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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
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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
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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
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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
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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
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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
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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=
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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,
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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
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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.
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