OZONE DEPLETION FAQ PART II: Chlorine and Bromine in the Stratosphere Copyright 1993 Robert Parson This part deals not with ozone depletion per se (that is covered in Part I) but rather with the sources and sinks of chlorine and bromine in the stratosphere. Special attention is devoted to the evidence that most of the chlorine comes from the photolysis of CFC's and related compounds. Caveat: I am not a specialist. In fact, I am not an atmospheric chemist at all - I am a physical chemist studying gas-phase processes who talks to atmospheric chemists. These files are an outgrowth of my own efforts to educate myself about this subject. I have discussed some of these issues with specialists but I am solely responsible for everything written here, including any errors. This document should not be cited in publications off of the net; it should instead be used as a pointer to the published literature. Corrections and comments are welcomed. - Robert Parson Associate Professor Department of Chemistry and Biochemistry, University of Colorado (for which I do not speak) parson_r@cubldr.colorado.edu rparson@rintintin.colorado.edu CONTENTS 1. THE STRATOSPHERE 1.1) What is the stratosphere? 1.2) How is the composition of air described? 1.3) How does the composition of air change with height? (Or, "CFC's are heavier than air - so how can they get into the stratosphere?") 2. CHLORINE IN THE STRATOSPHERE 2.1) Where does the Chlorine in the stratosphere come from? 2.2) What is the evidence for anthropogenic sources? 2.3) What is the trend of stratospheric chlorine concentration? 2.4) In what molecules is stratospheric chlorine found? 2.5) What happens to organic chlorine compounds in stratosphere? 2.6) How is chlorine removed from the atmosphere? 2.7) What are the possible sources of the HCl in the _stratosphere_? 2.8) What is the source of HCl in the troposphere? 2.9) How is chlorine distributed in the stratosphere? 2.10) Which source of stratospheric chlorine is supported by this evidence? 2.11) How do we know that the CFC's in the stratosphere are being photolyzed? 2.12) How do the CFCs produced in the Northern Hemisphere get to the Antarctic? 2.13) Isn't it true that volcanoes put much more chlorine into the stratosphere than CFCs? 2.14) How much chlorine comes from rockets and Space Shuttle launches? 3. BROMINE IN THE STRATOSPHERE 3.1) Is bromine important to the ozone destruction process? 3.2) How does bromine affect ozone concentrations? 3.3) Where does the bromine come from? 4. REFERENCES ================================================================= 1. THE STRATOSPHERE 1.1) What is the stratosphere? The stratosphere extends from about 15 km to 50 km (the precise altitude of the lower boundary, known as the tropopause, varies between ~10 and ~18 km, depending upon latitude and season.) In the stratosphere temperature _increases_ with altitude, due to the absorption of UV light by oxygen and ozone. This creates a global "inversion layer", that is, a layer where temperature increase with altitude. This means that the density of air decreases particularly rapidly with height, which impedes vertical motion into and within the stratosphere. The word "stratosphere" has the same root as the word "stratification" or layering. The stratosphere is often compared to the "troposphere", which is the atmosphere below about 15 km. The prefix "tropo" refers to change: the troposphere is the part of the atmosphere in which weather occurs. This results in relatively rapid mixing of tropospheric air. 1.2) How is the composition of air described? (What is a 'mixing ratio'?) The density of the air in the atmosphere depends upon altitude, and in a complicated way because the temperature also varies with altitude. It is therefore awkward to report concentrations of atmospheric species in units like g/cc or molecules/cc. Instead, it is convenient to report the relative number of molecules - the number of molecules of a given component in a small volume, relative to the total number of molecules in that volume. Chemists usually call this a mole fraction, but atmospheric scientists have taken tocalling it a "mixing ratio". Typical units for trace species are parts-per-billion by volume (ppbv); we will abbreviate this to ppb. (The 'by volume' reflects "Avogadro's Law": for an ideal gas mixture, equal volumes contain equal numbers of molecules.) 1.3) How does the composition of air change with height? (Or, "CFC's are heavier than air - so how can they get into the stratosphere?") In the earth's troposphere and stratosphere, most _stable_ chemical species are well-mixed - their mixing ratios are independent of altitude. If a species' mixing ratio changes with altitude, some kind of physical or chemical transformation is taking place. That last statement may seem surprising - one might expect the heavier molecules to dominate at lower altitudes. The mixing ratio of Krypton (mass 84), then, would decrease with altitude, while that of Helium (mass 4) would increase. In reality, however, molecules do not segregate by weight in the troposphere or stratosphere. The relative proportions of Helium, Nitrogen, and Krypton are unchanged up to about 80 km. Why is this? Vertical transport in the troposphere takes place by convection and turbulent mixing. In the stratosphere and in the next layer up, the "mesosphere", it takes place by "eddy diffusion" - the gradual mechanical mixing of gas by small scale motions. These mechanisms do not distinguish molecular masses. Only at much higher altitudes do mean free paths become large enough that _molecular_ diffusion dominates and gravity is able to separate the different species. Experimental measurements of the fluorocarbon CF4 verify this homogeneous mixing. CF4 has an extremely long lifetime in the stratosphere - probably many thousands of years. The mixing ratio of CF4 in the strat. was found to be 0.056-0.060 ppb from 10-50 km, with no overall trend. [Zander et al. 1992] Sometimes that part of the atmosphere in which the chemical composition of stable species does not change with altitude is called the "homosphere". The homosphere includes the troposphere, stratosphere, and the next layer up, the "mesosphere". The upper regions of the atmosphere are then referred to as the "heterosphere". 2. CHLORINE IN THE STRATOSPHERE 2.1) Where does the Chlorine in the stratosphere come from? ~80% from CFC's and related manmade organic chlorine compounds (eg. CCl4) ~15-20% from methyl chloride (CH3Cl), most of which (~3/4) is natural. A few % from inorganic sources, including volcanic eruptions. [WMO 1991] [Solomon] [AASE] [Rowland 1989,1991] [Wayne] 2.2) What is the evidence for anthropogenic sources? The numbers above come from measurements of the altitude and time dependence of the natural and manmade chlorine- and fluorine-containing compounds in the troposphere and stratosphere. The mixing ratios of manmade compounds are almost independent of altitude in the troposphere and drop off rapidly in the stratosphere. The mixing ratios of inorganic chlorine compounds drop off rapidly in the troposphere, then _increase_ rapidly in the stratosphere, suggesting that they are being produced there by photolysis of the organic chlorine compounds. At the bottom of the stratosphere nearly all of the chlorine is organic, at the top it is all inorganic, suggesting a quantitative conversion from one to the other. At the same time, the total amount of fluorine in the stratosphere has been increasing. The details are presented in the next few sections. 2.3) What is the trend of stratospheric chlorine concentration? The total amount of chlorine in the stratosphere has increased by a factor of 2.5 since 1975 [Solomon] During this time period the known natural sources have shown no such increases. On the other hand, emissions of CFC's and related manmade compounds have increased enormously. The major sink for the CFC's has been firmly established to be UV photolysis in the stratosphere. In 1989, the concentrations of the major CFC's in the troposphere were increasing at about 4% per year. [WMO 1991]. 2.4) In what molecules is stratospheric chlorine found? Let us divide up the chlorine compounds in the stratosphere into "organic" (i.e. carbon-containing) and "inorganic". The major inorganic compound in both the troposphere and stratosphere is Hydrogen Chloride, (HCl); in the lower stratosphere there is a strong runner-up, Chlorine Nitrate (ClONO2). These are called "chlorine reservoirs" - they do not themselves react with ozone, but they generate a small proportion of chlorine-containing radicals which do. The various chlorine-containing compounds are chemically active to varying extents, and a complex chemical equilibrium involving the concentrations of the various species and the local pressure and temperature results. The major organic chlorine compounds that reach the stratosphere are: ChloroFluoroCarbons, CF2Cl2 (CFC-12), CFCl3 (CFC-11), CF2ClCFCl2 (CFC-113); HydroChloroFluoroCarbons such as CHClF2 (HCFC-22); Carbon Tetrachloride, CCl4; Methyl Chloroform, CH3CCl3 and Methyl Chloride, CH3Cl (also called Chloromethane). Only the last has a large natural source; it is produced biologically in the oceans and chemically from biomass burning. The CFC's and CCl4 are nearly inert in the troposphere, and have lifetimes of 50-200+ years. Their major "sink" is photolysis by UV radiation. [Rowland 1989, 1991] The hydrogen-containing halocarbons are more reactive, and are removed in the troposphere by reactions with OH radicals. This process is slow, however, and they live long enough (1-20 years) for a large fraction to reach the stratosphere. There are many other organic chlorine compounds, natural and manmade, but they are either produced in small quantities or have short tropospheric lifetimes, and they have no relevance for stratospheric ozone depletion. Vinyl chloride, for example, is produced in much larger quantities than any of the CFC's, but it is quickly destroyed in the troposphere [Brasseur and Solomon]. 2.5) What happens to organic chlorine compounds in the stratosphere? The organic chlorine compounds are dissociated by UV radiation having wavelengths near 230 nm. Since these wavelengths are also absorbed by oxygen and ozone, the organic compounds have to rise high in the stratosphere in order for this photolysis to take place. The initial (or, as chemists say, "nascent") products are a free chlorine atom and an organic radical, for example: CFCl3 + hv -> CFCl2 + Cl The chlorine atom can react with methane to give HCl and a methyl radical: Cl + CH4 -> HCl + CH3 Alternatively, it can react with ozone and nitrogen oxides: Cl + O3 -> ClO + O2 ClO + NO2 -> ClONO2 (There are other pathways, but these are the most important.) The other nascent product (CFCl2 in the above example) undergoes a complicated sequence of reactions that also eventually lead to HCl and ClONO2. Most of the inorganic chlorine in the stratosphere resides in one of these two "reservoirs". The immediate cause of the Antarctic ozone hole is an unusual sequence of reactions, catalyzed by polar stratospheric clouds, that "empty" these reservoirs and produce high concentrations of ozone-destroying Cl and ClO radicals. [Wayne] [Rowland 1989, 1991] 2.6) How is chlorine removed from the atmosphere? The major chlorine reservoir, HCl, is very soluble in water, and is quickly washed out of the troposphere; its lifetime there is estimated to be 1-7 days. On the other hand, its stratospheric lifetime is about 2 years, with the principal sink being transport back down to the troposphere. 2.7) What are the possible sources of the HCl in the _stratosphere_? There are three possibilities: i. It can drift up from the troposphere. ii. It can be produced in the stratosphere, as the end product of photolysis of the organic chlorine compounds, as described above. 3. It can be injected into the stratosphere by a large volcanic eruption. These can be distinguished by measuring how the HCl mixing ratio varies with altitude. In case (1), we expect to see a more-or-less uniform distribution through the stratosphere. In case (2) we expect to see the HCl mixing ratio _increase_ strongly with altitude in the stratosphere, since there is more short-wavelength UV, and thus faster photolysis of the organic compounds, the higher you go. In particular we ought to be able to correlate the concentrations of organic and inorganic chlorine compounds. We can develop a simple model for this if we assume that photolysis instantaneously converts an organochlorine molecule into its final inorganic products. Such a picture would be correct if the chemical reactions following photolysis were very fast compared to vertical transport. (As we have seen above this is only partially true: photolysis pops off a single Cl atom which reaches its final destination quickly, but the remaining Cl atoms are removed by a sequence of slower reactions. This turns out not to be such a serious problem, however, and we can compensate for it in part by measuring the reaction intermediates and including them in the chlorine budget.) If we do assume that the chemistry is fast compared to vertical transport, then we ought to find that at any altitude the mixing ratios of Cl from all species should add up to aconstant, with the relative proportion of inorganic Cl increasing with altitude until eventually there is no organic chlorine left. In case (3) we expect to see irregular behavior: the HCl should be concentrated in the region of the volcanic plume immediately after the eruption, and then diffuse out to a more uniform distribution. We will deal with case (3) in more detail in another section, as a number of complications arise in connection with it. 2.8) What is the source of HCl in the troposphere? The principal source of HCl in the troposphere is acidification of salt spray - reaction of atmospheric sulfuric and nitric acids with chloride ions in aerosols. At sea level, this leads to an HCl mixing ratio of 0.05 - 0.45 ppb, depending strongly upon location (e.g. smaller values over land.) As mentioned above, however, condensation of water vapor efficiently removes HCl from the upper troposphere; in-situ and spectroscopic measurements that the HCl mixing ratio is down below 0.1 ppb at elevations above 3 km, and less than 0.04 ppb at 13.7 km. [Vierkorn-Rudolf et al. ] [Harris et al.] 2.9) How is chlorine distributed in the stratosphere? Over the past 20 years an enormous effort has been devoted to identifying sources and sinks of stratospheric chlorine. The concentrations of the major species have been measured as a function of altitude, by "in-situ" methods ( e.g. collection filters carried on planes and balloons) and by spectroscopic observations from aircraft, balloons, satellites, and the Space Shuttle. The basic trends have been clear since the early 1980's, and they confirm that photolysis of CFC's and related compounds is indeed the major source (scenario (2) above). The HCl distribution in the stratosphere differs markedly from that in the troposphere. The HCl mixing ratio _increases_ rapidly with altitude up to about 35 km, above which it increases more slowly, up to 55 km and beyond. This was noticed as early as 1976 [Farmer et al.] [Eyre and Roscoe] and has been confirmed repeatedly since. The other important inorganic chlorine compound in the stratosphere, Chlorine Nitrate, ClONO2, also increases rapidly in the lower strat., then falls off at higher altitudes. These results strongly suggest that the HCl in the stratosphere is being _produced_ there, not drifting up from below. Let us now look at the organic compounds. Again, there is an enormous body of data, all of which shows that the mixing ratios of the CFC's and CCl4 are _nearly independent of altitude_ in the troposphere, and _decrease rapidly with altitude_ in the stratosphere. The mixing ratios of the more reactive hydrogenated compounds such as CH3CCl3 and CH3Cl drop off somewhat in the troposphere, but also show a much more rapid decrease in the stratosphere. The drop-off in organic chlorine correlates nicely to the increase in inorganic chlorine, confirming the hypothesis that CFC's are being photolyzed as they rise high enough in the stratosphere to experience enough short-wavelength UV. And, _at the bottom of the stratosphere almost all of the chlorine is organic_. (This fact by itself is strong evidence that HCl diffusing up from the troposphere is _not_ the major source of chlorine in the stratosphere.) [Fabian et al. ] [Zander et al. 1987] [Zander et al. 1992] [Penkett et al.] For example, the following is extracted from Tables II and III of [Zander et al. 1992]; they refer to 30 degrees N Latitude in 1985. I have rearranged the tables and rounded some of the numbers, and the arithmetic in the second table is my own. Organic Chlorine, Mixing ratios in ppb Alt., CH3Cl CCl4 CCl2F2 CCl3F CHClF2 CH3CCl3 C3F3Cl3 COFCl km 12.5 .580 .100 .310 .205 .066 0.096 0.021 0.004 15 .515 .085 .313 .190 .066 0.084 0.019 0.010 20 .350 .035 .300 .137 .061 0.047 0.013 0.035 30 - - .030 - .042 - - 0.029 40 - - - - - - - - Inorganic Chlorine and Totals, Mixing ratios in ppb Alt., HCl ClONO2 ClO HOCl || Total Cl, Total Cl, Total Cl Inorganic Organic km 12.5 - - - - - 2.63 2.63 15 .065 - - - 0.065 2.50 2.56 20 .566 .212 - - 0.778 1.78 2.56 30 1.452 1.016 .107 .077 2.652 0.131 2.78 40 2.213 0.010 .234 .142 2.607 - 2.61 (The complete tables give results every 2.5 km from 12.5 to 55km, and also contain a similar inventory of the fluorine compounds. Standard errors on total Cl were estimated to be 0.02-0.04 ppb.) 2.10) Which source of stratospheric chlorine is supported by this evidence? We see that the _total_ Cl concentration is roughly constant at 2.5-2.7 ppb, suggesting that all but about 0.2 ppb has been accounted for. There is nearly quantitative conversion of organic chlorine in the lower stratosphere to inorganic chlorine in the upper stratosphere. Of course this approach - adding up mixing ratios at fixed altitude - is oversimplified. Making it truly quantitative requires a lot of work, accounting for vertical and horizontal transport time scales and complex chemistry. When all of these factors are built into atmospheric models, reasonably good agreement is achieved for the altitude dependence of the major chlorine compounds. [McElroy and Salawich]. We conclude that most of the inorganic chlorine in the stratosphere is produced there, as the end product of photolysis of the organic chlorine compounds. 2.11) How do we know that the CFC's in the stratosphere are being photolyzed? The previous argument - CFC mixing ratios decrease with altitude, inorganic chlorine mixing ratios increase with altitude - certainly suggests that one is being transformed into the other. But there is direct evidence as well: i. Increasing concentrations of HF in the stratosphere - a factor of 4 between 1978 and 1989 [Zander et al. 1990] HF is the major reservoir for Fluorine in the stratosphere, just as HCl is the major chlorine reservoir. The Fluorine budget, as a function of altitude, adds up in much the same way as the Chlorine budget. There are some discrepancies in the lower stratosphere; model calculations predict _less_ HF than is actually observed. [Zander et al. 1992]. ii. Observation of reaction intermediates such as COF2 and COFCl. These are formed when the photolysis products react with oxygen. 2.12) How do the CFCs produced in the Northern Hemisphere get to the Antarctic? Vertical transport into and within the stratosphere is slow. It takes more than 5 years for a CFC molecule released at sea level to rise high enough in the stratosphere to be photolyzed. North-South transport, on the other hand, is faster - there is a bottleneck in the tropics (it can take a year or two to get across the equator) but there is still plenty of time. CFC's are distributed uniformly as a function of latitude [Singh et al.]. 2.13) Isn't it true that volcanoes put much more chlorine into the stratosphere than CFCs? Short Reply: No. They account for at most a few percent of the chlorine in the stratosphere. Long reply: This is one of the most persistent myths in this subject. As is so often the case, there is a grain of truth at the root of the myth. Volcanic gases are rich in Hydrogen Chloride, HCl. As we have discussed, this gas is very soluble in water and is removed from the troposphere on a time scale of 1-7 days, so we can dismiss quietly simmering volcanoes as a stratospheric source, just as we can neglect sea salt acidification and other natural sources of HCl. Remember, the mixing ratio of HCl _decreases_ with altitude in the troposphere, and _increases_ with altitude in the stratosphere. This rules out all processes in which HCl slowly drifts upward from the troposphere. It does not, however, rule out a _major_ volcanic eruption, which could in principle inject HCl directly into the stratosphere. What is a "major" eruption? There is a sort of "Richter scale" for volcanic eruptions, the so-called "Volcanic explosivity index" or VEI. Like the Richter scale it is logarithmic; an eruption with a VEI of 5 is ten times bigger than one with a VEI of 4. To give a sense of magnitude, I list below the VEI for some familiar recent and historic eruptions: Eruption VEI Stratospheric Aerosol, Megatons (Mt) Kilauea 0-1 - Erebus, 1976-84 1-2 - Augustine, 1976 4 0.6 St Helen's, 1980 5 (barely) 0.55 El Chichon, 1982 5 12. Pinatubo, 1991 5-6 20 - 30 Krakatau, 1883 6 50 (est.) Tambora, 1815 7 80-200 (est.) Roughly speaking, an eruption with VEI>3 can penetrate the stratosphere. An eruption with VEI>5 can send a plume up to 25km, in the middle of the ozone layer. Such eruptions occur about once a decade. Since the VEI is not designed specifically to measure a volcano's impact on the stratosphere, I have also listed the total mass of stratospheric aerosols (mostly sulfates) produced by the eruption. Weak, passively degassing volcanoes such as Kilauea and Erebus are far too weak to penetrate the stratosphere, but large eruptions like El Chichon and Pinatubo need to be considered in detail. [Smithsonian] [Symonds et al.] [Sigurdsson] [Pinatubo] [WMO 1988] Before 1982, there were no direct measurements of the amount of HCl that an explosive eruption put into the stratosphere. There were, however, estimates of the _total_ chlorine production from an eruption, based upon such geophysical techniques as analysis of glass inclusions trapped in volcanic rocks. There was much debate about how much of the emitted chlorine reached the stratosphere; estimates ranged from < 0.03 Mt/year [Cadle] to 0.1-1.0 Mt/year [Symonds et al.]. During the 1980's emissions of CFC's and related compounds contributed >1.2 Mt of chlorine per year to the atmosphere. [Prather et al.] This results in an annual flux of >0.3 Mt/yr of chlorine into the stratosphere. The _highest_ estimates ofvolcanic emissions - upper limits calculated by assuming that _all_ of the HCl from a major eruption reached _and stayed_ in the stratosphere - were thus of the same order of magnitude as human sources. (There is no support whatsoever for the claim - occasionally heard in the popular media - that a _single_ volcanic eruption produces ~500 times as much chlorine as a year's worth of CFC production. This wildly inaccurate number is the result of an editorial mistake in a scientific encyclopedia). It is very difficult to reconcile these upper limits with the altitude and time-dependence of stratospheric HCl. The volcanic contribution to the upper stratosphere should come in sudden bursts following major eruptions, and it should initially be largest in the vicinity of the volcanic plume. Since vertical transport in the stratosphere is slow, one would expect to see the altitude profile change abruptly after a major eruption, whereas it has maintained more-or-less the same shape since it was first measured in 1975. One would also not expect a strong correlation between HCl and organochlorine compounds if volcanic injection were contributing ~50% of the total HCl. If half the HCl has an inorganic origin, where is all that _organic_ chlorine going? The issue has now been largely resolved by _direct_ measurements of the stratospheric HCl produced by El Chichon, the most important eruption of the 1980's, and Pinatubo, the largest since 1912. It was found that El Chichon injected *0.04* Mt of HCl [Mankin andCoffey]. The much bigger eruption of Pinatubo produced less [Mankin, Coffey and Goldman], - in fact the authors were not sure that they had measured _any_ significant increase. Analysis of ice cores leads to similar conclusions for historic eruptions [Delmas]. The ice cores show significantly enhanced levels of sulfur following major historic eruptions, but no enhancement in chlorine, showing that the chlorine produced in the eruption did not survive long enough to be transported to polar regions. It is clear, then, that even though major eruptions produce large amounts of chlorine in the form of HCl, most of that HCl either never enters the stratosphere, or is very rapidly removed from it. Recent model calculations [Pinto et al.] [Tabazadeh and Turco] have clarified the physics involved. A volcanic plume contains approximately 1000 times as much water vapor as HCl. As the plume rises and cools the water condenses, capturing the HCl as it does so and returning it to the earth in the extensive rain showers that typically follow major eruptions. HCl can also be removed if it adsorbed on ice or ash particles. Model calculations show that more than 99% of the HCl is removed by these processes, in good agreement with observations. In summary: *Older indirect estimates of the contribution of volcanic eruptions to stratospheric chlorine gave results that ranged from much less than anthropogenic to somewhat larger than anthropogenic. It is very difficult to reconcile the larger estimates with the altitude distribution of inorganic chlorine in the stratosphere, or its steady increase over the past 20 years. Nevertheless, there was an important scientific question here that needed to be resolved by _direct_ measurements in the stratosphere. These measurements have been carried out, and they show that the volcanic contribution is small. *Claims that volcanoes produce more stratospheric chlorine than human activity arise from the careless use of old scientific estimates that have subsequently been refuted by observation. *Claims that a single recent eruption injected ~500 times a year's CFC production into the stratosphere have no scientific basis whatsoever. To conclude, we need to say something about Mt. Erebus. In an article in _21st Century_ (July/August 1989), Rogelio Maduro claimed that this Antarctic volcano has been erupting constantly for the last 100 years, emitting more than 1000 tons of chlorine per day. This claim was repeated in Dixy Lee Ray's _Trashing the Planet_. "21st Century" is published by Lyndon LaRouche's political associates, although LaRouche himself usually keeps a low profile in the magazine. Mt. Erebus has in fact been simmering quietly for over a century but the 1000 tons/day of HCl only applies to an especially active period between 1976 and 1983. By late 1984 emissions had dropped by an order of magnitude, and have remained at low levels since; HCl emissions _at the crater rim_ were 107 tons/day (0.04 Mt/year) in 1986. [Kyle et al.]. (According to a recent report in _Science_, 11 June 1993, Kyle says that emissions are now down to 0.015 Mt/year.) Since this is a passively degassing volcano (VEI=1-2 in the active period), very little of this HCl reaches the stratosphere. The Erebus plume never rises more than 0.5 km above the volcano, and in fact the gas usually just oozes over the crater rim. Indeed, one purpose of the measurements of Kyle et al. was to explain high Cl concentrations in Antarctic snow. The only places where I have ever seen Erebus described as a source of stratospheric chlorine is in LaRouchian publications and in articles and books that, incredibly, consider such documents to be reliable sources. 2.14) How much chlorine comes from rockets and Space Shuttle launches? Very little. In the early 1970's, when very little was known about the role of chlorine radicals in ozone depletion, it was suggested that HCl from solid rocket motors might have a significant effect upon the ozone layer - if not globally, perhaps in the immediate vicinity of the launch. It was immediately shown that the effect was negligible, and this has been repeatedly demonstrated since. Each shuttle launch produces about 68 metric tons of chlorine as HCl; a full year's worth of shuttle and solid- rocket launches produces about 725 tons. This is negligible compared to chlorine emissions in the form of CFC's and related compounds (1.2 million tons/yr in the 1980's, of which ~0.3 Mt reach the stratosphere each year). [Prather et al.] [WMO 1991] 3. BROMINE 3.1) Is bromine important to the ozone destruction process? Br is present in much smaller quantities than Cl, but it is much more destructive on a per-atom basis. There is a large natural source; manmade compounds contribute about 40% of the total. 3.2) How does bromine affect ozone concentrations? Bromine concentrations in the stratosphere are ~150 times smaller than chlorine concentrations. However, atom-for-atom Br is 10-100 times as effective as Cl in destroying ozone. (The reason for this is that there is no stable 'reservoir' for Br in the stratosphere - HBr and BrONO2 are very easily photolyzed so that nearly all of the Br is in a form that can react with ozone. Contrariwise, F is innocuous in the stratosphere because its reservoir, HF, is extremely stable.) So, while Br is less important than Cl, it must still be taken into account. Interestingly, the principal pathway by which Br destroys ozone also involves Cl: BrO + ClO -> Br + Cl + O2 Br + O3 -> BrO + O2 Cl + O3 -> ClO + O2 ---------------------------------- Net: 2 O3 -> 3 O2 [Wayne p. 164] [Solomon] so reducing stratospheric chlorine concentrations will, as a side-effect, slow down the bromine pathways as well. 3.3) Where does the bromine come from? The largest source of stratospheric Bromine is methyl bromide, CH3Br. Much of this is natural (as with CH3Cl), but 30 - 60% is manmade. [Khalil et al.] It is widely used as a fumigant. Another important source is the family of "halons". Like CFC's these compounds have long atmospheric lifetimes (72 years for CF3Br) and very little is lost in the troposphere. [Wayne p. 167]. At the bottom of the stratosphere the total Br mixing ratio is ~20 ppt, of which ~8 ppt is manmade. [AASE] Uncertainties in these numbers are relatively larger than for Cl, because the absolute quantities are so much smaller, and we should expect to see them change. 4. REFERENCES FOR PART II A remark on references: they are neither representative nor comprehensive. There are _hundreds_ of people working on these problems. For the most part I have limited myself to papers that are (1) widely available (if possible, _Science_ or _Nature_ rather than archival sources such as _J. Geophys. Res._) and (2) directly related to the "frequently asked questions". (In this part, I have had to refer to archival journals more often than I would have liked, since in many cases that is the only place where the question is addressed in satisfactory detail.) Readers who want to see "who did what" should consult the review articles listed below, or, if they can get them, the extensively documented WMO reports. General Reading: [Graedel and Crutzen] T. E. Graedel and P. J. Crutzen, _Atmospheric Change: an Earth System Perspective_, Freeman, 1993. [Rowland 1989] F. S. Rowland, "Chlorofluorocarbons and the depletion of stratospheric ozone", _Am. Sci._ _77_, 36, 1989. -------------------------------- Books and Review Articles: [Brasseur and Solomon] G. Brasseur and S. Solomon, _Aeronomy of the Middle Atmosphere_, 2nd Edition, D. Reidel, 1986. [McElroy and Salawich] M. McElroy and R. Salawich, "Changing Composition of the Global Stratosphere", _Science_ _243, 763, 1989. [Rowland 1991] F. S. Rowland, "Stratospheric Ozone Depletion", _Ann. Rev. Phys. Chem._ _42_, 731, 1991. [Solomon] S. Solomon, "Progress towards a quantitative understanding of Antarctic ozone depletion", _Nature_ _347_, 347, 1990. [Wayne] R. P. Wayne, _Chemistry of Atmospheres_, 2nd. Ed., Oxford, 1991. [WMO 1988] World Meteorological Organization, _Report of the International Ozone Trends Panel_, Report # 18 [WMO 1991] World Meteorological Organization, _Scientific Assessment of Ozone Depletion: 1991_, Report # 25 ----------------------------- More specialized articles: [AASE] End of Mission Statement, second airborne arctic stratospheric expedition, NASA 30 April 1992. [Cadle] R. Cadle, "Volcanic emissions of halides and sulfur compounds to the troposphere and stratosphere", J. Geophys. Res. _80_, 1651, 1975] [Delmas] R. J. Delmas, "Environmental Information from Ice Cores", _Reviews of Geophysics_ _30_, 1, 1992. [Eyre and Roscoe] J. Eyre and H. Roscoe, "Radiometric measurement of stratospheric HCl", _Nature_ _266_, 243, 1977. [Fabian et al. 1979] P. Fabian, R. Borchers, K.H. Weiler, U. Schmidt, A. Volz, D.H. Erhalt, W. Seiler, and F. Mueller, "Simultaneously measured vertical profile of H2, CH4, CO, N2O, CFCl3, and CF2Cl2 in the mid-latitude stratosphere and troposphere", J. 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