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

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Subject: Ozone Depletion FAQ Part I: Introduction to the Ozone Layer
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Summary: This is the first of four files dealing with stratospheric
         ozone depletion. It provides scientific background for
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* Copyright 1993 Robert Parson                                        *
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 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
the remaining parts which are more specialized. Part II deals
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. I emphasize
physical and chemical mechanisms rather than biological effects,
although I make a few remarks about the latter in part IV. For
completeness, some questions have been included in more than
one part.

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.

| _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.
| 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
  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?

2.  THE OZONE LAYER

 2.1) How is ozone created?
 2.2) How much ozone is in the ozone layer,
      and what is a "Dobson Unit"?
 2.3) What is the concentration of ozone in the stratosphere?
 2.4) How does the ozone layer work?
 2.5) What sorts of natural variations does the ozone layer show?
 2.6) What are CFC's?  [See Part II for more detail]
 2.7) How do CFC's destroy ozone?
 2.8) What about HCFC's and HFCs? Do they destroy ozone?
 2.9) *IS* the ozone layer getting thinner (outside antarctica) ?
 2.10) Is middle-latitude ozone loss due to CFC emissions?
 2.11) Do Space Shuttle launches damage the ozone layer?
 2.12) Will commercial supersonic aircraft damage the ozone layer?
 2.13) What is being done about ozone depletion, and what can we
       expect to see?

3. REFERENCES
_________________________________________________________________


1.  THE STRATOSPHERE

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.  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 precise location of the
boundary between these regions, called the tropopause, 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]


1.2) How is the composition of air described?
    (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 fraction 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 (ppbv). ( 'By volume'
reflects "Avogadro's Law": for an ideal gas mixture, equal volumes
contain equal numbers of molecules.) [Wayne] [Graedel and Crutzen]


2. THE OZONE LAYER

2.1) How is ozone created?

 Ozone is formed naturally in the upper stratosphere by
short-wavelength UV radiation. Wavelengths less than ~240
nanometers  are absorbed by oxygen  molecules, which dissociate to
give O atoms. The O atoms combine with other O2  molecules to form
ozone:

 O2 + hv -> O + O  (wavelength < 240 nm)
 O + O2 -> O3


2.2) How much ozone is in the ozone layer?
      (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.

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.


2.3) What is the concentration of ozone 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 (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.


2.4) How does the ozone layer work?

 UV light with wavelengths between 240 and 320 nm is absorbed by
ozone, giving an O atom and and 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.

(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).


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.

 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 into account the dynamics of the atmosphere. (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
example:

    Location                          Column thickness, Dobson Units

                                        Jan     Apr     Jul     Oct

 Huancayo, Peru (12 degrees S) :        255     255     260     260
 Aspendale, Aus. (38 degrees S):        300     280     335     360
 Arosa, Switz.  (47 degrees N):         335     375     320     280
 St. Petersburg, FSU (60 degrees 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. 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.) [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.


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), and
persist for short periods, 1-2 years or less.


 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 the 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:

  CF2Cl2 (CFC-12),
  CFCl3 (CFC-11), and
  CF2ClCFCl2 (CFC-113).

 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
include:

 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 organochlorine 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.


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 tens of 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.
Each Br atom can destroy _millions_ of ozone molecules before it is
removed. 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]


2.8) 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.

The impact of a compound on stratospheric ozone is usually
measured by its "ozone depletion potential", defined as the
steady-state limit of the amount of ozone destroyed by the
halocarbon, relative to the amount destroyed by CFC-12. HCFC's
generally have ozone depletion potentials around 0.01-0.1, so that
in the long time limit a typical HCFC will have destroyed 1-10% as
much ozone as CFC-12. This measure can sometimes be misleading,
however. 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 their chlorine more
quickly. Just as short-lived radioisotopes are more intensely
radioactive than long-lived ones, those HCFC's that do reach the
stratosphere deplete ozone more quickly than CFC's. The short-term
effects are therefore larger than one would predict from the ozone
depletion potential alone, and the long-term effects correspondingly
smaller. This must be taken into account when substituting HCFC's
for CFC's. [Solomon and Albritton]

 HFC's, hydrofluorocarbons, contain no chlorine at all, and hence
have an ozone depletion potential of zero. The most 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.


2.9) *IS* the ozone layer getting thinner (outside antarctica) ?

 So it seems, although so far the effects 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
remains.  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,
being somewhat larger in the Southern Hemisphere. There is no
significant depletion in the tropics; over the US, Europe, and
Australia 4%/decade is typical. 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.)

 Since 1991 these trends have accelerated. Satellite and
ground-based measurements now show 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.], and there is evidence that this depletion
has persisted into the summer months. This decline overwhelms the
effect of the solar cycle; 1991 was a solar maximum, while the 1992
results are already below those for the 1986 solar minimum.
Sulfate aerosols from the July 1991 eruption of Mt. Pinatubo may
be the cause of this latest spike; 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.
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.]


2.10) Is the middle-latitude ozone loss due to CFC emissions?

That's the majority opinion, although not everyone agrees. The
present effects are too small to allow a watertight case to be
made (as _has_ been made for the far larger, but localized,
depletions in the Antarctic Ozone hole; see Part III.). Other
possible causes are being investigated. To quote from [WMO 1991],
p. 4.1:

 "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 recent UARS measurements of ozone and ClO in the Northern
Hemisphere find a correlation between enhanced ClO and depleted
ozone, which further supports this hypothesis. [Waters et al.]

 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 '60's and '70's. Present
controls on CFC emissions are designed to avoid possibly large
amounts of ozone depletion 30 years from now, not to remediate the
depletion that has taken place up to now.


2.11) Do Space Shuttle launches damage the ozone layer?

No. 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-fueled rocket motors might have a significant effect upon
the ozone layer - if not globally, perhaps in the immediate vicinity
of the launch. It was quickly 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-fueled 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 million tons 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".


2.12) 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 model calculations indicate that a fleet of high-speed civil
transports would deplete the ozone layer by <1%. [WMO 1991]

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]

(Aside: subsonic aircraft do sometimes enter the stratosphere;
however they stay very low and do not appreciably affect its
chemistry.)

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
shown - 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 less than 1%. 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]

..................................................................
_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.)
....................................................................


 2.13) 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.) 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 closed-cycle systems.

 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
in the first decade of the 21st century, and to slowly decline
thereafter.

 Model calculations predict that ozone levels, averaged over the
year and over the Northern hemisphere, will fall to about 4% below
1980 levels in the first decade of the 21st century if the
protocols are obeyed. Very little depletion is expected in the
tropics, so correspondingly larger losses - more than 6% - are
expected at middle and high latitudes. These same models have
systematically _underestimated_ ozone depletion in the past, so
significantly larger losses are expected. In fact, 4% global
year-averaged ozone depletion was _already_ measured in 1993 [
Gleason et al.] although this may be a transient caused by
Mt. Pinatubo's eruption in July 1991. After 2010 the ozone layer
will slowly recover over a period of 20 years or so, although the
form of the recovery is strongly model-dependent. [WMO 1991]
I have no results at hand for the southern hemisphere; if current
trends continue ozone depletion will be more serious there. The
antarctic ozone hole is expected to last until 2050 or so. This
does not take into account the possibility of global warming,
which by cooling the stratosphere could make ozone depletion more
serious both at mid latitudes and in polar regions.

 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.

___________________________________________________________________

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.


Introductory Reading:

[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.

----------------------------
Books and Review Articles:

[Brasseur and Solomon] G. Brasseur and S. Solomon, _Aeronomy of
the Middle Atmosphere_, 2nd. Edition, D. Reidel, 1986

[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.

[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_,
 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.

-----------------------------------
More Specialized:

[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.

[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.

[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.