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OCR for page 33
5
The Role of Halocarbons in
Stratospheric Ozone Depletion
F. SHERWOOD ROWLAND
University of California, Irvine
This presentation will cover two topics: (1) halocarbons in the
atmosphere and (2) the measurement of ozone.
Starting in 1978, my research group made gas chromatographic
measurements of trichiorofluoromethane (CCI3F, known as CFC-ll)
with air samples from many locations in both hemispheres that were
judged to be sufficiently remote from local emission sources. Other
research groups had collected similar data beginning as early as
1970. A set of our measurements of CCI3F from 1979 shows only a
small hemispheric difference in the lower atmosphere (Figure 5-1~.
Although about 95 percent of the chiorofluorocarbons (CFCs) are
released in the Northern Hemisphere, the redistribution between
the hemispheres is rapid enough that the Southern Hemisphere lags
behind the Northern by only about 10 percent. Measurements from
later summers show an increase at all latitudes. As part of the Global
Atmospheric Gas Experiment (GAGE), intensive measurements have
been made with automatic gas chromatography operating at five
stations most of the time since July 1978. These GAGE measure-
ments through 1983 for dichIorodifluoromethane (CCI2F2, known as
CFC-12) in Ireland and Tasmania (Cunnold et al., 1986) are shown
in Figure 5-2, together with flask data, from the Oregon Graduate
Center, for January measurements in Oregon and at the South Pole
(Rasmussen and Khalil, 1986~. All of the data show the level of
33
OCR for page 34
34
200
1 90
180
-
_~ 170
160
1 50
F. SHERWOOD ROWLAND
' ' ' ' ' ' 1 ' l l , , ,
SUMMER 1979
8 ° ° o o
8 GO o
o o o
_ o
°O o
o
,, , I I I I 1 1 , , ~ ~ II
90 60 30 o 30 60 90
N LATITUDE S
FIGURE 5-1 Tropospheric concentration (in pptv) of trichlorofluoromethane
(CCl3F) as a function of latitude for summer 1979. (Note nonzero origin of
ordinate.)
CFCs rising rapidly during the 1980s, with the Southern Hemisphere
lagging behind the Northern Hemisphere by about 10 percent.
In order to estimate the average lifetime of these molecules in
the atmosphere, one needs to know the amount being put into the
atmosphere for comparison with the amount that is still there. For
CCI2F2, the estimate of its mean life is more than 100 years, and this
is believed to be a fairly typical value for CFCs. Since the average
CFC molecule has been in the atmosphere only about 10 to 12 years,
90 percent of them are still there. The estimated mean life for CCI3F
is given as 75 years.
We ran a numerical calculation in which CCI3F release was as-
sumed to grow exponentially until 1976, remain constant for 15 years
until 1991, and then drop to zero after that. At 75 years after 1991,
the amount of chlorine compound remaining in the stratosphere had
dropped to 37 percent and at 150 years (i.e., 2160 A.D.) had declined
to 13 percent of the 1991 amount. Most of the other CFCs have
somewhat longer lifetimes, up to 140 years, so that an appreciable
fraction may still remain in the atmosphere even after 200 years,
given the above scenario. All indications are that the chlorine atoms
OCR for page 35
l
ROLE OF HALOCARBONS
400
360
320
CM
IL
280
240
200
1 975
.
1~1
,1~
,'
NORTH 0 1 l i u 1
,1l ,l I SOUTH
18
11.
. 11e
~1
.
.
.
1 980
1 985
YEAR
35
FIGURE 5-2 Concentration (in pptv) of dichloroditluoromethane (CC12F2)
from 1978 to 1985 for GAGE measurements in Ireland (vertical lines labeled
"North") and Tasmania (vertical lines labeled "South") (Cunnold et al., 1986),
together with January flask data obtained in Oregon (upper dots) and at the
South Pole (lower dots) (Rasmussen and Khalil, 1986~. (Note nonzero origin of
ordinate.)
OCR for page 36
36
1 .0
0.5
F. SHERWOOD ROWLAND
CIX
at '`
40 Km / \
I
\~;75 YEARS
AT- 19.6 YEARS
-
-
-
-
-
-
-
t~ /INPUT
0.0
1960 19~0 2000 2020 2040 2060 2080 2100 2120 2140 2160
YEAR
I , 1
FIGURE 5^3 Concentrations (in ppbv) as a function of year of two typical
stratospheric chlorinated molecules, one with a mean life of 75 years and the
other with a mean life of 19.6 years. These concentrations are based on an
assumed input of CFCs to the atmosphere (increasing to 1976, then constant
to 1991, then no further emissions) as shown in the lower left of the figure.
(Adapted from Rowland and Molina, 1976.)
released from CFCs are going to be with us for a long time, even if
release of CFCs is discontinued tomorrow.
A related point concerns what the Montreal Protocol calls the
"ozone depletion potential. Consider a chlorinated molecule with a
mean life of 75 years and another with a mean life of 20 years (Fig-
ure 5-3~. If their respective effects on ozone depletion are compared
as a function of time, the difference between them does not become
large until more than 50 years have passed (Rowland and Molina,
1976), by which time the 20-year compound will be largely gone.
If CFC emissions are assumed to continue into the future at a
constant rate, stratospheric chlorine compounds will continue to in-
crease (as shown in Figure 5-3~. In 1974, the Northern Hemisphere
contained chlorine compounds at about 1.8 ppbv; this has now in-
creased to about 3.5 ppbv. Continued release at 1986 rates will result
in increases to over 5.0 ppbv by the year 2000 (Figure 5-4~. If, in-
stead, we assume a 20 percent reduction of CFC emissions in 1994
OCR for page 37
ROLE OF HALOCARBONS
7.0
6.0
5.0
m 40
'in,
c' 3.0
2.0
1 .0
0.0 _
1950
, _
CON STA NT EM I SS I ON,
NO CONTROLS
MONTREAL PROTOCOL
COMPLETE CFC
P H A SEOUT
//
-
1960 1970 1980 1990 2000
YEAR
37
OTHERS
CC12FCCI F2
(CFC- I 13)
CC 12F2
(CFC -12)
- CC 13F
( CF C - I I )
CH3CC13
CC14
CH3C I
FIGURE 5-4 Increases in concentrations (in ppbv) of stratospheric chlorinated
molecules assuming (1) continued release of CFCs at 1986 rates (solid curves),
(2) a 20 percent reduction in release rates in 1994 and an additional reduction
of 30 percent in 1999 (dashed curves), and (3) a complete phaseout of CFC
emissions over a 10-year period beginning in 1989 (heavy solid curve).
and a further 30 percent reduction in 1999, as the Montreal Protocol
requires, the preclicted change in time will be different, assuming
that all countries obey the protocol. Finally, if we assume a complete
phaseout of CFC emissions over a lO~year period beginning in 1989,
OCR for page 38
38
F. SHERWOOD ROWLAND
TABLE 5-1 Trends in Total Ozone
Percent Change
Reference
am_
1970-1978 +0.28 ~ 0.67 Reinsel et al., 1981
1970-1979 +1.5 ~ 0.5 St. John et all, 1982
1970-1979 +0.1 i 0~55 Bloomfield et al., 1983
1970-1983 =0.003 ~ 1.12 per decade Reinsel et al., 1984
(-0.14 ~ 1.08) per decade
with sunspot series in model
Source: World Meteorological Organization-National Aeronautics and Space
Administration, Washington, D.C. (1986~.
the amount of stratospheric chlorine compounds will still continue to
increase for some decades.
The second topic ad^dressed in this presentation is ozone measure-
ments. The WMO-NASA report Atmospheric Ozone ~ 985 (WMO-
NASA, 1986) summarized recent calculations of trends in total global
ozone. The trends (through 1983) are only slightly, if at all, down-
ward (Table 5-1~. However, the trend estimates given in the WMO-
NASA report are misleading, as explained below.
The Arosa, Switzerland, station has the longest record of ground-
based measurements of total ozone, made since 1931 with a Dobson
spectrometer. Arosa is in the Swiss Alps at 47°N latitude. Neil
Harris, University of California at Irvine, has examined the monthly
averages of the data taken daily at Arosa. The amount of strato-
spheric ozone at Arosa, and generally in the north temperate zone,
varies seasonally, with a peak in March or April and a minimum in
October or November. The standard deviation of the measurements
is very large during the winter season and comparatively small in the
summer (Figure ~5~. If one compares the data for each month for
the periods 1931 to 1969 and 1970 to 1986, one sees that there is less
ozone, on the average, during the winter months in the later period
(Figure 5-6~. The greatest difference is observed for the month of De-
cember, with a loss that substantially exceeds the standard deviation
for the data.
Measurements at Bismarck, North Dakota, which is also at 47°N
latitude, began in 1963. We formed two sets of data, each 11 years
long, one for 1965 through 1975 and the other for 1976 through
1986. A length of 11 years was chosen to permit comparison over two
successive solar cycles. There ~ a wintertime loss of ozone during the
OCR for page 39
ROLE OF HALOCARBONS
400
-
~n
At
O 350
m
o
-
At
o
~ 300
o
250
JAN MAR
1
1
MAY JUL SEP NOV
MONTH
39
FIGURE 5-5 Mean monthly total column ozone measurements (Dobson units;
includes data from Dutsch, 1984) at Arosa, Switzerland, for the period August
1931 through July 1986. Standard deviations from the mean of individual
monthly data are also shown.
second period at Bismarck, similar to the loss reflected in the Arosa
results. The wintertime toss also shows up in data from Caribou,
Maine, which is also at 47°N. Hence, a wintertime depletion of ozone
in the last decade or so has been observed for numerous northern
stations.
Numerous factors are believed to affect the concentration of
stratospheric ozone. The solar sunspot cycle affects the ozone con-
centration because there is increased UV radiation at around 200 nm
during the sunspot maximum. Radiation of this wavelength can split
OCR for page 40
40
F. SHERWOOD ROWLAND
o
-
~n
Z -5
o
In
m
0 -10
Cal
-
OCR for page 41
ROLE OF HALOCARBONS
41
stations, which correlate so well after 1977, cross one another ear-
lier in a manner suggesting a problem with the continuing absolute
calibrations at one or both stations. Long-term calibration problems
such as these make the data at some stations not very useful for the
determination of trends in ozone concentrations over a 20- or 30-year
period.
In order to determine long-term ozone trends, Rumen Bojkov
from the Atmospheric Environment Service in Canada, Peter Bloom-
field, a statistician from North Carolina State University, Neil Harris,
and T have compiled data from all the Dobson stations and the So-
viet stations by latitude bands. (The Soviet stations use a different
instrument, the M-83, with somewhat different characteristics but
qualitatively similar measurement techniques.) The data span the
period 1965 to 1986 and are reported in terms of a "ramp" fit to the
22 years of data. The statistical analysis includes variations from the
solar cycle and the QBO, plus an assumed linear change after 1969
from an otherwise constant value from 1965 to 1969. The data have
been recorded as percentage changes over the 17-year period 1969 to
1986, from the linear ramp coefficients.
The results, reported by the Ozone Trends Pane! (Watson et al.,
1988), were as follows, on a monthly basis:
1. Between 53°N and 64°N (Figure 5-7a): not much change
in July to September but very substantial decreases in December to
March, similar to the Arosa data results. The QBO had a 2 per-
cent variation in the statistical analysis, and the solar cycle showed
1.8 percent more ozone at the solar maximum than at the solar min-
imum. (These variations were removed from the data in order to
study the long-term trends.)
2. Between 40°N and 52°N (Figure 5-7b): again, a marked
difference between summer and winter trends. The QBO and solar
cycle are again apparent.
3. Between 30°N and 39°N (Figure 5-7c): less difference be-
tween summer and winter trends, but all months show decreases
in ozone, some of them large enough to be statistically significant,
including decreases observed for July.
Regression coefficients were also calculated including successive years
of data from 1965 through 1980, 1981, 1982, and so on, up through
1986. The coefficients show some variation with the additional years
of data for the 53°N to 64°N zone and for the 30°N to 39°N zone, but
none of the changes appears to be statistically significant. A negative
OCR for page 42
42
5;
O
++tit t
a
t t
- 5
-10
. _
- 1 5 _
5 _
o
5
- 1 0
5 r
O
_5
- 1 0
- 1 5
F. SHERWOOD RO WLAND
PERCENT |
OZONE CHANGE t
53°N ~ 64°N
1969 ~ 1986
I 1 1 1 1 1 1 1 1 1 1 1
JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN
b
t , , f
PERCENT t t
OZONE CHANGE
40-N - 52°N
1969 - 1986
-15
+ , ~ I I ~ ~ t
JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN
C
t
~ " ' + t ~ I t t ~
PERCENT
OZONE CHANGE
30- N - 39°N
1969 - 1986
JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN
FIGURE 5-7 Percent
change in total column
ozone between 1969 and
1986 as a function of
month for three Northern
Hemisphere latitude
bands. Estimates of
uncertainty are shown by
vertical lines. See text for
additional explanation.
OCR for page 43
ROLE OF HALOCARBONS
43
trend in the regression coefficients appears in several months for the
40°N to 52°N band (Figure ~8~. Detailed examination shows the
coefficients tending to be more negative with added years of data,
suggesting that the ozone decreases may not be quite linear but may
tend toward steeper trends in later years. The great E! Chichon
volcanic eruption in 1982 does not seem to have had any large effect
on the regression coefficients (Dutsch, 1985~.
In conclusion, the amount of measured ozone loss in the sum-
mer is in reasonable agreement with theory, but the amount of loss
poleward of 40°N latitude in winter is greater than that indicated by
theoretical calculations. The statistical analysis suggests that there
is something missing from the atmospheric models of ozone depletion
that affects ozone levels in the Northern Hemisphere in winter. A
speculation is that stratospheric ice clouds in the arctic region are
having an effect somewhat similar to their effect in the Southern
Hemisphere, even though the meteorological conditions are very dif-
ferent in the two hemispheres. The fact that the wintertime ozone
decrease in the Northern Hern~sphere diminishes gradually at lower
latitudes, rather than abruptly as in the Southern Hemisphere, sug-
gests that atmospheric circulation is causing dilution in the Northern
Hemisphere.
In the statistical analyses that were reported previously, e.g., the
"no statistical change" result in the WMO-NASA report (WMO-
NASA, 1986), the assumption was routinely made that the amount
of Tong-term change was independent of month. Therefore, statis-
tical analysts tried to fit all months with a trend having the same
slope. Because the statistical reproducibility was much greater in
the summer months than in the winter months (see Figure 5-5), the
calculations tended to emphasize the summer months in the com-
bined data and led to the conclusion that ozone concentrations were
not changing much overall. Our studies show that, when the winter
data are analyzed separately, a significant loss of ozone has occurred
during the winter months.
(In response to a question about the comparison of Dobson
ground instrument data with satellite data): The basic problem with
calibrating the satellite data is the known fact that the instrument's
diffuser plate has been degracling under bombardment from the sun
since launch in October 1978. Calibration is carried out periodically
when the satellite passes over a ground-based Dobson instrument
site. The ground-based instruments, in turn, are calibrated with the
OCR for page 44
l
44
LL
z
2
o
(D
-2
o
lo
3
2
o
U) O
m
o
C)
F. SHERWOOD ROWLAND
LATITUDE BAND 40°N- 52°N
MONTHLY L I NEAR REGRESSION COEFFICIENTS
WITH SUCCESSIVE YEARS OF DATA, 1965 TO
1980 198 1 1982 1983 1984 1985 1986
:~ 11I
o DEC
o JAN
o FEB
o MAR
~O APR
-2 ~ ~ ~ MAY
3 _
- 4
FIGURE 5-8 Computed regression coefficients for linear change in ozone con-
centrations after 1969, using data for the periods 1965 to 1980, 1965 to 1981,
and 1965 to 1982 through to 1986 for each calendar month for the latitude band
40°N to 52°N. Estimates of uncertainty are shown by vertical lines. See text
for additional explanation.
OCR for page 45
ROLE OF HALOCARBONS
o
-2
3
~_4
llJ
2
en
He
1
o
at) O
m
o
C)
- 1
-2
3
4
LATITUDE BAND 40°N-52°N
MONTHLY LINEAR REGRESSION COEFFICIENTS
WITH SUCCESSIVE YEARS OF DATA, 1965 TO
1980 198 1 1982 1983 1984 1985 1986
11t Tll tll Ili ITI ~
T
FIGURE 5-8 (continued).
45
o JUN
O JUL
o AUG
o SEP
O OCT
o NOV
ll Ti! ITT TIT
OCR for page 46
46
F. SHERWOOD ROWLAND
worId-standard instrument at Mauna Loa, and the satellite instru-
ment is also compared directly with the Manna Loa instrument on
some overpasses. from these comparisons, it is possible to estimate
that the total degraciation in the satellite instrument since October
1978 has been about 3.5 percent.
Comparing the average readings from 1979 to 1980 with those
from 1986 to 1987 (two years are used to remove the ejects of the
QBO) shows a loss of ozone in most parts of the world. However,
because the solar cycle went from a maximum in the period 1979 to
1980 to a minimum in 1986, a general decline is predicted during this
time period. The Dobson data, on the other hand, span 22 years or
more, and the effect of the solar cycle can be statistically removed
from the data. The solar cycle cannot be reliably removed from the
satellite data.
.
(In answer to a question about the role of tropospheric ozone in
the total ozone measurements): The fraction of ozone in the tropm
sphere is approximately 10 percent of the total. There are indications
that the amount of tropospheric ozone has been increasing at a rate
of about 1 percent per year. Hence, the tropospheric contribution
is increasing total ozone at a rate of about ~ percent per decade. If
correct, this means that the stratospheric ozone losses are somewhat
greater than the total ozone column measurements indicate because
of the increase in tropospheric ozone. However, there is some un-
certainty, because it Is not well known how uniform the amount and
rate of increase of tropospheric ozone are globally.
(In answer to a question about the Umkehr ozone data): The
Ozone Mends Pane! (Watson et al., 1988) has analyzed the Umkehr
data and finds a loss of ozone at 40 km of -9 percent, compared to
a loss of -3 percent as measured by the Stratospheric Aerosol and
Gas Experiment (SAGE) satellite instrument. Theory, depending
on latitude and season, predicts a loss of from -5 to-12 percent.
So, there is plausible agreement but also concern over whether the
Umkehr and SAGE instruments are measuring the same thing.
REFE:RENCE:S
.
Bloomfield, P., G. Oehlert, M.L. Thompson, and S. Zeger. 1983. A frequency
domain analysis of trends in Dobson total ozone records. J. Geophys. Res.
88:8512-8522.
Cunnold, D.M., R.G. Prinn, R.A. Rasmussen, P.G. Simmonds, F`.N. Alyea, C.A.
Cardelino, A.J. Crawford, P.J. Freer, and R.D. Rosen. 1986. Atmospheric
lifetime and annual release estimates for CFC4 and CF2CI2 from 5 years
- of ALE data. J. Geophys. Res. 91:10797-10817.
Dutsch, H.U. 1984. An update of the Arosa ozone series to the present using a
statistical instrument calibration. Q. J. R. Meteorol. Soc. 110:1079-1096.
OCR for page 47
ROLE OF HALOCARBONS
47
Dutsch, H.U. 1985. Total ozone in the light of ozone soundings, the impact of
E1 Chichon. Pp. 263-268 in Atmospheric Ozone (Eds. C.S. Zerefos and E.
Ghazi). D. Reidel Co., Dordrecht, The Netherlands.
Rasmussen, R.A., and M.A.K. Khalil. 1986. Atmospheric trace gases: trends
and distributions over the last decade. Science 232:1623-1624.
Reinsel, G., G.C. Tiao, M.N. Wang, R. Lewis, and D. Nychka. 1981. Statistical
analysis of stratospheric ozone data for the detection of trend. Atmos.
Environ. 15:1569-1577.
Reinsel, G., G.C. Tiao, J.L. DeLuisi, C.L. Mateer, A.J. Miller, and J.E.
Frederick. 1984. Analysis of upper stratospheric Umkehr ozone profile
data for trends and the effects of stratospheric aerosols. J. Geophys. Res.
89:4833-4840.
Rowland, F.S., and M.J. Molina. 1976. Estimated future atmospheric concen-
trations of CC13F (Fluorocarbon-11) for various hypothetical tropospheric
removal rates. J. Phys. Chem. 80:2049-2056.
St. John, D., W.H. Bailey, W.H. Fellner, J.M. Minor, and R.D. Sull. 1982. Time
series analysis of stratospheric ozone. Commun. Stat., Part A 11:1293-1333.
Watson, R.T., M.J. Prather, and M.J. Kurylo. 1988. Present State of Knowl-
edge of the Upper Atmosphere 1988: An Assessment Report. NASA
Reference Publication No. 1208. National Aeronautics and Space Admin-
istration, Washington, D.C.
World Meteorological Organization-National Aeronautics and Space Adminis-
tration (WMO-NASA). 1986. Atmospheric Ozone 1985: Assessment of
Our Understanding of the Processes Controlling Its Present Distribution
and Change. Global Ozone Research and Monitoring Project, Report No.
16,3 vole., WMO, Geneva.
l
Representative terms from entire chapter:
sherwood rowland
