|
Items in cart [1] |
Questions? Call 888-624-8373 |
| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 19
4
Stratospheric Ozone Depletion:
Antarctic Processes
ROBERT T. WATSON
National Aeronautics and Space Administration
This paper describes the current understanding of the antarctic
ozone hole phenomenon. A later speaker, James Anderson, will
expand on the role of chlorine in causing the hole to occur. This talk
addresses two questions: (1) What are the observations of ozone that
define how large and ~deep" the antarctic hole is? and (2) What is
our present understanding of cause and effect? As Daniel Albritton
noted, the Montreal Protocol did not explicitly take the appearance
of the hole into account. However, the occurrence of the hole at
about that time served as a major driving force to get the Europeans
to view ozone as a serious issue and to get them to the table. The
U.S. position at the Montreal meeting was bred on the assumption
that we did not know the cause of the antarctic ozone hole, although
by then we recognized that an ozone hole existed. It is worth noting
that the appearance of the ozone hole was an unexpected event in
the sense that the models referred to by Albritton did not predict the
hole. The ozone hole has made modelers realize that stratospheric
modeling needs further work and has rekindled scientific interest in
the problem of stratospheric ozone.
The ozone over Antarctica had, by October 1987, been reduced
by more than 50 percent of its 1979 value (Watson et al., 1988; Figure
4-~. I.ocally, depletion was as great as 95 percent between 15 and
20 km altitude (Figure 4-2~. Not only was the ozone level the lowest
on record in 1987, but the seasonal period of depletion also lasted
19
OCR for page 20
20
,I /~/
~ \\~\\'
cn
o
-~ ~J,:g
(0 /'//
o
i..
' \~M - \ _~ '/~?
"'~_
I
o
cn
~ 5~-~/Ji
o
I.
, .l,\\
\~7
.
Ott,,,
OCR for page 21
I
o
cn
.'''"''.
'i,--
.~: )~ .~. ,~JU .,J5/l
'I'//
/~/#
I::
LN ~ -. //lll\; I I
//,
o ~
._
'_=
To ."
._ ~
4= d
~ W
·~=
O ~
I_, ._
3
R
° ~ ~
Ad I
o ~ o
d o
~ · - .=
_ ED
o ~
~ ~ I
- I, ·-
$-
~, :^ °
r3
W ~ ~
o
d
I,, ._ ~
P4 ~ ~
· _ en .
~ O ~
o _.
d ~ c'
,=
p d o
4
Id '',
~ ~ 3
is O .m
21
OCR for page 22
22
ROBERT T. WATSON
the longest. The ozone hole itself did not fully disappear until late
November or early December, 1987. This long-lasting minimum may
have had significant consequences for the ecosystems in the antarctic
region. The solar elevation angle is comparatively low by October,
when the hole was at its deepest, but is much higher in November,
when the ultraviolet (UV) effect might be stronger at the surface.
The ozone amount was also the lowest on record at all latitudes
south of 60°S latitude in 1987. E urthermore, the occurrence of strong
depletion was a year-Ion" phenomenon south of 60°S and was not
confined to the spring season as in preceding years, although the
greatest depletion occurred during the Southern Hemisphere spring.
Therefore it is no longer a relatively local, isolated effect (Figure 4-3~.
For 1987, the measurements show little in the way of an ozone
hole as late as August 17. After that, the ozone hole developed
rapidly, especially after September 5, so that by October 5, the
ozone over the middle of Antarctica had dropped from 320 Dob-
son units (DU) to 120 DU. The monthly average October mean of
ozone decreased from about 300 DU in 1979 to 120 DU in 1987
over the middle of Antarctica. Additionally, the amount of ozone
in the horseshoe-shaped maximum that extends out to at least 60°S
decreased by around 100 DU in 1987, compared to 1979.
Two ozonesondes were obtained on October 6 and 9, 1987, at
the U.S. Palmer Peninsula station (64°S latitude). This station is
located near the edge of the region of low ozone. On October 6, when
the edge of the strongly depleted region was poleward of the Paimer
station, the ozone showed a fairly normal vertical profile. Three days
later, the edge of the chemically disturbed and depleted region moved
northward past the station, and the profile then showed a decrease
of around 95 percent between 15 and 20 km. Other ozonesonde data
from the South Pole, McMurdo, and Halley Bay stations, stations
that were continually in the polar vortex region of depletion, show
an almost complete disappearance of ozone after October 5. Hence,
the ozone hole was a continent-wide phenomenon extending out to
around the latitude of the Palmer station, where a steep horizontal
gradient of ozone exited.
A series of Nimbus 7 Total Ozone Mapping Spectrometer
(TOMS) satellite images shows some recovery of ozone in the antarc-
tic hole by November 15, 1987, but the ozone amount remained below
175 DU over most of the continent (Figure 4-4a,b). By November 29,
the minimum had moved from the polar region to over the Weddell
Sea, surrounded by a large region of less than 200 DU. By about
OCR for page 23
o
~-- 1 1 1 1 1 1 1 ~°
J
l
ll
1 1
oo 0
cD ~
~ o
~ z
cM
1
'''t
) o
o c~cM ~
(UJ~) sanllllv
~o
0 0
J
\
6 0
~D
C~
,: \t\ 1
,
' N"~.
I '- ~
~ff
LO o Ln O
CO CO CM CM
(w~) 3anllllv
O Ln
llJ
tr:
cn
cn
llJ
~:
CL
J
~:
-
6
Q
o 111
z
o
N
o
o
o
CM
-
O ~
CD
cn
LL
O ~
O J
~ ,<
-
6
o llJ
o
o
d
o
._
4=
C~
4=
U2
o
p
o
*
U]
d
o
CO
o
~q
o
v
·_'
a,
o
a,
~0
d
._
cn
a)
o
~q
o
o
~q
o
s~
-
._
4=
c~
~ -
23
OCR for page 24
24
90
70
50
30
llJ
~ 1 0
-
6 -10
-30
-50
-70
-90
ROBERT T. WATSON
v&~"2 ;;'i';'';';'''-;';1 -- ~7
1' '. :'.. '.... 1. ,.'. ' :. ', it'. : ~ ~ 1 ~ ~ ~ o 1
...... .: .~ I ~ ~ l
~7~ -~47(
Q
_- ~
-6
J 03-
~o~
i
(
/
--4 ~\~ ~-it/ 1
,/-S ~°~ 1
. .- :::::::::::: ::::::::::: :::::::::::::::::: ~ ~ ~ {ADA=_ ~ \ \ ~
~ /2~) t;~, 1- 1 - ~1\~1~1111
J F M A M J J A S O N D
MONTH
FIGURE 4-3 Percentage total column ozone changes, (1986-1987) minus (1979-
1980), from the TOMS instrument as a function of latitude and month. Shaded
areas are regions of no data during the polar night.
December 5, the hole had filled considerably, with a minimum of
about 250 DU located well out over the Weddell Sea not far from
the southern tip of South America. The relatively high sun angle of
November and December coupled with this shift in the location of
the ozone minimum likely resulted in a significant UV impact on the
aquatic ecosystem of that region.
Further comparison of 1987 with earlier years indicates a pro-
gressively more rapid decrease of ozone during September in the later
years. This fact is in agreement with a chemical hypothesis that, as
more chlorine is- added to the system, the rate of seasonal ozone
decrease becomes greater.
OCR for page 25
OZONE DEPLETION: ANTARCTIC PROCESSES
25
A comparison of 1986 plus 1987 TOMS data minus 1979 plus
1980 data (2-year averages are used to remove possible effects of the
quasi-biennial oscilIation) shows that at least a 5 percent decrease
in ozone occurred at all seasons south of about 50°S latitude. The
satellite data have been corrected for drift relative to the ground-
based stations and are believed to be correct to within ~ percent, but
possibly 2 percent in the immediate South Polar region.
TOMS data from the Northern Hemisphere show a decrease in
ozone from 1979 through 1985. This is consistent with an increase in
trace gases, primarily chIorofluorocarbons (CFCs), and a decline in
solar output (solar maximum in 197~1980, minimum in 1985-1986~.
From 1985 to 1987, the ozone curve flattened or increased slightly,
consistent with increased solar output after 1985 that countered
the effects of increasing trace gases. In the Southern Hemisphere,
however, the data show a continued decrease after the 1985 solar
minimum (Figure 4-5~. This is consistent with the concept that the
antarctic ozone hole phenomenon causes a dilution effect throughout
much of the Southern Hemisphere.
We now turn to the question of what is causing the antarctic
ozone hole. The unique meteorology of the antarctic region in the
winter and spring seasons results in the development of a strong
polar vortex that consists of an air mass that ~ largely isolated
from air farther north. Within the vortex, temperatures become cold
enough to form stratospheric ice crystals. The ice crystab then
allow unusual chemical interactions among nitrogen, hydrogen, and
chlorine atoms. The weight of observational evidence indicates that
the chlorine is primarily responsible for the ozone hole. Without
chlorine in the antarctic stratosphere, there would be no ozone hole.
(Here "hole" refers to a substantial reduction below the naturally
occurring concentration of ozone over Antarctica.)
The relevant chemical reactions occur within the polar vortex.
The vortex is not a uniform cylinder but has a shape that varies
with altitude and is strongest and most isolated above the 40~K
isentropic surface, around 15 km and above. Below 15 km, there
is considerably more exchange of air between the midIatitudes and
the polar regions. Hence, it is easier to explain behavior above
15 km in terms of chemistry than below 15 km, where atmospheric
dynamics has an important role. Behavior below 15 km is still largely
unexplained and is a matter of active research.
The National Aeronautics and Space Administration (NASA),
the National Oceanic ant] Atmospheric Administration (NOAA),
OCR for page 26
26
ROBERT T. WATSON
8/1 7/87
.
9/ 5/87
, · - ~ _
"I'''"
10/ 5/87
, - , _ - ·,
, I---- ~
,"'~/~''\'t'.
i....
)QO~
I,,
FIGURE 4-4a Daily Southern Hemisphere maps (polar orthographic projec
tion) of total ozone measured by TOMS for the indicated dates. Period shown
includes the formation of the ozone minimum in 1987.
the Chemical Manufacturers Association, and the National Science
Foundation collaborated on ground-based expeditions to McMurdo
in both 1986 and 1987 and provided extremely important results.
These included observations of chlorine dioxide by Susan Solomon,
which served as a very good indicator of perturbed chlorine chemistry,
as well as measurements of very low nitrogen dioxide, showing that
the atmosphere appears to be denitrified.
~ participated in an aircraft expedition that was based in Puntas
Arenas, Chile. We flew a DC-8 all over the antarctic continent during
a 6-week period. We used remote sensors looking upward to obtain
total column measurements of a wide variety of gases. We had a UV
OCR for page 27
OZONE DEPLETION: ANTARCTIC PROCESSES
1 1/15/87
, ;-, - ~
I,
1 o/ 1 s/87
I''
1 1/29/87 1 2/ 5/87
, · . . - · · · .
_ ~
--- ! _ --'
27
FIGURE 4-4b Daily Southern Hemisphere maps (polar orthographic projec-
tion) of total ozone measured by TOMS for the indicated dates. Period shown
includes the breakup of the ozone minimum in 1987.
spectrometer similar to the one that Susan Solomon took to the ice
to look at chlorine dioxide and bromine monoxide (BrO). We had a
lidar instrument to look at both ozone and the aerosols. The ER-2
aircraft flew as-high as 18.5 km in geometric altitude and measured
several quantities: bromine and chlorine monoxide (James Anderson,
Harvard University) to determine if the chIorine-bromine chemistry
was perturbed, ozone (Walt Starr, NASA, and Mike Proffit, NOAA),
total amount of nitrogen compounds (NOy) in the atmosphere to
determine whether they were enhanced or depleted (David Fahey,
NOAA), whole air samples for CFCs, methane (CH4), and nitrous
oxide (N2O) (Leroy Heidt, National Center for Atmospheric Research
OCR for page 28
28
6
4
A
~2
L,J
it_
A
o
2
c:,
ROBERT T. WATSON
1 1 1 1 1 1 1 1
l
.
· ,,; I.,
A.!, by. ';
_ ~ an'
.. a. .
~ I1. '' ,';
'`~;'':; ijIl''
1 :
-2 _
- 4
NTD
ad.:
6
An-_
- - ~ I;: Or ~
· I\,'
_.~ ,., . ~ ~
I:
SLOPE 7 YEARS' -0.40 % per year
SLOPE 9 YEARS- -0.32 ~ per year
_
, :
,. ,
1 1 1 1 1 1 1
- _ ~; t~ ~
or,, ~ w.-.;: _
. ; I`.;
.u .
. . ~ .
7 B 79 80 81 82 83
YEAR
84 85 86 87 88
FIGURE 4-5 Total column ozone integrated from 53°S to 53°N latitude, from
1979 to 1987, as determined from TOMS data, shown as percent deviation from
an arbitrary reference level.
(NCAR)) to determine rising or sinking motions in the air mass, and
a large number of aerosol measurements, as well as a system to
measure the temperature lapse rate (Bruce Gary, Jet Propulsion
Laboratory (IMPLY.
The data from this ensemble of instruments are being used to
test the various hypotheses that have been proposed. These theories
include the solar theory, whereby periodically the amount of nitrogen
compounds is enhanced. These enhanced levels can catalytically
destroy ozone in the lower stratosphere. This theory, if correct,
implies that levels of oxides of nitrogen should be elevated; besides,
the ozone hole should occur cyclically. The data showed that this
theory is completely and utterly wrong. The oxides of nitrogen were
measured as being unusually low. Some other theories that require
an increase in nitrogen compounds are likewise incorrect.
The fluorocarbon-halon theory suggests that there should be a
OCR for page 29
OZONE DEPLETION: ANTARCTIC PROCESSES
29
change in the partitioning of chlorine from the inactive forms of
chlorine, namely hydrochloric acid and chlorine nitrate, into the
active forms of chlorine, namely chlorine atoms and chlorine oxide
radicals. Therefore, Anderson's tests were critical for deterrn~ning
whether the chlorine oxide (and bromine oxide) abundances were
enhanced.
Another theory, advanced by K.-K. Tung, requires a change
from downward to upward motions over Antarctica in association
with other circulation changes. If this is correct, one should see
enhanced levels of tropospheric trace gases such as nitrous oxide and
methane in the lower and middle stratosphere. Therefore, the Heidt
measurements were critical for this purpose.
The ER-2 aircraft could not climb higher than 18.5 km because
of the very cold, dense atmosphere and the need to carry a lot of
fuel for safety reasons. Also, it did not range farther south than
72°S latitude, about midway down the Palmer Peninsula. Therefore,
many of the measurements were made close to the inside edge of the
polar vortex. It would have been scientifically desirable to have flown
higher and farther southward in the vortex. In any event, our flights
from Puntas Arenas to 72°S and back were useful for comparing
conditions just inside the vortex to those outside.
One of our first flights was made on August 23, 1987. Water
vapor dropped from about 3 ppm outside the vortex to about half
this value inside, indicating that the atmosphere inside the vortex
was dehydrated. Ozone changes were only slight across the vortex
boundary. However, the abundance of the chlorine monoxide radical
(ClO) increased from about 10 parts per trillion by volume to about
500 parts per trillion. The nitrogen compounds (except for nitrous
oxide) dropped from 8 to 10 parts per billion by volume (ppbv) to
only 1.5 to 2 ppbv. Thus, the vortex atmosphere on this date was
dehydrated, denitrified, and highly enriched in chlorine oxides, but
with little effect on ozone levels.
By the end of our mission on September 22, the polar vortex
atmosphere was still dehydrated and denitrified, the chlorine oxides
had increased to about 1 ppbv, and the ozone concentration had
dropped to less than half of its value outside the vortex. Therefore, it
appears that a significant amount of time, approximately 1 month, is
required for the chlorine oxides to destroy ozone. The concentrations
of bromine oxides within the vortex were in the range of 3 to 5 parts
per trillion on all flights. These small concentrations, compared to
the chlorine oxide concentrations, imply that whereas chlorine oxides
OCR for page 30
30
ROBERT T. WATSON
play a major role in destroying stratospheric ozone by the ClO-ClO
mechanism, the proposed ClO-BrO mechanism for destroying ozone
is of relatively minor importance, accounting for less than 10 percent
of the total ozone depletion.
The DC-8 flew nearly to the South Pole and obtained many bulk
column measurements. These show that, in crossing inside the po-
lar vortex, nitrogen dioxide drops off significantly, nitric acid peaks
around 70°S and then drops, chlorine nitrate reaches a maximum at
the edge of the vortex, and hydrochloric acid fails off significantly
within the vortex. The JPL and NCAR measurements are in excel-
lent agreement with each other. The chlorine nitrate results from the
presence of both ClO and nitrogen dioxide; hence it maximizes near
the vortex boundary, where ClO is increasing rapidly but nitrogen
dioxide is decreasing rapidly.
Calculations by Anderson show that ozone depletion at the 410-
and 420-K isentropic surfaces between August 23 and September 22
can be almost entirely explained by the amount of ClO present if
one assumes that the ClO-ClO mechanism is effective. At the 36~K
surface, the calculated ozone loss is somewhat less than the observed
loss. At least we can say that above about the 40~K level, there
does seem to be enough ClO to explain the observed ozone loss.
A number of measurements were obtained of particles, ranging
from sulfuric acid particles (less than 0.1 micron) through nitric acid
tribydrate (0.5 micron) to relatively large ice crystals (2 to 3 mi-
crons). These measurements tend to support the current hypothesis
of how chlorine oxide concentrations become enhanced in the polar
stratosphere.
Measurements of nitrous oxide and methane obtained at an al-
titude of about 18 km and near the inner edge of the vortex did not
give any evidence of upward vertical motions. Since the ER-2 flights
did not penetrate poleward of 72°S, one cannot make a blanket state-
ment that upward motions did not occur anywhere within the polar
vortex region. However, these data, in conjunction with the other
data, suggest that upward vertical motions do not play an important
role in the ozone depletion process.
In summary, chlorine ~ intimately involved in the depletion of
ozone, and most of the ozone lo" can be explained quantitatively
on a chemical bash. All theories, especially the solar theory, that
require elevated concentrations of oxides of nitrogen are incorrect,
and the apparent absence of large-scale upward motions suggests
that the K.-K. Tong type of theory is wrong as well.
OCR for page 31
OZONE DEPLETION: ANTARCTIC PROCESSES
31
(In answer to a question about ice crystals and temperature):
Since 1984, there has been an increase in the persistence of polar
stratospheric clouds (PSCs) at 16, 18, and 20 km, with the PSCs
persisting through October. This is possibly consistent with the
temperature getting colder through October. We have looked at the
temperature record, and there is no evidence for a change in strato-
spheric temperature from 1979 to 1987 in August or September, when
the hole forms, but the stratosphere appears to be about 8°C colder
in October and November at the 100 mb level over Antarctica. This
implies that the temperature change is a result of ozone depletion
rather than a cause of it. Since it is colder in October than formerly,
PSCs seem to be persisting longer now.
Question: What would be the first signs of damage to the biota
in Antarctica from increased UV radiation?
Answer: Presumably, the first sign would be a die-off of the
phytoplankton and then the krill in the surrounding waters. Un-
fortunately, there are no long-term records of the phytoplankton
population over the last 20 to 40 years, so a good comparison cannot
be made. Laboratory studies suggest that enhanced leveb of UV
would be quite catastrophic to the phytoplankton and krill life in the
region, but such measurements may not properly represent how the
natural system works.
Question: How effective will the Montreal Protocol be in reduc-
ing the severity of the antarctic ozone hole?
Answer: There was no antarctic ozone hole from 1965 to 1970
with chlorine at a concentration of 2 ppbv. There is a huge antarctic
ozone hole today with chlorine at 3 ppbv, and there is evidence
that the ozone hole is enlarging and spreading. Under the Montreal
Protocol, the concentration of chlorine will certainly rise to at least
5 ppbv and possibly to as high as 8 or 9 ppbv. Therefore, ~ believe that
the protocol will do absolutely nothing to protect the antarctic region.
The ozone hole may get worse, and there will be more hemispheric,
and possibly global, ramifications. If policymakers believe that we
should protect ozone over Antarctica, then it is quite clear that the
Montreal Protsco! will have to be revised and the measures made
much more stringent.
Question: How is the edge of the southern polar vortex defined?
Answer: ~ have heard two different definitions of the vortex.
They are based on the location of the steepest potential vorticity
gradient and the location of the jet of maximum winds, which is
about 5 to 8° of latitude wide. Some define the inner edge of the
OCR for page 32
l
32
ROBERT T. WATSON
wind jet an the limit of the vortex air. This appeared to correspond
closely to the boundary of the chemically perturbed region, and we
usually encountered it around 68 to 70°S latitude. The alternate
definition is the equatorward side of the belt of maximum winds,
which extends to about 60°S. This definition is not consistent with
our measurements. A related question is: How fast does air move
across the polar vortex boundary? Several scientists are looking at
this question.
Question: At lower latitudes where there are no stratospheric
ice crystals, is the role of ice mimicked by other aerosols such as
volcanic dust?
Answer: The next two papers address that question. The evi-
dence that ~ have seen from laboratory studies indicates that liquid
sulfuric acid particles will not provide such an efficient surface for
heterogeneous chemistry, partly because the rate of reaction proceeds
more slowly compared to that with ice crystals, and partly because
the typical density of the sulfuric acid aerosols is less than that for
ice crystals over Antarctica. Therefore, the likelihood of significant
heterogeneous chemistry appears to be less at lower latitudes.
REFERENCE
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.
Representative terms from entire chapter:
polar vortex
