Res. _98_, 12783, 1993.
[Hofmann and Solomon] D. J. Hofmann and S. Solomon, "Ozone
destruction through heterogeneous chemistry following the
eruption of El Chichon", J. Geophys. Res. _94_, 5029, 1989.
[Hofmann et al. 1994a] D. J. Hofmann, S. J. Oltmans, W. D. Komhyr,
J. M. Harris, J. A. Lathrop, A. O. Langford, T. Deshler,
B. J. Johnson, A. Torres, and W. A. Matthews,
"Ozone Loss in the lower stratosphere over the United States in
1992-1993: Evidence for heterogeneous chemistry on the Pinatubo
aerosol", Geophys. Res. Lett. _21_, 65, 1994.
[Hofmann et al. 1994b] D. J. Hofmann, S. J. Oltmans, J. M. Harris,
J. A. Lathrop, G. L. Koenig, W. D. Komhyr, R. D. Evans, D. M. Quincy,
T. Deshler, and B. J. Johnson,
"Recovery of stratospheric ozone over the United States in the winter
of 1993-94", Geophys. Res. Lett. _21_, 1779, 1994.
[Kerr et al.] J. B. Kerr, D. I. Wardle, and P. W. Towsick,
"Record low ozone values over Canada in early 1993",
Geophys. Res. Lett. _20_, 1979, 1993.
[Khalil and Rasmussen] M.A.K. Khalil and R. Rasmussen, "The Global
Sources of Nitrous Oxide", _J. Geophys. Res._ _97_, 14651, 1992.
[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.
[Ravishankara et al.] A. R. Ravishankara, A. A. Turnipseed,
N. R. Jensen, S. Barone, M. Mills, C. J. Howard, and S. Solomon,
"Do Hydrofluorocarbons Destroy Stratospheric Ozone?",
_Science_ _263_, 71, 1994.
[SAGE II] Special Section on the Stratospheric Aerosol and Gas
Experiment II, _J. Geophys. Res._ _98_, 4835-4897, 1993.
[Solomon and Albritton] S. Solomon and D.L. Albritton,
"Time-dependent ozone depletion potentials for short- and long-term
forecasts", _Nature_ _357_, 33, 1992.
[Stolarski et al.] R. Stolarski, R. Bojkov, L. Bishop, C. Zerefos,
J. Staehelin, and J. Zawodny, "Measured Trends in Stratospheric
Ozone", Science _256_, 342 (17 April 1992)
[Waters et al.] J. Waters, L. Froidevaux, W. Read, G. Manney, L.
Elson, D. Flower, R. Jarnot, and R. Harwood, "Stratospheric ClO and
ozone from the Microwave Limb Sounder on the Upper Atmosphere
Research Satellite", _Nature_ _362_, 597, 1993.
[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_,