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Last-modified: 20 Dec 1997
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Subject: How to get this FAQ

These files are posted to the newsgroups sci.environment, sci.answers,
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Many individuals have archived copies on their own servers, but these
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-----------------------------

Subject: Copyright Statement

***********************************************************************
* Copyright 1997 Robert Parson                                        *
*                                                                     *
* This file may be distributed, copied, and archived. All such copies *        
* must include this notice, the preceding instructions on how         *
* to obtain a current version, and the paragraph below entitled       *
* "Caveat."  If this document is transmitted to other networks or     *
* stored on an electronic archive, I ask that you inform me. I also   *
* ask you to keep your archive up to date; in the case of world-wide  *
* web pages, this is most easily done by linking to one of the        *
* archives listed above instead of storing local copies. Finally, I   *        
* request that you inform me before including any of this information *
* in any publications of your own. Students should note that this     *
* is _not_ a peer-reviewed publication and may not be acceptable as   *
* a reference for school projects; it should instead be used as a     *
* pointer to the published literature. In particular, all scientific  *
* data, numerical estimates, etc. should be accompanied by a citation *
* to the original published source, not to this document.             *
***********************************************************************


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

Subject: General 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
  Associate Professor
  Department of Chemistry and Biochemistry
  University of Colorado (for which I do not speak)

  rparson@spot.colorado.edu
  Robert.Parson@colorado.edu

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

Subject: Dedication

This FAQ is dedicated to the memory of Carl J. Lydick, who was one of
the first people to read it through carefully and who helped me to clarify
my presentation. Carl was not a scientist, but he had a profound 
understanding of and love for science and an outstanding talent for
presenting scientific results in non-technical language.

-----------------------------
 
Subject: TABLE OF CONTENTS

 How to get this FAQ
 Copyright Statement
 General remarks
 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
     Introductory Reading
     Books and Review Articles
     More Specialized References
     Internet Resources

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

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 ~9 and ~18 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 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 someone says the mixing ratio of hydrogen chloride at 3 km
is 0.1 ppbv, he 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. The lower and middle atmosphere are thus
said to be "well mixed."
[Chamberlain and Hunten] [Wayne] [Wallace and Hobbs]

Experimental measurements of the fluorocarbon CF4 demonstrate 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 stratospheric 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 
make ozone: 

 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 average thickness
of the ozone layer over the US come out to be about 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 spectrophotometer measures the intensity solar UV radiation at
four wavelengths, two of which are absorbed by ozone and two of
which are not [Dobson 1968b].  These instruments are still in use
in many places, although they are gradually being replaced by the more 
elaborate Brewer spectrophotometers. 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 (see http://www.umnw.ethz.ch/LAPETH/doc/totozon.html)
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. 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 stratosphere.

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

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 actually determines the form 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. For example:

    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].
Day-to-day fluctuations can be quite large (as much as 60 DU at high
latitudes). 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 1968a] [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 with a period of about two years. 
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
influence ozone levels, by producing nitrogen oxides in the upper
stratosphere and mesosphere. This can have a marked, though
short-lived, effect on ozone _concentrations_ at very high altitudes,
but the effect on total column ozone is usually small since most of
the ozone is found in the lower and middle stratosphere.  Ozone can
also be depleted by a major volcanic eruption, such as El Chichon in 
1982 or 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.
[McCormick et al. 1995]. This too is a transient effect, lasting 2-3 years.

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

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:

Trichlorofluoromethane, CFCl3 (usually called CFC-11 or R-11);
Dichlorodifluoromethane,  CF2Cl2 (CFC-12 or R-12); and
1,1,2 Trichlorotrifluoroethane, CF2ClCFCl2 (CFC-113 or R-113).

"R" stands for "refrigerant". One occasionally sees CFC-12 referred 
to as "F-12", and so forth; the"F" stands for "Freon", DuPont's trade
name for these compounds. 

In discussing ozone depletion, "CFC" is occasionally used to 
describe 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, R-22);
 Carbon Tetrachloride (tetrachloromethane), CCl4;
 Methyl Chloroform (1,1,1 trichloroethane), CH3CCl3 (R-140a);
 and Methyl Chloride (chloromethane), 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 substantia 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]

The catalytic destruction of ozone by Cl-containing radicals was first
suggested by Richard Stolarski and Ralph Cicerone in 1973. However,
they were not aware of any large sources of stratospheric chlorine. 
In 1974 F. Sherwood Rowland and Mario Molina  realized that CFC's
provided such a source. [Molina and Rowland 1974][Rowland and Molina 1975]
For this and for their many subsequent contributions to stratospheric
ozone chemistry Rowland and Molina shared the 1995 Nobel
Prize in Chemistry, together with Paul Crutzen, discoverer of the NOx 
cycle. (The official announcement from the Swedish Academy can be found
on the web at http://www.nobel.se/announcement95-chemistry.html .)
 
-----------------------------

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 is 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
a few halocarbons [Solomon and Albritton] [WMO 1991]

Compound               Formula         Ozone Depletion Potential  

                                     10 yr  30 yr   100 yr   Steady State

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