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OCR for page 15
Chapter 1
CURRENT STATUS
INTRODUCTION
This chapter reviews recent changes in the state of
understanding of the chemical and physical processes that
determine the effect of human activities on concentrations
of stratospheric ozone. The report is motivated by a
continuing need to assess the potential effects on
stratospheric ozone of chlorofluorocarbons (CFCs) and
other chemicals, as prescribed in the Clean Air Act, as
amended (42 USC 7450). The topic has been the subject of
intense study during the past decade; our report builds
on that work, most notably on previous studies by the
National Research Council (NRC 1975, 1976b, 1977, 1978,
1979b) and the National Aeronautics and Space
Administration (NASA) (Hudson and Reed 1979). To prepare
our assessment, we relied on our professional knowledge,
on a concurrent technical review prepared under the
auspices of NASA, the Federal Aviation Administration,
the National Oceanic and Atmospheric Administration, and
the World Meteorological Organization (WMO) (Hudson et
al. 1982), and on a series of topical reviews prepared at
our request by technical consultants. The consultants'
reports are contained in Appendixes A to F.
PROCESSES DETERMINING OZONE CONCENTRATIONS
Ozone (O3) is formed in the stratosphere by reaction of
atomic oxygen (O) with diatomic molecular oxygen (O2).
The process is initiated by photolysis of O2, that is,
the dissociation of O2 into atomic oxygen by absorption
of solar ultraviolet radiation at wavelengths below 240
nanometers (nm). Photolysis of O2 occurs mainly at
altitudes above 25 km.
15
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16
According to current understanding, approximately 1
percent of the ozone created in the stratosphere is
removed by transport to the troposphere; the remaining 99
percent is destroyed by chemical reactions in the
stratosphere that re-form ozone into O2. The net
effect of these chemical reactions is either the
combination of ozone with atomic oxygen to form O2,
represented by the equation
O + O3 ~
2o2 r
or the combination of two ozone molecules represented by
O3 + O3 ~ 3O2
I
II
These equations represent the net results of a number of
complex sets of reactions catalyzed by a variety of gases
and chemical radicals present in the stratosphere in
trace amounts.
Important examples of sets of reactions summarized by
process I are
C1 + O3 + C10 + O2 (la)
C10 + O ~ C1 + O2, (lb)
NO + O3 ~ NO2 + O2
NO2 + O ~ NO + O2,
OH + O3 ~ HO2 ~ O2
0 + HO2 + OH + O2
Process I may also proceed by the direct path
O + O3 ~ 2O2
(2a)
(2b)
(3a)
(3b)
(4)
These reactions are limited by the availability of oxygen
atoms and therefore occur mainly at altitudes above 25
km. The reactions that limit the rates at which chains
1, 2, and 3 proceed are (lb), (2b), and (3b),
respectively.
Process II summarizes reaction schemes in which atomic
oxygen is not limiting, for example,
OH + O3 ~ HO2 + O2
HO2 + O3 ~ OH + 2O2.
(5a)
(5b)
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17
Reactions (5) account for most of the ozone lost below 25
km in current models. The chemistry of the lower strato-
sphere is complex, however (Appendix A), and one cannot
exclude additional reaction schemes involving oxides of
nitrogen and chlorine (NOX, ClOX) and oxidation
products of hydrocarbons such as methane (CH4)~
Ozone removed from the stratosphere by transport to
the troposphere is ultimately lost by chemical reactions
in the gas phase or at the earth's surface.
The spatial and temporal distribution of the
concentration of ozone reflects a dynamic balance among
the processes that form and remove ozone (Figure 1.1).
According to current understanding, photolysis of O2
provides a global source of ozone of 50,000 million
metric tons per year, with more than 90 percent of this
amount formed above 25 km. Most of this ozone is removed
by reactions represented by process I. At altitudes
between 25 km and 45 km, reaction (2b) accounts for
roughly 45 percent of the ozone removed while reactions
(lb) and (4) each account for about 20 percent and
reaction (3b) for 10 percent (S.C. Wofsy, Harvard
University, private communication, 1982). About 1
percent of stratospheric ozone, 600 million metric tons
per year, is removed below 25 km by process II, with a
similar amount being lost by physical transport to the
troposphere.
Only 30 percent of global ozone is stored at altitudes
above 25 km, reflecting the relatively short chemical
lifetime of ozone at high altitudes. The rest is
contained in the region below 25 km, and more than 70
percent of the amount below 25 km is found at latitudes
above 30°. The abundance of ozone below 25 Am is
determined by the balance between transport from the
chemically more active region at higher altitudes and
losses to the troposphere; its distribution is regulated
by atmospheric motions.
Adding to the stratosphere substances that destroy
ozone has the effect of creating a new balance between
production and removal processes in which the total
abundance of ozone is reduced. For example, stratospheric
concentrations of chlorine monoxide (C1O) and nitrogen
dioxide (NO2) may be increased as a result of emissions
of CFCs and nitrous oxide (N2O) from human activities.
The effects are persistent. A typical CFC molecule,
CF2C12 for example, survives for approximately 75
years in the atmosphere before it is decomposed by
sunlight releasing its constituent chlorine atoms in the
OCR for page 18
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OCR for page 19
19
stratosphere.
_~ _" A chlorine atom can affect recombination
of between 104 and 105 ozone molecules during its
lifetime in the stratosphere (on the order of two years)
before it returns to the troposphere, mainly as hydro-
chloric acid (HC1). A similar situation holds for N2O.
Approximately 10 percent of N2O molecules released to
the atmosphere decompose by paths leading to production
of stratospheric nitric oxide (NO), and subsequently
NO2, by reaction (2a). The average NOX molecule also
removes between 104 and 105 ozone molecules before it
returns to the troposphere, after its typical two-year
residence in the stratosphere. Current theoretical
models lead us to conclude that the dependence of ozone
concentration on altitude will also change, the net
effect being a redistribution of ozone from higher to
lower altitudes. Quantitative estimates of these effects
have varied somewhat over the past decade (Appendix A).
Perturbations by Chlorine
Currently, approximately 3 parts per billion (ppb) of the
lower stratosphere consists of chlorine bound in organic
molecules such as methyl chloride (CH3C1), carbon
tetrachloride (CC14), and CFCs (Hudson and Reed 1979,
Hudson et al. 1982). Table 1.1 indicates the abundances
of the more prevalent species; only methyl chloride is
known to have natural origins. The table also shows
estimates of current rates of release of man-made
compounds found in the lower stratosphere.
Halocarbons decompose under the influence of sunlight
at altitudes above 20 km; the fractional abundances
(mixing ratios) of halocarbons (in ppb)
are observed to
decrease with increasing altitude (Appendix C). The
chlorine produced by decomposition of halocarbons is
converted to inorganic species, including HC1, chlorine
nitrate (ClNO3), C1O, and atomic chlorine (C1). Hydro-
chloric acid is the major reservoir for chlorine at
altitudes above 25 km (Appendix C). Concentrations of
C1, C1O, and HC1 have been observed in the stratosphere;
observations and predictions of theoretical models are in
general agreement, although some difficulties remain
(Appendix D), as we shall see.
Computer calculations using current understanding and
incorporating new data on rates of several important
reactions (Appendixes C and D) suggest that continued
release of the CFCs, CF2C12 and CFC13, at rates
_ , , ~. ~ ~ ~
\
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20
TABLE 1.1 Concentration in the Lower Stratosphere and Release Rates of Major
Sources of Chlorine in the Stratosphere
Rate of Release
Concentration (ppb) a (million metric
.
Compound Molecular Chlorine tons of C1 per year)
Methyl chloride (CH3 C1) 0.62 0.62 2b
F-12 (CF2C12) 0.30 0.60 o.lgc
Fell (CFC13) 0.18 0.54 o.2oc
Carbon tetrachloride (CC14) 0.13 0.52 0.0s3d
Methyl chloroform (CH3CC13) 0.1 1 0.33 0 3Se
aHudson et al. (1982).
bAbout 85 to 90 percent of CH3 C1 is naturally produced, the remainder being attr~b
uted to industrial sources (Cicerone 1981). The total release rate varies slowly in time
because of the large contributions of natural sources.
C1980 release rate from "World Production and Release of Chlorofluorocarbons 1 1
and 12 through 1980," Chemical Manufacturers Association Fluorocarbon Program
Panel, July 29, 1981. Release rate has decreased by about 20 percent from the peak
rate of 1974.
dl 976 release rate (NRC 1 979b). The release rate is apparently relatively constant,
although somewhat uncertain.
eNeely and Plonka (1978).
prevalent in 1977 would ultimately cause a net decrease
of total global ozone roughly between 5 percent and 9
percent assuming no other perturbations (Hudson et al.
1982). We regard a representative result to be 7 percent
tAPpendix C). This would result in a smaller steady
state reduction in ozone than reported in NRC (1979b),
which was 16.5 percent with a 95 percent probability that
the true value lies between 5 percent and 28 percent.
bother models current in 1979 gave reductions ranging
from 15 percent to 18 percent tHudson and Reed 1979).
Estimates have fluctuated between roughly 5 percent and
20 percent over the past eight years as models have been
refined (Appendix A).) The steady state reduction would
be reached asymptotically in times on the order of a
century. Calculations now indicate that the reduction
would occur almost entirely at altitudes above 35 km, in
the region of the stratosphere where the ozone concentra-
tion is determined primarily by chemical processes, with
a smaller, partially compensating increase in ozone
concentrations at lower altitudes. The current result
obtains for both 1- and 2-dimensional models and further
differs from that prevalent in 1979 in that earlier
calculations showed regions of reduction both above and
below 35 km.
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21
The differences between current findings and those
reported in 1979 are the result of refinements in the
values for the rates of several reactions affecting the
concentration of the hydroxyl radical (OH) (Appendixes C
and D). The refinements are the result of improved
laboratory measurements (Hudson et al. 1982). OH is
important because the concentration of C1O in the lower
stratosphere is particularly sensitive to it. Results of
model calculations using current values for these reaction
rates are in good agreement with observations of C1O for
altitudes below 35 km (Appendixes C and D), whereas
models using the reaction rates favored in 1979 give
concentrations of C1O a factor of 3 higher than observed
values in this range. The new reaction rates have not
changed greatly the results of calculations for altitudes
above 35 km, however, so that the amounts of reduction in
ozone above 35 km obtained in the 1979 and current models
are about the same. The models continue to indicate lower
concentrations of C1O in the stratosphere above 40 km
than are observed. We shall return to this discrepancy.
Increased attention to effects of releases of methyl
chloroform (CH3CC13) on stratospheric ozone is
warranted because of the growing use of this compound, an
industrial solvent. The release rate increased by a
factor of about 50 between 1958 and 1978 (Neely and
Plonka 1978).
Perturbations by Oxides of Nitrogen
The chemically active oxides of nitrogen in the strato-
sphere (such as NO2) are thought to arise mainly from
photooxidation of N2O. N2O is formed naturally by
bacteria in soil and water. As indicated earlier,
reactions involving NO2 account for about 45 percent of
the ozone removed in the stratosphere between 25 km and,
45 km.
The human influence on the global cycle of fixed
nitrogen is thought to be significant and increasing (NRC
1978). The global atmospheric concentration of N2O
appears to have increased by 2.7 percent (from 292 ppb in
1964 to 300 ppb in 1980) over the past 16 years (Weiss
1981, Weiss and Craig 1976). The concentration of N2O
in the atmosphere is likely to continue to increase with
increases in emissions associated with agricultural
practices, disposal of human and animal wastes, and
possibly combustion; but we cannot say how or on what
time scale.
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22
An increase in N2O concentrations of about 30
percent in the absence of other perturbations could cause
a reduction in global ozone of an amount comparable with
the 7 percent reduction currently estimated due to
continued emissions of CF2C12 and CFC13 at 1977
rates, also taken as the sole perturbation. This estimate
is based on current model calculations that indicate
that, should the concentration of N2O double in the
absence of other perturbations, total global ozone would
decline between 10 percent and 16 percent (Hudson et al.
1982).
Early attention to human influences on the strato-
sphere focused on effects of NOX released by high-
flying aircraft (NRC 1975). Models then and now suggest
that an input of NOk at altitudes above about 20 km
should lead to reduction in stratospheric ozone. A
source of NOX at lower altitude, associated for example
with subsonic commercial aviation, can modify local
chemistry such as to cause an increase in tropospheric
ozone. It has been suggested that reductions in the
column of ozone above the earth's surface due to
reductions in stratospheric ozone may be masked to some
extent by increases in tropospheric ozone attributable to
subsonic jets and urban smog.
Assessment of the impact on stratospheric ozone due to
a combination of perturbations requires investigation of
specific cases since the effects are not simply
additive. Hudson et al. (1982) report the results of
several studies of the effects of doubling atmospheric
N2O concentrations and continuing releases of CFCs at
1977 rates, both separately and in combination. The
Lawrence Livermore National Laboratory (LLNL) model, for
example, indicated a reduction of 12.5 percent due to
doubling N2O with a reduction of 12.9 percent due to
the combination of perturbations. The LLNL model gives a
reduction of 5.0 percent for CFC releases alone. Another
model, from Atmospheric and Environmental Research, Inc.,
gives reductions of 9.5 percent for doubling N2O, 6.1
percent for continuing CFC releases, and 13.0 percent for
the combination. The results may be misleading, however,
since current trends suggest a considerably longer time
scale for doubling atmospheric concentrations of N2O
than for reaching the steady state reduction due to
continued emissions of CFCs.
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23
Perturbations by Other Species
Stratospheric ozone may be affected by human activity in
a number of other ways. Of greatest potential concern
are changes in concentrations of carbon dioxide (CO2),
water vapor (H2O), and perhaps methane (CH4).
The well-documented increase in atmospheric
concentrations of CO2 is directly attributable to
combustion of fossil fuels and wood. This increase is
expected to lead to a global warming of the atmosphere
near the surface of the earth but is expected to cause a
reduction in the temperature of the stratosphere (Fels et
al. 1980).
Lower stratospheric temperatures would have at least
two effects. First, the chemical removal processes
affecting ozone that were described earlier are sensitive
functions of temperature, being less efficient at lower
temperature. Consequently, with lower temperature the
equilibrium concentration of ozone would be higher.
Current models incorporating this effect suggest that the
steady state reduction in total ozone due to continuing
emissions of CFCs at 1977 rates would change from 5
percent to 9 percent to between 4 percent and 6 percent
if global CO2 were doubled concurrently (Hudson et al.
1982). (Global CO2 has increased by about 6 percent in
the past 22 years.)
The possible second effect of lower statospheric
temperatures resulting from increased CO2 is a thermally
driven change in stratospheric water vapor (H2O) caused
by a change in the temperature of the tropical tropopause.
Dissociation of H2O provides the source of hydrogen
radicals, and these radicals play a key role in strato-
spheric chemistry regulating abundances of both active
NOk and C1X species in addition to their contributions
to reactions (3) and (5). A complete model for strato-
spheric chemistry should include a description of H2O
interactions, a requirement beyond current capability.
Stratospheric ozone may also vary in response to
changes in concentrations of CH4, which plays an
important role in reaction (lb) by regulating the
partitioning of chlorine between HC1 and C1O (Hudson et
al. 1982). Recent reports (Rasmussen and Khalil 1981)
suggest increases in global concentrations of CH4, but
likely future changes and their consequences are unknown.
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24
CURRENT STATUS OF MODELS OF THE STRATOSPHERE
Theoretical models of stratospheric chemistry cannot be
validated by measurements of total ozone only, owing to
the diversity of factors, natural and man-made, that may
affect ozone concentrations. Comparison of calculated
and observed values for the concentrations of important
trace species and radicals--such as OH, ClO, NO2, and
atomic oxygen--must play a central role in any orderly
strategy for validating models.
In general terms, agreement in detail between the
predictions of theoretical models and observations is
excellent. For example, changes in reaction rates since
1979 have resulted in substantial agreement between
theory and observation for ClO below 35 km. There are,
however, three areas in which discrepancies remain. The
discrepancies may or may not point to significant
difficulties in modeling. Similarly, agreement between
modeling results and observation of C10, while
encouraging, need not imply validity of the model at
lower altitudes.
The improved agreement between observed and calculated
concentrations of C10 in the lower stratosphere may be
attributed mainly to changes in rate constants for
reactions affecting OH. Concentrations of OH in current
models are lower than values obtained in 1979, with the
result that a larger fraction of Clx is now found as
HC1. The chemistry of the lower stratosphere is complex,
however. Agreement between model and observed values of
C10 in the lower stratosphere should be considered
necessary but not sufficient for validation. A more
extensive and demanding test would require comparison of
theoretical and observed profiles of other radicals,
particularly OH.
To improve understanding of stratospheric chemistry
also requires that attention be directed to the
assumptions of the models and to the measurements against
which models are tested. High-quality measurements are
obviously prerequisite to validation of models.
Confidence in observations of critical species is
enhanced by using a number of independent, inter-
calibrated techniques, each relying on different physical
properties. Validation of measurement technique is
difficult since concentrations of the important
atmospheric species may vary in time and space on scales
that are not well understood. Validation procedures
involve coordinated studies in the field requiring
considerable logistical support.
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25
The assumptions of models are of two types: (1) input
~ ~ conditions, reaction rates, and
(2) the reaction schemes incor
data on environmental
other parameters, and
porated into the model. There are
still uncertainties
about the appropriateness of some assumptions common in
current models. For example, the rate for reaction of OH
with HO2, an important path for removal of hydrogen
radicals, remains uncertain despite extensive and
continuing efforts in the laboratory. There are other
reactions in need of similar clarification. Models are
sensitive to assumptions about the abundance and
distribution of stratospheric H2O; the underlying
physical and chemical processes that regulate this key
parameter are not well understood. It is difficult to
rule out the possibility of an important role for species
not now included in models, and, if history is a guide,
there may well be future surprises in this area. Models
for the stratosphere have been adjusted over the past
decade in just this manner to include gases such as C10
(1974), ClNO3 (1976), and HOC1 (1978) (Appendix A,
Figure A.1), and there is current discussion of a
possible participation of sodium (Kolb and Elgin 1976,
Murad et al. 1981). Progress in recognition of missing
species or reactions occurs through a combination of
laboratory, field, and theoretical studies, the normal
practice of validating models and resolving discrepancies.
As was noted earlier, there is reasonable agreement
between model calculations and observations for C10 in
the lower stratosphere. Currently, however, there is a
discrepancy between theory and observation for C10 in the
region above 35 km, where chlorine-mediated catalysis is
most important. The average value for the concentration
of C10 measured by Anderson and co-workers (see Appendix
D) near 40 km is almost a factor of 2 larger than the
value calculated from models. Furthermore, theory and
experiment give different dependences of the concentration
of C10 on altitude in the upper stratosphere. The C10
discrepancy is particularly important because it occurs
at altitudes where ozone is most sensitive to perturba-
tions caused by CFCs (Appendix D, Figure D.52).
Extensive ground-based observations of NO2 have been
made over a range of latitudes by J. Noxon (see Appendix
C), revealing a sharp spatial discontinuity in concentra-
tion in the winter with very low concentrations poleward
of the discontinuity. Thus far, no theroretical model
has been able to explain this phenomenon.
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26
A third area of discrepancy between current models and
observations is in concentrations of CFC13 and
CF2C12 at altitudes above about 20 km (see Appendix
C, Figure C.ll). Observed values are substantially lower
than predicted values. The difference could be due to
errors in model simulation of ultraviolet radiance in the
lower stratosphere, which, if true, would imply that the
CFCs have shorter residence time in the lower strato-
sphere. The issue is not resolved and requires
continuing attention.
Nevertheless, the extent of agreement between
measurement and theory is encouragingly good.
MONITORING AND ASSESSMENT OF TRENDS
Measurements of the total amount of ozone above a unit
area of the earth's surface (called total column ozone)
are essential for assessing the human influence on ozone
(as well as the potential effects of changes in ozone on
humans and other organisms). As detailed in Appendix A,
total column ozone fluctuates on a variety of spatial and
temporal scales owing to natural causes; these fluctua-
tions tend to mask possible systematic changes due to
man-made perturbations. For example, current models for
single and combined perturbations predict a reduction of
total column ozone over the past decade of less than 1
percent, but a change of this magnitude cannot be
distinguished from fluctuations due to other causes
(Appendixes D and E).
Models of the stratosphere predict that the largest
reductions in ozone due to releases of CFCs should occur
near 40 km. Reductions should therefore be most readily
detectable at this altitude. Current models suggest that
ozone concentrations at 40 km should have decreased by
several percent over the past decade. There have been
reports in the press that an effect of this order has
been detected in data from satellite experiments (see,
for example, Science, Sept. 4, 1981, pp. 1088-1089). The
community of atmospheric scientists has not yet had the
opportunity to scrutinize this evidence, which must
therefore be regarded as preliminary (Appendix F).
Our ability to detect trends in ozone in the future
will depend on the availability of consistent, high-
quality data taken over long time intervals. Improvements
in the current monitoring systems are feasible and clearly
needed. For example, it is vitally important to improve
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27
and enhance systems for monitoring ozone profiles in the
upper stratosphere that could provide a valuable early
indication of systematic changes In ozone aue co emend ~ Ural
of CFCs or N2O, but existing data in the upper strato-
sphere are inadequate for this purpose. It is also
imperative to continue, and desirable to expand, the
high-quality monitoring of total ozone by Dobson
spectrophotometers.
THE QUESTION OF EARLY DETECTION
A notable feature of the ozone issue is that a reduction
due to increases in the tropospheric concentrations of
CFCs or N2O, once it has taken place, is expected to
persist for more than 100 years even if the practices
that caused it are stopped immediately. It is therefore
important to detect an anthropogenic effect at the
Three methods currently exist
,
earliest possible time.
for this purpose.
1. Measurement of total ozone. Relying on
measurement of total ozone has the following advantages
(Appendix E): There exists a relatively long historical
base (30 to 50 years) of data. Ground-based
instrumentation is available and may be readily
complemented by observations from satellites. Finally,
total ozone is most directly related to one of the
consequences of depletion that is of concern, the
possibility of enhanced exposure to ultraviolet radiation
at the ground (Part II). Since, however, the reduction
due to CFCs is expected to be concentrated at high
altitudes, measurements of total column ozone are less
sensitive indicators of an anthropogenic effect than are
measurements of ozone profiles.
2. Measurement of ozone at high altitudes. The
advantages of this method derive from the theoretical
result that changes in ozone due to CFCs are predicted to
be largest at high altitudes. Changes in the spatial
distribution of ozone may be important for understanding
the second major consequence of depletion that is of
concern, the possibility of climate change (Appendixes
and C). The disadvantages stem from the difficulty of
making the measurements, whose quality and stability are
inferior to those of total ozone (Hudson and Reed 1979,
Hudson et al. 1982). Satellite data are particularly
subject to changes in calibration of instruments, which
-- B
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28
cannot be refurbished; ground-based measurements by the
Umkehr method give poor height resolution and are subject
to perturbations by hazes and stratospheric particulate
matter. Partly because of these difficulties, the data
base is relatively small and somewhat fragmented (Appendix
F). Ozone measurements using satellites would have the
desirable attribute of obtaining temporal and spatial
distributions that would be useful in validating 2- and
3-dimensional models.
3. Measurement of key radicals involved in chemical
removal processes. Measurements~of spatial and temporal
profiles of important species such as C10 and OH may be
combined with chemical models for assessment of trends
and their causes, such that the dependence on specific
models can be relatively slight. This method is in
principle the most sensitive, but it is also the least
direct.
The last approach is regarded by many experts as
having already shown the effect of chlorine of human
origin, mainly connected with emissions of CFCS. But
this conclusion would be more firmly established with
more direct confirmation, as discussed in the previous
section. Ideally, all three types of measurement should
be integrated (with due regard to their sensitivity) in a
strategy for early detection of anthropogenic effects.
UNCERTAINTY
Quantitative estimates of the uncertainties inherent in
current estimates of reductions in ozone due to emissions
of CFCs and N2O are difficult to obtain. The ability
to make quantitative estimates of uncertainty depends
both on what we know and on what we do not know. Such
estimates employ professional judgments about the
importance of various factors and the sensitivity of the
results to potential changes in understanding.
Our major concern in estimating uncertainties in our
understanding of stratospheric ozone is with the
possibility that some key process or processes may be
missing from current models. In an orderly scientific
strategy, continuing development of models on the basis
of an ongoing comparison with observational data is
expected. Progress is stimulated by the existence of
discrepancies or uncertainties and tends to occur in more
or less discrete steps rather than uniformly. Our
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29
understanding of the lower stratosphere has improved over
the past two years as a result of developments that may
be attributed at least in part to efforts to resolve
earlier (and larger) discrepancies between observed and
computed values for the concentration of C1O. Agreement
between observed and computed values of C1O is now
satisfactory below 35 km, but, as noted earlier, there
continues to be a serious discrepancy at higher altitudes.
This disagreement illustrates the difficulty of estimating
limits of uncertainty for current estimates for reduction
in ozone due to CFCs.
For example, observed values of C1O at higher altitudes
are larger than calculated values, suggesting that the
long-term reduction in ozone could be correspondingly
larger. One can, however, conceive of speculative
chemical schemes that could suggest a stratosphere less
vulnerable to perturbations.
In circumstances such as this, the usual ways of
estimating uncertainty (using mathematically rigorous
procedures) are not applicable.
professional judgment.
Instead we rely on
The predictions of the current
chemical scheme have been cross-checked against observed
atmospheric data in many ways, and the agreement in
general is quite good. As stated earlier, a representa-
tive estimate of potential steady state reduction of
global ozone due to continued releases of CFCs at the
1977 rate in the absence of other perturbations is 7
percent. There continue to be, however, important
discrepancies between theory and observation.
Our opinions are divided on whether there are
sufficient scientific grounds to estimate the effect of
resolving one of the discrepancies, that of C1O in the
upper stratosphere, on calculations of ozone reduction.
We agree that we do not know enough at this time to make
a quantitative judgment of the uncertainty associated
with the other major discrepancies, NO2 at high
latitudes and lifetime of CFCs in the stratosphere above
20 km.
Those of us who believe there are grounds to judge the
effect of resolving the C1O issue conclude that our
estimate of ozone reduction from CFC emissions should not
change by more than a factor of 2.
Those of us unwilling to offer quantitative estimates
of uncertainty hold the conviction that no rigorous
scientific basis exists for such statements. We are
concerned by implications of the discrepancies noted
earlier. These discrepancies should be resolved in the
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next few years by orderly application of the scientific
method with appropriate interaction between theory and
observation. We see no reason to prejudge the result of
this process.
Research during the past several years has enhanced
our understanding of the factors affecting stratospheric
ozone. Development of the field is progressing rapidly.
We anticipate further developments in both observation
and modeling in the next few years that will result in
considerable improvement in our understanding, both
clarifying and reducing uncertainties.
FINDINGS
1. Our understanding of the stratosphere has advanced
considerably in the past two years. Progress is
significant in all areas with improvements in our ability
to model the system in more than 1 dimension, with
impressive achievements in techniques for measurement of
chemical reactions in the laboratory, and with major
advances in our ability to measure concentrations of
important trace species in the atmosphere. We note her
that the success of the research is due in no small part
to the breadth of the scientific effort involving
scientists from many countries with support from both
private and governmental sources. We expect continued
improvement in understanding of the chemistry and
dynamics of ozone reduction to result from research
currently under way, planned, and proposed.
2. The concern regarding the possibility of reduction
e
in stratospheric ozone due to CFCS remains, although
current estimates for the effect are lower than results
given in NRC (1979b). The change in estimates of ozone
reduction reflects improvements in our understanding of
chemical processes in the stratosphere below 35 km.
There has been no significant change in results obtained
by models for the stratosphere above 35 km. The major
impact of CFCs is predicted for the height range of 35 km
to 45 km.
3. The chlorine species C1 and C10 participate in a
series of chemical reactions that destroy ozone. The
radical C10 has been measured in the stratosphere in
significant amounts and is believed to be primarily of
human origin. Our current understanding indicates that
if-production of CFCs continues into the future at the
rate existing in 1977, the steady state reduction in
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total ozone, in the absence of other perturbations, would
be between 5 percent and 9 percent. Previous estimates
fluctuate between roughly 5 percent and 20 percent, with
those current in 1979 ranging from 15 percent to 18
percent. Latest results also suggest that CFC releases
to date should have reduced the total ozone column by
less than 1 percent.
4. According to current understanding, increases of
N2O in the stratosphere would result in reductions in
total ozone. with the largest effects occurring in the
lower stratosphere. Although concentrations of N2O in
the stratosphere appear to be increasing, we cannot
reliably project the future course of N2O sources. If,
however, the concentrations of N2O in the atmosphere
were to double, in the absence of other perturbations,
current models suggest that the steady state reduction in
the total ozone would be between 10 percent and 16
percent.
5. On the whole, there have been substantial
improvements in the agreement between model predictions
and observed profiles of trace species in the past
several years. Three exceptions are still a cause for
concern: Above 40 km, more C1O is observed than is
predicted by current theory; the behavior of NOX in
winter at near-polar latitudes is unexplained; and
concentrations of CFCS in the stratosphere above 20 km
are lower than predicted by the models.
6. Examination of historical data (extending back 30
to 50 years) has not yet shown a significant trend in
total ozone '~hat can be ascribed to human activities.
Current models of combinations of pollutants suggest that
a reduction of total ozone to date from human activities
would be less than 1 percent. No detectable trend would
be expected on the basis of these results.
7. Data on total ozone should not be used alone to
guide decisions on whether to take action to prevent
future changes in stratospheric ozone. Although an
important guide, analysis of trends in total ozone cannot
by itself reveal causes of ozone reductions or
increases. Such analysis, together with measurement of
altitude profiles of trace species and ozone and
theoretical modeling, offers promise of understanding
causes of ozone changes and the consequences of
alternative actions in response.
8. The impact of CFCs should be assessed in the
context of a broad understanding of the variety of ways
in which human activity can alter stratospheric
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composition. Ozone may be reduced by increasing levels
of CFCs and N2O, but reductions might be offset in part
by higher concentrations of CO2 and perhaps CH4.
Human activities have already increased the amounts of
CO2 and CFCS in the atmosphere, and from the known
release rates, further increases can be confidently
expected. In addition, there is evidence that N2O and
CH4 concentrations are also increasing. A special
reason for concern about perturbations potentially caused
by CFCs and N2O is the long lifetime of these gases in
the atmosphere, of the order of 50 to 150 years. Even if
the releases of these gases were reduced, the atmosphere
would not recover until far in the future.
RECOMMENDATIONS
In light of our findings, we believe it is important
to maintain a competent, broadly based research program
that includes a long-term commitment to monitoring
programs. The research effort should extend over at
least two solar cycles (of 11 years each) to distinguish
between changes induced by variations in the sun from
those associated with man. Accordingly, we make the
following recommendations:
1. The national research program, including
atmospheric observations, laboratory measurements, and
theoretical modeling, should maintain a broad perspective
with some focus on areas of discrepancy between theory
and observation. A coordinated research program to
understand the spatial and temporal distributions of key
species and radicals merits highest priority.
Observations should be extended to include studies of the
equatorial and polar regions.
2. The global monitoring effort should include both
ground-based and satellite observations of total ozone
and of concentrations of ozone above 35 km, where theory
indicates the largest reductions might occur. We also
need data to define the variability of stratospheric
temperature and water vapor. We regard sound,
satellite-based systems for stratospheric observations as
essential.
3. Potential emissions of a number of relevant gases,
in addition to CFCs and N2O, and their consequences for
stratospheric ozone should be thoroughly evaluated and
assessed. It is important that we understand current and
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potential rates of emissions of these compounds and the
effects these emissions might have on ozone in addition
to understanding emissions and effects of CECs. There is
observational evidence that atmospheric concentrations of
N2O and CO2 are increasing. Models should be
developed to describe the combined effects on strato-
spheric ozone of future changes in releases of all
relevant gases, such as CFCs, N2O, CO2, CB4,
CH3C1, and CH3CC13.
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Representative terms from entire chapter:
stratospheric ozone
