Reduced Antarctic Ozone Depletions in a Model with Hydrocarbon Injections

Motivated by increased losses of Antarctic stratospheric ozone and by improved understanding of the mechanism, a concept is suggested for action to arrest this ozone loss: injecting the alkanes ethane or propane (E or P) into the Antarctic stratosphere. A numerical model of chemical processes was used to explore the concept. The model results suggest that annual injections of about 50,000 tons of E or P could suppress ozone loss, but there are some scenarios where smaller E or P injections could increase ozone depletion. Further, key uncertainties must be resolved, induding initial concentrations of nitrogen-oxide species in austral spring, and several poorly defined physical and chemical processes must be quantifed. There would also be major difficulties in delivering and distributing the needed alkanes.

Cosmochim. Aca 54, 2869Aca 54, (1990. 23. These data were modeled using a computer program donated by R. H. Becker. 24. D. D.OayonLPlanw Sc. Coafd X, 165 (1989). 25 Motivated by increased losses of Antarctic stratospheric ozone and by improved understanding ofthe mechanism, a concept is suggested for action to arrest this ozone loss: injecting the alkanes ethane or propane (E or P) into the Antarctic stratosphere. A numerical model of chemical processes was used to explore the concept. The model results suggest that annual injections of about 50,000 tons of E or P could suppress ozone loss, but there are some scenarios where smaller E or P injections could increase ozone depletion. Further, key uncertainties must be resolved, induding initial concentrations ofnitrogen-oxide species in austral spring, and several poorly defined physical and chemical processes must be quantifed. There would also be major difficulties in delivering and distributing the needed alkanes.

LAARGE LOSSES OF ATMOSPHERIC
ozone are occurring over Antarctica each austral spring (1,2). Ozone losses have also been observed (3) at middle and higher latitudes in both hemispheres for the period 1979 to 1990. South of 600S the ozone loss rate was more than 0.5% per year; a reasonable interpretation is that mixing of ozone-poor air from the Antarctic stratosphere is causing this wider impact (3). R J. Cicerone and S. Eliott, Department of Geosciences, University of California, Irvine, CA 92717. R. P. Turco, Department of Atmospheric Sciences and Institute for Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024. 22 NOVEMBER 1991 Episodes of reduced ozone have also been observed over Australia and New Zealand (4). There are also indications that the size and severity of the Antarctic ozone hole could increase (5) and that the hole will form each year for the next 100 years even if CFC releases are controlled (6).
The threat of expanded future impacts of the Antarctic ozone hole leads one to search for measures that could prevent them. Here we explore a concept for mitigating ozonehole formation. It is based on recent gains in scientific understanding of the processes responsible for ozone loss in the polar winter stratosphere, induding increased knowledge of polar stratospheric douds [PSCs (7)].
We explore whether injections of certain hydrocarbons into the lower stratosphere during an optimal time period each year could prevent ozone-destroying reactions in the austral spring. The annual sequence of physical and chemical events that occur during the formation ofthe ozone hole indudes a several week period during which stratospheric Cl atoms are activated for ozone attack. Our idea is to immobilize this active chlorine through the rapid reaction of Cl atoms with simple alkanes like propane, as in Cl + C3H8 --HC1 + C3H7 (1) Because the concentrations of active chlorine are as large as 2 parts per billion (ppb by volume) of the local atmosphere during the period of ozone destruction, we expect that it would be necessary to raise the concentration of propane (or ethane) artificially to at least 2 ppb. In this report we test this idea with a model ofAntarctic ozone-layer chemistry. Before presenting results of the model calculations we must describe the processes that we are simulating.
The formation of PSCs is now known to be essential to the formation of the ozone hole over Antarctica (7). PSCs cause dehydration and denitrification of polar air and activate inert chlorine species (HCl and ClONO2) into photolytically unstable chlorine species (C12, CINO2, and HOC1) that are transformed by sunlight into ozonereactive species (Cl and C1G). The morphological properties -of polar stratospheric douds are well-defined by satellite observations (8) and by optical and physical evidence; there are two broad categories. Type I PSCs consist of an aerosol haze of micrometer-sized nitric acid ice particles composed of HNO3 and H20 in roughly a molar ratio of 1: 3, respectively (9). Type II PSCs are composed of larger (>10 pm) water-ice crystals.
Type I PSCs begin to form at temperatures near 195 K, generally late in the austral fall, when the southern polar vortex has formed and cooling within the vortex has occurred in the absence of strong solar or longwave heating. These PSCs continue to form well into the austral spring. Type II PSCs condense at lower temperatures (s187 K), at the frost point of water vapor in the polar stratosphere. Type II douds therefore appear later in the winter season and dissipate earlier in the spring than type I douds. Type I haze dominates during the early part ofthe Antarctic winter season, but the proportion of type II clouds increases as cooling progresses. Type II particles cause dehydration, and both type I and type II clouds appear to cause denitrification of the polar vortex. Chlorine activation seems to occur primarily on type I PSCs. The systematic dehydration and denitrification of the polar winter stratosphere can affect the persistence of PSCs in the late winter and early spring, and ozone depletion can affect the stability and breakup of the vortex at that time (10). The complex sequence of events that prepares the Antarctic atmosphere for ozone depletion at first spring light is depicted schematically in Fig. 1.
The key heterogeneous chemical reactions that are catalyzed by PSCs are: HCl + ClONO2 --C12(g) + HNO3 (2) HC1 + N205 --CINO2(g) + HNO3 (3) ClONO2 + H20 -O HOCI(g) + HNO3 (4) N205 + H20 -> 2HNO3 (5) In these equations, all species are assumed to be on the surfaces ofPSC particles where the reactions occur; species that readily desorb into the gas phase are indicated by "g." We performed our calculations with a tested model (11) of stratospheric gasphase photochemistry including about 130 reactions, to which we added a detailed C2H6 photooxidation sequence (12) that includes peroxyacetylnitrate and its reactions. We also added the four important heterogeneous reactions (reactions 2 to 5 above). Gas-phase reaction rates were taken from Demore et al. (13) and reaction efficiencies on ice surfaces were taken from recent laboratory measurements (14,15).
Ethane (E) was chosen as a representative hydrocarbon because its chemistry is relatively simple and its degradation channels are well-defined. We have also simulated injections of propane (P), which is even more efficient at scavenging chlorine, using the C2H6 sequence with rate constants for C3H8 hydrogen abstraction. Model data 18)] requires certain assumptions concerning autumn concentrations and heterogeneous processing. We used results from twodimensional models to estimate autumnal levels of nitrogen-and chlorine-containing species (19,21,22) because high-latitude measurements are scarce during polar fall (19,20). Calculations generally have not included heterogeneous decomposition of stratospheric N205 as it moves poleward; therefore, our adopted fall NOQ values may represent upper limits.
Two methods were used to quantify the effect of processing that might occur on particles before sunrise. First, reactions 2 through 5 were included in a one-dimensional microphysical simulation of PSC formation (15) along with laboratory determinations of heterogeneous efficiencies on types I and II particles (14,23), and the processing of N205, HC1, and CIONO2 were calculated explicitly. Reactions 2 and 3 reached completion, yielding our baseline scenario B. We also considered simple titrations of nitrogen and inorganic chlorine species from initial levels (19,21), leading to a number of possible alternative solutions. We focus here on models B and H in Table  2. Model B is most consistent with the detailed microphysical calculations, whereas model H maximizes the ratio of active chlorine to nitrogen. Neither model B nor H is inconsistent with data from Antarctic measurement campaigns (19,21,24). Indeed both models reflect conditions at the time of the 1987 ozone hole, including mole fractions of total inorganic chlorine (1. were assumed to terminate on 1 September, a rough average date for final PSC evaporation (25); HNO3 amounts were set to equal or exceed the HNO3 vapor pressure over NAT (26). Control simulations with both models B and H produced large ozone depletions during days 230 to 275 (Fig. 2), although not as large as the 95% depletions that have been observed on occasion (16). We eventually explored eight scenarios spanning a range of permutations of CIOX and NON, not all ofwhich yielded simulated ozone holes (27). Model responses to additions of E and P varied (Fig. 2). In model H the addition of 1.8 ppb E or P prevented a considerable amount of ozone loss; further increases to 3.6 ppb were even more effective. This desirable response is due to the absence of NOG at the beginning ofsunlight (column 2 of Table 2). The results of model B show a  Table 2. Initial (8 August) concentrations at 15 km. Concentrations of key species used to initialize the photochemistry were obtained from two dark processing scenarios. In one scenario (model H), chlorine and nitrogen species were simply titrated by reactions 2 to 5, and relative efficiencies were assumed to be defined by sticking coefficients. In the second scenario (model B), a detailed one-dimensional microphysics-heterogeneous chemistry model was used to estimate presunrise concentrations (15); also see text. Total inorganic chlorine was taken to be 1.8 ppb in autumn, partitioned as 1.5 ppb HCI and 0.3 ppb ClONO2 (19)(20)(21) (Fig. 3A), the response upon the addition ofhydrocarbons is similar to that of model B, but with 1.2 ppb NO. available (Fig. 3B), the response is a decreased ozone loss, as for model H. In models without added hydrocarbons, the ozone loss is larger than was calculated for 1987 conditions (Fig. 2) when there was less chlorine. The results ofFigs. 2 and 3 are potentially sensitive to many assumptions; we have tested some of these. When we maintained PSCs and heterogeneous processing until 1 October in the model with no added E or P, the ozone holes were deeper and lasted longer. Responses to E or P injections remained sensitive to NOY amounts but were more encouraging than those of Fig. 2. Reducing the equivalent bimolecular rate constant for reaction 2 from 3 x 10-14 cm3/s by a factor of 10 did not affect the results substantially. Note also that, if the sequestering of Cl atoms by reaction 1 prevents formation of C1O, the effects of bromine could also be diminished even though the addition of alkanes will not convert Br atoms to HBr [the reaction ofBr atoms with E and P to produce HBr is endothermic (13)]. The effects of bromine are diminished because the attack of bromine on ozone proceeds through the reaction of BrO with CIO (28).
Other serious uncertainties remain. The present model calculations rest on current understanding of the mechanism of ozonehole formation; many specific processes and their rates must be clarified. Processes that control the Antarctic vortex, its temperature, and PSC formation are not well understood nor are the mechanisms that determine the extent of denitrification and NO. and NOY distributions (29). Atmospheric concentrations of N205 and other NOQ compounds must be measured more completely, and the absence of background E and P should be verified. Significant questions remain about the mechanisms and rates of heterogeneous chemical reactions that activate chlorine and denitrify the stratosphere. If, for example, HCI and HOCl react quickly on PSC surfaces to form C12 and H20 (30), there may be a pathway to continue to produce Cl atoms in the absence of ClNO3. This pathway could reduce the effectiveness of alkane additions. Furthermore, because concentrations of NO, and Cl, species should increase with altitude above 15 km (19,21,22), the potential effectiveness of E or P should be examined at other altitudes. Also, the sensitivity of the results to the date of PSC evaporation should be tested further and the response of air parcels that experience intermittent sunlight during winter should be modeled.
Although these initial calculations are encouraging, it is not clear whether such an intervention would be feasible. In our calculations, it was assumed that hydrocarbons could be delivered in the required quantities uniformly throughout a 2 x 107 km2 region of about 5 km depth (15 to 20 km altitude). It may be possible to deliver the gross quantity, say 50,000 tons ofE, to an altitude of 15 km with a fleet of several hundred large airplanes, for example, but it would be very difficult to assure that the E would mix adequately with air in the Antarctic polar stratosphere. Inside the winter vortex of the Antarctic stratosphere, vertical mixing would likely require significantly more than one month, although horizontal mixing might be accomplished within a month (31). If the hydrocarbons were to be injected into the confines of the vortex, then either the delivery system would have to distribute the material over the volume, or one would have to allow adequate time for slow internal mixing. The possibility that the vortex region exchanges and processes air from lower latitudes (32) could lead to some losses of the added alkanes. If instead the gases would be introduced into the stratosphere before the vortex sets up (when there is more mixing), much larger quantities would be needed, and their chemical fate would be more uncertain. Experiments can be imagined with vertically thin atmospheric layers wherein the injected hydrocarbons would be consumed (reaction 1) and the present concept could be tested. Before any actual injection experiment is undertaken there are many scientific, technical, legal, and ethical questions to be faced, not the least of which is the issue of unintended side effects (33).
stratosphere as a result of fast reactions with OH radicals and with Cl atoms. After sunrise in the polar lower stratosphere these alkanes could survive for perhaps a year, but transport to warmer regions (where OH attack is faster) or to sites with more OH should limit their survival times to some tens of ins) and the induction and accumulation of proteinase inhibitors and lytic enzymes such as chitinase and 0-1,3-glucanase. The success of the plant in warding off phytopathogen invasion appears to depend on the coordination between the different defense strategies and the rapidity of the overall response (2).
Chitinase catalyzes the hydrolysis of chitin, a 3-1,4-linked polymer of N-acetyl-D-glucosamine and a major component of the cell wall of most filamentous fungi except the Oomycetes (3). Although chitinase is generally found at low or basal levels in healthy plants, its expression is increased