Science
Stratospheric science undergoing change Sophisticated atmospheric measurements and reaction kinetics studies and better modeling are strengthening stratospheric chemistry
Rudy M. Baum C&EN, San Francisco
Current scientific understanding, expressed in both one- and twodimensional models, indicates that if production of two CFCs—CF2CI2 and CFCI3—were to continue into the future at the rate prevalent in 1977, the steady-state reduction in total global ozone, in the absence of other perturbations, could be between 5 and 9%. Comparable results from models prevalent in 1979 ranged from 15 to 18%. From the 1982 National Academy of Sciences report, "Causes and Effects of Stratospheric Ozone Reduction: An Update." This revision of the NAS prediction of potential ozone depletion owing to release of chlorofluorocarbons (CFCs) represents one manifestation of a change in the nature of stratospheric chemistry: The science is outgrowing its crises. Although a handful of scientists have been studying stratospheric chemistry for about 50 years, the discipline came into its own only in the late 1960s and early 1970s as a result of two perceived threats to what has become known as the ozone layer—a region of relatively high ozone concentration between 20 and 40 km above Earth's surface. That ozone absorbs the vast majority of solar ultraviolet radiation and, as such, protects living organisms from the damage it could do. On the basis of what was essentially paper chemistry, atmospheric scientists realized that two of society's activities could alter the amount of ozone in the stratosphere. One was the plan to build a fleet of about 500 supersonic transport jet airplanes. Such aircraft fly in the stratosphere
and their engines release large effect. As indicated in the NAS reamounts of nitrogen oxides that cat- port, the predicted ozone depletion alytically destroy ozone. The other refers to a steady-state situation. activity was the use of CFCs in aerosol Such a steady state will not occur for cans as propellants and in refrigera- 50 to 100 years for CFCs. For nitrogen tion systems. The SSTs never were oxides—which are increasing for built, more for economic than envi- reasons other than SSTs—the time ronmental reasons, but the possibility scale is as long or longer. that their operation could destroy The crises, rather, were crises of ozone prompted intensive research in information. Quite simply, very little stratospheric chemistry. The con- was known about the trace species troversy about CFCs assumed crisis that paper chemistry predicted could proportions in the mid-1970s and the have serious environmental effects. compounds entered the public con- Scientists were hard pressed to give sciousness and the legislative process, confident, or even realistic, predicas--well-as spurring yet more re- tions about the effects of society's search. activities on stratospheric ozone. If there were perceived crises, they Predictions were important, howwere certainly not crises of immediate ever, because, for the same reasons
Temperature defines regions of the atmosphere The atmosphere is divided into four regions based on the profile of temperature as a function of altitude. Troposphere: the region from the surface of Earth to about 12 km. It is heated from below by solar radiation absorbed at Earth's surface, so its temperature decreases with altitude. Because this leads to cold air masses overlying warmer air masses, the troposphere is unstable and has rapid convective motions. This gives rise to what is commonly referred to as weather and results in rapid vertical mixing of gases that enter the troposphere. The troposphere is the only region where both homogeneous and heterogeneous processes are important in determining chemistry. Stratosphere: the region from about 12 to 50 km. In this region, ozone absorbs solar ultraviolet radiation with wavelengths from about 200 to 300 nm leading to a permanent temperature inversion. In other words, warmer air masses over cooler air masses. This makes the stratosphere very stable toward vertical mixing. Because of this, substances that are injected into the stratosphere tend to remain there. Mésosphère: the region from about 50 to 85 km. In the mésosphère, the very low pressures do not allow enough ozone to be formed from molecular oxygen to maintain the temperature inversion. The temperature structure results in considerable vertical mixing.
Thermosphère: also known as the ionosphere, the region from about 85 km to the top of Earth's atmosphere. Here, ionizing ultraviolet radiation and x-rays (radiation with wavelengths shorter than 120 nm) are absorbed by just about everything. The reactions of the subsequent ions and electrons produce kinetic energy. Temperature increases as a function of altitude because most of such radiation is absorbed at the" higher altitudes.
Ozone concentration, molecules per cc 10 1 0
0
100
1011
200
1012
300
10 1 3
400
500
Temperature, Κ
Sept. 13, 1982 C&EN
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Science Many key stratospheric reactions involve radicals Stratospheric chemistry involves a be wildering number of chemical reactions and catalytic cycles. Current computer models incorporate about 140 reac tions. In terms of predicting the effect on ozone of a given chemical perturbation, the primary question is the balance among various species, which interact extensively. Ozone is formed when ultraviolet ra diation dissociates molecular oxygen:
catalytic cycles that destroy ozone, such as the following:
H0 2 + NO — OH + N0 2
CI + 0 3 — CIO + 0 2
Longer catalytic cycles that result in ozone destruction involve more than one family of radicals. Two examples are the following:
0 2 + ru> — 2 0
0 3 + Ο — 20 2 (net)
HOCI + hi> - • HO + CI
0 + 02 - * 03
OH + 0 3 ~* H0 2 + 0 2
0 3 + 0 3 - * 30 2 (net)
Ultraviolet radiation also can disso ciate ozone:
H0 2 + Ο — OH + 0 2
0 3 + h^ - * 0 2 + 0( 1 D)
0 3 + Ο - * 20 2 (net)
It is this reaction that is responsible for ozone's absorption of ultraviolet radia tion that otherwise would reach Earth's surface. The electronically excited oxygen atom, 0( 1 D), formed in the photodisso ciation of ozone is responsible for the formation of odd nitrogen radicals and odd hydrogen radicals. It reacts with ni trous oxide (N 2 0), which has a number of natural and anthropogenic sources:
Because the atomic oxygen that reacts in the second reaction in each of the above cycles otherwise would have formed ozone, the net effect of each cycle is the destruction of two ozone molecules. The reactive species are removed by reactions that produce stable species that either remain in the stratosphere or are water soluble and can be rained out:
0( 1 D) + N 2 0 — 2NO
CI + CH4 -> HCI + CH3
It also reacts with water vapor: 1
0 3 + Ο — 20 2
(net)
NO + 0 3 -» N0 2 + 0 2 N0 2 -h Ο -> NO + 0 2
CI + H0 2 — HCI + 0 2 N0 2 + OH -> HON02 (nitric acid)
Free atomic chlorine is produced by the photodissociation of compounds with long atmospheric lifetimes such as man-made chlorofluorocarbons or nat urally occurring methyl chloride:
OH + H0 2 — H20 + 0 2 Photodissociation or reaction with other reactive species can reverse these reactions:
CCI 2 F 2 + hp - > CCJF2 + CI
OH + HCI — H20 + CI
CCI3F + ru> - > CCI2F + CI
HON02 + ru> -> OH + N0 2
The odd nitrogen and hydrogen radi cals and atomic chlorine participate in
that the effects would not manifest themselves for many years, it would take many years to correct them if they occurred. As is often the case in such situations, the need for quick answers led to hurried science. Not only were there many questions, but the techniques for answering the questions largely did not exist. Stratospheric science is much calmer today. The latest downward revision of the prediction of ozone depletion due to CFCs is part of it. More important, however, is that techniques have been developed to C&EN Sept. 13, 1982
CI + 0 3 -» CIO + 0 2 CIO + H0 2 - * HOCI + 0 2 OH + 0 3 — H0 2 + 0 2
CI + 0 3 - * CIO 4- 0 2 NO + 0 3 — N0 2 + 0 2
0( D) + H 2 0 - * 2 0 H
CH3CI + hv —> CH 3 + CI
22
CIO + Ο - * CI + 0 2
OH + CO — C0 2 + H
Some reactions simply act to change a radical into another radical in the same family of radicals:
measure many of the important stratospheric compounds and the rates at which they react. A data base has been established, at least ten uously. Models to tie the information together are more sophisticated. The result is that stratospheric science has shifted from what one scientist de scribes as a response mode to a more systematic examination of its meth odology. Essentially, stratospheric science can be broken down into three rela tively simple questions: What are the concentrations of the reactive, trace
CIO + N0 2 — CION02 CION02 + hv — N0 3 4- CI N0 3 + hf — NO + 0 2 0 3 + 0 3 - * 30 2 (net) And so on. To get a sense of the task facing stratospheric scientists, one must remember that these species are present at part-per-million, part-perbillion, and part-per-trillion mixing ratios in the stratosphere. Their concentrations vary as a function of altitude. The reac tions are taking place at the low tem peratures and pressures of the strato sphere. Many of the reactions involve ultraviolet radiation, the intensity of which also varies as a function of alti tude. Because of this, different catalytic cycles and different species dominate the destruction (or production) of ozone at different altitudes. Some of the species have never been observed in the atmosphere, either because they are present at very low concentrations or because suitable techniques have not yet been developed.
constituents of the stratosphere and how did they get there? How fast do they react? What are the overall consequences of the interplay of those reactions? Three distinct, but very comple mentary, lines of research have evolved to answer those questions. One group of scientists has flown a variety of instruments on airplanes, rockets, and the gondolas of balloons and has developed ground-based measurement techniques to try to answer the first. Another group, gasphase kineticists, has harnessed pri-
marily two techniques to study the rates of the many reactions, most of them involving radicals, that affect ozone. Information from both of these groups is used by the third group in computer models of the stratosphere and its chemistry to make predictions such as the one in the NAS report and, more important, to see if the various pieces all fit together. Although the basic questions are straightforward, answering them has not been and still isn't. "The distinctive thing about the stratosphere is that it is just within our scientific capabilities to understand it," says James G. Anderson, professor of atmospheric chemistry at Harvard University. "By understanding it, I mean well enough to predict its subsequent development. It is different from the economy or the weather because the independent variables are definable and observable, particularly the free radicals that drive it." Anderson has been developing insitu methods of measuring stratospheric trace species for about 10 years. Airborne measurement techniques can be divided into two groups: In-situ methods analyze a collected sample of air; long-path methods use the sun as a light source and measure absorption or emission characteristics of a species over about 200 km of atmosphere. Anderson will fly his latest device sometime during the next several weeks. Formally called the reel-down experiment, Anderson generally refers to it as the yo-yo. The device is essentially a large winching platform that will be carried to an altitude of about 45 km by balloon. On the winch's spool is 20 km of nylonwrapped Kevlar filament that can support about 400 lb. A package of in-situ measurement instruments is attached to the filament and lowered through the stratosphere at any desired rate. As it is lowered, it charges battteries on the winching platform to allow the package to be raised back up. "Historically, we have taken these instruments up to the upper stratosphere, dropped them on a parachute, and taken one snapshot of the system," Anderson says. "No matter how hard we try and how much practice we get in the lab running these things on simulated flow systems, it is still an archaic way of doing science." Anderson points out that in the past, anomalous measurements, although they might indicate something important, could not be verified. With his reel-down device, as soon as a measurement is obtained that varies
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from what a ground-based computer predicts should be in that region, he can stop the instrument package and sample several times to verify the observation. The goal is to measure several, related species simultaneously. The "royal flush" of measurement, Anderson says, would be atomic oxygen, ozone, atomic chlorine, chlorine oxide radical (CIO), hydroxyl radical (OH), hydroperoxyl radical (H0 2 ), nitrogen oxide radical (NO), and nitrogen dioxide radical (NO2). A parallel effort in Anderson's lab has been the development of an atomic copper vapor laser system to detect OH and H0 2 . "OH is really the demon," Anderson says. "Although we have made some observations of OH in the middle-upper stratosphere, those observations simply aren't good Anderson: latest device is yo-yo enough to discriminate between different models." The laser will be the heart of an instrument package that will accomplish Anderson's royal flush of measurements but, at least in its present configuration, it will be too heavy to be used on the reel-down experiment. "The tactic is to march along on two parallel tracks—one is the reel-down experiment to tie down corners of the argument, and the other is the heavy-lift experiment that can make all of the measurements but can do it only once." The natural, sometimes significant, variability of the stratosphere has caused difficulties for stratospheric scientists hoping to use measurements to test the predictions of models. The problem is that the model calculations are inherently average numbers whereas observations are not. Anderson suggests that Watson: not sure of quality of data that variability is a blessing now that techniques to measure a number of chemically related species simultaneously are becoming available. The idea is this: "If chlorine extracted from the chlorofluorocarbon molecule is in fact responsible for destroying ozone," Anderson explains, "and if we are able to measure CIO, which is the rate-limiting free radical, simultaneously with ozone in a tiny volume of air, then as CIO concentration goes up, the ozone concentration had better go down in that same volume. That is, watching the covariance between those two species is the only way of proving, apart from any chemical arguments, that the existence of the chlorine radical in a given volume element leads to the destruction of ozone." Anderson also views the atmospheric measurements as a way to Howard: haven't pushed models to limit short-circuit some lengthy computer Sept. 13, 1982 C&EN
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Science calculations. Although he admits that the computer modelers object, he believes that six or seven, rate-limiting, free radical reactions can give a reasonable approximation of the destruction side of the ozone equation. "The model has to work very hard to calculate the distribution of the OH radical," he says, "which is really the pivotal radical. But if you observe the distribution of that radical in the stratosphere, then you have allowed the atmosphere to do the calculation for you." Another example of similar research is being tested by scientists at the National Center for Atmospheric Research (NCAR) and the National Oceanic & Atmospheric Administration (NO A A), both in Boulder, Colo. Brian A. Ridley, a Canadian chemist currently working at NCAR, Mack McFarland, a scientist with NOAA's Aeronomy Laboratory, and coworkers have developed the first instrument for measuring NO and NO2 radicals simultaneously in the stratosphere. The instrument has been tested twice and, according to Ridley, so far the data look promising. "If I want to compare the amount of NO with the amount of NO2, which should be related," Ridley says, "I don't want to make that comparison by somebody measuring NO2 with a long-path optical method and somebody else measuring NO with an insitu instrument. That's been recognized for a long time, but it has been limited by the capabilities to do it." The instrument package the group has developed measures NO, NO2, ozone, and temperature. The heart of it is a chemiluminescence device for measuring both NO and NO2. Ridley has been working with a chemiluminescence device to measure NO alone since 1971. The device measures the chemiluminescence from the reaction of NO with ozone. Nitrogen dioxide (and other nitrogen species of importance in the stratosphere) does not undergo a similar chemiluminescence reaction. The technique used by the current device, based on an idea first proposed by NOAA scientist Dieter Kley, uses an intense light source with wavelengths between 320 and 400 nm to dissociate N 0 2 to form NO. That NO, along with stratospheric NO, then enters the chamber, where it reacts with ozone. Ridley currently is working on a device to measure the photodissociation rate of NO2 in the stratosphere that the scientists hope to add to the instrument package by the end of the year. That package will be able to measure the critical species in the catalytic cycle by which nitro24
C&EN Sept. 13, 1982
are two possible reasons for those differences: One is that the atmosphere is variable; the other is that one or more of the techniques used to make the measurements are inaccurate. "We've spent a lot of time building state-of-the-art instrumentation to make some very difficult measurements, but we have not put a lot of effort into understanding the quality of the data," Watson says. "So I'm acting as a sort of cheerleader with some other people to coordinate this intercomparison. " For the intercomparison, 17 scientists from seven nations will fly experiments. Four balloons will be launched from the National Science Balloon Facility in Palestine, Tex., as close to each other as possible. "We've got seven people measuring HC1, 10 measuring NO2, eight measuring NO, Molina: complex biomolecular reactions and so on. There're always more than five people measuring one of these gen species destroy ozone in the major constituents with a wide range of techniques," Watson says. "We'll stratosphere. The problem, according to Ridley, knock out the excuse we've always is that the chemiluminescence reac- had for a range of measurements, tion produces a broad-band signal. namely atmospheric variability." "When you make these measure- Another set of devices will be flown in ments you have to be very careful that 1983. you can convince yourself, and espeThe data from a previous, similar cially that you can convince others, intercomparison on stratospheric that the signal you get is really just water vapor still are being analyzed, NO. In the past, it has been easy to according to Watson, but the results whip together a chemiluminescence so far indicate a combination of atinstrument and go up and make mospheric variability and some measurements and just say it is an problems with accuracy. Watson upper limit for NO. That was proba- thinks the same may turn out to be bly good enough five years ago, but true of this larger intercomparison. now you really want to know whether Watson says that, so far, he has had it is 10 ppt or 50 ppt. Knowing that it excellent cooperation from all of the is less than 50 ppt isn't good enough scientists involved in the project. "It's anymore." really been rather pleasing. It may not Another program of simultaneous be quite so pleasing when they all see measurements that will be carried out they are different, and that means this fall is aimed at testing the quality that some are right and some are of data more than it is to get infor- wrong. But at the moment they all mation on related species. The Na- have their egos on the line and are tional Aeronautics & Space Admin- willing to risk it." istration is sponsoring, along with the Watson also points out that, alChemical Manufacturers Association though intercomparison is the priand the Commission of the European mary focus of the experiment, with 17 Community, an intercomparison of instruments measuring a number of various long-path techniques for species, the data should help conmeasuring several stratospheric trace strain the model predictions. "If we species. In a sense, it is mundane get good quality data, it easily will be science at its best, and stratospheric the most severe test of the models scientists agree that it has become ever, just because we are making more essential to carry out such experi- measurements at the same time and ments. place than has ever been done be"We're not sure of the quality of fore." our data," says Robert Watson, proWatson points out that, once the gram manager for NASA's strato- quality of the data is understood, it spheric ozone office. The measured can be compared to the theoretical concentrations of most stratospheric predictions of the models. However, species vary outside the stated ex- such comparisons must be apperimental error bars, he says. There proached carefully. "One might
The Surface Contingent
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