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Appendix A PERTURBATIONS OF THE STRATOSPHERE AND OZONE DEPLETION Ralph J. Cicerone National Center for Atmospheric Research Boulder, Colorado HISTORICAL BACKGROUND Central to the concern that man's activities can modify the atmospheric ozone layer is the concept of chemical catalysis. A substance is a catalyst if it serves to drive or enhance a process or reaction--without itself being consumed in the process. In the earth's strato- sphere, nitric oxide, NO, can catalyze the destruction of ozone through the cyclic chain reactions: NO + O O3 + he NO2 + O NO2 + O2 O2 + O NO + O2 O3 + O3 + he ~ 3O2 (net). The corresponding catalytic chain reaction involving chlorine atoms proceeds through: C1 + O3 + O3 + he C10 + O C10 + O2 O2 + O C1 + O2 O3 + O3 + he + 3O2 (net). In these reactions, NO and C1 are not consumed as they destroy O3 because they are regenerated in the last reaction of the cycle. Chemical catalysis can be an extremely efficient process; some industrial catalysts mediate millions of cyclical reactions before they themselves require regeneration. The number of times that the catalytic cycle proceeds is called the chain length. With a chain length of 10,000 one can see how a 145
146 substance present at part per billion concentrations is capable of chemically controlling another substance present at 10 parts per million. me realizations that (a) efficient reactions like these were occurring in the natural stratosphere and (b) certain pollutants could mimic nature's ozone-destroying catalytic cycles were nearly simultaneous and have helped measurably to improve our understanding of the natural atmosphere and of man's potential for perturbing it. The research of Crutzen (1970, 1971) and Johnston (1971) showed that natural nitrogen oxides and aircraft-injected NO could have important roles in counterbalancing natural ozone production and providing extra, artificial ozone- destroying capacity, respectively. Earlier the need for identifying unspecified natural loss processes for strato- spheric ozone had been noted by Hampson (1964) and Hunt (1966), who based their work on earlier theory from Bates and Nicolet. The proposal (Berkner and Marshall 1967) that the evolution of life on the exposed earth surface began with the formation of the ultraviolet light- absorbing ozone screen, coupled with the realization that extant or planned human activities could destroy some of the ozone and a generally growing environmental awareness caused scientists to respond seriously to suggestions of stratospheric chemical perturbations. The ongoing release of synthetic chlorofluorocarbons, shown by Molina and Rowland (1974) to be capable of delivering chemically effective amounts of ozone-destroying chlorine atoms to the stratosphere, remains in 1981 the largest and most plausible threat. The biological UV-shield function of atmospheric ozone has focused attention on chemical pollutants capable of reducing the total amount of ozone in a vertical column of the atmosphere. Proposed fleets of stratospheric supersonic aircraft (releasing NO and H2O), space shuttle rockets (releasing HC1), the use of bromine-containing chemicals, the surface release of N2O from agricultural nitrogen-fertilizer usage and from some types of fuel combustion, and the emissions to the air of certain chlorinated solvents have been proposed as possible ozone reducers. Research in the United States and elsewhere (as documented in earlier NRC and NASA reports) has focused too narrowly on possible reductions in the total vertical column of ozone in the stratosphere--probably because of the UV shield that ozone provides to life on earth. Too little emphasis has been placed on inquiring whether ozone spatial redistribu- tions (in altitude and latitude) can result from man's
147 activities. Climatic effects could ensue from large redistributions. Further, tropospheric ozone (about 10 percent of total atmospheric ozone) has been relatively unstudied. Until recently, the prevailing view has been that the only interesting tropospheric photochemistry involving O3 takes place in highly polluted urban areas. Better understanding of tropospheric chemistry and more complete photochemical kinetic data bases are changing this view, and consequently a fuller concept of man's impact on atmospheric ozone is emerging. NATURAL PERTURBATIONS TO ATMOSPHERIC OZONE Both the chemical and the dynamical forces that control the atmospheric ozone distribution are subject to natural perturbations and variations. It is important to under- stand the consequent natural ozone responses and variability; cause-effect relations must be fathomed if we are to be able to predict ozone changes. The very existence of natural variations affects our ability to detect secular trends in ozone. On human time scales the most pertinent natural pertur- bations to atmospheric ozone appear to arise from: solar proton events, relativistic electron bombardments, quasi- biennial oscillation (and temperature change) effects, temperature changes on other time scales, and, possibly, 11-year solar W irradiance changes and volcanic chemical inputs. In August of 1972, a burst of solar protons entered the high-latitude atmosphere. High-latitude ozone amounts were observed to decrease almost simultaneously by the Nimbus 4 B W instrument (Heath et al. 1977, Reagan et al. 1981); these decreases persisted for several weeks. The first theoretical analysis of the effects of this solar proton event (Heath et al. 1977 ) found good quali- tative agreement between observed O3 decreases and those calculated in a 2-dimensional parameterized trans- port model including only the direct chemistry, i.e., ion-pair production by proton impact, dissociative recombination to yield NO, then NO-catalyzed O3 depletion. Recently, Solomon and Crutzen (1981) have expanded the computational model's chemistry to include chlorine chemistry and the production of hydrogen oxides (HOx) by the arriving protons. They also included the expected temperature-decrease feedback in their model. Their calculated O3 decreases due to the solar protons
148 agreed very well with the measured decreases except above 50 km. A further analysis has been performed by McPeters et al. (1981), who have provided certain corrections to the Heath et al. (1977) B W ozone data. The revised August 1972 data on the ozone perturbation evidently agree more closely with Solomon and Crutzen's calculated ozone reductions. McPeters and coworkers also analyzed two earlier (1971) smaller solar-proton events after which high-altitude ozone was observed to be depleted; the measured O3 depletions were larger than McPeters and co-workers calculated with their photochemical model. All of these investigations, when combined with earlier studies of a 1969 solar proton event and observed ozone reductions (Weeks et al. 1972, Swider and Kene she a 1973), have utilized a natural NOx-injection event to establish that NOX catalytic cycles do reduce ozone in the middle and upper stratosphere. Natural variations in the solar W output may have influenced stratospheric ozone during the recent past when Dobson instruments and satellite instruments have measured ozone. While there is no argument in principle that W irradiance changes would modulate ozone amounts, there is disagreement over the reality of solar cycle variations in W irradiance. Recently, Brasseur and Simon (1981) have expressed this concern, reviewed earlier calculations and presented new calculations of altitude, latitude, and temporal problems to be expected from solar-cycle-related W changes. A more empirical approach has been taken by Keating et al. (1981) and Reber and Huang (1982). From the monthly global average ozone amounts derived from Nimbus IV B W measurements (1970-1977), Tolson (1981) and Keating et al. (1981) sought an empirical relationship between solar W output (as indicated by the 10.7-cm radiowave flux) and global total ozone. They found a very high correlation between the two; this strong correlation suggested a causal relationship. An independent analysis by Reber and Huang (1982) shows that much of this correlation is due to a secular trend in both. Further, the remaining correla- tion maximizes for a zero time lag or for ozone changes one month preceding the 10.7-cm flux change. Coupled with the uncertainty (several references cited by Reber and Huang) in the stability of the BUV instrument for total ozone measurements over this seven-year time period, firm conclusions about relationships between total ozone and solar UV seem impossible at this time (Reber and Huang 1982). Thus, while photochemical theory
149 calls for such a relationship, it has been difficult to observe. Perturbations to stratospheric ozone can also be caused by altered circulation patterns or temperature fields. Episodic phenomena such as sudden stratospheric warmings should affect ozone, but more interesting for our present purpose are those large-scale temperature changes that can be sustained for a year or more. Recently, Angell has extended earlier analyses that have found a significant cooling of the upper stratosphere (cited in Hudson et al. (1982)). In the 46- to 55-km region there has been about a 5°K cooling between 1971 and 1980; a less pronounced cooling is evident down to 36 km. Such a cooling should have led to slightly larger local ozone concentrations. These were not observed by Heath with the NIMBUS IV B W instrument. reported ozone decreases of nearly 9 percent at the 40-km level from 1971 to 1977 (NASA/WMO Stratosphere Workshop, Hampton, Virginia, May 18-22, 1981; see also Science, September 4, 1981, pp. 1088-1089). It is also important to recognize the possibility of a large natural change in tropospheric ozone, both because the ozone column would be affected and because of possible climatic effects that could ensue. Data suggest that northern hemispheric tropospheric ozone has increased substantially in the last decade; this is discussed below. Instead, he Finally, although explosive volcanoes can in principle affect stratospheric ozone by direct injections of water and chlorine, there are no indications of measurable effects due to volcanoes during the life of the Dobson instrument network. A related question involves the ability of stratospheric dust to confound the Dobson measurement technique (Dave et al. 1981, De Luisi et al. 1975). One would feel more confident with a complete absorption spectrum rather than discrete wavelength pairs at which absorption is measured. MAN'S IMPACT: ASSESSMENT AND UNCERTAINTY IN 1981 Of all the potential anthropogenic influences on atmospheric ozone the continued release of chlorofluoro- carbons 11 and 12 and of trichloroethane remains in 1981 that of most immediate and apparently largest concern. The anticipated magnitude of the effect continues to change as the laboratory photochemical data base grows. It is worth noting that there have been few, if any,
150 results reported from coupled meteorological/chemical model calculations. Our estimates of man's impact on ozone due to fluorocarbon release continue to be based on models that have not changed conceptually since before the fluorocarbon problem was identified. Further, even the most elegant and difficult field measurements such as the C1 and C1O profile measurements of Anderson et al. (1980) and those of total chlorine in the lower strato- sphere by Berg et al. (1980) have not altered the initial view of this environmental problem. Field measurements in general have substantiated all elements of the original Molina-Rowland hypothesis; quantitative adjustments to the size of the ozone perturbation have arisen frequently from new or changed laboratory kinetic data. If we focus on the expected reduction in total ozone due to continued release of CF2C12 and CFC1~ at 1~h=; ~ 1 Ode; :~^r~ll~l r:~^c icon it Zi 1 ~ -arc-- -I ~- -any- `~~~ ~ -- Meal r we see that major changes have resulted from altered chemical reaction rates and from the inclusion of previously omitted reactions and species (e.g., ClNO3 and HNO4). The calculations that produced Figure A.1 were performed with 1-dimensional photochemical models with parameter- ized transport. Since 1980 there have also been similar 2-dimensional models that have been able to include as 20 z o 0 15 z LL co ~ 10 cr: LU z 5 c: A: c 20 ~ A / CFM Release ~ ~~~~~~~ ~- __ N2O Doubling 15 .' 10 _ 5 1974 1975 1976 1977 1978 1979 RATE CONSTANT Cl OH+ REVISIONS HO2 CIONO2 NO + HO2 2ppb Clx I' 1 980 1 98 1 __~ OH + HNO3 HNO4 + he OH + HNO4 OH + H2O2 OH + HO2 FIGURE A.1 Brief schematic history of the estimates of the steady state column ozone reduction due to (a) continued release of CF2C12 and CFC13 at 1975 annual rates, and (b) doubling of N2 O (from 300 to 600 ppb). More detail on reasons for changes between 1979 and 1981 is in Hudson et al. (1982, Chapter 3~.
151 many chemical processes as the preexisting 1-dimensional models. The change in globally averaged total ozone amounts due to CF2C12 and CFC13 from the available 2-dimensional models is very close to that from 1- dimensional models (see Chapter 3 of Hudson et al. (1982)). As Figure A.1 indicates, since 1979 there has ~ . .~ ~ ~ a been a considerable downward revision or one preen cues steady state ozone depletion. With currently accepted chemical reaction rates one calculates steady state, globally averaged ozone reductions of perhaps 6 percent. The principal chemical data changes since 1979 are: (i) a faster rate for OH + HNO3 ~ H2O + NO3 especially at low temperatures, (ii) a faster rate for OH + H2O2 ~ H2O + HO2, (iii) slower photolysis of HNO4 than previously measured, (iv) faster reaction of OH + HNO4 + products, presumably H2O + NO2 + O2 than originally estimated, and (v) upward revision of the OH + HO2 rate constants. Key references for these recent changes are: (i) Wine et al. (1981); (ii) Keyser (1981) and Kaufman (1980); (iii) Molina and Molina (1981); (iv) and (v) NASA/JPL Kinetics Panel (1981). While these changes have the effect of lowering calculated OH (and C1O) concentrations below 30 km and thus permitting more favorable comparison with Anderson's C1O measurements below 30 km than before (Cicerone and Walters 1980, Duewer and Wuebbles 1980, Sze and Ko 1981), they are not the final word (see next paragraph). It would not be surprising if the best estimates of column ozone changes due to CF2C12 and CFC13 and N2O increases continue to oscillate as on Figure A.1. More detail on the chemical reaction rates that have changed since 1979 and the effect each change has had on ozone-depletion predictions can be found in Chapter 3 of Hudson et al. (1982). The effects of each of the processes mentioned above as well as discussion of recent findings on the reactions HO2 + C1O ~ products and C1O + NO2 ~ products (isomers of C1NO3) are spelled out in that report. The remaining uncertainties in every one of these processes except OH + H2O2 ~ H2O + HO2 are considerable. Unfortunately, most of these processes involve working with notoriously difficult laboratory systems, e.g., any study of HNO4 properties and the reactions of the radicals like OH + HO2. Besides the laboratory kinetic uncertainties one must also note that measurements of most of these apparently important polyatomic species in the atmosphere have not yet been achieved: there has been no positive detection
152 of ClONO2 (chlorine nitrate), H2O2, HOC1, HNO4, or N2O5. In this regard, one must state that there is considerable remaining uncertainty in ozone-depletion estimates. As with most scientific questions, in this case there is uncertainty on the high and low sides, i.e., if certain predicted species do not actually exist or certain reactions proceed at predicted rates, the curves on Figure A.1 could go in either direction. To make matters worse, these key polyatomic species are predicted to exist (and to mediate the critical chemistry) in the lower to middle stratosphere, precisely where physical transport in dynamical meteorological systems is important and simple photochemistry is not the controlling factor. Accordingly, it appears that the resolution of present uncertainties involving chlorine nitrate, HOC1, HNO4, H2O5, H2O2, etc., will require not only difficult laboratory measurements but much more complete coupling of chemical and dynamical meteorology models. Phrased in the terminology of chemical catalysis, we must be able to calculate the catalytic chain lengths of the chlorine and nitrogen oxide chains and the effectiveness of the methane-oxidation ozone production reactions. Such a calculation must accurately account for (a) processes that can interrupt catalytic chains that form, for example, HNO4 or HOC1 or ClONO2, and (b) meteorological motions that can rapidly move the reacting chemicals to locations with different pressures and temperatures. The possibility of a separate anthropogenic effect on atmospheric ozone has been raised by Liu et al. (1980). In a research report concerned with the natural origins of tropospheric ozone they found evidence that ozone produced photochemically in the upper troposphere where subsiding stratospheric NOX encounters rising nyaro- carbons. If so, then the NOX emitted by commercial and military subsonic aircraft should lead to ozone production near the 10 km (flight altitude) level. Liu et al. (1980) calculated that increased subsonic air traffic could have increased northern hemispheric tropospheric ozone by about 15 percent from 1970 to 1980. Such an increase, while important in its own right, would also amount to a 1.5 percent increase in total overhead ozone. This increase could mask a 1.5 percent decrease in the stratospheric ozone column. At eight of nine northern hemisphere stations where tropospheric ozone profiles are measured regularly there was a measured increase of about the predicted amount (Liu et al. 1980). This apparent
153 increase in northern hemispheric tropospheric ozone has also been discussed by Pittock (see Appendix F) and in the 1981 NASA/WMO Stratosphere Report (Hudson et al. 1982). From all these sources it appears clear that an increase might have occurred but the present state of measurement capability for tropospheric ozone above the surface is poor enough to be unable to settle this question. The changes in our photochemical reaction schemes and data of the past two years have had relatively little effect on our view of the upper stratosphere. One still expects ozone at the 40-km level to be strongly attacked by chlorine compounds. No serious doubt at all exists on this point; one must state that a stong perturbation of the upper stratosphere is under way. However, because of the large uncertainties in the region below 30 km, mostly with the polyatomic species mentioned above, one cannot say with much confidence what the total column ozone depletion will be eventually. It is possible that as the upper stratospheric ozone decreases and NATO and COD increase, there could be extra ozone production below about 25 km so that the vertical column of ozone could be changed only slightly. In this event there would probably be a significant redistribution of ozone in latitude and altitude, leading to concern over climatic effects. Two other human activities need updating. First, in the case of atmospheric N2O, Weiss (1981) has shown through measurements that N2O has increased by about 0.2 percent per year since 1976 and most likely at a similar rate since 1963. The relative contributions of combustion-produced N2O and fertilizer-produced N2O are not yet clear although Weiss's data can be explained roughly by the former. Second, atmospheric detonation of nuclear explosives has been examined once again, and it appears as before that there are significant uncertainties in estimating the NO yields (and thus the chemical effects) of such explosions (McGhan et al. 1981). RECOMMENDATIONS FOR RESEARCH Although the exact size of the effect has proven difficult to predict, the hypothesis that continued chlorofluoro- carbon release will have a significant global impact on atmospheric ozone appears correct--it has withstood over seven years of reexamination. Because of the need for
154 industry and government to make decisions on production and regulation and because other global anthropogenic pollutants (e.g., NOx and N2O) remain items of concern, further atmospheric chemistry research is indicated. The goals of the research areas listed below are (i) to understand relevant atmospheric chemistry and meteorology well enough to permit better prognostic mathematical models, (ii) through measurements, to better characterize the atmosphere's chemical behavior, and (iii) to obtain more accurate and precise measurements of atmospheric ozone to permit variations and trends to be detected earlier and more clearly. 1. Continue the operation, maintenance, and data analysis of the WMO/NOAA Dobson instrument ozone measurements. 2. Develop improved ground-based instrumentation for measurements of the ozone column. It should be feasible to take entire absorption spectra for ozone determinations rather than the isolated measurements at a few discrete wavelengths. We are fortunate to have the Dobson instruments, but one suspects that it is possible to improve accuracy and precision with modern techniques (the Dobson instrument was invented in 1927). 3. Develop improved methods for measurements of tropospheric ozone. Both lidar and stable chemical sensors seem like good prospects. 4. Continue and expand, if possible, in situ measurements of key chemical species and the ratios of key reactive species in spatial regions where the reactants are important and where photochemical time constants are smaller than those for transport. 5. Accelerate the development of mathematical models of atmospheric chemistry with coupled atmospheric dynamical fluid motions. 6. Encourage extant models to focus on more complicated scenarios, e.g., increasing CFMs and increasing CO2 and increasing N2O, CH4, and CH3CC13. 7. Accelerate research on climatological effects of redistribution of atmospheric ozone and of trace gas increases. 8. Continued monitoring of background concentrations of CF2C12, CFC13, N2O, and CH4. While no evidence exists for tropospheric removal processes of CF2C12, CFC13, or N2O, it is very important to obtain a continuous record of their concentrations. The
155 preparation and stable maintenance of calibrated standards for each of these gases is still an important research problem deserving commitment of government and industrial resources and scientific talent. . 9. Improved satellite sensors and continued data- reduction effort focused toward stratospheric ozone and trace-constitutent monitoring are needed. 10. Expanded high-altitude (upper stratospheric) whole-air sampling is needed to obtain vertical profiles of H2O, CH4, and N2O and other stable trace gases. These are needed to provide ground truth values for overflights of satellite sensors and to begin to acquire a climatology of the upper stratosphere for multi- dimensional models to employ in validation tests. REFERENCES Anderson, J.G., H.J. Grassl, R.E. Shetter, and J.J. Margitan (1980) Stratospheric free chlorine measured by balloon-borne in situ resonance fluorescence. Journal of Geophysical Research 85:2869-2887. Berg, W.W., P.J. Crutzen, F.E. Grahek, S.M. Gitlin, and W.A. Sedlacek (1980) First measurements of total chlorine and bromine in the lower stratosphere. Geophysical Research Letters 7:937-940. Berkner L.W. and L.L. Marshall (1967) The rise of oxygen in the earth's atmosphere with notes on the martian atmosphere. Advances in Geophysics 12:309-331. Brasseur, G. and P.C. Simon (1981) Stratospheric chemical and thermal response to long-term variability in solar UV irradiance. Journal of Geophysical Research 86:7343-7362. Cicerone, R.J. and S. Walters (1980) NO2-Catalyzed Removal of Stratospheric BOX. Paper presented at Fourteenth Informal Conference on Photochemistry, March 31. (unpublished) Crutzen, P.J. (1970) The influence of nitrogen oxides on the atmospheric ozone content. Quarterly Journal of the Royal Meteorological Society 96:320-325. Crutzen, P.J. (1971) Ozone production rates in an oxygen- hydrogen-nitrogen oxide atmosphere. Journal of Geophysical Research 76:7311-7327. Dave, J.V., C.L. Mateer, and J.J. De Luisi (1981) An examination of the effect of haze on the short Umkehr method for deducing the vertical distribution of ozone. Pages 222-229, Proceedings of the Quadrennial \
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