OOL
World Meteorological Organization
Global Ozone Research and Monitoring Project Report No. 37
SOENTIFIG ASSESSMENT OF
OZONE DEPLETION: 1994
EXECUTIVE SUMMARY
National Oceanic and Atmospheric Administration
National Aeronautics and Space Administration
United Nations Environment Programme
World Meteorological Organization
Reprinted by the U.S. Environmental Protection Agency ','..:
Recycled/Recyclable Printed with Vegetable Oil Based Inks on 100% Recycled Paper (50% Posteonsumer)
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TABLE OF CONTENTS
EXECUTIVE SUMMARY
SCIENTIFIC ASSESSMENT OF OZONE DEPLETION: 1994
PREFACE..... ,...:.....;......"....; ;... .7. ........: 5
EXECUTIVE SUMMARY ..... ;, i... ....;.. .... 1
COMMON QUESTIONS ABOUT OZONE ,....; 19
LIST OF INTERNATIONAL AUTHORS,. CONTRIBUTORS, AND REVIEWERS .. 29
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PREFACE
The present document contains key summaries from the Scientific Assessment of Ozone Depletion:-1994. The full
assessment report will be part of the information upon which the Parties to the United Nations Montreal Protocol will
base their future decisions regarding protection of the stratospheric ozone layer.
Specifically, the Montreal Protocol on Substances.That Deplete the Ozone Layer states (Article 6): "... the Parties
shall assess the control measures ... on the basis of available scientific, environmental, technical, and economic infor-
mation." To provide the mechanisms whereby these assessments are conducted, the Protocol further states: ".'-.. the
Parties shall convene appropriate panels of experts" and "the panels will report their conclusions ... to the Parties."
Three assessment reports have_been prepared during 1994 to be available to the Parties in advance of their meeting
in 1995, at which they will consider the need to amend or adjust the Protocol. The two companion reports to the
scientific assessment focus on the environmental and health effects of ozone layer depletion and on the technology and
economic implications of mitigation approaches. '" ...
The scientific assessment summarized in the present document is the latest in a series of seven scientific reports
prepared by the world's leading experts in the atmospheric sciences and under the international auspices of the World
Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP). The chronology of
those scientific assessments and the relation to the international policy process are summarized as follows: .
. 1981
1985 .
1987
1988
1989
1990
1991
1992
1992
1994
(1995)
Policy Process -
Vienna Convention
Montreal Protocol
London Amendment
Copenhagen Amendment,
Vienna Amendment (?) -
Scientific Assessment .
The Stratosphere 1981 Theory and Measurements.
WMO No. 11. ,
Atmospheric Ozone 1985: 3 vol. WMO No. 16.
. International Ozone Trends Panel Report 1988.
2vol. WMO No. 18! '
Scientific Assessment of Stratospheric Ozone:
1989. 2 vol. WMO No. 20. .
Scientific Assessment of Ozone Depletion: 1991..
WMO No. 25.
Methyl Bromide: Its Atmospheric Science, Technology, and '
Economics (Assessment Supplement). UNEP (1992).
Scientific Assessment of Ozone Depletion: 1994.
WMO No. 37.
The genesis of Scientific Assessment of Ozone Depletion: 1994 occurred at the 4th meeting of the Conference of the
Parties to the Montreal Protocol in Copenhagen, Denmark, in November 1992, at which the scope of the scientific needs
of the Parties was defined. The formal planning of the present report was a workshop that was held on 11 June 1993 in
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Virginia Beach, Virginia, at which an international steering group crafted the outline and suggested scientists from the
world community to serve as authors. The first drafts of the chapters were examined at a meeting that occurred on 2 - 4
March 1994 in Washington, D.C., at which the authors and a small number of international experts improved the coor-
dination of the text of the chapters. -
The second draft was sent out to 123 scientists worldwide for a mail peer review. These anonymous comments
were considered by the authors. At a Panel Review Meeting in Les Diablerets, Switzerland, held on 18 - 21 July 1994,
the responses to these mail review comments were proposed by the authors and discussed by the 80 participants. Final
changes to the chapters were decided upon, and the Executive Summary contained herein was prepared by the partici-
pants.
The group also focused on a set of questions commonly asked about the ozone layer. Based upon the scientific
understanding represented by the assessments, answers to these common questions were prepared and are also included
here. ,
As the accompanying list indicates, the Scientific Assessment of Ozone Depletion: 1994 is the product of 295
scientists from the developed and developing world1 who contributed to its preparation and review (230 scientists
prepared the report and 147 scientists participated in the peer review process).
What follows is a summary of their current understanding of the stratospheric ozone layer and its relation to hu-
mankind.
1 Participating were Argentina, Australia, Austria, Belgium, Brazil, Canada, Chile, Cuba, Czech Republic, Denmark, Egypt, France, C-ermany,
Greece, Hungary, India, Iran, Ireland, Israel, Italy, Japan, Kenya, Malaysia, New Zealand, Norway, Poland, Russia, South Africa, Sweden, Switzer-
land, Taiwan, The Netherlands, The People's Republic of China, United Kingdom, United States of America, and Venezuela.
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EXECUTIVE SUMMARY
Recent Major Scientific Findings and Observations
- The laboratory investigations, atmospheric observations, and theoretical and modeling studies of the past few years
have provided a deeper understanding of the human-influenced and natural chemical changes in the atmosphere and
their relation to the Earth's stratospheric ozone layer and radiative balance of the climate system. Since the last interna-
tional scientific assessment of the state of understanding, there have been several key ozone-related findings,
observations, and conclusions: ;
The atmospheric growth rates of several major ozone-depleting substances have slowed, demonstrating the
expected impact of,the Montreal Protocol and its Amendments and Adjustments. The abundances of the
chlorofluorocarbons (CFCs), carbon tetrachloride, methyl chloroform, and halons in the atmosphere have been
monitored at global ground-based sites since about 1978. Over much of that period, the annual growth rates of
these gases have been positive. However^ the data of recent years clearly show that the growth rates of CFC-11, '
CFC-l2,'halon-1301, and halon-1211 are slowing down. In particular, total tropospheric organic chlorine in-
creased by only about 60 ppt/year (1.6%) in 1992, compared to 110 ppt/year (2.9%) in ,1989. Furthermore,
tropospheric bromine in halons increased by only about 0.25 ppt/year in 1992, compared to about 0.85 ppt/year in
1989. The abundance of carbon tetrachloride is actually decreasing. The observed trends in total tropospheric
organic chlorine are consistent with reported production data, suggesting less emission than the maximum al-
lowed under the Montreal Protocol and its Amendments and Adjustments. Peak total chlorine/bromine loading in
; the troposphere is expected to occur in 1994, but the stratospheric peak will lag by about 3 - 5 years.. Since the
stratospheric abundances of chlorine and bromine are expected to continue to grow for a few more years, increas-
ing global ozone losses are predicted (other things being equal) for the .remainder of the decade, with gradual
recovery in the 21st century. ' - ,
The atmospheric abundances of several of the CFG substitutes are increasing, as anticipated. Withphaser
out" dates for-the CFCs and other ozone-depleting substances now fixed by international agreements, several
hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) are being manufactured and used as substi- -
tutes. ,The atmospheric growth of some of these compounds (e.g., HCFC-22) has been'observed for several years,
and the growth rates of others (e.g., HCFC-142b and HCFC-141b) are now being monitored. Tropospheric
chlorine in HCFCs increased by 5 ppt/year in 1989 and about 10 ppt/year in 1992. ,
Record low global ozone levels were measured over the past,two years. Anomalous ozone decreases were
observed in the midlatitudes of both hemispheres in 1992 and 1993. The Northern Hemispheric decreases were
larger than those in the Southern Hemisphere. 'Globally, ozone values were i - 2% lower than would be expected
from an extrapolation of the trend prior to 1991, allowing for solar-cycle and quasi-biennial-oscillation (QBO)
effects. The 1994 global ozone levels are returning to values closer to thoser expected from the longer-term
downward trend.
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The stratosphere was perturbed by a major volcanic eruption. The eruption of Mt. Pinatubo in 1991 led to a
large increase in sulfate aerosol in the lower stratosphere throughout the globe. Reactions on sulfate aerosols
resulted in significant, but temporary, changes in the chemical partitioning that accelerated the photochemical
ozone loss associated with reactive hydrogen (HOX), chlorine, and bromine compounds in the lower stratosphere
in midlatitudes and polar regions. Absorption of terrestrial and solar radiation by the Mt. Pinatubo aerosol result-
ed in a transitory rise of 1°C (globally averaged) in the lower-stratospheric temperature and also affected the
distribution of ozone through circulation changes. The observed 1994 recovery of global'ozone is qualitatively
consistent with observed gradual reductions of the abundances of these volcanic particles in the stratosphere.
Downward trends in total-column ozone continue to be observed over much of the globe, but their magni-
tudes are underestimated by numerical models. Decreases in ozone abundances of about 4-5% per decade at
midlatitudes in the Northern and Southern Hemispheres continue to be observed by both ground^based and satel-
lite-borne monitoring instruments. At midlatitudes, the losses continue to be much larger during winter/spring
than during summer/fall in both hemispheres, and the depletion increases with latitude, particularly in the South-
ern Hemisphere. Little or no downward trends are observed in the tropics (20°N - 20°S). While the current two-
dimensional stratospheric models simulate the observed trends quite well during some seasons and latitudes, they
underestimate the trends by factors of up to three in winter/spring at mid- and high latitudes. Several known
atmospheric processes that involve chlorine and bromine and that affect ozone in the lower stratosphere are
difficult to model and have not been adequately incorporated into these models.. -
Observations have demonstrated that halogen chemistry plays a larger role in the chemical destruction of
ozone in the midlatitude lower stratosphere than expected from gas phase chemistry. Direct in situ measure-
ments 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 from 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.
The conclusion that anthropogenic chlorine and bromine compounds, coupled with surface chemistry on
natural polar stratospheric particles, are the cause of polar ozone depletion has been further strengthened.
Laboratory studies have provided a greatly improved understanding of how the chemistry on the surfaces of ice,
nitrate, and sulfate particles can increase the abundance of ozone-depleting forms of chlorine in the polar strato-'
spheres. Furthermore, satellite and in situ observations of the abundances of reactive nitrogen and chlorine
compounds have improved the explanation of the different ozone-altering properties of the Antarctic and Arctic.
The Antarctic ozone "holes" of 1992 and 1993 were the most severe on record. The Antarctic ozone "hole"
has continued to occur seasonally every year since its advent in the late-1970s, with the occurrences over the last
several years being particularly pronounced. Satellite, balloon-borne, and ground-based monitoring instruments
revealed that the Antarctic ozone "holes" of 1992 and 1993 were the biggest (areal extent) and deepest (minimum
amounts of ozone overhead), with ozone being locally depleted by more than 99% between about 14 -19 km in
October, 1992 and 1993. It is likely that these larger-than-usual ozone depletions could be attributed, at least in
part, to sulfate aerosols fromMt. Pinatubo increasing the effectiveness of chlorine- and bromine-catalyzed ozone
destruction. A substantial Antarctic ozone "hole" is expected to occur each austral spring for many more decades
because stratospheric chlorine and bromine abundances will approach the pre-Antarctic-ozone-"hole" levels
(late-1970s) very slowly during the next century. ,
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Ozone losses have been detected in the Arctic whiter stratosphere, and their links to halogen chemistry
have been established. Studies in the Arctic lower stratosphere have been expanded to include more widespread
observations of ozone and key reactive species. In the late-winter/early-spring period, additional chemical losses
of ozone up to 15 - 20% at some altitudes are deduced from these observations, particularly in the winters of 19917
2 and 1992/3.: Model calculations constrained by the observations are also consistent with these losses, increasing'.
the confidence in the role of chlorine and bromine in ozone destruction. The interannual variability in the photo-
chemical and dynamical conditions of the Arctic polar vortex continues to limit the ability to predict ozone
changes in future years. ' ' , ,
The link between a decrease in stratospheric ozone and an increase in surface ultraviolet (UV) radiation
has been further strengthened. Measurements of UV radiation at the surface under clear-sky conditions show
that low overhead ozone yields high UV radiation and in the amount predicted by radiative-transfer theory: Large
increases of surface UV are observed in Antarctica and the southern part of South America during the period of
the seasonal ozone "hole." Furthermore, elevated surface UV levels at mid-to-high latitudes were observed in the
NorthenrHemisphere in 1992 and. 1993, corresponding to the low ozone levels of those years. However, the lack
of a decadal (or longer) record of accurate monitoring of surface UV levels and the variation introduced by clouds;
and other factors have precluded the unequivocal identification of a long-term trend in surface UV radiation.
Methyl bromide continues to be viewed as a significant ozone-depleting compound. Increased attention has
been focused upon the ozone-depleting role of methyl bromide. Three potentially major anthropogenic sources of
atmospheric methyl bromide have been identified (soil fumigation, biomass burning, and the exhaust of automo-
biles using leaded gasoline), in addition to the natural oceanic source. Recent laboratory studies have confirmed
the fast rate for the BrO + HC>2 reaction and established a negligible reaction pathway producing HBr, both of
which imply greater ozone losses due to emissions of compounds containing bromine. While the magnitude of
the atmospheric photochemical removal is well understood, there are significant uncertainties in quantifying the
oceanic sink for atmospheric methyl bromide. The best estimate for the overall lifetime of atmospheric methyl
bromide is 1.3 years, with a range of 0.8 -1.7 years. The Ozone Depletion Potential (ODP) for methyl bromide is
calculated to be about 0.6 (relative to an ODP of 1 forCFC-11). " ' ,
, Stratospheric ozone losses cause a global-mean negative radiative forcing. In the 1991 scientific assessment,
it was pointed out that the global ozone losses that were occurring in the lower stratosphere'caused this region to
cool and result in less radiation reaching the surface-troposphere system. Recent model studies have strengthened
this picture. A long-term global-mean cooling of the lower stratosphere of between 0.25 and 0.4°G/deeade has
been observed over the last three decades. Calculations indicate that, on a global mean, the ozone losses between
1980 and 1990 offset about 20% of the radiative forcing due'to-the well-mixed greenhouse-gas increases during
that period (i.e., carbon dioxide, methane, nitrous oxide, and halocarbons). . .
Tropospheric ozone, which is a greenhouse gas, appears to have increased in many regions of the Northern
Hemisphere. Observations show- that tropospheric ozone, which is formed by chemical reactions involving
pollutants, has increased above many locations in the Northern Hemisphere over the last 30 years. However, in
the 1980s, the trends were variable, being small or nonexistent. In the Southern Hemisphere, there are insufficient
data to draw strong inferences, At the South Pole, a decrease has been observed since the mid-1980s. Model
simulations and limited observations suggest that tropospheric ozone has increased in the Northern Hemisphere
since pre-industrial times. Such changes would augment the radiative'forcing from all other greenhouse gases by
.about 20% over the same time period, . -;
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The atmospheric residence times of the important ozone-depleting gases, CFC-11 and methyl chloroform,
and the greenhouse gas, methane, are now better known. A reconciliation of observed concentrations with
known emissions using an atmospheric model has led to a best-estimate lifetime of "50 years for CFC-11 and 5.4
years for methyl chloroform, with uncertainties of about 10%. These lifetimes provide an accurate standard for
gases destroyed only in the stratosphere (such as CFCs and nitrous oxide) and for those also reacting with tropo-
spheric hydroxyl radical, OH (such as HCFCs and HFCs), respectively. Recent model simulations of methane
perturbations and a theoretical analysis of the tropospheric chemical system that couples methane, carbon monox-"
ide, and OH have demonstrated that methane perturbations decay with a lengthened time scale in a range of about
12 - 17 years, as compared with the 10-year lifetime derived from the total abundance and losses. This longer
response time and other indirect effects increase the estimate of the effectiveness'of emissions of methane as a
greenhouse gas by a factor of about two compared to the direct-effect-only values given in the 1991 assessment.
Supporting Scientific Evidence and Related Issues
OZONE CHANGES IN THE TROPICS AND MTOLATITUDES AND THEIR INTERPRETATION
Analysis of global total-column ozone data through early 1994 shows substantial decreases of ozone in all sea-
sons at midlatitudes (30° - 60°) of both hemispheres. For example, in the middle latitudes of the Northern
Hemisphere, downward trends of about 6% per decade over 1979 -1994 were observed in winter and spring and
about 3% per decade were observed hi summer and fall. In the Southern Hemisphere, the seasonal difference was
somewhat less, but the midlatitude trends averaged a similar 4% to 5% per decade. There are no statistically
significant trends in the tropics (20°S - 20°N). Trends through 1994 are about 1% per decade more negative in the
Northern Hemisphere (2% per decade in the midlatitude winter/spring in the Northern Hemisphere) compared to
those calculated without using data after May 1991. At Northern midlatitudes, the downward trend in ozone
between 1981 -1991 was about 2% per decade greater compared to that of the period 1970 -1980.
Satellite and ozonesonde data show that much Of the downward trend in ozone occurs below 25 km (i.e., in the
lower stratosphere). For the region 20 - 25 km, there is good agreement between the trends from the Stratospheric
Aerosol and Gas Experiment (SAGE I/II) satellite instrument data and those from ozonesondes, with an observed
annual-average decrease of 7 ± 4% per decade from 1979 to 1991 at 30° - 50°N latitude. Below 20 km, SAGE
yields negative trends as large as 20 ± 8% per decade at 16 - 17 km, while the average of available midlatitude
ozonesonde data shows smaller negative trends of 7 ± 3% per decade. Integration of the ozonesonde data yields
total-ozone trends consistent with total-ozone measurements. In the 1980s, upper-stratospheric (35 - 45 km)
ozone trends determined by the data from SAGE I/II, Solar Backscatter Ultraviolet satellite specttometer
(SBUV), and the Umkehr method agree well at midlatitudes, but less so in the tropics. Ozone declined 5 -10%
per decade at 35 - 45 km between 30°T 50°N and slightly more at southern midlatitudes. In the. tropics at 45 km,
SAGE I/n and SBUV yield downward trends of 10 and 5% per decade, respectively.
Simultaneous in situ measurements of a suite of reactive chemical species have directly- confirmed modeling
studies imply ing that the chemical destruction of ozone in the midlatitude lower stratosphere is more strongly
influenced by HOX and halogen chemistry than NOX chemistry. The seasonal cycle of CIO in the lower strato-
sphere at midlatitudes in both hemispheres supports a role for in situ heterogeneous perturbations (i.e., on sulfate
aerosols), but does not appear consistent with the timing of vortex processing or dilution. These studies provide
key support for the view that sulfate aerosol chemistry plays an important role in determining midlatitude chem-
ical ozone destruction rates.
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The model-calculated ozone depletions in the upper stratosphere for 1980 - 1990 are in broad agreement with the
measurements. Although these model-calculated ozone depletions did not consider radiative feedbacks and tem-
perature trends, including these effects is not likely to reduce the predicted ozone changes by more than 20%.
Models including the chemistry involving sulfate aerosols and polar stratospheric clouds (PSCs) better simulate ,
the observed total ozone depletions of the past decade than models that include only gas phase reactions. How-
ever, they still underestimate the ozone loss by factors ranging from 1.3 to 3.0.
Some unresolved discrepancies between observations and models exist for the partitioning of inorganic chlorine
species, which could impact model predictions of ozone trends, These occur for the G1O/HC1 ratio in the upper
stratosphere and the fraction of HC1 to total inorganic chlorine in the lower stratosphere. . .
The transport of ozone-depleted air from polar regions has the potential to influence ozone concentrations at
middle latitudes. While there are uncertainties about the importance of this process relative to in situ chemistry
for midlatitude ozone loss, both directly involve ozone destruction by chlorine- and bromine-catalyzedjeactions.
Radiosonde and satellite data continue to.show a long-term cooling trend in globally annual-average lower-strato-
spheric temperatures , of about '0.3 >0.4°C per decade over the last three decades. .Models suggest that ozone
depletion is the major contributor to this trend.
Anomalously large downward ozone trends have been observed in midlatitudes of both hemispheres in 1992 and
, 1993 (i.e., the first two years after the eruption of Mt. Pinatubo), with Northern-Hemispheric decreases larger than
those of the Southern Hemisphere. Global-average total-ozone levels in early 1993 were about ,1 % to 2% below
that expected from the long-term trend and the particular phase of the solar and QBO cycles, while peak decreases
of about 6 - 8% from expected ozone levels were seen over 45 - 60°N. In the first :half of/1994, ozone levels
returned to values closer to those expected from the long-term trend. ~" - . - ;
The sulfur gases injected by Mt. Pinatubo led to large enhancements in stratospheric sulfate aerosol surface areas
(by a. maximum factor of about 30 - 40 at northern midlatitudes within a year after the, eruption), which have
subsequently declined. , ;
Anomalously low ozone was measured at altitudes below 25 km at a Northern-Hemispheric midlatitude station in
1 992 and 1993 and was correlated with observed enhancements in sulfate-aerpsol surface areas, pointing towards
a causal link. . .
Observations indicate that the eruption of,Mt. Pinatubo did not significantly increase the HC1 content of the
stratosphere. . -
The recent large ozone changes at midlatitudes are highly likely to have been due, at least in part, to the greatly
increased sulfate aerosol in the lower stratosphere following Mt. Pinatubo. Observations and laboratory studies
have demonstrated the importance of heterogeneous hydrolysis of ^Os on sulfate aerosols in the atmosphere.
Evidence suggests that C1ONO2 hydrolysis also occurs on sulfate aerosols under cold conditions. Both processes
perturb the chemistry in such a way as to increase ozone loss through coupling with the anthropogenic chlorine
and bromine loading of the stratosphere. .
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Global mean lower stratospheric temperatures showed a marked transitory rise of about 1°C following the erup-
tion of Mt. Pinatubo in 1991, consistent with model calculations. The warming is likely due to absorption of
radiation by the aerosols.
POLAR OZONE DEPLETION
In 1992 and 1993, the biggest-ever (areal extent) and deepest-ever (minimum ozone below 100 Dobson units)
ozone "holes" were observed in the Antarctic. These extreme ozone depletions may have been due to the chem-
ical perturbations caused by jsulfate aerosols from Mt. Pinatubo, acting in addition to the well-recognized chlorine
and bromine reactions on polar stratospheric clouds.
Recent results of observational and modeling studies reaffirm the role of anthropogenic halocarbon species in
Antarctic ozone depletion. Satellite observations show a strong spatial and temporal correlation of CIO abun-
dances with ozone depletion in the Antarctic vortex. In the Arctic winter, a much smaller ozone loss has been
observed. These losses are both consistent with photochemical model calculations constrained with observations
from in situ and satellite instruments. .
Extensive new measurements of HC1, CIO, and C1ONO2 from satellites and in situ techniques have confirmed the
picture of the chemical processes responsible for chlorine activation in polar regions and the recovery from those
processes, strengthening current understanding of the seasonal cycle of ozone depletion in both polar regions.
New laboratory and field studies strengthen the confidence that reactions on sulfate aerosols can activate chlorine
under cold conditions, particularly those in the polar regions. Under volcanically perturbed conditions when
aerosols are enhanced, these processes also likely contribute to ozone losses at the edges of PSC formation
regions (both vertical and horizontal) just outside of the southern vortex and in the Arctic. ''.''.
Satellite measurements have confirmed that the Arctic vortex is much less denitrified than the Antarctic, which is
likely to be an important factor in determining the interhemispheric differences in polar ozone loss.
Interannual variability in the photochemical and dynamical conditions of the vortices limits reliable predictions of
future ozone changes in the polar regions, particularly in the Arctic.
COUPLING BETWEEN POLAR REGIONS AND.MroLATiruDEs
Recent satellite observations of long-lived tracers and modeling studies confirm that, above 16 km, air near the
center of the polar vortex is substantially isolated from lower latitudes, especially in the Antarctic.
Erosion of the vortex by planetary-wave activity transports air from the vortex^edge region to lower latitudes.
Nearly all observational and modeling studies are consistent with a time scale of 3 - 4 months to replace a substan-
tial fraction of Antarctic vortex air. The importance of this transport to in situ chemical effects for midlatitude
ozone loss remains poorly known. . . ;
Air is readily transported between polar regions and midlatitudes below 16 km. The influence of this transport on
midlatitude ozone loss has not been quantified.
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TROPOSPHERIC OZONE
There is observational evidence that tropospheric ozone (about 10% of the total-column ozone) has increased in
the Northern Hemisphere .(north of 20°N) over the past three decades. The upward trends are highly regional.
They are smaller in the 198(0s than in the 1970s and may be slightly negative at some locations. European
measurements at surface sites also indicate a doubling in the lower-tropospheric ozone concentrations since ear-
lier this century. At the South Pole, a decrease has been observed since the midrl980s. Elsewhere in the Southern
Hemisphere, there are insufficient data to draw strong inferences. . - - .
There is strong evidence that ozone levels in the boundary layer over the populated regions of the Northern
Hemisphere are enhanced by more than 50% due to photochemical production from anthropogenic precursors,
and'that export of ozone from North America is a significant source for the North Atlantic region during summer.
It has also been shown that biomass burning is a significant source of ozone (and carbon monoxide) in the tropics
' during the dry season. -
' An increase in UV-B radiation (e.g., from stratospheric ozone loss) is expected to decrease tropospheric ozone in
the background atmosphere, but, in some cases, it will increase production of ozone in the more polluted regions.
Model calculations predict that a 20% increase in methane concentrations would result in tropospheric ozone
increases ranging from 0.5 to 2.5 ppb in the tropics and the northern midlatitude summer, and an increase in the
methane residence time to about 14 years (a range of 12 - 17 years). Although there is a high degree of consis-
tency in the global transport of short-lived tracers within .three-dimensional chemical-transport models, and a
general agreement in the computation of photochemical rates affecting tropospheric ozone, many processes con-
trolling tropospheric ozone are not adequately represented or tested in the models, hence limiting the accuracy of
these results. *
TRENDS IN SOURCE GASES RELATING TO OZONE CHANGES
CFCs, carbon tetrachloride, methyl chloroform, and the halons are major anthropogenic source gases for strato-
. spheric chlorine and bromine, and hence stratospheric ozone destruction. Observations from several monitoring
; networks worldwide have demonstrated slowdowns in growth rates of these species that are consistent .(except for
carbon tetrachloride) with expectations based upon recent'decreases in emissions. In addition, observations from
several sites have revealed accelerating growth :rates of the CFC substitutes, HCFC-22, HCFC-141b, and HCFC-
142b, as expected from their increasing use. , - '
Methane levels in the atmosphere affect tropospheric and stratospheric ozone levels. Global methane increased
by 7% over about the past decade. However, the 1980s were characterized by slower growth rates, dropping from
.approximately 20 ppb per year in 1980 to about 10 ppb per year by the end of the decade. Methane growth rates
slowed dramatically in 1991 and 1992, but the very recent data suggest that they have started to increase hi late
1993. The cause(s) of this behavior are not known, but it is probably due to changes in methane sources rather
than sinks.
. Despite the increased methane levels, the total amount of carbon .monoxide in today's atmosphere is less than it
was a decade ago. Recent analyses of global carbon monoxide data show that tropospheric levels grew from the
early 1980s toabout 1987 and have declined from the late 1980s to the present. The cause(s) of this behavior have
,' not been identified.
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CONSEQUENCES OF OZONE CHANGES
The only general circulation model (GCM) simulation to investigate the climatic impacts of observed ozone
depletions between 1970 and 1990 supports earlier suggestions that these depletions reduced the model-predicted
warming due to well-mixed greenhouse gases by about 20%. This is consistent with radiative forcing calcula-
tions. ' . , -'".-
Model simulations suggest that increases in tropospheric ozone since pre-industrial times may have made signif-
icant contributions to the greenhouse forcing of the Earth's climate system, enhancing the current total forcing by
about 20%"compared to that arising from the changes in the well-mixed greenhouses gases over that period.
Large increases in ultraviolet (UV) radiation have been observed in asspciation with the ozone hole at high south-
ern latitudes. The measured UV enhancements agree well with model calculations.
. Clear-sky UV measurements at midlatitude locations in the Southern Hemisphere are significantly larger than at
a corresponding site in the Northern Hemisphere, in agreement with expected differences due to ozone column
and Sun-Earth separation.
Local increases hi UV-B were measured in 1992/93 at mid- and high latitudes in the Northern Hemisphere. The
spectral signatures of the enhancements clearly implicate the anomalously low ozone observed in those years,
rather than variability of cloud cover or tropospheric pollution. Such correlations add confidence to the ability to
link ozone changes to UV-B changes over relatively long time scales.
Increases in clear-sky UV over the period 1979 to 1993 due to observed ozone changes are calculated to be
greatest at short wavelengths and at high latitudes. Poleward of 45°,'the increases are greatest in the Southern
Hemisphere. ,
Uncertainties in calibration, influence of tropospheric pollution, and difficulties,of interpreting data from broad-
band instruments continue to preclude the unequivocal identification"of long-term UV trends. However, data
from two relatively unpolluted sites do appear to show UV increases consistent with observed ozone trends.
Given the uncertainties of these studies, it now appears that quantification of the natural (i.e., pre-ozone-reduc-
tion) UV baseline has been irrevocably lost at mid- and high latitudes.
Scattering of UV radiation by stratospheric aerosols from the Mt. Pinatubo eruption did not alter total surface-UV
levels appreciably. . '
RELATED PHENOMENA AND ISSUES .
Methyl Bromide
Three potentially major anthropogenic sources of methyl bromide have been identified: (i) soil fumigation: 20 to
60 ktons per year, where new measurements reaffirm that about 50% (ranging from 20 - 90%) of the methyl
bromide used as a soil fumigant is released into the atmosphere; (ii) biomass burning: 10 to 50 ktons per year; and
(iii) the exhaust of automobiles using leaded gasoline: 0.5 to 1.5 ktons per year or 9 to 22 ktons per year (the two
studies report emission factors that differ by a factor of more than 10). In addition, the one known major natural
source of methyl bromide is oceanic, with emissions of 60 to 160 ktons per year.
14
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Recent measurements have confirmed that there is more methyl bromide in the Northern Hemisphere than hi the.
Southern Hemisphere, with an interhemispheric ratio of 1.3. , -:
There are two known sinks for atmospheric methyl bromide; (i) atmospheric, with a lifetime of 2.0 years (1.5 to
2.5 years); and (ii) oceanic, with an estimated lifetime of 3.7 years (1.5 to 10 years). The overall best estimate for -
the lifetime-of atmospheric methyl bromide is 1.3 years, with a range of 0.8 to 1.7 years. An overall lifetime of
less than 0.6 years is thought to be highly unlikely because of constraints imposed by the observed interhemi-
spheric ratio and total known emissions. . 's '."""'
The chemistry of bromine-induced stratospheric ozone destruction is now better understood. Laboratory mea-
surements have confirmed the fast rate for the BrO + HC>2 reaction and have established a negligible reaction
pathway producing HBr, both of which imply greater ozone losses due to emissions of compounds containing
bromine. Stratospheric measurements show that the abundance of HBr is less than 1 ppt. '
- Bromine.is estimated to be about 50 times more efficient than chlorine in destroying stratospheric ozone on a per-
atom basis. The QDP for methyl bromide is calculated to be about 0.6, based on an overall lifetime of 1.3 years.
An uncertainty analysis suggests that the ODP is unlikely to be less than 0.3.
Aircraft
Subsonics: Estimates indicate that present subsonic aircraft operations may be significantly increasing trace
species, (primarily NOX, sulfur dioxide, and soot) at upper-tropospheric altitudes in the North-Atlantic flight cor-
ridor. Models indicate that the NOX emissions from the current subsonic fleet produce upper-tropospheric ozone
inereases-as much as several percent, maximizing at northern rnidlatitudes. Since the results of these rather
complex models depend critically on NOX chemistry and since the tropospheric NOX budget is uncertain, little
confidence should be put in these quantitative model results at the present time. !
Supersonics: Atmospheric effects of supersonic aircraft depend on the number of'aircraft, the altitude of opera-
tion, the exhaust emissions, and the background chlorine and aerosol loadings. Projected fleets of supersonic
transports would lead to significant changes in trace-species concentrations, especially in the North-Atlantic
flight corridor. Two-dimensional model calculations of the impact of a projected fleet (500 aircraft, each emitting
15 grams of-NOx per kilogram of fuel burned at Mach 2.4) in a stratosphere with a chlorine loading of 3.7 ppb,
imply additional (i.e., beyond those from halocarbon losses) annual-average ozone column decreases of
0.3 - 1.8% for the Northern Hemisphere. There are, however, important uncertainties in these model results,
especially in the stratosphere below 25 km: The same models fail to reproduce the observed ozone trends in the
stratosphere below 25 km between 1980 and 1990, Thus, these models may not be properly including mecha-
nisms that are important in this crucial altitude range. -" ,
Climate Effects: Reliable quantitative estimates of the effects of aviation emissions on climate are not yet avail-
able. Some initial estimates indicate that the climate effects of ozone changes resulting from subsonic aircraft
emissions may be comparable to those resulting from their COi emissions. . .
-.15
-------
Ozone Depletion Potentials (ODPs)
If a substance containing chlorine or bromine decomposes in the stratosphere, it will destroy some ozone.
HCFCs have short tropospheric lifetimes, which tends to reduce their impact on stratospheric ozone as compared
to CFCs and halons. However, there are substantial differences in ODPs among various substitutes. The steady-
state ODPs of substitute compounds considered in the present assessment range from about 0.01-0.1.
Tropospheric degradation products of CFC substitutes will not lead to significant ozone loss in the stratosphere.
Those products will not accumulate in the atmosphere and will not significantly influence the ODPs and Global
Warming Potentials (GWPs) of the substitutes.
Trifluoroacetic acid, formed in the atmospheric degradation of HFC-134a, HCFC-123, and HCFC-124, will enter
into the aqueous environment, where biological, rather than physico-chemical, removal processes may be effec-
tive.
It is known that atomic fluorine (F) itself is not an efficient catalyst for ozone loss, and it is concluded that the
F-containing fragments from the substitutes (such as CF^OK) also have negligible impact on ozone. Therefore,
ODPs of HFCs containing the CF3 group (such as HFC-134a, HFC-23, and HFC-125) are likely to be much less
than 0.001.
New laboratory measurements and associated modeling studies have confirmed that perfluorocarbons and sulfur
hexafluoride are long-lived in the atmosphere and act as greenhouse gases.
The ODPs for several new compounds, such as HCFC-225ca, HCFC-225cb, and.CF3I, have been evaluated using
both semi-empirical and modeling approaches, and are found to be 0.03 or less.
Global Warming Potentials (GWPs)
Both the direct and indirect components of the GWP of methane have been estimated using model calculations.
Methane's influence on thehydroxyl radical and the resulting effect on the methane response time lead to substan-
tially longer response times for decay of emissions than OH removal alone, thereby increasing the GWP. In
addition, indirect effects including production of tropospheric ozone and stratospheric water vapor were consid-
ered and are estimated to range from about 15 to 45% of the total GWP (direct plus indirect) for methane.
GWPs, including indirect effects of ozone depletion, have been estimated for a variety of halocarbons, clarifying
the relative radiative roles of ozone-depleting compounds (i.e., CFCs and halons); The net GWPs of halocarbons
depend strongly upon the effectiveness of each compound for ozone destruction; the halons are highly likely to
have negative net GWPs, while those of the CFCs are likely to be positive over both 20- and 100-year time
horizons. ' . '
Implications for Policy Formulation
The research findings of the past few years that are summarized above have several major implications as scientific
input to governmental, industrial, and other policy decisions regarding human-influenced substances that lead to deple-
tion of the stratospheric ozone layer and to changes of the radiative forcing of the climate system:
16
-------
The Montreal Protocol and its Amendments and Adjustments are reducing the impact of anthropogenic
halocarbons on the ozone layer and should eventually eliminate this ozone depletion. Based on assumed
compliance with the amended Montreal Protocol (Copenhagen, 1992). by all nations, the stratospheric chlorine
abundances will continue to grow from their current levels (3.6 ppb) to a peak of about 3.8 ppb around (he turn of
the century. The future total bromine loading will depend upon choices made regarding future human production
and emissions of methyl bromide. After around the turn of the century, the levels of stratospheric-chlorine and
brornine will begin a decrease that will continue into the 21st and 22nd centuries. The rate of decline is dictated
by the long residence times of the CFCs, carbon tetrachloride, and halons. Global ozone losses and the Antarctic
ozone "hole" were first discernible in the late 1970s and are predicted to recover in about the .year 2045, other
things being equal. The recovery of the ozone layer would have been impossible without the Amendments and
Adjustments to the original Protocol (Montreal, 1987). . . - ,
Peak global ozone losses are expected to occur during the next several years. The ozone layer will be most
affected by human-influenced perturbations and susceptible to natural variations in the period around the year
1998, since the peak stratospheric chlorine and bromine abundances are expected to occur then. Based on extrap-
olation of current trends, observations suggest that the maximum ozone loss, relative to the late 1960s, will likely
be: ...'.'. :...' " . . ' V ;
(i) about 12-13% at Northern midlatitudes in winter/spring (i.e., about 2.5% above current levels);
(ii) about 6 -7% at Northern midlatitudes in summer/fall (i.e., about 1.5% above current levels); and
. (iii) about 11% (with less certainty).at Southern midlatitudes on a year-round basis (i.e.', about 2.5% above
current levels).
Such changes would be accompanied by 15%, 8%, and 13% increases, respectively, in surface erythemal radia-
tion, if other influences such as clouds remain constant. Moreover, if there were to be a major volcanic eruption
like that of Mt. Pinatubo, or if an extremely cold and persistent Arctic winter were to occur, then the ozone losses
a'hd UV increases could be larger in individual years. . -..-.-.'"'
Approaches to lowering stratospheric chlorine and bromine abundances are limited. Further controls on
ozone-depleting substances would not be expected to significantly change the timing or the magnitude of the peak
stratospheric halocarbon abundances and hence peak ozone loss. However, there are four approaches that would
steepen the initial fall from the peak halocarbon levels in the early decades of the next century:
(i) If emissions of methyl bromide from agricultural, structural, and industrial activities were to be eliminated
in the year 2001, then the integrated effective future chlorine loading above the 1980 level (which is related
to the cumulative future loss of ozone) is predicted to be 13% less over the next 50 years relative to full
compliance to the Amendments and Adjustments to the Protocol. ; ,
(ii) -. If emissions of HCFCs were to be totally eliminated by the year 2004, then the integrated effective future
chlorine loading above the 1980 level is predicted to be 5% less over the next 50 years relative to full
compliance with the Amendments and Adjustments to the Protocol. ~ '..'.'.
(iii) If halons presently contained in existing equipment were never released to the atmospnere,-then the inte-
grated effective future chlorine loading above the 1980 level is predicted to be 10% less over the next 50
years relative to full compliance with the Amendments and Adjustments to the Protocol.
(iv) If CFCs presently contained in existing equipment were never released to the atmosphere, then the integrat-
ed effective future chlorine loading above the 1980 level is predicted to be 3% less over the next 50 years
relative to full compliance with the Amendments and Adjustments to the Protocol.
17
-------
Failure to adhere to the international agreements will delay recovery of the ozone layer. If there were to be
additional production of CFCs at 20% of 1992 levels for each year through 2002 and ramped to zero by 2005
(beyond that allowed for countries operating under Article 5 of the Montreal Protocol), then the integrated effective
future chlorine loading above the 1980 level is predicted to be 9% more over the next 50 years relative to full
compliance to the Amendments and Adjustments to fhe Protocol. . - ,
Many of the substitutes for the CFCs and halons are also notable greenhouse gases. Several CFC and halon
substitutes are not addressed under the Montreal Protocol (because they do not deplete ozone), but, because they
are greenhouse gases, fall under the purview of the Framework Convention on Climate Change. There is a wide
range of values for the Global Warming Potentials (GWPs) of the HFCs (150 - 10000), with about half of them
having values comparable to the ozone-depleting compounds they replace. The perfluorinated compounds, some
of which are being considered as substitutes, have very large GWPs'.(e.g., 5000 - 10000). These are examples of
compounds whose current atmospheric abundances are relatively small, but are increasing or could increase in the
future. ,
Consideration of the ozone change will be one necessary ingredient in understanding climate change. The
extent of our ability to attribute any climate change to specific causes will likely prove, to be important scientific"
input to decisions regarding predicted human-induced influences on the climate system. Changes in ozone since
pre-industrial times as a result of human activity are believed to have been a significant influence on radiative
forcing; this human influence is expected to continue into the foreseeable future.
18
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COMMON QUESTIONS ABOUT OZONE
Ozone is exceedingly rare in our atmosphere,
averaging about 3 molecules of ozone for
every ten million air molecules. ;Nonethe-:
less, atmospheric ozone plays vital roles that belie its
small numbers. This Appe'ndix-to the World Meteoro-
logical Organization/United Nations Environment:
Programme, (WMO/UNEP) Scientific Assessment of
Ozone Depletion: 1994 answers some of the questions
that are most commonly asked about ozone and the
changes that have been occurring in recent years. These .
common questions and their answers were discussed by
the 80 scientists from 26 countries who participated in
the Panel Review Meeting of the Scientific Assessment of
Ozone Depletion:,1994. Therefore, this information' is
presented by a large group of experts.from the interna-
tional scientific community. . . '
Ozone is mainly found in two regions of the Earth's atmo-
sphere. Most ozone (about 90%) resides in a layer
between approximately 10 and 50 kilometers (about 6 to
30 miles) above the Earth's surface, in the region of the
' atmosphere called the stratosphere. This stratospheric
ozone is commonly known as the "ozone layer." The re-
maining ozone is in the lower region of the atmosphere,
the troposphere, which extends from the Earth's surface
up to about 10 kilometers. The figure" below shows this
distribution of ozone in-the atmosphere.
While the ozone in these two regions is chemically iden-
tical (both consist of three oxygen atoms and have the
chemical formula "03"), the ozone molecules have very
, different effects on humans and other living things de-
pending upon their location.
Stratospheric ozone plays a beneficial role by absorbing
most of the biologically damaging, ultraviolet sunlight
called UV-B, allowing only a small amount to reach the
, Earth's surface. The absorption of UV radiation by ozone
creates a source of heat, which actually forms the strato-
sphere itself (a region in which the temperature rises as
.one goes to higher altitudes).. Ozone thus plays a key
role in .the temperature structure of the Earth's^ atmo-
sphere. Furthermore, without the filtering action of the
ozone layer, more of the Sun's UV-B radiation would -
penetrate the atmosphere and would reach the Earth's
surface in greater amounts. Many experimental studies
of plants and animals, and clinical studies of humans,
have shown the harmful effects of excessive exposure to
UV-B radiation (these are discussed in the WMO/UNEP
reports on impacts of ozone depletion, which are com-
panion documents to the WMO/UNEP scientific assess-
ments:of ozone depletion).
At the planet's surface, ozone comes into direct contact
with life-forms and displays its destructive side.. Be-
cause ozone reacts strongly with other molecules, high
levels are toxic to. living systems and can severely dam-
age the tissues of plants and animals. Manystudies
have documented the harmful.effects of ozone on crop
production, forest growth, and human health. The sub-
stantial negative effects of surface-level tropospheric
ozone from this direct toxicity contrast with the benefits
of the additional filtering of UV-B radiation that it pro-.
vides. ' . ' ,- -_"'.
With these-dual aspects of ozone come two separate en-
vironmental issues, controlled by different forces in the
atmosphere., lathe troposphere, there is concern about
increases in ozohe^ Low-lying ozone is a key component
of smog, a familiar problem in the atmosphere of many
cities around the world. Higher than usual amounts of
surface-level ozone are now increasingly being observed
in rural areas as well. However, the ground-level ozone
concentrations in the smoggiest cities are very much
smaller than the concentrations routinely found in the
stratosphere. ' .- " . ~
There is widespread scientific and'public interest and
concern about losses of stratospheric ozone. Ground-
based and satellite , instruments have measured
decreases in the amount of stratospheric ozone in our
atmosphere. Over some parts of Antarctica, up to 60% of
the total overhead amount of ozone (known as the~"col-
umn.ozone") is depleted.during September and October.
This.phenomenpn has come to be known as the Antarctic-
"ozone hole." Smaller,'but still significant, stratospheric .
, decreases have been seen at other, more-populated re-
gions of the Earth. Increases in^surface UV:B radiation
have been observed in association with decreases in
stratospheric ozone. ." " .
The scientific evidence, accumulated over more than two
decades of study by the international research communi-
ty, has shown that human-made chemicals are '
responsible for the observed depletions of the ozone lay-
er over Antarctica and-likely play a major role in global
ozone losses. The ozone-depleting compounds contain
various combinations of the chemical elements chlorine,
fluorine, bromine, carbon, and hydrogen, and are often
described by the general term halocarbons. The com-
19
-------
pounds that contain only carbon, chlorine, and fluorine
are called chlorofluorocarbons, usually abbreviated as
CFCs. CFCs, carbon tetrachloride, and methyl chloro-
form are important human-made ozone-depleting gases
that have been used in many applications including re-
frigeration, air conditioning, foam blowing, cleaning of
electronics components, and as solvents. Another im-
portant group of human-made halocarbons is the
halons, which contain carbon, bromine, fluorine, and (in
some cases) chlorine, and have been mainly used as fire
extinguishants. Governments have decided to discon-
tinue production of CFCs, halons, carbon tetrachloride,
and methyl chloroform, and industry has developed
more "ozone-friendly" substitutes.
Two responses are natural when a new problem has been
identified: cure and prevention. When the problem is the
destruction of the ^stratospheric ozone layer, the corre-
sponding questions are: Can we repair the damage
already done? How can we prevent further destruction?
Remedies have been investigated that could (i) remove
CFCs selectively from our atmosphere, (ii) intercept
ozone-depleting chlorine before much depletion has tak-
en place, or (Hi) replace the ozone lost in the stratosphere
(perhaps by shipping the ozone from cities that have too
much smog or by making new ozone). Because.ozone
reacts strongly with other molecules, as noted above, it
is too unstable to be made elsewhere (e.g., in the smog
of cities) and transported to the stratosphere. When the
huge volume of the Earth's atmosphere and the magni-
tude of global stratospheric ozone depletion are carefully
considered," approaches to cures quickly become much
too expensive, impractical, and potentially damaging to
the global environment. Prevention involves the interna-
tionally agreed-upon : Montreal Protocol and its
Amendments and Adjustments, which call for elimina-
tion of the production and use of the CFCs and other
ozone-damaging compounds within .the next few years.
As a result, the ozone layer is expected to recover oyer
the next fifty years or so as the atmospheric concentra-
tions of CFCs and other ozone-depleting compounds
slowly decay.
The current understanding of ozone depletion and its re-
lation to humankind is discussed in detail by the leading
scientists in the world's ozone research community in the
Scientific Assessment of Ozone Depletion: 1994. The
answers lo the common questions posed below are
based upon that understanding and on the information
given in earlier WMO/UNEP reports.
Atmospheric Ozone
Stratospheric Ozone
(The Ozone Layer)
Tropospheric Ozone
Contains 90% of Atmospheric
Ozone
Beneficial Role:
Acts, as Primary UV Radiation
Shield
Current Issues:
- Long-term Global
Downward Trends
- Springtime Antarctic Ozone
Hole Each Year "
Contains 10% of Atmospheric
Ozone .
Harmful Impact: Toxic Effects
on Humans and Vegetation
Current Issues:
- Episodes of High Surface
Ozone in Urban and
Rural Areas
0 5 10 15 20 25
Ozone Amount
(pressure, milli-Pascals)
20
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How Can Chloroiluorocarbons (CFCs) Get to the Stratosphere
If They're Heavier than Air? ;"
Although the CFC molecules are indeed several times
heavier than air, thousands of measurements have been
made from balloons, aircraft, and satellites.demonstrat-.
ing that the CFCs are actually present in the stratosphere.
The atmosphere-is not stagnant. Winds mix the atmo-;
sphere,to altitudes far above the top of the stratosphere
much faster than molecules can settle according to their
weight. Gases such as CFCs that are insoluble in water
and relatively unreactiveln the lower atmos'phere (below
about 10 km) are quickly mixed and therefore reach the
stratosphere regardless of their weight.
Much can be learned about the atmospheric fate of com-
pounds from .the measured changes in concentration
versus altitude. For example, the two gases carbon tet-
rafluoride (CF4, produced mainly as a by-product of the
manufacture of aluminum) and CFC-11 (CCI3F, used in a
variety of human activities) are both much heavier than
air. Carbon tetrafluoride is completely unreactive in the
lower 99.9% of the atmosphere, and measurements
show it to be nearly uniformly distributed throughout the
atmosphere as shown in the figure. There have also'been
measurements over the past two decades.of several other
completely unreactive gases, one lighter than air (neon)
ana1 some heavier than air (argon, krypton);'which show
that they also mix upward uniformly through the strato-
sphere regardless of their weight, just.as observed with
carbon tetrafluoride. CFC-11 is unreactive in the lower
atmosphere .(below about. 15 km) and is similarly uni-
formly mixed there, as shown. The abundance of
CFC-11 decreases as the gas reaches higher altitudes,
where it is broken down by high energy solar ultraviolet
radiation. Chlorine released from this breakdown of
CFC-11 and other'CFCs remains -in the stratosphere for
several years, where it destroys many thousands of mol-
ecules of ozone.
Measurements of CFC-11 and CF4
40
10
_o
15
"O
30
20
< 10
Stratosphere
0.0.1
0-1
1.0
10.0 100; 1000
AAA/V
I
Troposphere
JL'
Atmospheric Abundance
( in parts per trillion )
21
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What is the Evidence that Stratospheric Ozone
is Destroyed by Chlorine and Bromine?
Laboratory studies show that chlorine (Cl) reacts very
rapidly with ozone. They also show that the reactive
chemical chlorine oxide (CIO) fprmedjn that reaction
can undergo further processes which regenerate the
original chlorine, allowing the sequence to be repeated
very many times (a "chain reaction"). Similar reactions
also take place between bromine and ozone.
But do these ozone-destroying reactions occur in the real
world? All of our accumulated scientific experience dem-
onstrates that if the conditions of temperature and
pressure are like those in the laboratory studies, the
same chemical reactions will take place in nature. How-
ever, many other reactions including those_of other
chemical species are often also taking place simulta1
neously in the stratosphere, making the connections
among the changes difficult to untangle. Nevertheless,
whenever chlorine (or bromine) and ozone are found to-
gether in the stratosphere, the ozone-destroying
reactions must be taking place.
Sometimes a small number of chemical reactions is so
important in the natural circumstance that the connec-
tions are almost as clear as in laboratory experiments.
Such a situation occurs in the Antarctic stratosphere dur-
ing the springtime formation of the ozone hole, During
August and September 1987 - the end of winter and be-
ginning of spring in the Southern Hemisphere-aircraft
equipped with many different instruments for measuring
a large number of chemical species were flown repeated-
ly over Antarctica. Among the chemicals measured were
ozone and chlorine oxide, the reactive chemical identi-
fied in the laboratory as one of the participants in the
ozone-destroying chain reactions. On the first flights
southward from the southern tip of South'America, rela-
tively high concentrations of ozone were measured
everywhere over Antarctica. By mid-September, howevT
er, the instruments recorded low concentrations of ozone
in regions where there were high concentrations of chlo-
rine oxide and vice versa, as shown in the figure. Flights
later in September showed even less'ozone over Antarc-
tica, as the chlorine continued to react with the
stratospheric ozone.
Independent measurements made by these and other in-
struments on this and other airplanes, from the ground,
from balloons, and from satellites have provided a de-
tailed understanding of the chemical reactions going on
in the Antarctic stratosphere. Regions with high concen-
trations of reactive chlorine reach temperatures so cold
(less than approximately -80°C, or -112°F) that strato-
spheric clouds form, a rare occurrence except during the
polar winters. These clouds facilitate other chemical re-
actions that allow the release of chlorine in sunlight. The'
chemical reactions related to the clouds are now well
understood through study under laboratory conditions
mimicking those found naturally. Scientists are working
to understand the role of such reactions of chlorine and
bromine at pther latitudes,-and the involvement of parti-
cles of sulfuric acid from volcanoes or other sources.
Measurements of Ozone and Reactive Chlorine
from a Flight into the Antarctic Ozone Hole
63 . 64 65 66 67
Latitude (Degrees South)
22
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Does Most of the Chlorine in the Stratosphere
Gome from Human or Natural Sources?
Most of the chlorine in the stratosphere is there as a re-
sult of human activities. -
Many compounds containing chlorine are released at the.
ground, but those that dissolve in water cannot reach
stratospheric altitudes.. Large quantities'of chlorine are
released from evaporated ocean spray as sea salt (sodi-
' urn chloride) aerosol. However, because sea salt
dissolves in water, this chlorine quickly is taken up in
clouds or in ice, snow, or rain droplets and does not
reach the stratosphere. Another ground-level source of
chlorine is its use in swimming pools and as household
bleach. When released, this chlorine is rapidly convert-
ed to forms that dissolve in water and therefore are
removed from the lower atmosphere, never reaching the
stratosphere in significant amounts. Volcanoes can emit
large quantities of hydrogen chloride, but this gas is rap-
idly converted to hydrochloric acid in rain water, ice, and'
snow and does not reach the stratosphere. Even in ex-
plosive volcanic plumes that rise high in the atmosphere,
nearly all of the hydrogen chloride is scrubbed out in
precipitation before reaching stratospheric altitudes. -
[n contrast, human-made halocarbons - such as CFGs,
carbon tetrachloride (CCU) and methyl chloroform
(CHsCCIs) - are.not soluble in'water, do not react with
snow or other natural surfaces, and are not broken'down
chemically in the lower atmosphere. While the exhaust
from the Space Shuttle and from some rockets does in-
ject some chlorine~directly into the stratosphererthis
'input is.very small (less than one percent of the annual
input from .halocarbons in the present stratosphere, as-
suming nine Space Shuttle and six Titan IV rocket
launches per year). , ,
Several pieces of evidence combine to establish human-
made halocarbons as the primary source of stratospheric
chlorine. First, measurements (see the figure below)
have shown that the chlorinated species that rise to the
stratosphere:are primarily manufactured compounds
(mainly CFCs, carbon tetrachloride, methyl chloroform,
and the HCFC substitutes for.CFCs), together with small
amounts of hydrochloric acid (HCI) and methyl .chloride
(CHsCI) which are partly natural in origin. The. natural
contribution now is much smaller than that from human
activities, as shown in the figure below. Second, in 1985
and 1992 researchers measured nearly-all known gases
containing chlorine in the stratosphere. They found that
.human emissions of halocarbons plus the much smaller
contribution from natural sources could account for all of
the stratospheric chlorine compounds. Third, the in-
crease in total stratospheric chlorine measured between"
1985 and 1992 corresponds with,the known increases in'
concentrations of human-made halocarbons during that
time. - - -
Primary Sources of Chlorine Entering the Stratosphere
Entirely ,
Human-
Made
Natural
Sources
Contribute
23
-------
Can Changes in the Sun's Output Be Responsible
for the Observed Changes in Ozone?
Stratospheric ozone is primarily created by ultraviolet
(UV) light coming from the Sun, so the Sun's output af-
fects the rate at which ozone is produced. The Sun's
energy release (both as UV light and as charged particles
such as electrons and protons) does vary, especially
over the well-known 11-year sunspot cycle. Observa-
tions over several solar cycles (since the 1960s) show
that total global ozone levels decrease by 1-2% from the
maximum to the minimum of a typical cycle. Changes in
the Sun's output cannot be responsible for the observed
long-term changes in ozone, because these downward
trends are much larger than 1-2%. Further/during the
period since 1979, the Sun's energy output has gone
from a maximum to a minimum in 1985 and back
through another maximum in 1991, but the trend in
ozone was downward throughput that time. The ozone
trends presented in this and previous international sci-
entific assessments have been obtained by evaluating
the long-term changes in ozone concentrations after ac-
counting for the solar influence (as has been done in the
figure below).
Global Ozone Trend (60°S-60°N)
1°
g
1-4
0)
-6
1980 1982 1984 1986 1988
Year
1990
1992
1994
24
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When Did the Antarctic Ozone Hole First Appear?
The Antarctic ozone hole is a new'phenomenon. The fig-
ure shows that observed ozone over the British Antarctic
Survey station at Halley Bay, Antarctica first revealed ob-
vious decreases in the early 1980s compared to data
obtained since 1957. The ozone hole is formed reach
year when there is a sharp decline (currently up to 60%)
in the total ozone over most of Antarctica for a period of
about two months during Southern Hemisphere spring
(September and October), Observations from three other
stations in Antarctica, also covering several decades, re-
veal similar progressive, recent decreases in springtime
ozone. The ozone hole has been shown to result from
destruction of stratospheric ozone by gases containing
chlorine and bromine, whose sources are mainly hu-
man-made halo-carbon gases.
Before the stratosphere' was affected by human-made
chlorine and bromine, the naturally occurring springtime
ozone levels over Antarctica were about 30-40% lower
than springtime ozone levels over the Arctic. This natu-
ral difference between Antarctic and Arctic conditions'.
was first observed in the late 1950s by Dobson. It stems
from the exceptionally cold temperatures and different
winter wind patterns within the Antarctic stratosphere as
..compared "to the Arctic. This is not at all the same phe-
nomenon as the marked downward trend in total ozone in
recent years referred to asvthe ozone hole and shown in
the figure below. . :
Changes in stratospheric meteorology cannot explain
the ozone hole. Measurements show that wintertime
Antarctic stratospheric temperatures of past decades
have not changed prior to the development of the hole
each September. Ground, aircraft, and satellite measure-
ments have provided, in contrast, clear evidence of the
importance of the chemistry of .chlorine and bromine
originating from* human-made compounds .in depleting
Antarctic ozone in recent years.
A single report of extremely-low-Antarctic winter ozone.in
one location in 1958 byan unproven technique has been
shown to be completely Inconsistent with the measure-
ments depicted here and with all credible measurements
of total ozone.
Historical Springtime Total Ozone Record
for Halley Bay, Antarctica (76°S)
400
§ 300
JS
o
Q
200
October Monthly Averages
I955
I965
I975
Year
I985
I995
25
-------
Why is the Ozone Hole Observed over Antarctica
When CFCs Are Released Mainly in the Northern Hemisphere?
Human emissions of CFCs do occur mainly in the North-
ern Hemisphere, with about 90% released in the
latitudes corresponding to Europe, Russia, Japan, and
North America. Gases such as CFCs that are insoluble in
water and relatively unreactive are mixed within a year or
two throughout the lower atmosphere (below about 10
km). The CFCs in this well-mixed air rise from the lower
atmosphere into the stratosphere mainly in tropical lati-
tudes. Winds then move this air poleward ^ both north
and south - from the tropics, so that air throughout the
stratosphere contains nearly the same amount of chlo-
rine. However, the meteorologies of the two polar
regions are very different from each-other because of
major differences at the Earth's surface. The South Pole
is part of a very large land mass (Antarctica) that is com- ,
pletely surrounded by ocean. These conditions produce
very low stratospheric-temperatures which in turn lead to
formation of clouds (polar stratospheric clouds). The
clouds that form at low temperatures lead to chemical
changes that promote rapid ozone loss during Septem-
ber and October of each year, resulting in the ozone hole.
In'contrast, the Earth's surface in the northern polar re-
gion lacks the land/ocean symmetry characteristic of the
southern polar area. As a consequence, Arctic strata-.
spheric air is; generally much warmer than in the
Antarctic, and fewer clouds form there. Therefore,, the
ozone depletion in the Arctic is much less than in the
Antarctic.
Schematic of Antarctic Ozone Hole
1979
1986
1991
26
-------
Is the Depletion of the Ozone Layer Leading to an Increase in
Ground-Level Ultraviolet Radiation?
The Sun emits light over a wide range of energies, with
about two percent given off in the form qf high-energy,
-ultraviolet (UV) radiation. Some of this.UV radiation
(UV-B) is especially effective in causing damage to living'.
things, including sunburn, skin cancer, and eye damage
for humans. The amount of solar UV radiation received
at any particular location on the Earth's surface depends
upon the position of the Sun above the horizon, on the
amount of ozone in the atmosphere, and upon local
" cloudiness and pollution. Scientists agree that in the ab-
sence of changes in clouds or pollution, decreases in
atmospheric ozone will increase ground-level UV radia-
tion. - ' - - . - - .. - '
.The largest decreases in ozone during the last decade
have been observed over Antarctica, especially'during
each September and October when the "ozone hole".
forms. During the last several years, simultaneous mea-
surements of UV radiation and total ozone have-been
made at several Antarctic stations. As shown in the fig-
ure below, when the ozone, amounts decrease, UV-B
increases. Because of the ozone hole, the UV-B intensity
at Palmer Station; Antarctica, in late October, 1993, was
more intense.than found at San Diego, California, at any
time during all of 1993. '_ : , .
In areas where small ozone depletion has been observed,
UV-B increases are more difficult to detect/Detection of
UV trends associated with ozone decreases can also'be
complicated by changes in Cloudiness or by local pollu-
tion, as well as by difficulties in keeping the detection
instrument in precisely the same condition over many
years. Prior to the late 1980s, instruments with the nec-
essary accuracy and stability for measurement of small
longrterm trends in ground-level UV-B were not em-
ployed. Recently, however, such instruments have been
used in the Antarctic because of the very large changes
in ozone being observed there. When high-quality mea-~
surements have been made in other areas far from major
cities and their associated air pollution, decreases ifi
ozone have regularly been accompanied by increases in
UV-B. The data from urban locations with older, less
specialized instruments provide much less reliable infor-
mation, especially because, .good simultaneous
measurements are not available for any changes in
cloudiness or local pollution. . -',...-
Increases in Erythema! (Sunburning) UV Radiation
Due to Ozone Reductions
.g
.12
"a 150%
c 100%
50%
§ 0%
1/5
O
South Pole, Antarctica
Feb 1991 - Dec 1992
-60% -50% '-40% -30% -20% r!0%
- Change In Ozone
( Spring vs. Autumn, for-the Same Solar Angle)
0%
-- 27 :
-------
How Severe Is the Ozone Depletion Now,
and Is It Expected to Get Worse?
Scientific evidence shows that ozone depletion caused
by human-made chemicals is continuing and is expected'
to persist until chlorine and bromine levels are reduced.
Worldwide monitoring has shown that stratospheric
ozone has been decreasing for the past two decades or
more. Globally averaged losses have totaled about 5%
since the mid-1960s, with cumulative losses of about
10% in the winter and spring and 5% in the summer and
autumn over locations such as Europe, North America,.
and Australia. Since the late-1970s, an ozone "hole" has
formed in Antarctica each Southern Hemisphere spring
(September / October), in which up to 60% of the total
ozone is depleted. The large increase in atmospheric
concentrations of human-made chlorine and bromine
compounds is responsible for the formation of the Ant-
arctic ozone hole, and the weight of evidence indicates
that it also plays a major role in midlatitude ozone deple-
tion.
During 1992 and 1993 ozone in many locations dropped
to record low values: springtime depletions exceeded
20% in some populated northern midlatitude regions,
and the levels in the Antarctic ozone hole fell to the low-
est values ever recorded. The unusually large ozone
decreases of 1992 and 1993 are believed to be related, in
part, to the volcanic eruption of Mount Pinatubo in the
Philippines during 1991. This eruption produced large
Ozone-Damaging Stratospheric Chlorine/Bromine
I5000
12000
*C
tl 9000
8.
S- 6000
i
2 3000
No .
Protocol /
/' /Montreal
/ Protocol
1950 1975 2000 2025 2050 2075 2100
Year
amounts of stratospheric sulfate aerosols that temp'orari-
.ly increased the ozone depletion caused by human-made
chlorine and bromine compounds. Recent observations
have shown that as those aerosols have been swept out
of the stratosphere, ozone concentrations have returned
to the depleted levels consistent with the downward trend
observed before the Mount Pinatubo eruption.
In 1987 the recognition of the potential for chlorine and
bromine to destroy stratospheric ozone led to an interna-
tional agreement (The United Nations Montreal Protocol.
on Substances that Deplete the Ozone Layer) to reduce
the global production of ozone-depleting substances..
Since then, new global observations of significant ozone
depletion have prompted amendments to strengthen the
treaty. The 1992 Copenhagen Amendments call for a ban
on production of the most damaging compounds by
1996. The figure shows past and projected future strato,-
spheric abundances of chlorine and bromine: (a) without
the Protocol; (b) under the Protocol's original provi-
sions; and (c) under the Copenhagen Amendments now
in force. Without the Montreal Protocorand its Amend-
ments, continuing human use of CFCs and other
compounds would have tripled the stratospheric abun-
dances of chlorine and bromine by about the year 2050,
Current scientific understanding indicates that such in-
creases would have led to global ozone depletion very
much larger than observed today. In contrast, under cur-
rent international agreements, which are now reducing
and will eventually eliminate human emissions of ozone-
depleting gases, the stratospheric abundances of
chlorine and bromine are expected to reach their maxi-
mum within a few years and then slowly decline. All
other things being equal, the ozone layer is expected to
return to normal by the middle of the next century.
In summary, record low ozone levels have been observed
in recent years, and substantially larger future global de-
pletions in ozone would have been highly likely without
reductions ia human emissions of ozone-depleting gas-
es. However, worldwide compliance with current
' international agreements is rapidly reducing the yearly
emissions of these compounds. As these emissions
cease, the ozone layer will gradually improve over the
next several decades. The recovery of the ozone layer
will be gradual because of the long times required for
CFCs to be removed from the atmosphere.
28
-------
LIST OF INTERNATIONAL AUTHORS,
CONTRIBUTORS, AND REVIEWERS
Assessment Co-chairs
Darnel L. Albritton, Robert T. Watson, and Piet J.Aucamp
Chapters and Lead Authors
Parti. Observed Changes in Ozone and Source Gases
Chapter 1. Ozone Measurements (NeilR.P. Harris)
Chapter 2. Source Gases: Trends and Budgets (Eugenia Sanhueza)
Part 2. Atmospheric Processes Responsible for the Observed Changes in Ozone
Chapter 3. Polar Ozone (David W. Fahey)
Chapter 4. Tropical and Midlatitude Ozone (Roderic L. Jones)
Chapters. Tropospheric Ozone (Andreas Volz-Thomas and Brian A.Ridley)
PartS. Model Simulations of Global Ozone
Chapter 6. Model Simulations of Stratospheric Ozone (Malcolm K.W. Ko) .
Chapter 7. Model Simulations of Global Tropospheric Ozone (Frode Stordal)
Part 4. Consequences of Ozone Change
Chapter 8. Radiative Forcing and Temperature Trends (Keith P. Shine)
Chapter 9. Surface Ultraviolet Radiation (Richard L. McKenzie)
Part 5. Scientific Information for Future Decisions ', -
. Chapter 10. Methyl Bromide (Stuart A. Penkett)
Chapter 11. Subsonic and Supersonic Aircraft Emissions (Andreas Wahner and Marvin A. Getter)
Chapter 12. Atmospheric Degradation of Halocarbon Substitutes (RA. Cox)
Chapter 13. Ozone Depletion Potentials, Global Warming.Potentials, and
Future Chlorine/Bromine Loading (Susan Solomon and Donald J. Wuebbles)
Coordinators: Common Questions About Ozone '
Susan Solomon NOAA Aeronomy Laboratory - US
" F. Sherwood Rowland University of California at Irvine US . ,
Daniel L. Albritton
Marc Allaart
Fred N. Alyea
Gerard Ancellet
Meinrat O. Andreae
James K. Angell
Frank Arnold
Authors, Contributors, and Reviewers
NO AA Aeronomy Laboratory
Koninklijk Nederlands Meteorologisch Instituut
Georgia Institute of Technology,
Centre National de la Recherche Scientifique
Max-Planck-Institut fur Chernie
NO A A Air Resources Laboratory
Max-Planck-Institut fur Kernphysik
US
The Netherlands
/ '. ,-US
France
Germany
::. US
Germany
29
-------
Roger Atkinson
Elliot Atlas
Piet J. Aucamp
Linnea M. Avallone
Helmuth Bauer
Slimane Bekki
Tibor B&rces
T. Berntseh
Lane Bishop
Donald R. Blake
N.J. Blake
Mario Blumthaler
Greg E. Bodeker
Rumen D. Bojkov
Charles R. Booth
Byron Boville
Kenneth P. Bowman
Geir Braathen
Guy P. Brasseur
Carl Brenninkmeijer
Christoph Briihl
William H. Brune
James H. Butler
Sergio Cabrera
Bruce A. Callander
Daniel Cariolle
Richard P. Cebula
William L. Chameides
Sushil Chandra
Marie-Lise Chanin
J. Christy
Ralph J. Cicerone
G.J.R. Coetzee
Peter S. Connell
D. Considine
R.A. Cox
Paul J. Crutzen
Derek N. Cunnold
John Daniel
Malgorzata Deg<5rska
John J. DeLuisi
DirkDeMuer
Frank Dentener
Richard G. Derwent
Terry Deshler
Susana B. Diaz
Russell Dickerson
University of California at Riverside
National Center for Atmospheric Research
Department of Health
University of California at Irvine .
Forschungszentram fur Umwelt u. Gesundheit
University of Cambridge
Hungarian Academy of Sciences
Universitetet I Oslo
Allied Signal -
University of California at Irvine
University of California at Irvine . .
University, of Innsbruck
University of Natal/NIWA . ' ' . '."...
World Meteorological Organization
Biospherical Instruments
National Center for Atmospheric-Research
Texas A&M University
Norsk Institutt for Luftforskning .
National Center for Atmospheric Research
National Institute of Water and Atmospheric Research
Max-Planck-Institut fur Chemie
Pennsylvania State University .
NOAA Climate Monitoring and Diagnostics Laboratory
Um'versidad de Chile
United Kingdom Meteorological Office
Meteo-France, Centre National de Recherches Meteprologiques
Hughes STX
Georgia Institute of Technology
NASA Goddard Space Flight Center
Centre National de la Recherche Scientifique
University of Alabama at Huntsville
University of California at Irvine
Weather Bureau . -
Lawrence Livermore National Laboratory
NASA Goddard Space Flight Center
National Environmental Research Council
Max-Planck-Institut fiir Chemie
Georgia Institute of Technology"
NOAA Aeronomy Laboratory/CIRES
Polish Academy of Sciences
NOAA Ail Resources Laboratory
Institut Royal Meteorologique de Belgique
Wageningen Agricultural University
UK Meteorological Office
University of Wyoming
Austral Center of Scientific Research (CADIC/CONICET)
University of Maryland
- US
US
South Africa
US
Germany
UK
Hungary
Norway
;. ' US
US
us
Austria
South Africa
Switzerland
US
, us ,
. us
Norway
.... us
New Zealand
Germany
US
US
Chile
UK
France
US
US
us
France
US
us.
South Africa
- " US
"US
UK
Germany
, US
US
Poland
- US
. Belgium
The Netherlands
UK
US
Argentina
'".'..' US
30
-------
J. Dignoii
Ed Dlugokencky
.Anne R. Douglass
Tom Duafala
James E. Dye
Dieter H. Ehhalt
James W. Elkins
Christine Ennis
D.Etheridge
David W. Fahey
T. Duncan A. Fairlie
Donald A. Fisher
Jack Fishman
Eric L. Fleming
Frank Flocke ' ;
Lawrence E. Flynn
P.M. de F. Forster
James Franklin
Paul J. Fraser
John E- Frederick
Lucien Froidevaux
J.S. Fuglestvedt
Reinhard Furrer .
Ian E. Galbally
Brian G. Gardiner
Marvin A. Geller
Hartwig Gernandt
James F. Gleason
Sophie Godin
Amram Golombek
Uhich Gorsdorf
Thomas E. Graedel
Claire Granier
William B. Grant
L.J. Gray
William L. Grose
J. Gross
A.S. Grossman
Alexander Gruzdev
James E. Hansen
Neil R.P. Harris
Shiro Hatekeyama
D.A. Hauglustaine
Sachiko Hayashida
G.D. Hayman
Kjell Henriksen
Ernest Hilsenrath
Lawrence Livermore National Laboratory
NOAA Climate Monitoring and Diagnostics Laboratory
NASA Goddard Space Flight Center
Methyl Bromide Global Coalition
National Center for Atmospheric Research
Forschurigszentrum Jiilich
NOAA Climate Monitoring and Diagnostics Laboratory
NOAAAeronomyLaboratory/CIRES ,
CSIRO Division of Atmospheric Research,
NOAA Aeronomy Laboratory
NASA Langley Research Center '' " ' '
E.I. DuPont de Nemours and Company
NASA Langley Research Center , ' .
Applied Research Corporation . .
Forschungszentrum Jiilich
Software Corporation of America
University of Reading
SolvayS.A.
CSIRO Division of Atmospheric Research
University of Chicago
California Institute of Technology/Jet Propulsion Laboratory
Center for International Climate & Energy Research
Freie Universitat Berlin
CSIRO Division of Atmospheric Research
British Antarctic Survey . ,
State University of New York at Stony Brook
Alfred Wegener Institut "'._.-'
NASA Goddard Space Flight Center
Centre National de la Recherche Scientifique
Israel Institute for Biological Research
, Deutscher Wetterdienst
AT&T Bell Laboratories
National Center for Atmospheric Research
NASA Langley Research Center
SERC Rutherford Appleton Laboratory
NASA Langley Research Center
Max-Planck-Institut fur Chemie
Lawrence Livermore National Laboratory
Russian Academy of Sciences
NASA Goddard Institute for Space Studies
European Ozone Research Coordinating Unit
National Institute for the Environment ' .
Centre National de la Recherche Scientifique
Nara Women's University .- -.
Harwell Laboratory/AEA Environment and Energy
University of Troms0. ~
NASA Goddard Space Flight Center
US
'us
us
us
us.
Germany
US
US
Australia
- US
US
. . ' . us
us
/us
Germany
-US
UK
Belgium
Australia
US
US
.Norway
Germany
Australia
UK
US
Germany
- -' -us;
France,
Israel
.Germany
US
- . US
" us'
-us;
Germany
US
Russia
. .- US
'UK
. Japan
France
Japan
UK
Norway
US-
31
-------
David J. Hofmann
Stacey M. Hollandsworth
James R. Holton
Lon L. Hood
0ystein Hov
Carleton J. Howard
Robert D. Hudson
D. Hufford
Linda Hunt
Abdel M. Ibrahim
Mohammad Ilyas
Ivar S.A. Isaksen
Tomoyuki Ito
Charles H. Jackman
Daniel J. Jacob
Colin E. Johnson
Harold S. Johnston
Paul V. Johnston
Roderic L. Jones
Torben S. J0rgensen
Maria KanaMdou
IgorL. Karol
Prasad Kasibhatla
Jack A. Kaye
Hennie Kelder
James B. Kerr
M.A.K.Khalil
Vyacheslav Khattatov
Jeffrey T.Kiehl
Stefan Kinne
D. Kinnison
VolkerKirchhoff
Malcolm K-W.Ko
UlfKShler
Walter D. Komhyr
Yutaka Kondo
JanuszW. KrzyScin
Antti Kulmala
Michael J. Kurylo
Karin Labitzke
MurariLal
K.S. Law
G. LeBras
Yuan-Pern Lee
Franck Lefevre
Jos Lelieveld
Robert Lesclaux
NOAA Climate Monitoring and Diagnostics Laboratory
Applied Research Corporation
University of Washington
University of Arizona ,
Universitetet I Bergen
NOAA Aeronomy Laboratory
University of Maryland
Environmental Protection Agency
NASA Langley Research Center
Egyptian Meteorological Authority
University of Science Malaysia
Universitetet I Oslo
Japan Meteorological Agency
NASA Goddard Space Flight Center
Harvard University '
UK Meteorological Office/AEA Technology
University of California at Berkeley
National Institute of Water & Atmospheric Research
University of Cambridge
Danish Meteorological Institute ,
Centre National de la Recherche Scientifique
A.I. Voeikov Main Geophysical Observatory
Georgia Institute of Technology
NASA Goddard Space Flight Center
Koninklijk Nederlands Meteorologisch Instituut
Atmospheric Environment Service
Oregon Graduate Institute of Science and Technology
Central Aerological Observatory
National Center for Atmospheric Research
NASA Ames Research Center ,
Lawrence Livermore National Laboratory
Institute) Nacional de Pesquisas Espaciais
Atmospheric and Environmental Research, Inc.
Deutscher Wetterdienst .
NOAA Climate Monitoring and Diagnostics Laboratory
Nagoya University -
Polish Academy of'Sciences '
World Meteorological Organization
NASA Headquarters/NIST
Freie Universitat Berlin
Indian Institute of Technology
University of Cambridge
Centre National de la Recherche Scientifique
National Tsing Hua University
Meteo France, Centre National de Recherches Meteorologiques
Wageningen University
Universite de Bordeaux 1
US
US
us
US
Norway
US
US
.US
US
Egypt
Malaysia
Norway
Japan
US
US
UK
US
New Zealand
.'UK-
Denmark
France
Russia
US..
US
The Netherlands
Canada
US
Russia
US
Germany
US
Brazil
US
Germany
' US
Japan
Poland
Switzerland
US
Germany
India
. ' UK
France
Taiwan
France
The Netherlands
France
.32
-------
Joel S. Levine '
Joel Levy
J. Ben Liley
Peter Liss
David H. Lister
Zeaobia Litynska
Shaw C. Liu
Jennifer A. Logan
Nicole Louisnard
Pak Sum Low
Daniel Lubin
. Sasha Madronich
Jerry Mahlman
Gloria L. Manney
Huiting Mao
W. Andrew Matthews
Konrad Mauersberger
Archie McCulloch
Mack McFarland
Michael E. Mclntyre
Richard L. McKenzie
Richard D. McPeters
Gerard Megie
Paulette Middleton
AlvirrJ. Miller
IgorMokhov
Mario Molina
Geert K. Moortgat
Hideaki Nakane
Paul A. Newman
Paul C. Novelli
Samuel J. Oltmans
Alan O'Neill
Michael Oppenheimer
S. Palerrni
Ken Patten
Juan Carlos Pelaez
Stuart A. Penkett
Joyce Penner
Thomas Peter
Leon F. Phillips
Ken Pickering
R. Bradley Pierce
S. Pinnock
Michel Pirre
Giovanni Pitari
Walter G. Planet
NASA Langley Research Center
NOAA Office of Global Programs --- ' ;
National-Insitutute of Water & Atmospheric Research'
University of East Anglia : ;
Defence Research Agency ' ...
Centre of Aerology . .. .
NOAA Aeronomy Laboratory
.Harvard University
Office National d'Etudes et de Recherches Aerospatiales
United Nations Environment Programme
University of California at San Diego ,
National Center for Atmosplieric Research
NOAA Geophysical Fluid Dynamics Labbratory
California Institute of Technology/Jet Propulsion Laboratory
State:University of New York at Albany/ASRC
National Institute of Water & Atmospheric Research' - _.
Max-Planck-Institut fiir Kernphysik .
ICI Chemicals and Polymers Limited ,
E.I. DuPont de Nemours and Company :
University of Cambridge . - .
National Institute of Water & Atmospheric Research
NASA Goddard Space Flight Center
Centre National de la Recherche Scientifique
Science and Policy Associates
NOAA National Meteorological Center -
Institute of Atmospheric Physics
Massachusetts Institute of Technology
Max-Planck-Institute fiir Chemie
National Institute for Environmental Studies '.../,
NASA Goddard Space Flight Center
University of Colorado .
- NOAA Climate Monitoring~and Diagnostics Laboratory
University of Reading - , . ;
Envkonmental Defense Fund
Universita'degli Studi-i'Aquila -.'."'
Lawrence Livermore National Laboratory
Instituto de Meteorologia ' -',
University of East Anglia
. Lawrence Livermore National Laboratory .
Max-Planck-Institut fiir Chemie
University of Canterbury ,
NASA Goddard Space Flight Center :
NASA Langley Research Center
University of Reading , _.
Centre National de la Recherche Scientifique
, Universita' degli Studi-l'Aquila ,
NOAA National Environmental Satellite, Data and Information
US
-.-".' us
~ " New Zealand
; UK
' .-; UK
Poland
; US
.us
- France
Kenya
'.-' -us
'' '.-,- -us
. - .us
'; -'. US
us
.New Zealand
Germany
UK
US
' - - -UK-
New Zealand
"- -.. .us
France
. " ..US"
. " -:. US
Russia
- ' ' . US
Germany
Japan
'---.. US
' US
us
UK
US
Italy
',;; US
Cuba
UK-
US
Germany
New Zealand
-. " .-.-- .us
,--'. us
UK
France
Italy
Service ' US
33
-------
R. Alan Plumb
Jean-Pierre Pommereau
Lamont R. Poole
Michael J. Prather
Margarita Pr6ndez
Ronald G. Prinn
Joseph M. Prospero
John A. Pyle
Lian Xiong Qiu
Richard Ramaroson
V. Ramaswamy
William Randel
Philip J. Rasch
A.R. Ravishankara
WilliamS. Reeburgh
C.E. Reeves
J. Richardson
Brian A. Ridley
David Rind
Curtis P. Rinsland
Aidan E. Roche
Michael O. Rodgers
Henning Rodhe
Jose M. Rodriguez
M. Roemer
Franz Rohrer
Richard B. Rood
F. Sherwood Rowland
Colin E. Roy
Jochen Rudolph
James M. Russell HI
Nelson Sabogal
Karen Sage
Ross Salawitch
Eugenio Sanhueza
K.M. Sarma
Tom Sasaki
Sue M. Schauffler
Hans Eckhart Scheel
Ulrich Schmidt
Rainer Schmitt
Ulrich Schumann
M.D. Schwarzkopf
Gunther Seckmeyer
Jonathan D. Shanklin
Keith P. Shine
H.W.'Sidebottom
Massachusetts Institute of Technology US
Centre National de la Recherche Scientifique " . . France
NASA Langley Research Center .US
University of California at Irvine ., US
Universidad de Chile " . Chile
Massachusetts Institute of Technology , US
University of Florida . US
University of Cambridge - UK
Academia Sinica China
Office National d'Etudes et de Recherches A6rospatiales France
NOAA Geophysical Fluid Dynamics Laboratory/Princeton University . US
National Center for Atmospheric Research US
National Center for Atmospheric Research US
NOAA Aeronomy Laboratory US
University of California at Irvine , . ', US
University of East Anglia UK
NASA Langley Research Center US
National Center for Atmospheric Research US
NASA Goddard Institute for Space Studies US
NASA Langley Research Center . US
Lockheed Corporation . US
Georgia Institute of Technology US
Stockholm -University - Sweden
Atmospheric and Environmental Research, Inc. US
TNO Institute of Environmental Sciences The Netherlands
Forschungszentrum Jiilich , Germany
NASA Goddard Space Flight Center . US
University of California at Irvine , US
Australian Radiation Laboratory .- . Australia
Forschungszentrum Jiilich , . Germany
NASA Langley Research Center , , US
United Nations Environment Programme Kenya
NASA Langley Research Center ' US
Harvard University US
Institute Venezolano de Investigaciones Qentificas . . Venezuela
United Nations Environment Programme '"'.. Kenya
Meteorological Research Institute Japan
National Center for Atmospheric Research US
Fraunhofer Institut fur Atmospharische Umweltforschung Germany
Forschungszentrum Jiilich , Germany
Meteorologie Consult . , Germany
DLR Institut fiir Physik der Atmosphare Germany
NOAA Geophysical Fluid Dynamics Laboratory US
Fraunhofer Institute for Atmospheric Environment Germany
British Antarctic Survey . UK
University of Reading UK
University College Dublin \ Ireland
34
-------
P. Simmonds . . .
Paul C.Simon
Hanwant B. Singh
Paula Skfivankova
Hennan Smit
Susan Solomon
Johannes Staehelin
Knut Stamnes
L. Paul Steele
Leopoldo Stefanutti
Richard S. Stolarski
FrodeStordal
A. Strand
B.H. Subbaraya
Nien-Dak Sze .
Anne M. Thompson
XueX.Tie
Margaret A. Tolbert
Darin W. Toohey
RalfTourni-
Michael Trainer
Charles R. Trepte
Adrian Tuck
R.VanDorland
Karel Vanicek
Geraint Vaughan
Guido Visconti
Andreas Volz-Thomas
Andreas Wahner
Wei-Chyung Wang
David I. Wardle
David A. Warrilow
Joe W. Waters
Robert T. Watson
E.G. Weatherhead
Christopher R. Webster
D. Weisenstein
Ray F.Weiss
Paul Wennberg
Howard Wesoky
Thomas M.L. Wigley
Oliver Wild
Paul H. Wine
Peter Winkler
Steven C. Wpfsy
Donald J. Wuebbles
Vladimir Yushkov.
University of Bristol.
Institut d'Aeronomie Spatiale de Belgique
NASA Ames Research Center
Czech Hydrometeorological Institute
Forschungszentrum Jiilich.
NOAA Aeronomy Laboratory
Eidgenossische Technische Hochschule Zurich -
University of Alaska ' ,
CSIRO Division of Atmospheric Research ,
InstitutO di Riccrea sulle Onde Elettromagnetiche del CNR
NASA Goddard Space Flight Center
Norsk Institutt for Luftforskning
University of Bergen
Physical Research Laboratory ,
Atmospheric and Environmental Research, Inc.
NASA Goddard Space Flight Center '
National Center for Atmospheric Research
University of Colorado
University of California at Irvine
University of Cambridge -..'.
NOAA Aeronomy Laboratory
NASA Langley Research Center ,
NOAA Aeronomy Laboratory
Kdninklijk Nederlands Meteorologisch Instituut
Czech Hydrometeorological Institute
University of Wales .
Universita1 degli Studi-l'Aquila
Forschungszentrum Jiilich
Forschungszentrum Jiilich
State University.of New York at Albany/ASRC
Atmospheric Environment Service
UK Department of the Environment
.California Institute of Technology/Jet Propulsion Laboratory
Office of Science and Technology Policy
NOAA Air Resources Laboratory
California Institute of Technology/Jet Propulsion Labpratory
Atmospheric and Environmental Research, Inc.
Scripps Institution of Oceanography
Harvard University
National Aeronautics and Space Administration
University Corporation for Atmospheric Research
University of Cambridge
Georgia Institute of Technology -
Deutscher Wetterdienst
Harvard University :
University of Illinois . - .
Central Aerological Observatory .
.UK
Belgium
US
Czech Republic
Germany
US
Switzerland
' US
- ' Australia
Italy
US
Norway
. Norway
India
: ' US
' ', .US
us
us
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UK
US
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us
The Netherlands
Czech Republic
UK
Italy
Germany
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Canada
UK
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us
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Russia
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Ahmed Zand
Rudi J. Zander
Joseph M. Zawodny
Reinhard Zellner
Christos Zerefos
Xiu Ji Zhou
Tehran University
University of Liege
NASA Langley Research Center
Universitat Gesamthochschule Essen
Aristotle University of Thessaloniki
Academy of Meteorological Science
Iran
Belgium
US
Germany
Greece
China
Sponsoring Organizations Liaisons
Rumen D. Bojkov World Meteorological Organization Switzerland
K.M. Sarma United Nations Environment Programme Kenya
Daniel L. Albritton National Oceanic and Atmospheric Administration US
Michael J. Kurylo National Aeronautics and Space Administration US,
Coordinating Editor
Christine A. Ennis NOAA Aeronomy Laboratory/CIRES
US
Jeanne S. Waters
Editorial Staff
NOAA Aeronomy Laboratory
US
Publication Design and Layout
University of Colorado at Boulder Publications Service:
Elizabeth C. Johnston
Patricia L. Jensen
Andrew S. Knoedler , .
Conference Coordination and Documentation
Rumen D. Bojkov ' World Meteorological Organization Switzerland
Marie-Christine Charri6re World Meteorological Organization France
Christine A. Ennis NOAA Aeronomy Laboratory/CIRES US
Jeanne S. Waters NOAA Aeronomy Laboratory US
Conference Support
Flo M. Ormond Birch and Davis Associates, Inc. US
Kathy A. Wolfe Computer Sciences Corporation US
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