Subject: Ozone Depletion FAQ Part IV: UV Radiation and its Effects
From: rparson@spot.colorado.edu (Robert Parson)
Date: 11 Oct 1996 19:07:06 GMT
Archive-name: ozone-depletion/uv
Last-modified: 1 September 1996
Version: 5.7
-----------------------------
Subject: How to get this FAQ
These files are (usually) posted monthly, towards the end of the month.
The current versions are stored on several archives:
A. World-Wide Web
(Limited) hypertext versions, with embedded links to some of the on-line
resources cited in the faqs, can be found at:
http://www.cs.ruu.nl/wais/html/na-dir/ozone-depletion/.html
http://www.cis.ohio-state.edu/hypertext/faq/usenet/ozone-depletion/top.html
http://www.lib.ox.ac.uk/internet/news/faq/sci.environment.html
The ohio-state version has the nicest format, but it sometimes falls
behind. The Utrecht version has the best record for staying up to date.
Plaintext versions can be found at:
ftp://rtfm.mit.edu/pub/usenet/news.answers/ozone-depletion/
ftp://ftp.uu.net/usenet/news.answers/ozone-depletion/
----
B. Anonynmous ftp
To rtfm.mit.edu, in the directory /pub/usenet/news.answers/ozone-depletion
To ftp.uu.net, in the directory /usenet/news.answers/ozone-depletion
Look for the four files named intro, stratcl, antarctic, and uv.
----
C. Regular email
Send the following messages to mail-server@rtfm.mit.edu:
send usenet/news.answers/ozone-depletion/intro
send usenet/news.answers/ozone-depletion/stratcl
send usenet/news.answers/ozone-depletion/antarctic
send usenet/news.answers/ozone-depletion/uv
Leave the subject line blank.
If you want to find out more about the mail server, send a
message to it containing the word "help".
I have found a number of copies of the faqs tucked away in various corners
of the net, but many of them are seriously out-of-date. The archives listed
above usually get the latest version within a few days of its being posted.
-----------------------------
Subject: Copyright Notice
***********************************************************************
* Copyright 1996 Robert Parson *
* *
* This file may be distributed, copied, and archived. All such *
* copies must include this notice and the paragraph below entitled *
* "Caveat". Reproduction and distribution for personal profit is *
* not permitted. 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 the master at the *
* ohio-state http URL 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. *
***********************************************************************
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Subject: General Remarks
This file deals with the physical properties of ultraviolet
radiation and its biological consequences, emphasizing the
possible effects of stratospheric ozone depletion. It frequently
refers back to Part I, where the basic properties of the ozone
layer are described; the reader should look over that file first.
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 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.
will probably do, but also the worst that he could possibly do.
There have been surprises, mostly unpleasant, in this field in the
past, and there are sure to be more in the future. In general,
_much_ less is known about biological effects of UV-B than about
the physics and chemistry of the ozone layer.
-----------------------------
Subject: Caveats, Disclaimers, and Contact Information
| _Caveat_: I am not a specialist. In fact, I am not an atmospheric
| scientist at all - I am a physical chemist studying gas-phase
| reactions who talks to atmospheric scientists. In this part in
| particular I am well outside the range of my own expertise.
| I have discussed some aspects of this subject with specialists,
| but I am solely responsible for everything written here, including
| any 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: TABLE OF CONTENTS
How to get this FAQ
Copyright Notice
General Remarks
Caveats, Disclaimers, and Contact Information
TABLE OF CONTENTS
1.) What is "UV-B"?
2.) How does UV-B vary from place to place?
3.) Is UV-B at the earth's surface increasing?
4.) What is the relationship between UV and skin cancer?
5.) Is ozone loss to blame for the melanoma upsurge?
6.) Does UV-B cause cataracts?
7.) Are sheep going blind in Chile?
8.) What effects does increased UV have upon plant life?
9.) What effects does increased UV have on marine life?
10.) Is UV-B responsible for the amphibian decline?
REFERENCES FOR PART IV
Introductory Reading
Books and General Review Articles
More Specialized References
-----------------------------
Subject: 1.) What is "UV-B"?
"UV-B" refers to UV light having a wavelength between 280 and
320 nm. These wavelengths are on the lower edge of ozone's UV
absorption band, in the so-called "Huggins bands". They are
absorbed by ozone, but less efficiently than shorter wavelengths
("UV-C"). (The absorption cross-section of ozone increases by more
than 2 orders of magnitude between 320 nm and the peak value at
~250 nm.) Depletion of the ozone layer would first of all result
in increased UV-B. In principle UV-C would also increase, but it is
absorbed so efficiently that a very large depletion would have to
take place in order for significant amounts to reach the earth's
surface. UV-B and UV-C are absorbed by DNA and other biological
macromolecules, inducing photochemical reactions. UV radiation with
a wavelength longer than 320 nm is called "UV-A". It is not
absorbed by ozone, but it is not usually thought to be especially
dangerous. (See, however, question #6.)
For a good introduction to many aspects of UV and UV measurements, see
the web page for Biospherical Instruments:
http://www.biospherical.com/research/uvhome.htm
-----------------------------
Subject: 2.) How does UV-B vary from place to place?
A great deal. It is strongest at low latitudes and high altitudes.
At higher latitudes, the sun is always low in the sky so that it takes
a longer path through the atmosphere and more of the UV-B is absorbed.
For this reason, ozone depletion is likely to have a greater impact on
_local_ ecosystems, such as terrestrial plants and the Antarctic
marine phytoplankton, than on humans or their livestock. UV also
varies with altitude and local cloud cover. These trends can be seen
in the following list of annually-averaged UV indices for several US
cities [Roach] (units are arbitrary - I don't know precisely how this
index is defined though I assume it is proportional to some integral
over the UV-b region of the spectrum)
Minneapolis, Minnesota 570
Chicago, Illinois 637
Washington, DC 683
San Francisco, California 715
Los Angeles, California 824
Denver, Colorado 951
Miami, Florida 1028
Honolulu, Hawaii 1147
The effect of clouds on local UV-B irradiance is not straightforward
to determine. While the body of a cloud attenuates the radiation,
scattering from the sides of a cumulus cloud can actually enhance it.
[Mims and Frederick 1994.]
In comparing UV-B estimates, one must pay careful attention to
exactly what is being reported. One wants to know not just whether
there is an increase, but how much increase there is at a particular
wavelength, since the shorter wavelengths are more dangerous.
Different measuring instruments have different spectral responses,
and are more or less sensitive to various spectral regions. [Wayne,
Rowland 1991]. Wavelength-resolving instruments, such as the
spectroradiometers being used in Antarctica, Argentina, and Toronto,
are particularly informative, as they allow one to distinguish the
effects of ozone trends from those due to clouds and aerosols.
[Madronich 1993] [Kerr and McElroy]. When wavelength-resolved
data are available, they are frequently convolved with an "action
spectrum" that is relevant for a particular biological influence.
Thus the "erythemal action spectrum", designed to estimate the
tendency of UV radiation to redden human skin, places less emphasis
on short wavelengths that the action spectrum designed to estimate
the tendency of UV to damage DNA. When the ozone column overhead
decreases by 1%, erythemal UV increases by about 1% while DNA-damaging
UV increases by about 2.5%. [Madronich 1993] The widely-used broadband
Robertson-Berger meter has a spectral response that is close to
the erythemal action spectrum.
-----------------------------
Subject: 3.) Is UV-B at the earth's surface increasing?
Yes, in some places; no, in some others; unknown, in most.
There is very little data on long-term UV trends, primarily because
with very few exceptions UV monitoring operations of the requisite
sensitivity did not exist until very recently. (See the US
Department of Agriculture's UV Monitoring Program web page,
http://uvb.nrel.colostate.edu/UVB/uvb_climate_network.html.)
Measurements over a period of a few years cannot establish long-term
trends, although they can be used in conjunction with ozone measurements
to quantify the relationship between surface UV-B intensities and
ozone amounts.
Very large increases, by as much as a factor of 2-3, have been seen
within the Antarctic ozone hole. [Frederick and Alberts] [Stamnes et
al.] UV-B intensity at Palmer Station (65 degrees S. Lat.) in late
October 1993 exceeded *summertime* UV-B intensity at San Diego,
California. [WMO 1994] At Ushaia at the tip of South America, the
noontime UV-B irradiance in the austral summer is 45% above what would
be predicted were there no ozone depletion. [Frederick et al. 1993]
[Bojkov et al. 1995] The effect is to expose Ushaia to UV intensities
that are typical of Buenos Aires.
Small increases, of order 1% per year, have been measured in the
Swiss Alps. [Blumthaler and Ambach] These _net_ increases are small
compared to natural day-to-day fluctuations, but they are actually
a little larger than would be expected from the amount of ozone
depletion over the same period.
In urban areas of the US, erythemal UV-B showed no significant increase
(and in most cases actually decreased a little) between 1974 and
1985. [Scotto et al.]. This is probably due to increasing urban
pollution, including low-level ozone and aerosols. [Grant]
Tropospheric ozone is actually somewhat more effective at absorbing UV
than stratospheric ozone, because UV light is scattered much more in
the troposphere, and hence takes a longer path. [Bruehl and Crutzen]
Increasing amounts of tropospheric aerosols, from urban and industrial
pollution, may also offset UV-B increases at the ground. [Liu et al.]
[Madronich 1992, 1993] [Grant] There have been questions about the
suitability of the instruments used by Scotto et al.; they were not
designed for measuring long-term trends, and they put too much weight
on regions of the UV spectrum which are not appreciably absorbed by
ozone in any case. [WMO 1989] Nevertheless it seems clear that so far
ozone depletion over US cities is small enough to be largely offset by
competing factors. Tropospheric ozone and aerosols have increased in
rural areas of the US and Europe as well, so these areas may also be
screened from the effects of ozone depletion.
Several studies [Kerr and McElroy] [Seckmayer et al.] [Zerefos et
al.] have presented evidence of short-term UV-B increases at northern
middle latitudes (Canada, Germany, and Greece), associated with the
record low ozone levels seen in these areas in the years 1992-93. As
discussed in Part I, these low ozone levels are probably due to
stratospheric sulfate aerosols from the 1991 eruption of Mt.Pinatubo;
such aerosols change the radiation balance in the stratosphere,
influencing ozone production and transport, and accelerate the
conversion of inactive chlorine reservoir compounds into
ozone-destroying ClOx radicals. The first mechanism is purely natural,
while the second is an example of a natural process enhancing an
anthropogenic mechanism since most of the chlorine comes ultimately
from manmade halocarbons. (High UV levels associated with low ozone
levels were also reported in Texas [Mims 1994, Mims et al. 1995],
however in this case the low ozone is attributed to unusual
climatology rather than chemical ozone destruction.) One cannot
deduce long-term trends from such short-term measurements, but one can
use them to help quantify the relationship between stratospheric ozone
and surface UV-B intensities under real world conditions. Measurements
in Toronto, Canada [Kerr and McElroy] over the period 1989-93 found
that UV intensity at 300 nm increased by 35% per year in winter and 7%
per year in summer. At this wavelength 99% of the total UV is
absorbed, so these represent large increases in a small number, and do
not represent a health hazard; nevertheless these wavelengths play a
disproportionately large role in skin carcinoma and plant damage.
_Total_ UV-B irradiance, weighted in such a way as to correlate with
incidence of sunburn ("erythemally active radiation"), increased by 5%
per year in winter and 2% per year in summer. These are not really
"trends", as they are dominated by the unusually large, but temporary,
ozone losses in these regions in the years 1992-1993 (see part I), and
they should not be extrapolated into the future. Indeed, [Michaels et
al.] have claimed that the winter "trend" arises entirely from a brief
period at the end of March 1993 (they do not discuss the summer
trend.) Kerr and McElroy respond that these days are also reponsible
for the strong decrease in average ozone over the same period, so that
their results do demonstrate the expected link between total ozone and
total UV-B radiation. UV-B increases of similar magnitude were seen
in Greece for the period 1990-1993 [Zerefos et al.] and in Germany
for the period 1992-93. [Seckmeyer et al.]
Indirect evidence for increases has been obtained in the Southern
Hemisphere, where stratospheric ozone depletion is larger and
tropospheric ozone (and aerosol pollution) is lower. Biologically
weighted UV-B irradiances at a station in New Zealand were 1.4-1.8
times higher than irradiances at a comparable latitude and season in
Germany, of which a factor of 1.3-1.6 can be attributed to differences
in the ozone column over the two locations [Seckmeyer and McKenzie].
Record low ozone columns measured at Mauna Loa during the winter
of 1994-95 were accompanied by corresponding increases in the ratio
of UV-B to UV-A [Hofmann et al. 1996.]
The satellite-borne Total Ozone Mapping Spectrometer (TOMS) actually
measures the UV radiation that is scattered back into space from the
earth's atmosphere. [Herman et al. 1996] have combined ozone and
reflectivity data from TOMS with radiative transfer calculations to
arrive at an estimate of the ultraviolet flux at the surface. The
estimates are validated by comparison with ground-based UV measurements.
The advantage of this technique is that it gives truly global
coverage; the disadvantage is that it is indirect. Herman et al.
estimate that during the period 1979-92 UV irradiance, weighted for
DNA damage, increased by ~5% per decade at 45 degrees N latitude,
~7% per decade at 55 N, and ~10% per decade at 55 S. The increases
occurred primarily in spring and early summer.
-----------------------------
Subject: 4.) What is the relationship between UV and skin cancer?
Most skin cancers fall into three classes, basal cell carcinomas.
squamous cell carcinomas, and melanomas. In the US there were
500,000 cases of the first, 100,000 of the second, and 27,600 of
the third in 1990. [Wayne] More than 90% of the skin carcinomas in
the US are attributed to UV-B exposure: their frequency varies
sharply with latitude, just as UV-B does. The mechanism by which UV-B
induces carcinomas has been identified - the pyrimidine bases
in the DNA molecule form dimers when they absorb UV-B radiation.
This causes transcription errors when the DNA replicates, giving
rise to genetic mutations.[Taylor] [Tevini] [Young et al.] [Leffell
and Brash]. Fortunately, nonmelanoma skin cancers are
relatively easy to treat if detected in time, and are rarely fatal.
Fair-skinned people of North European ancestry are particularly
susceptible; the highest rates in the world are found in Queensland,
a northerly province of Australia, where a population of largely
English and Irish extraction is exposed to very high natural UV
radiation levels.
[Madronich and deGruijl] have estimated the expected increases in
nonmelanoma skin cancer due to ozone depletion over the period 1979-1992:
Lat. % ozone loss % increase in rate, % increase in rate,
1979-1992 basal cell carcinoma squamous cell carcinoma
55N 7.4 +-1.3 13.5 +-5.3 25.4 +-10.3
35N 4.8 +-1.4 8.6 +-4.0 16.0 +-7.6
15N 1.5 +-1.1 2.7 +-2.4 4.8 +-4.4
15S 1.9 +-1.3 3.6 +-2.6 6.5 +-4.8
35S 4.0 +-1.6 8.1 +-3.6 14.9 +-6.8
55S 9.0 +-1.5 20.4 +-7.4 39.3 +-15.1
Of course, the rates themselves are much smaller at high latitudes,
where the relative increases in rates are large. These estimates do
not take expected changes in lifestyle into consideration.
Malignant melanoma is much more dangerous, but its connection with UV
exposure is not well understood. [van der Leun and de Gruijl] [Ley].
There seems to a correlation between melanomas and brief, intense
exposures to UV (long before the cancer appears.) Melanoma incidence
is correlated with latitude, with twice as many deaths (relative to
state population) in Florida or Texas as in Wisconsin or Montana, [Wayne]
but this correlation does not necessarily imply a causal
relationship. There is some evidence that UV-A, which is not absorbed
by ozone, may be involved. [Skolnick] [Setlow et al.] [Ley] There is
a good summary [De Gruijl 1995] in the electronic journal _Consequences_,
at http://www.gcrio.org/CONSEQUENCES/summer95/impacts.html
-----------------------------
Subject: 5.) Is ozone loss to blame for the melanoma upsurge?
A few physicians have said so, but most others think not.
[Skolnick] [van der Leun and de Gruijl]
First of all, UV-B has not, so far, increased very much, at least
in the US and Europe.
Second, melanoma takes 10-20 years to develop. There hasn't been
enough time for ozone depletion to play a significant role.
Third, the melanoma epidemic has been going on since the 1940's.
Recent increases in rates may just reflect better reporting, or
the popularity of suntans in the '60's and '70's. (This becomes
more likely if UV-A is in fact involved.)
-----------------------------
Subject: 6.) Does UV-B cause cataracts?
While the evidence for this is indirect, it is very plausible.
The lens of the eye is a good UV-filter, protecting the delicate
structures in the retina. Too much UV burns the lens, resulting in
short-term "snowblindness", but the cumulative effects of prolonged,
repeated exposure are not fully understood. People living in naturally
high UV environments such as Bolivia or Tibet do have a high incidence
of cataracts, and in general cataracts are more frequently seen at lower
latitudes. [Tevini] [Zigman] For more on this, see [De Gruijl 1995]
at http://www.gcrio.org/CONSEQUENCES/summer95/impacts.html
-----------------------------
Subject: 7.) Are sheep going blind in Chile?
If they are, it's not because of ozone depletion.
For a short period each year, the edge of the ozone hole passes
over Tierra del Fuego, at the southern end of the South American
continent. This has led to a flurry of reports of medical damage
to humans and livestock. Dermatologists claim that they are seeing
more patients with sun-related conditions, nursery owners report
damage to plants, a sailor says that his yacht's dacron sails have
become brittle, and a rancher declares that 50 of his sheep,
grazing at high altitudes, suffer "temporary cataracts" in the
spring. (_Newsweek_, 9 December 1991, p. 43; NY Times, 27 July
1991, p. C4; 27 March 1992, p. A7).
These claims are hard to believe. At such a high latitude,
springtime UV-B is naturally very low and the temporary increase
due to ozone depletion still results in a UV fluence that is well
below that found at lower latitudes. Moreover, the climate of
Patagonia is notoriously cold and wet. (There is actually more of
a problem in the summer, after the hole breaks up and ozone-poor
air drifts north. The ozone depletion is smaller, but the
background UV intensity is much higher.) There may well be effects
on _local_ species, adapted to low UV levels, but even these are
not expected to appear so soon. It was only in 1987 that the hole
grew large enough to give rise to significant UV increases
in southern Chile, and cataracts and malignant melanomas take many
years to develop. To be sure, people do get sunburns and
skin cancer even in Alaska and northern Europe, and all
else being equal one expects on purely statistical grounds such
cases to increase, from a small number to a slightly larger number.
All else is definitely not equal, however - the residents are now
intensely aware of the hazards of UV radiation and are likely to
protect themselves better. I suspect that the increase in
sun-related skin problems noted by the dermatologists comes about
because more people are taking such cases to their doctors.
As for the blind sheep, a group at Johns Hopkins has investigated
this and ascribes it to a local infection ("pink eye"). [Pearce]
This is _not_ meant to dismiss UV-B increases in Patagonia as
insignificant. Damage to local plants, for example, may well emerge
in the long term, as the ozone hole is expected to last for 50
years or more. The biological consequences of UV radiation are real,
but often very subtle; I personally find it hard to believe that
such effects are showing up so soon, and in such a dramatic fashion.
Ozone depletion is a real problem, but this particular story is a red
herring.
-----------------------------
Subject: 8.) What effects does increased UV have upon plant life?
Generally (though not exclusively) harmful, but hard to quantify.
Many experiments have studied the response of plants to UV-B radiation,
either by irradiating the plants directly or by filtering out some
of the UV in a low-latitude environment where it is naturally high.
The artificial UV sources do not have the same spectrum as solar
radiation, however, while the filtering experiments do not
necessarily isolate all of the variables, even when climate
and humidity are controlled by growing the plants in a greenhouse.
Out of some 200 agricultural plants tested, more than half show
sensitivity to UV-B increases. The measured effects vary markedly
from one species to another; some adapt very readily while others are
seriously damaged. Even within species there are marked differences;
for example, one soybean variety showed a 25% growth reduction under a
simulated ozone depletion of 16%, whereas another variety showed no
significant yield reduction. The general sense seems to be that
ozone depletion amounting to 10% or more could seriously affect
agriculture. Smaller depletions could have a severe impact on local
ecosystems, but very little is known about this at present.
I have not investigated the literature on this in detail, not
being a biologist. Interested readers should consult [Tevini and
Teramura], [Bornman and Teramura], or the book by [Tevini] and
the references therein. If any botanist out there would like to write
a summary for this FAQ, please let me know.
-----------------------------
Subject: 9.) What effects does increased UV have on marine life?
Again, generally harmful but hard to quantify. Seawater is
surprisingly transparent to UV-B. In clear waters radiation at 315
nm is attenuated by only 14% per meter depth. [Jerlov]. Many marine
creatures live in surface waters, and they have evolved a variety
of methods to cope with UV: some simply swim to lower depths, some
develop protective coatings, while some work at night to repair the
damage done during the day. Often these natural mechanisms are
triggered by _visible_ light intensities, in which case they
might not protect against an increase in the _ratio_ of UV to visible
light. Also, if a photosynthesizing organism protects itself by
staying at lower depths, it will get less visible light and produce
less oxygen. An increase in UV-B can thus affect an ecosystem
without necessarily killing off individual organisms.
Many experiments have been carried out to determine the
response of various marine creatures to UV radiation; as with land
plants the effects vary a great deal from one species to another,
and it is not possible to draw general conclusions at this stage.
[Holm-Hansen et al.] We can assume that organisms that live in tropical
waters are safe, since there is little or no ozone depletion there, and
that organisms that are capable of living in the tropics are probably
safe from ozone depletion at high latitudes since background UV
intensitiesat high latitudes are always low. (One must be careful
with the second inference if the organism's natural defenses are
stimulated by visible light.) The problems arise with organisms
that have adapted to the naturally low UV levels of polar regions.
In this case, we have a natural laboratory for studying UV
effects: the Antarctic Ozone hole. (Part III of the FAQ discusses
the hole in detail.) The outer parts of the hole extend far out
into the ocean, beyond the pack ice, and these waters get
springtime UV-B doses equal to or greater than what is
seen in a normal antarctic summer. [Frederick and Alberts] [Smith
et al.]. The UV in shallow surface waters is effectively even
higher, because the sea ice is more transparent in spring than in
summer. There has been speculation that this UV could cause a
population collapse in the marine phytoplankton, the microscopic
plants that comprise the base of the food chain. Even if the plankton
are not killed, their photosynthetic production could be reduced.
Laboratory experiments show that UV-A and UV-B do indeed inhibit
phytoplankton photosynthesis. [Cullen and Neale] [Cullen et al.]
In one field study, [Smith et al.]. measured the photosynthetic
productivity of the phytoplankton in the "marginal ice zone" (MIZ),
the layer of relatively fresh meltwater that lies over saltier
deep water. Since the outer boundary of the ozone hole is
relatively sharp and fluctuates from day to day, they were able to
compare photosynthesis inside and outside the hole, and to
correlate photosynthetic yield with shipboard UV measurements.
They concluded that the UV-B increase brought about an overall
decrease of 6-12% in phytoplankton productivity. Since the "hole"
lasts for about 10-12 weeks, this corresponds to an overall decrease
of 2-4% for the year. The natural variability in phytoplankton
productivity from year to year is estimated to be about + or - 25%,
so the _immediate_ effects of the ozone hole, while real, are far
from catastrophic. To quote from [Smith et al.]: "Our estimated
loss of 7 x 10^12 g of carbon per year is about three orders
of magnitude smaller than estimates of _global_ phytoplankton
production and thus is not likely to be significant in this
context. On the other hand, we find that the O3-induced loss to a
natural community of phytoplankton in the MIZ is measurable and the
subsequent ecological consequences of the magnitude and timing of
this early spring loss remain to be determined." It appears, then,
that overall loss in productivity is not large.
The cumulative effects on the marine community are not known. The
ozone hole first became large enough to expose marine life to large
UV increases in 1987, and [Smith et al.] carried out their survey in
1990. Ecological consequences - the displacement of UV-sensitive
species by UV-tolerant ones - are likely to be more important than
a decline in overall productivity, although they are poorly
understood at present. [McMinn et al.] have examined the relative
abundance of four common phytoplankton species in sediment cores from
the fjords of the Vestfold hills on the Antarctic coast. They conclude
that compositional changes over the past 20 years (which should include
effects due to the ozone hole) cannot be distinguished from long-term
natural fluctuations. Apparently thick coastal ice protects the
phytoplankton in these regions from the effects of increased UVB;
moreover, these phytoplankton bloom after the seasonal hole has closed.
McMinn et al. emphasize that these conditions do not apply to ice-edge
and sea-ice communities.
For a general review, see [Holm-Hansen et al.]
-----------------------------
Subject: 10.) Is UV-B responsible for the amphibian decline?
UV-B may be part of the story, although it is unlikely to be the
principal cause of this mysterious event.
During the past decade, there has been a widespread decline in
amphibian populations [Livermore] [Wake]. The decline appears to be
global in scope, although some regions and many species appear to be
unaffected. While habitat destruction is undoubtedly an important
factor, many of the affected species are native to regions where
habitat is relatively undisturbed. This has led to speculation that
global perturbations, such as pesticide pollution, acid deposition,
and climate change, could be involved.
Recently, [Blaustein et al.] have investigated the effects of UV-B
radiation on the reproduction of amphibians living in the Cascade
Mountains of Oregon. In their first experiment, the eggs of several
amphibian species were analyzed for an enzyme that is known to
*repair* UV-induced DNA damage. The eggs of the Cascades frog,
R. cascadae, and of the Western toad, Bufo Boreas, showed low levels
of this enzyme; both species are known to be in serious decline
(R. Cascadae populations have fallen by ~80% since the 1970's [Wake].)
In contrast, much higher levels of the enzyme are found in the eggs of
the Pacific Tree Frog, _Hyla Regilla_, whose populations do not appear
to be in decline.
Blaustein et al. then studied the effects of UV-B upon the
reproductive success of these species in the field, by screening the
eggs with a filter that blocks the ambient UV. Two control groups were
used for comparison; in one no filter was present and in the other a
filter that *transmitted* UV-B was put in place. They found that for
the two species that are known to be in decline, and that showed low
levels of the repair enzyme, filtering the UV dramatically increased
the proportion of eggs surviving until hatch, whereas for the species
that is not in decline and that produces high levels of the enzyme,
filtering the UV made little difference. Thus, both the laboratory and
the field experiments suggest a correlation between amphibian declines
and UV sensitivity, albeit a correlation that at present is based on a
very small number of species and a limited time period.
Contrary to the impression given by some media reports, Blaustein and
coworkers did *not* claim that ozone depletion is "the cause" of the
amphibian decline. The decline appears to be world-wide, whereas ozone
depletion is restricted to middle and high latitudes. Also, many
amphibian species lay their eggs under dense canopies or underground
where there is little solar radiation. So, UV should be regarded
as one of many stresses that may be acting on amphibian populations.
-----------------------------
Subject: REFERENCES FOR PART IV
A remark on references: they are neither representative nor
comprehensive. There are _hundreds_ of people working on these
problems. For the most part I have limited myself to papers that
are (1) widely available (if possible, _Science_ or _Nature_ rather
than archival journals such as _J. Geophys. Res._) and (2) directly
related to the "frequently asked questions". 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
[Graedel and Crutzen] T. E. Graedel and P. J. Crutzen,
_Atmospheric Change: an Earth System Perspective_, Freeman, NY 1993.
[Leffell and Brash] D. J. Leffell and D. E. Brash, "Sunlight and Skin
Cancer", _Scientific American_ July 1996, p. 52.
[Roach] M. Roach, "Sun Struck", _Health_, May/June 1992, p. 41.
[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 General Review Articles
[Chamberlain and Hunten] J. W. Chamberlain and D. M. Hunten,
_Theory of Planetary Atmospheres_, 2nd Edition, Academic Press, 1987
[De Gruijl 1995] F. R. de Gruijl, "Impacts of a Projected Depletion
of the Ozone Layer", _Consequences_ _1_, #2, 1995, on the web at
URL http://www.gcrio.org/CONSEQUENCES/summer95/impacts.html
[Dobson] G.M.B. Dobson, _Exploring the Atmosphere_, 2nd Edition,
Oxford, 1968.
[Mukhtar] H. Mukhtar, editor: _Skin Cancer: Mechanisms and Human
Relevance_, CRC series in dermatology, CRC, 1995.
[Rowland 1991] F. S. Rowland, "Stratospheric Ozone Depletion",
_Ann. Rev. Phys. Chem._ _42_, 731, 1991.
[Tevini] M. Tevini, editor: _UV-B Radiation and Ozone Depletion:
Effects on humans, animals, plants, microorganisms, and materials_
Lewis Publishers, Boca Raton, 1993.
[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: 1989_
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 #37.
[Young et al.] _Environmental UV Photobiology_, Ed. by A. R. Young,
L. O. Bjorn, J. Mohan, and W. Nultsch, Plenum, N.Y. 1993.
-----------------------------
Subject: More Specialized References
[Blaustein et al.] A. R. Blaustein, P. D. Hoffman, D. G. Hokit,
J. M. Kiesecker, S. C. Walls, and J. B. Hays, "UV repair and
resistance to solar UV-B in amphibian eggs: A link to population
declines?", _Proc. Nat. Acad. Sci._ _91_, 1791, 1994.
[Blumthaler and Ambach] M. Blumthaler and W. Ambach, "Indication of
increasing solar ultraviolet-B radiation flux in alpine regions",
_Science_ _248_, 206, 1990.
[Bojkov et al. 1995] R. D. Bojkov, V. E. Fioletov, and S. B. Diaz,
"The relationship between solar UV irradiance and total ozone from
observations over southern Argentina", _Geophys. Res. Lett._ _22_,
1249, 1995.
[Bornman and Teramura] J. F. Bornman and A. H. Teramura, "Effects of
Ultraviolet-B Radiation on Terrestrial Plants", in [Young et al.]
[Bruehl and Crutzen] C. Bruehl and P. Crutzen, "On the
disproportionate role of tropospheric ozone as a filter against
solar UV-B radiation",_Geophys. Res. Lett._ _16_, 703, 1989.
[Cullen et al.] J. J. Cullen, P. J. Neale, and M. P. Lesser, "Biological
weighting function for the inhibition of phytoplankton photosynthesis by
ultraviolet radiation", _Science_ _258_, 646, 1992.
[Cullen and Neale] J. J. Cullen and P. J. Neale, "Ultraviolet Radiation,
ozone depletion, and marine photosynthesis", _Photosynthesis Research_
_39_, 303, 1994.
[Frederick and Alberts] J.E. Frederick and A. Alberts, "Prolonged
enhancement in surface ultraviolet radiation during the Antarctic
spring of 1990", _Geophys. Res. Lett._ _18_, 1869, 1991.
[Frederick et al. 1993] J.E. Frederick, P.F. Soulen, S.B. Diaz,
I. Smolskaia, C.R. Booth, T. Lucas, and D. Neuschuler,
"Solar Ultraviolet Irradiance Observed from Southern Argentina:
September 1990 to March 1991", J. Geophys. Res. _98_, 8891, 1993.
[Grant] W. Grant, "Global stratospheric ozone and UV-B radiation",
_Science_ _242_, 1111, 1988. (a comment on [Scotto et al.])
[Herman et al. 1996] J. R. Herman, P. K. Bhatia, J. Ziemke, Z. Ahmad,
and D. Larko, "UV-B increases (1979-92) from decreases in total
ozone", _Geophys. Res. Lett._ _23_, 2117, 1996.
[Hofmann et al. 1996] D. J. Hofmann, S. J. Oltmans, G. L. Koenig,
B. A. Bodhaine, J. M. Harris, J. A. Lathrop, R. C. Schnell, J. Barnes,
J. Chin, D. Kuniyuki, S. Ryan, R. Uchida, A. Yoshinaga, P. J. Neale,
D. R. Hayes, Jr., V. R. Goodrich, W. D. Komhyr, R. D. Evans, B. J. Johnson,
D. M. Quincy, and M. Clark, "Record low ozone at Mauna Loa Observatory
during winter 1994-95: A consequence of chemical and dynamical
synergism?", Geophys. Res. Lett. _23_, 1533, 1996.
[Holm-Hansen et al.] O. Holm-Hansen, D. Lubin, and E. W. Helbling,
"Ultraviolet Radiation and its Effects on Organisms in Aquatic
Environments", in [Young et al.]
[Jerlov] N.G. Jerlov, "Ultraviolet Radiation in the Sea",
_Nature_ _166_, 112, 1950.
[Kerr and McElroy] J. B. Kerr and C. T. McElroy, "Evidence for Large
Upward Trends of Ultraviolet-B Radiation Linked to Ozone Depletion",
_Science_ _262_, 1032, 1993.
[Ley] R. D. Ley, "Animal Models for Melanoma Skin Cancer", in [Mukhtar].
[Livermore] B. Livermore, "Amphibian alarm: Just where have all the
frogs gone?", _Smithsonian_, October 1992.
[Liu et al.] S.C. Liu, S.A. McKeen, and S. Madronich, "Effect of
anthropogenic aerosols on biologically active ultraviolet
radiation", _Geophys. Res. Lett._ _18_, 2265, 1991.
[Lubin and Jensen] D. Lubin and E. H. Jensen, "Effects of clouds
and stratospheric ozone depletion on ultraviolet radiation trends",
_Nature_ _377_, 710, 1995.
[Madronich 1992] S. Madronich, "Implications of recent total
atmospheric ozone measurements for biologically active ultraviolet
radiation reaching the earth's surface",
_Geophys. Res. Lett. _19_, 37, 1992.
[Madronich 1993] S. Madronich, in [Tevini].
[Madronich 1995] S. Madronich, "The radiation equation" _Nature_ _377_,
682, 1995. (News and Views column.)
[Madronich and de Gruijl] S. Madronich and F. R. de Gruijl,
"Skin Cancer and UV radiation", _Nature_ _366_, 23, 1993.
[McMinn et al.] A. McMinn, H. Heijnis, and D. Hodgson, "Minimal effects
of UVB radiation on Antarctic diatoms over the past 20 years", _Nature_
_370_, 547, 1994.
[Michaels et al.] P. J. Michaels, S. F. Singer, and P. C.
Knappenberger, "Analyzing Ultraviolet-B Radiation: Is There
a Trend?", _Science_ _264_, 1341, 1994. (Technical Comment)
[Mims 1994] F. M. Mims III, "UV-B and ozone observations",
_Science_ _265_, 722, 1994. [Correspondence]
[Mims and Frederick 1994] F. M. Mims III and J. E. Frederick,
"Cumulus Clouds and UV-B", _Nature_ _371_, 291, 1994.
[Mims et al. 1995] F. M. Mims III, J. W. Ladd and R. A. Blaha,
"Increased solar ultraviolet-B associated with record low ozone
over Texas", _Geophys. Res. Lett._ _22_, 227, 1995.
[Pearce] F. Pearce, "Ozone hole 'innocent' of Chile's ills",
_New Scientist_ #1887, 7, 21 Aug. 1993.
[Scotto et al.] J. Scotto, G. Cotton, F. Urbach, D. Berger, and T.
Fears, "Biologically effective ultraviolet radiation: surface
measurements in the U.S.", _Science_ _239_, 762, 1988.
[Seckmeyer et al.] G. Seckmeyer, B. Mayer, R. Erb, and G. Bernhard,
"UV-B in Germany higher in 1993 than in 1992", _Geophys. Res. Lett._
_21_, 577-580, 1994.
[Seckmeyer and McKenzie] G. Seckmeyer and R. L. McKenzie,
"Increased ultraviolet radiation in New Zealand (45 degrees S)
relative to Germany (48 degrees N.)", _Nature_ _359_, 135, 1992.
[Setlow et al.] R. B. Setlow, E. Grist, K. Thompson and
A. D. Woodhead, "Wavelengths effective in induction of Malignant
Melanoma", PNAS _90_, 6666, 1993.
[Skolnick] A. Skolnick, "Is ozone loss to blame for melanoma
upsurge?" JAMA, _265_, 3218, June 26 1991.
[Smith et al.] R. Smith, B. Prezelin, K. Baker, R. Bidigare, N.
Boucher, T. Coley, D. Karentz, S. MacIntyre, H. Matlick, D.
Menzies, M. Ondrusek, Z. Wan, and K. Waters, "Ozone depletion:
Ultraviolet radiation and phytoplankton biology in antarctic
waters", _Science_ _255_, 952, 1992.
[Stamnes et al.] K. Stamnes, Z. Jin, and J. Slusser, "Several-fold
enhancement of biologically effective Ultraviolet radiation levels at
McMurdo Station Antarctica during the 1990 ozone 'hole'", _Geophys. Res.
Lett._ _19_, 1013, 1992.
[Taylor] J.-S. Taylor, "Unraveling the Molecular Pathway from Sunlight
to Skin Cancer", _Acc. Chem. Res._ _27_, 76-82, 1994.
[Tevini and Teramura] M. Tevini and A. H. Teramura, "UV-B effects
on terrestrial plants", _Photochemistry and Photobiology_, _50_,
479, 1989. (This issue contains a number of other papers dealing
with biological effects of UV-B radiation.)
[van der Leun and de Gruijl] J. C. van der Leun and F. R. de Gruijl,
"Influences of Ozone Depletion on Human and Animal Health", in [Tevini].
[Wake] D. B. Wake, "Declining Amphibian Populations", _Science_
_253_, 860, 1991.
[Zerefos et al.] C. S. Zerefos, A. F. Bias, C. Meleti, and I. C. Ziomas,
"A note on the recent increase of solar UV-B radiation over northern
middle latitudes", _Geophys. Res. Lett._ _22_, 1245, 1995.
[Zigman] S. Zigman, "Ocular Damage by Environmental Radiant Energy
and Its Prevention", in [Young et al.]
Subject: Ozone Depletion FAQ Part I: Introduction to the Ozone Layer
From: rparson@spot.colorado.edu (Robert Parson)
Date: 11 Oct 1996 19:03:52 GMT
Archive-name: ozone-depletion/intro
Last-modified: 1 September 1996
Version: 5.7
------------------------------
Subject: How to get this FAQ
These files are (usually) posted monthly, towards the end of the month.
The current versions are stored on several archives:
A. World-Wide Web
(Limited) hypertext versions, with embedded links to some of the on-line
resources cited in the faqs, can be found at:
http://www.cs.ruu.nl/wais/html/na-dir/ozone-depletion/.html
http://www.cis.ohio-state.edu/hypertext/faq/usenet/ozone-depletion/top.html
http://www.lib.ox.ac.uk/internet/news/faq/sci.environment.html
The ohio-state version has the nicest format, but it sometimes falls
behind. The Utrecht version has the best record for staying up to date.
Plaintext versions can be found at:
ftp://rtfm.mit.edu/pub/usenet/news.answers/ozone-depletion/
ftp://ftp.uu.net/usenet/news.answers/ozone-depletion/
----
B. Anonymous ftp
To rtfm.mit.edu, in the directory /pub/usenet/news.answers/ozone-depletion
To ftp.uu.net, in the directory /usenet/news.answers/ozone-depletion
Look for the four files named intro, stratcl, antarctic, and uv.
----
C. Regular email
Send the following messages to mail-server@rtfm.mit.edu:
send usenet/news.answers/ozone-depletion/intro
send usenet/news.answers/ozone-depletion/stratcl
send usenet/news.answers/ozone-depletion/antarctic
send usenet/news.answers/ozone-depletion/uv
Leave the subject line blank.
If you want to find out more about the mail server, send a
message to it containing the word "help".
I have found a number of copies of the faqs tucked away in various corners
of the net, but many of them are seriously out-of-date. The archives listed
above usually get the latest version within a few days of its being posted.
Utrecht has the best record of keeping up to date; the others sometimes
fall behind.
-----------------------------
Subject: Copyright Statement
***********************************************************************
* Copyright 1996 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 version (5.7) of the Ozone 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 the scientific method, 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://jwocky.gsfc.nasa.gov/multi/arosa1.gif)
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
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.
Most 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. When evaluating substitutes for CFC's, the "time-dependent
ozone depletion potential", discussed in the preceding section,
is more useful 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. 1994]) 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 was
no significant ozone loss in the tropics during this period. (See,
however, [Hofmann et al. 1996] for more recent trends which appear to
show a decline in some tropical stations.) The depletion is larger in
the winter months, smaller in the summer. [Stolarski et al.] [WMO 1994]
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.)
It should be noted that one high-latitude ground station (Tromso
in Norway) has found no long-term change in total ozone change
between 1939 and 1989. [Larsen and Henriksen][Henriksen et al. 1992]
The reason for the discrepancy is not known. [WMO 1994]
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.]. By February 1994 ozone over the United States had
recovered to levels similar to 1991, [Hofmann et al. 1994b] and in the
spring of 1995 they were down again, to levels lower than any previous
year other than 1993. [Bojkov et al. 1995] Sulfate aerosols from the
July 1991 eruption of Mt. Pinatubo are the most likely cause of the
exceptionally low ozone in 1993; 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] [McCormick et al. 1995]
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 was a transient phenomenon,
superimposed upon the slower downward trend identified before 1991.
-----------------------------
Subject: 2.11) Is the middle-latitude ozone loss due to CFC emissions?
That's the majority opinion, although it's not a universal opinion.
The present trends are too small and the atmospheric chemistry and
dynamics too complicated 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
(available on the Web at http://www.al.noaa.gov/WWWHD/pubdocs/WMOUNEP94.html)
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]
For a contrasting view, see [Henriksen and Roldugin].
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. [Prather et al. 1996]
-----------------------------
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. [Rowland and Molina 1975] (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 imply instability.
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 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. 1990] [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:
http://www.cis.ohio-state.edu/hypertext/faq/usenet/space/controversy/faq.html
-----------------------------
Subject: 2.14) Will commercial supersonic aircraft damage the ozone layer?
Short answer: Probably not. This problem is very complicated,
and a definitive 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 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: [Crutzen 1970]
NO + O3 -> NO2 + O2
NO2 + O -> NO + O2
-------------------------------
net: O3 + O -> 2 O2
(For this and other contributions to ozone research, Crutzen,
together with Rowland and Molina, was awarded the 1995 Nobel Prize
in Chemistry. The official announcement from the Swedish Academy is
available at http://www.nobel.se/announcement95-chemistry.html .)
Two years later, Harold S. Johnston 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] Very recently, a new complication
has appeared: _in situ_ measurements in the exhaust plume of a
Concorde aircraft flying at supersonic speeds indicate that the
ground-based estimates of NOx emissions are accurate, but that the
exhaust also contains large amounts of sulfate-based particulates
[Fahey et al. 1995]. Since reactions on sulfate aerosols are believed
to play an important role in halogen-catalyzed ozone depletion, it may
be advisable to concentrate on reducing the sulfur content of the
fuels that are to be used in new generations of supersonic aircraft,
rather than further reducing NOx emissions.
..................................................................
_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 concerns 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 (available on the world-wide web through
http://www.ciesin.org/TG/PI/POLICY/montpro.html ) 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 were also applied
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.
For more information about legal and policy issues, see the books by
[Benedick] and [Litvin], and the following web sites:
http://www.acd.ucar.edu/gpdf/ozone/index.html
http://www.epa.gov/docs/ozone/index.html
http://www.ciesin.org/TG/OZ/ozpolic.html
Recent NOAA measurements [Elkins et al. 1993] [Montzka et al. 1996]
show that the _rate of increase_ of halocarbon concentrations in the
atmosphere has decreased markedly since 1987. 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
1994 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 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 articles 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.
(I make an exception for the Nobel-Prize winning work of Crutzen,
Molina and Rowland.) 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.
[Rowland and Molina 1994] F. S. Rowland and M. J. Molina, "Ozone
depletion: 20 years after the alarm", _Chemical and Engineering
News_, 15 Aug. 1994, pp. 8-13.
[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 1968a] G. M. B. Dobson, _Exploring the Atmosphere_,
2nd Edition, Oxford, 1968.
[Dobson 1968b] G. M. B. Dobson, "Forty Years' research on atmospheric
ozone at Oxford", _Applied Optics_, _7_, 387, 1968.
[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.
[Litvin] K. T. Litvin, _Ozone Discourses_, Columbia 1994.
[McElroy and Salawich] M. McElroy and R. Salawich,
"Changing Composition of the Global Stratosphere",
_Science_ _243, 763, 1989.
[Rowland and Molina 1975] F. S. Rowland and M. J. Molina,
"Chlorofluoromethanes in the Environment",
Rev. Geophys. & Space Phys. _13_, 1, 1975.
[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 #37.
The Executive Summary of this report is available on the
World-Wide Web at http://www.al.noaa.gov/WWWHD/pubdocs/WMOUNEP94.html
-----------------------------
Subject: More Specialized References
[Bojkov et al. 1995] R. D. Bojkov, V. E. Fioletov, D. S. Balis,
C. S. Zerefos, T. V. Kadygrova, and A. M. Shalamjansky,
"Further ozone decline during the northern hemisphere winter-spring
of 1994-95 and the new record low ozone over Siberia",
Geophys. Res. Lett. _22_, 2729, 1995.
[Brasseur and Granier] G. Brasseur and C. Granier, "Mt. Pinatubo
aerosols, chlorofluorocarbons, and ozone depletion", _Science_
_257_, 1239, 1992.
[Crutzen 1970] P. J. Crutzen, "The influence of nitrogen oxides on the
atmospheric ozone content", _Quart. J. R. Met. Soc._ _90_, 320, 1970.
[Elkins et al. 1993] 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.
[Fahey et al. 1995] D. W. Fahey, E. R. Keim, K. A. Boering,
C. A. Brock, J. C. Wilson, H. H. Jonsson, S. Anthony, T. F. Hanisco,
P. O. Wennberg, R. C. Miake-Lye, R. J. Salawich, N. Louisnard,
E. L. Woodbridge, R. S. Gao, S. G. Donnelly, R. C. Wamsley,
L. A. Del Negro, S. Solomon, B. C. Daube, S. C. Wofsy, C. R. Webster,
R. D. May, K. K. Kelly, M. Loewenstein, J. R. Podolske, and K. R. Chan,
"Emission Measurements of the Concorde Supersonic Aircraft in the
Lower Stratosphere", _Science_ _270_, 70, 1995.
[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.
[Henriksen and Roldugin] K. Henriksen and V. Roldugin, "Total ozone
variations in Middle Asia and dynamics meteorological processes
in the atmosphere", _Geophys. Res. Lett._ _22_, 3219, 1995.
[Henriksen et al. 1992] K. Henriksen, T. Svenoe, and S. H. H. Larsen,
"On the stability of the ozone layer at Tromso", J. Atmos. Terr. Phys.
_55_, 1113, 1992.
[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.
[Hofmann et al. 1996] D. J. Hofmann, S. J. Oltmans, G. L. Koenig,
B. A. Bodhaine, J. M. Harris, J. A. Lathrop, R. C. Schnell, J. Barnes,
J. Chin, D. Kuniyuki, S. Ryan, R. Uchida, A. Yoshinaga, P. J. Neale,
D. R. Hayes, Jr., V. R. Goodrich, W. D. Komhyr, R. D. Evans, B. J. Johnson,
D. M. Quincy, and M. Clark, "Record low ozone at Mauna Loa Observatory
during winter 1994-95: A consequence of chemical and dynamical
synergism?", Geophys. Res. Lett. _23_, 1533, 1996.
[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.
[Larsen and Henriksen] S. H. H. Larsen and T. Henriksen,
"Persistent Arctic ozone layer", _Nature_ _343_, 134, 1990.
[McCormick et al. 1995] M. P. McCormick, L. W. Thomason, and
C. R. Trepte, "Atmospheric effects of the Mt Pinatubo eruption",
_Nature_ _373_, 399, 1995.
[Molina and Rowland 1974] M. J. Molina and F. S. Rowland,
"Stratospheric sink for chlorofluoromethanes: chlorine
atom-catalyzed destruction of ozone", _Nature_ _249_, 810, 1974.
[Montzka et al. 1996] S. A. Montzka, J. H. Butler, R. C. Myers,
T. M. Thompson, T. H. Swanson, A. D. Clarke, L. T. Lock, and
J. W. Elkins, "Decline in the Tropospheric Abundance of Halogen
from Halocarbons: Implications for Stratospheric Ozone Depletion",
_Science_ _272_, 1318, 1996.
[Prather et al. 1990] 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.
[Prather et al. 1996] M. J. Prather, P. Midgley, F. S. Rowland,
and R. Stolarski, "The ozone layer: the road not taken",
_Nature_ _381_, 551, 1996.
[Ravishankara et al. 1994] 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_,
171, 1992.
-----------------------------
Subject: Internet Resources
This list is preliminary and by no means comprehensive; it includes a
few sites that I have found particularly useful and which provide
good *starting points* for further exploration.
Lenticular Press publishes a multimedia CD-ROM (for Apple Macintosh)
containing ozone data and images, as well as a hypertext document similar
to this FAQ. For sample images and information about ordering the CD,
see http://www.lenticular.com/ Note that these samples are copyrighted
and may not be further distributed.
Probably the most extensive collection of online resources is that provided
by the Consortium for International Earth Science Information Network:
http://www.ciesin.org/TG/OZ/oz-home.html
It includes links to many other documents, including on-line versions
of some of the original research papers. At the present time it is
heavily under construction and many of the links are not yet established.
A very useful resource for both science and policy (including the
text of many legal documents) is the "SOLIS" (Stratospheric Ozone Law
Information and Science) page, created and maintained by
Gregory Dubois-Felsmann, at:
http://www.acd.ucar.edu/gpdf/ozone/index.html
The NOAA Aeronomy Lab: http://www.al.noaa.gov/ ,
has the text of the Executive Summary of the 1994 WMO Scientific
Assessment, http://www.al.noaa.gov/WWWHD/pubdocs/WMOUNEP94.html
The US Environmental Protection Agency has an ozone page that includes
links to both science and policy resources:
http://www.epa.gov/docs/ozone/index.html
Some of the more interesting scientific web pages include:
The Centre for Antarctic Information and Research (ICAIR) in New Zealand:
http://icair.iac.org.nz/ozone/index.html
Environment Canada:
http://ellesmere.ccm.emr.ca/ogd/ozone/english/html/menu.html
The TOMS home page: http://jwocky.gsfc.nasa.gov/
The EASOE home page: http://www.atm.ch.cam.ac.uk/images/easoe/
The UARS Project Definition page:
http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/UARS_project.html
The HALOE home page: http://haloedata.larc.nasa.gov/home.html
The British Antarctic Survey:
http://www.nbs.ac.uk/public/icd/ozone_pub/index.html
The Institute for Meteorology at the Free University of Berlin:
http://www.met.fu-berlin.de/~strato/ozon/ozon.html
The Climate Prediction Center's TOVS Total Ozone Analysis page:
http://nic.fb4.noaa.gov:80/products/stratosphere/tovsto/
The USDA UV-B Radiation Monitoring Program Climate Network,
http://uvb.nrel.colostate.edu/UVB/uvb_climate_network.html