Science
Scientists detail chlorofluorocarbon research Controversy continues on the compounds' ability to destroy ozone in the stratosphere, and thus reduce earth's UV shield
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169th ACS NATIONAL MEETING
With study groups being formed, hearings being held, and more articles appearing in the general press, events surrounding the chlorofluorocarbon controversy continue to accumulate. The focus of this controversy settled early this month on Philadelphia, where a Division of Physical Chemistry symposium on chlorine reactions and stratospheric ozone provided details of current research efforts. At a heavily attended press conference, symposium participants reflected the current scientific thinking about the problem. Meanwhile, during the same week, members of a panel formed by the National Research Council to assess stratospheric pollution were announced. Joining Dr. H. S. Gutowsky, director of the school of chemical sciences at the University of Illinois, who had previously been chosen as chairman (C&EN, March 31, page 14), are 11 scientists from the U.S., Canada, the U.K., and West Germany. The panel is charged with assessing the potential effects of stratospheric pollution from natural as well as manmade sources. With funding from the National Aeronautics & Space Administration, National Science Founda-
tion, Environmental Protection Agency, and National Oceanic & Atmospheric Administration, the panel will give principal attention to the largescale use of chlorofluoromethanes and emissions from NASA's upcoming space shuttle. The chlorofluorocarbon problem first surfaced last June when Dr. F. Sherwood Rowland and Dr. Mario Molina announced their theory of ozone destruction by chlorofluoromethane gas. However, the issue didn't draw major public attention until Rowland and Molina presented their findings at the American Chemical Society's national meeting last September in Atlantic City (C&EN, Sept. 23, 1974, page 27). Since then, the issue has been continually in the headlines and it has galvanized studies and research on numerous fronts. The main concern is with the chlorofluoromethanes fluorocarbon-11 (CFC13) and fluorocarbon-12 (CF2CI2)— FC-11 and FC-12. The problem arises with the extreme inertness of the compounds. Used as aerosol propellants and as refrigerants—perhaps best known are the Du Pont Freons—the compounds escape to the lower atmosphere. But because of their inertness, they do not decompose there. Slowly but inexorably they find their way into the stratosphere. There, after absorbing short-wave-length ultraviolet radiation (1900 to 2250 A), they decompose to free radical atomic chlorine and other products. The chlorine radical then acts as a catalyst and attacks ozone through a chain of reactions to form oxygen. That, at least, is the theory. What makes it an issue is that no reduction in the ozone layer by this means has been directly measured as yet, and the
question still remains as to whether chlorine atoms react in the stratosphere as they do in the lab. What makes the issue serious is that the stratospheric ozone layer is a shield against ultraviolet radiation and its reduction could result in increased incidences of skin cancer, weather modification, and agricultural damage. The fluorocarbon industry doesn't deny the seriousness of the problem. Indeed, as Dr. Raymond L. McCarthy, technical director of Du Pont's Freon products division, told the press conference at the ACS meeting, if the theoretical models are correct, there is no question that production of chlorofluoromethanes must end. But faced with the lack of proof, the industry urges caution lest severe economic dislocations be caused needlessly from a ban on the products. The models have been developed, McCarthy says, and the next step now is to make measurements. A number of competent experimenters, he notes, are developing techniques for measuring the CI and CIO species. Faced with the same lack of direct proof, many scientists are calling for a ban on production and use of chlorofluoromethanes. Armed with the theoretical models and experimental results, they contend that every year spent in discussion and additional research on the problem will result in a greater ultimate effect on ozone levels. Even significant errors in current calculations will have little effect on the eventual fate of the ozone layer, Rowland says. He maintains that worldwide manufacture of chlorofluoromethanes already is about 25 times the level that the atmosphere can absorb without significant reduction in average ozone concentration. Even if
Johnston, Rowland, Wofsy, McCarthy, Cicerone, and Kaufman (left to right) at ACS news conference on stratospheric ozone
Rowland: persist through next century
use of the chemicals ceased immediately, the long life of chlorofluoromethanes in the lower atmosphere would mean that the effects of current production wouldn't show up until later this century and would persist through the next century, he says. One problem with gathering direct proof of ozone depletion is that there is a natural variability of ozone in the stratosphere of possibly more than 10%. Thus, Dr. Ralph Cicerone of the University of Michigan told the press conference, if it is necessary to show actual harm, it is necessary to have a lot of harm. Scientists have come to realize in the past several months, he says, that scientific proof will be difficult and may take years. But, he says,
decision-makers do not have much room to hedge their bets. All the theoretical models, he points out, indicate that the effects of chlorofluorocarbons will persist for decades. In a like vein, Dr. Fred Kaufman, a chemical kineticist at the University of Pittsburgh and organizer of the Philadelphia symposium, notes that the problem involves a complex scheme in which it is difficult to measure the final product. But a decision must be made at some point, he says, and there is a large body of knowledge to rely on. Admitting to being bothered by the "man-from-Missouri" approach, he pleads that the show-me attitude not become excessive. "From the kineticist's point of view," he says, "there is nothing on the horizon that would change matters." Kaufman notes that many new experiments and measurements are being made. There is, he says, "a rapid convergence of various laboratories on the truth." His own research group, for example, described at the symposium experiments it is carrying out. Kaufman notes that the catalytic removal of "odd oxygen"—0 3 and O—by chlorine species formed in photolysis of chlorofluorocarbons in the stratosphere involves seven or eight key reactions. In these reactions, CI or CIO is formed or destroyed or interacts with "odd oxygen." The Pittsburgh group is in one stage or another of studying four of the reactions in a discharge-flow apparatus using resonance fluorescence detection of atom or radical species such as CI, O, or OH. The radicals are measured at the downstream end of a temperature-controlled tube at concentrations
upwards of one part in 100 million and at pressures about 1% of atmospheric. The results so far: • Work is completed on the reaction OH + HC1 — H 2 0 + CI, which regenerates reactive CI from relatively inert HC1. A total of 60 experiments show the reaction to be fast, in good agreement with measurements made by others using a different method. • Work is just about complete on the principal CI reaction in the stratosphere, CI + 0 3 — CIO 4- 0 2 . Some 73 experiments at 200 to 370° K have shown the reaction to be very rapid, although about 30% lower than the one reported room-temperature measurement. It is, the group says, a fast first step in the catalytic chain. • Work is in progress on the reaction CIO + NO — CI + N 0 2 , in which CI is regenerated. Initial experiments have shown it to be very rapid. • Work on the reaction CIO + O -* CI -I- O2 is just getting under way. All told, says the Pittsburgh group, the general conclusion that can be drawn at this stage is that none of the laboratory results so far contradict the seriousness of the projected ozone removal problem. Dr. Julian Heicklen of Pennsylvania State University's chemistry department and ionosphere research laboratory told the symposium of work carried out by his group on the photooxidation of chlorofluoromethanes. In studying the photodissociation of FC-11 and FC-12, the group found that the photodissociation efficiency to give chlorine atoms is unity. The group also measured the rates of attack of O on CF3CI, CF2CI2, CFCI3, and CCU and found the reactions to give CIO all the time.
Pollution by chlorofluorocarbons may parallel well-understood natural NOx cycle Catalytic cycle
NO + O, N0 2 + 0 2 N0 2 + UV -» NO + O O + 02-»03
NO + O3 • N0 2 + 0 2 N0 2 + 0 •NO + 0 , Net: 0 3 + O
•02 + 02
Net:
Zero
Neutral cycle CI + 0 3 - > C I O + 0 2 CIO + N O - * CI + N 0 9 N0 2 + UV- • NO + O
Catalytic cycle
Neutral cycle
CI +0 3 -»CIO + 0 2 CI0 + O - * C l + 0 , Net: 0 3 + O - • 0 „ + O,
0 + 0,Net:
Zero
Destroys 70% of ozone formed
N20
CH4
UV CF2CI2
HO
#
NO, 5% 1 million tons per year
100%
N20 N 2 0, N2
22
C&EN April 21, 1975
500,000 CF?CI 2 tons per year
Rainout in troposphere
Inert in troposphere
0 CITCIO
Stratosphere HNOa
N20
HC
ci;
0(1D)
Inert in troposphere
10% HCI Rainout in troposphere
CF 2 CI 2 NO", N0 2 in soil
Note: CF 2 C 1 3 represents chlorofluorocarbons.
The significance of the group's findings, Heicklen says, is that the chlorofluoromethanes are removed very efficiently to produce CI or CIO both by photodissociation and by 0 attack. In fact, he says, the rates of these processes are as fast as theoretically possible. Thus, the molecules are indeed a threat to the ozone layer. Dr. Harold S. Johnston of the University of California, Berkeley, says that more has been learned about the stratosphere in the past three years than was ever known before. He cites a three-year, $50 million study made by the Department of Transportation through its Climatic Impact Assessment Program. That program included research on the stratosphere, stratospheric ozone, and the vulnerability of stratospheric ozone to future fleets of supersonic transports (C&EN, Jan. 27, page 8). The new data from the DOT study, Johnston says, are directly applicable to the chlorofluorocarbon problem. Johnston draws a direct parallel between the now well-understood role of natural stratospheric nitrogen oxides and the problem of stratospheric pollution by chlorofluorocarbons. Nitrous oxide is released from the
NRC picks panel members for study of stratosphere Chairman, Dr. H. S. Gutowsky, director, school of chemical sciences, University of Illinois, Urbana Dr. Julius Chang, University of California, Lawrence Livermore Laboratory Dr. Robert Dickinson, National Center for Atmospheric Research, Boulder, Colo. Dr. James P. Friend, department of chemistry, Drexel University, Philadelphia Dr. Christian E. Junge, MaxPlanck Institut fur Chemie, Mainz, West Germany Dr. Frederick Kaufman, department of chemistry, University of Pittsburgh Dr. R. A. Marcus, department of chemistry, University of Illinois, Urbana Dr. George Pimentel, department of chemistry, University of California, Berkeley Dean H. I. Schiff, York University, Downsview, Ont. Dr. John H. Seinfeld, division of chemistry and chemical engineering, California Institute of Technology, Pasadena Dr. Brian Thrush, department of chemistry, Cambridge University, England Dr. Cheves Walling, department of chemistry, University of Utah, Salt Lake City
soil by natural processes. Inert in the lower atmosphere, it slowly moves into the stratosphere where it is partially converted to nitric oxide plus nitrogen dioxide (NO x ). The NO x destroys some 70% of the ozone, which is formed from solar radiation and oxygen. The global rate of formation of stratospheric NO x , Johnston notes, has been found to be 1 million tons per year. This rate, he says, constitutes a simple, firm reference point for comparison against possible stratospheric pollutants, including chlorofluorocarbons. Meanwhile, as studies are made and data come in, Rowland proposes pinning down the chlorofluorocarbon question with experiments that could be performed during the next few months. He suggests use of special techniques and equipment to detect the amounts of radioactive chlorine in fresh samples of stratospheric air. The radioactive chlorine is formed in small amounts in the stratosphere when molecules of trace argon are struck by cosmic rays. The radioactive chlorine decomposes quickly with a half-life of less than an hour. But if air samples were analyzed quickly for key compounds in the ozone destruction sequence, the presence of radioactive chlorine in the
compounds would constitute evidence that the ozone-depleting reactions are indeed occurring in the stratosphere. Rowland also proposes as a shortterm solution to the chlorofluoromethanes problem the substitution of FC-22 for FC-12. He notes that FC-22 (CHF2C1) is one of three commercial chlorofluorocarbons containing a C—H bond. Chlorofluorocarbons with this or C = C bonds can react, he says, with OH radicals in the lower atmosphere and be destroyed there, unlike chlorofluoromethanes. Even so, it would be just a shortterm solution. Rowland estimates that about 10% of FC-22 will reach the stratosphere and react there. But since it can release only one chlorine atom per molecule, compared to two for FC-12, the relative atmospheric hazard is half of 10%, or 5%. On this basis, he estimates that FC-22 is 20 times less hazardous than FC-12 per molecule. He notes, however, that considerable uncertainty remains in the calculations and the added safety factor could easily be 10 or 40 rather than 20. He also estimates that the safety factor for FC-21 (CHFCI2) and FC-31 (CH2FC1) should be even higher by another factor of five—that is, 50 to 200. •
Hypertensives work by several mechanisms 169th ACS NATIONAL MEETING The underlying causes of essential hypertension, or high blood pressure, are still largely unknown. But the chemical and physical changes that occur in the body with the onset of hypertension have been observed with some degree of precision. This knowledge has enabled pharmaceutical researchers to synthesize antihypertensive drugs to treat the symptoms of this disease that affects millions of people worldwide. By class, antihypertensive drugs fall roughly into three classes of compounds: adrenoceptor antagonists, peripherally acting compounds, and centrally acting compounds. Adrenoceptor antagonists inhibit the action of noradrenaline, a chemical messenger that carries sympathetic nerve impulses to the various organs of the body—particularly the heart and blood vessels, which play a key role in blood pressure control. Peripherally acting drugs work by increasing the degree of vasodilation—in other words, decrease the degree of blood vessel constriction in the hypertensive patient. Centrally acting antihypertensives, on the other hand, get right to the root of the problem, so to speak. They work on the vasomotor center of the lower brain, the section directly responsible for blood pressure control. Drugs acting by these several mechanisms were discussed at a Medicinal
Chemistry Division symposium on recent advances in antihypertensive therapy. Dr. Richard Clarkson of Imperial Chemical Industries' pharmaceutical division in the U.K. led off the program with a discussion of betaadrenoceptor antagonist compounds that show promise for human use. The initial search for a specific beta-adrenoceptor antagonist, Clarkson says, centered on a means of protecting the myocardium (heart muscle) from sympathetic nerve stimulation that would be useful for treating angina pectoris and for patients recovering from heart attacks. Trials with the early compounds synthesized for this purpose proved successful. But these drugs exhibited antihypertensive activity as well. One drug in particular, propranolol, in 1964 was discovered to have antihypertensive activity. It is now marketed in the U.S. as Inderal by Ayerst under license from ICI. Followup studies with other compounds of the propanolamine class, to which propranolol belongs, showed that antihypertensive activity is typical of the class. The specific mechanism by which the beta-blockers act, according to Clarkson, is still uncertain. But there is speculation that they work by one or a combination of modes. These include reduction of cardiac output; suppression of renin release (a proteolytic enzyme responsible for some forms of hypertension); by direct central effects on the brain area responsible for blood April 21, 1975 C&EN
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