|
Items in cart [0] |
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 56
7
Free Radicals in the
Earth's Atmosphere:
Measurement and Interpretation
JAMES G. ANDERSON
Harvard University
This talk deals with the more mechanistic aspects of the ozone
depletion problem and includes a discussion of three areas. The first
area deals with middle and upper stratosphere homogeneous gas-
phase catalysis. ~ will review some recent results that deal with the
fundamental chemical structure of the stratosphere and our under-
standing of the way these processes occur in the actual atmosphere.
The second area covers the topic already discussed by Robert Wat-
son: antarctic ozone depletion. Finally, ~ will report some results
obtained in early 1988 regarding aircraft flights in the Northern
Hemisphere that reached 61°N latitude.
Starting with the first topic, I will review our understanding
of the gas-phase processes that take place. Molina and Rowland
(1974) suggested that the couplet, chlorine atoms plus ozone react-
ing to form chlorine monoxide (ClO) plus oxygen followed by chlorine
monoxide reacting with oxygen atoms to reform chlorine atoms plus
oxygen in a catalytic cycle, constitutes a loss process for ozone in
the stratosphere. The rate-limiting step in that couplet is the slower
of the two reactions that controls the frequency of closure of the
catalytic cycle. A definition of the distribution of chlorine monox-
ide at micIlatitudes is now complete between the tropopause and
the stratopause. Combined measurements from aircraft and balloon-
borne instruments show that the top of the profile varies, we believe
in response to methane. This brings up the very crucial coupling
56
OCR for page 57
FREE RADICALS
57
between the catalytic cycle and the ability of the atmosphere to
eliver molecules from the tropopause region to the upper strato-
sphere. If circulation slows down and methane is not delivered to the
upper stratosphere, perhaps because of distorted heating patterns,
the efficiency of these mechanisms will change dramatically.
In the lower stratosphere, on the other hand, we find through
observed diurnal variations of the chlorine monoxide radical that
there is coupling between the nitrogen and chlorine families. As
Daniel Albritton has previously indicated, the presence of chlorine
in the atmosphere extracts radicals of the nitrogen family from the
atmosphere. In the normal atmosphere, unpolluted by chlorine rad-
icals, nitrogen oxides account for some 70 to 75 percent of globally
integrated ozone destruction. Thus in the midIatitudes there has
been somewhat of a canceling eject; this has had an impact on the
history of ozone studies. To properly treat the entire stratospheric
ozone system requires consideration of some 200 gas-phase reactions
coupled with transport processes. If we can observe free radicals in
situ, we can simplify the problem considerably. We can talk about
the effect of increases in chiorofluorocarbons (CFCs) continuing into
the next century. If other factors are held constant, the mixing ratio
of the sum of atomic chlorine and chlorine monoxide will be raised as
the CFCs reach the stratosphere, thereby raising the efficiency of the
rate of chlorine radical catalysis. The coupling with the nitrogen sys-
tem occurs when the chlorine monoxide plus nitrogen dioxide (NO2)
reaction occurs, forming chlorine nitrate (ClONO2~. The partition-
ing between hydrogen chloride and chlorine radicals is controlled in a
positive sense by the hydroxy! radical (OH). The more OH that exists
in the upper atmosphere, the higher will be the fractional amount of
total chlorine tied up in the chlorine radical form. The reverse is true
in the nitrogen system, in which OH reacting with nitrogen dioxide
forms nitric acid. The OH concentration in a highly simplified form
is established by a balance between excited oxygen atoms, O(iD), re-
acting with water and (at 30 km and below) the catalytic conversion
of OH back to water, with nitric acid as the catalytic agent.
So we can, in turn, examine the processes that we believe will
dominate into the next century by starting with increased CFCs.
The CFCs cause chlorine monoxide to increase, and the chlorine
monoxide in turn controls the nitrogen dioxide concentration through
the ClO plus NO2 reaction. The nitric acid concentration suffers as
a result. Reductions in the nitric acid concentration drive up the
OH concentration, leading to a positive feedback. The feedback
OCR for page 58
l
58
JAMES G. ANDERSON
is established first through the OH plus HC! reaction, and second
through the diminished nitric oxide (NO) concentration, which blocks
the channel from chlorine monoxide back to chlorine.
As we look into the future, we have to understand the coupling
between chlorine and nitrogen and also the processes that control
the OH concentration. In February 1988, we flew a series of aircraft
experiments with a chlorine monoxide instrument on board (see Fig-
ure 7-1) to map out the diurnal behavior of chlorine monoxide at
midIatitudes over California. The detection threshold of the instru-
ment is such that chlorine monoxide at 1 part per trillion is easily
detectable. We followed the build up of chlorine monoxide at sun-
rise over 2 orders of magnitude. We obtained similar measurements
in the evening, allowing us to identify the coupling reactions. The
first analysis with a mode! shows that, in fact, the dominant process
involved chlorine nitrate, from the reaction of nitrogen dioxide with
chlorine monoxide, and the photolysis of chlorine nitrate back into
free radical form. That tells us that the chlorine and nitrogen fami-
lies are indeed coupled, a fact that is crucial as the system is loaded
with increasing amounts of CFCs.
We turn to the next question, that of the OH radicals, and history
is again an important teacher. The sequence of integrated column
ozone depletion predictions, which began in 1976, traces a clear
reduction in column concentration of ozone after the introduction
of chlorine reactions in the prediction models. After 1978, when
agreement on the reactions used in models was reached, we see a
profound effect on the mode! results of increasing the reaction rate
constant of NO + HO2 ~ NO2 + OH, which increased the model-
calculated column integral of ozone depletion by more than a factor
of three. Subsequent refinements, such as the introduction of OH loss
terms (nitric acid plus OH and pernitric acid plus OH), had a further
impact on predicted column ozone depletion. The mode! predictions
can be checked by observing the concentration of OH in the lower
stratosphere.
To carry out that check, we flew an experiment during the sum-
mer of 1987 on a high-altitude (to 42.7 km) research balloon, which
used a high-repetition-rate copper vapor laser system that is con-
trolled by command from the ground through computer algorithms.
These allow tuning in a controlled way on a balloon platform, thus
permitting laboratory-quality experiments from these platforms. For
example, we can superpose the OH spectrum from a plasma dim
charge lamp with the output from the laser, defining precisely which
OCR for page 59
FREE RADICALS
NO INJECTOR
THERMISTOR \ UV MO~TORS PAT DETECTORS
ARRAYS ~ /~[-~ ORIFICE PLATE
L: ~
I NL E
DUCT
59
:3 ~r r r ~ (A -~
n n r
3 ~1
RE SONANCE
f LUORESCENCE
LIGHT SOURCES
BYPASS fLOW
TAROT TLE
VALVE S
:?1
NITRIC OXIt NE/AIR COMPUTER CONTROL
CONTROL DATA COLLECTION
AND SEWAGE AND STORAGE
-PI TOT ~ USE ~
EXIT
DUCT
FIGURE 7-1 Schematic of the instrument developed for the ER-2 aircraft to
observe chlorine monoxide (ClO) and bromine monoxide (BrO) in situ within
the antarctic polar vortex. (Adapted from Brune et al., 1989.)
rovibronic transition is being pumped during the atmospheric mea-
surement. This is simply one example of a number of cross-checks
that allow us to believe what we are observing. The outcome of the
measurement is as follows: if we look at the mixing ratio of OH as a
function of altitude in the crucial region between 22 and 30 km, we
can use a mode] by Ko, Tung, Weisenstein, arid Sze (1985) that in-
cludes nitric and pernitric acid. Figure 7-2 shows that observed OH
agrees closely with the model-calculated distribution in the lower
stratosphere. This implies that under present-day conditions, one
would expect less change in ozone for a given amount of chlorine.
But that is not true because OH, we now believe, is controlled by
nitric and pernitric acid, which means it is susceptible to linkage
through the chlorine-induced removal of nitrogen oxides. So, while
the OH concentration is low, it is susceptible to change with increas-
ing mixing ratios of total chlorine in the atmosphere. In conclusion,
many of the basic elements of our picture of midiatitude catalytic
destruction of ozone are now coming into focus.
In the meantime, we have the antarctic ozone hole, which now
OCR for page 60
l
60
JAMES G. ANDERSON
46
42
38 -
y
34
30
26-
22 - /' """1 ~l ~ 1,,,,
1 10 50
M I X I NO RATIO
· ANDERSON
1 - 12 - 7 6
4-26-77
4-20-77
7- 14-77
· HEAPS
10 - 27-83
10-80
~ ,1~ .
. · . .
7 ~
,1
~-
·A
.
a
~:~&
/ A&
·/
.
1 ' """1
200
PPT
FIGURE 7-2 Composite of in situ hydroxyl (OH) measurements by Heaps and
McGee (1985), Anderson (1976), and this work (with approximate error bars
shown). The calculation (Ko et al., 1985) shown by the solid curve corresponds
to simulation of the present-day atmosphere for July conditions at 30°N. Water
vapor is fixed in the model from 4.5 ppmv in the lower stratosphere to 6.0 ppmv
in the upper stratosphere.
is receiving significantly more focused attention. As Watson men-
tioned, their high-altitude ER-2 aircraft flew from the southern tip
of Chile (Pumas Arenas), ascended rapidly to 65,000 to 67,000 feet
(about 20 km), and then flew approximately horizontally to 72°S
latitucle. There, the aircraft descended to 45,000 feet (13.7 km)
and then climbed back up and returned to Punt a Arenas. The first
penetration that descent on August 23, 1987 into the polar vortex
OCR for page 61
FREE RADICALS
61
showed the chlorine monoxide concentration there to be increased by
a factor of 100 over the concentration measured above Puntas Arenas
and considered to be representative of midIatitude concentrations at
that time. The threshold of the instrument was 0.1 part per trillion,
and the response time was about 0.25 s; therefore, our confidence
in the measured profiles is very high. At 700 parts per trillion, the
measured amount of stratospheric chlorine monoxide corresponds to
approximately 200 times normal levels. (In the laboratory, where
we study reactions of these chlorine monoxide molecules, we use a
concentration of only about 5 parts per trillion to look at the kinetics
of the species that exist over Antarctica.)
The question is, what happened to the ozone over Antarctica?
On August 23, 1987, the ozone concentration was still unperturbed.
Whatever took place on the stratospheric ice crystal structures dur-
ing the dark winter months had little effect on the ozone concen-
tration. But after August 23, the sun penetrated the South Polar
vortex region for a small but increasing number of hours per day.
Three weeks later, on September 16, the aircraft profile through
the boundary of the polar vortex still showed a dramatic increase
in chlorine monoxide on the poleward side. Conversely, the ozone
level had dropped markedly on the poleward side. Thus, there was
a strong anticorrelation between chlorine monoxide and ozone levels
that had cleveloped during the 3-week period. These results are sum-
marized in Figure 7-3. The anticorrelation, by itself, does not prove
anything, but the change from an initial condition in which ozone
was unaffected by chlorine monoxide on August 23 to the strong
negative correlation on September 16 does indicate what kind of pro-
cess must be occurring. Actually, the behavior of the geochemical
system within the polar vortex is about as simple as any likely to be
encountered in the atmosphere.
Given 500 times the normal free radical concentration and the
strong anticorrelation between ozone and chlorine monoxide that
developed in 3 weeks, what are the fundamental mechanisms? There
are three possibilities. The one ~ will discuss in detail involves the
formation of a chlorine monoxide dimer. There are two forms: one
is a peroxide structure with chlorine atoms at a dihedral angle of
90°; the other is a ClO-ClO structure. Molina and Molina (1987)
have suggested the symmetrical (peroxy) dimer as the key structure
in the mechanism that may be important in the antarctic region. It
replaces the O plus ClO rate-limiting step with a pressure-dependent
dimerization step that is followed by photodissociation of that dimer
OCR for page 62
62
1 c,
(gOI S ~ INn) Old ELIXIR ~NOZO
53 ~ ~ US ~
Cal ~ Cal
_ _ o o
. . · · · ~· · ~
o
In
CD O
ADZ
O
o
-
C51
~ A
~ o
(8 Ot SIINn) OllV~ ONIXIW OlD
(g O! S l INn) O! ~ V~ ELIXIR 3~ZO
CD ~
. hi. . . ~
lo
. . . . ~
0
Cal
ADZ
~0
~o
r · · ~- ~r
O C~
C~
J
J
_ I~ (J)
CD O
O}
~ 2
,_\ - CD
LL
- O
_ ~n ~
J
C~
(ol Ot SllNfl) 011~8 ONIXIW 010
o
-
4=
.
-
oo
-
CO
L
,Q
R
U]
O
C~
4= ~
-
-
4.
_ ~
o
41) eq
a
4
O ~
4= %_
~ _ _
~ ~ ~V
3 ~ _
4= sq _'
,= ~ ~
p. ~ C5)
._
d C~
R
- sO" ~
- , eq
_ _ ~A
o
~ V ~
-~ _
4.
t_ ~ O
0= X
R O ~
.0 ~ O
O O~
m.,, O
O
~A~ ~ ~
~ O
~ O
o
4=
4=
OCR for page 63
FREE RADICALS
63
to form C] + ClOO. The other suggestions revolve around chlorine
and bromine and around chlorine and hydrogen.
We can analyze the effect of just the ClO + ClO _ ClOOC} step
by simply looking at the continuity equation for ozone. There are two
chemical contributions and a dynamical contribution. It is possible
to set upper limits on the flux divergence terms for ozone within the
vortex. We can tell that approximately 5 million molecules of ozone
are destroyed per cubic centimeter per second by chlorine monoxide
at midday. We can then calculate, in a very simple way, the expected
decrease in ozone because of the inherent symmetry of the problem.
We have air moving in a circumpolar pattern that deviates somewhat
in latitude, but the time constant of that deviation is small compared
to the time constant of ozone changes. So, we can take slices into the
rotating polar vortex "disk" and sample the chlorine monoxide and
ozone concentrations simultaneously. The kinetics are determined by
calculating the time rate of change of ozone bred on the observed
chlorine monoxide concentrations. We correct for the shape of the
ClO radical diurnal concentration variation and integrate over each
24-hour period, normalizing to the observed ClO concentration.
We have mapped out the temporal dependence of chlorine mon-
oxide on several potential temperature (isentropic) surfaces as a
function of latitude. (Potential temperature surfaces were used to
minimize the effects of atmospheric fluctuations.) We can calculate
the incremental ozone that is removed each day, based on the in
situ observation of ClO. We can take the rate constants for the
rate-limiting step in the dimerization cycle of ClO-ClO, put in the
observed ClO concentration, and compare the rate of that catalytic
cycle with the observed removal of ozone. This procedure may not
have the precision of a laboratory experiment, but it is very indicative
of the behavior of the system. In fact, for a continental-scale chemical
problem, the results are very straightforward. Figure 7-4 summarizes
the results of this comparison.
~ will pass quickly over the bromine contribution mentioned
earlier by Watson. Bromine ~ present in the vortex at concentrations
of about S parts per trillion. That corresponds to some 5 to 10
percent of the removal process due to chlorine. Chlorine was clearly
the important removal agent in 1987 and will remain so for many
years.
~ will finish by discussing our latest results for the Northern
Hemisphere. Again, Brune was the major collaborator here. If we
look at the wintertime latitude-versus-altitude dependence of chIo-
rine monoxide as far north as 61°N latitude and as high as 20 km,
OCR for page 64
l
64
7
6.5
6
E5.5
o5
-
cn 4.5
4
a, 3. 5
z
o
o 3
2.5
2
JAMES G. ANDERSON
~-
. e=420
, . , . , .
8/20 8/25 8/30
9/4 9/9 9/14 9/19 9/24
DATE
FIGURE 7-4 Comparison between calculated and observed rates of ozone
loss at a fixed potential temperature (which eliminates adiabatic variations)
throughout the course of the Airborne Antarctic Ozone Experiment (Anderson
et al., 1989b).
we note that chlorine monoxide is strongly perturbed from normal
conditions, with 60 parts per trillion at the edge of the polar strato-
spheric jet. Even at 32 to 35°N latitude, the amount is significantly
higher than summer levels. In fact, summer levels are generally ex-
ceeded to some extent poleward of 25°N latitude. If one compares the
recent winter data with mean July data, the difference at 36°N lati-
tude is about a factor of two. The 60 parts per trillion at 61°N is not
nearly as high a concentration as the 1,000 parts per trillion observed
in the Antarctic, but then the Northern Hemisphere measurements
probably did not extend all the way into the polar vortex.
(In answer to a question): ~ think that the lower temperatures in
the absence of sunlight in the Arctic are tying up the oxygenated ni-
trogen in the form of either nitric acid or nitrogen pentoxide (N2O5)
and leaving a large deficiency of nitric oxide and nitrogen dioxide. A
deficiency of nitrogen dioxide has been observed using ground-based
measurements in central Canada. This ~ probably a manifestation
of the gas-phase process that allows chlorine monoxide to increase in
OCR for page 65
FREE RADICALS
65
the absence of nitrogen dioxide. The degree to which the nitrogen
system is perturbed depends on the stability of the polar vortex,
which is clearly greater in the Southern Hemisphere than in the
Northern Hemisphere.
REFERENCES
Anderson, J.G. 1976. The absolute concentration of OH in the earth's strato-
sphere. Geophys. Res. Lett. 3:165-168.
Anderson, J.G., W.H. Brune, and M.J. Proffitt. 1989. Ozone destruction
by chlorine radicals in the antarctic vortex: The spatial and temporal
evolution of C1O - O3 anticorrelation based on in situ ER-2 data. J.
Geophys. Rue., Special Issue on Antarctic Ozone (in press).
Anderson, J.G., W.H. Brune, S.A. Lloyd, W.L. Starr, M. Loewenstein, and J.R.
Podolske. 1989. Kinetics of O3 destruction by C1O and BrO within the
antarctic vortex: An analysis based on in situ ER-2 data. J. Gleophys.
Res., Special Issue on Antarctic Ozone (in press).
Brune, W.H., J.G. Anderson, and K.R. Chan. 1989. In situ observations of
C1O in the Antarctic: ER-2 aircraft results from 54°S to 72°S latitude. J.
Geophys. Res., Special Issue on Antarctic Ozone (in press).
Heaps, W.S., and T.J. McGee. 1985. Progress in stratospheric hydroxyl
measurement by balloon-borne LIDAR. J. Geophys. Res. 90:7913-7921.
Ko, M.K.W., K.K. Tang, D.K. Weisenstein, and N.D. Sze. 1985. A zonal
mean model of stratospheric tracer transport in isentropic coordinates:
numerical simulations for nitrous oxide and nitric acid. J. Geophys. Res.
90:2313-2329.
Molina, L.T., and M.J. Molina. 1987. Production of chlorine oxide (C12O2)
from the self-reaction of the chlorine oxide (C1O) radical. J. Phys. Chem.
91:433.
Molina, M., and F.S. Rowland. 1974. Stratospheric sink for chlorofluo-
romethanes: chlorine atom catalyzed destruction of ozone. Nature 249:810-
812.
Representative terms from entire chapter:
polar vortex
