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Subject: Ozone Depletion FAQ Part II: Stratospheric Chlorine and Bromine

This article was archived around: 24 Dec 1997 20:49:55 GMT

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Archive-name: ozone-depletion/stratcl Last-modified: 16 Dec 1997 Version: 5.9
----------------------------- Subject: How to get this FAQ These files are posted to the newsgroups sci.environment, sci.answers, and news.answers. They are also archived at a variety of sites. These archives work by automatically downloading the faqs from the newsgroups and reformatting them in site-specific ways. They usually update to the latest version within a few days of its being posted, although in the past there have been some lapses; if the "Last-Modified" date in the FAQ seems old, you may want to see if there is a more recent version in a different archive. Many individuals have archived copies on their own servers, but these are often seriously out of date and in general are not recommended. A. World-Wide Web (Limited) hypertext versions, with embedded links to some of the on-line resources cited in the faqs, can be found at: http://www.faqs.org/faqs/ozone-depletion/ http://www.cis.ohio-state.edu/hypertext/faq/usenet/ozone-depletion/top.html http://www.lib.ox.ac.uk/internet/news/faq/sci.environment.html http://www.cs.ruu.nl/wais/html/na-dir/ozone-depletion/.html Plaintext versions can be found at: ftp://rtfm.mit.edu/pub/usenet/news.answers/ozone-depletion/ ftp://ftp.uu.net/usenet/news.answers/ozone-depletion/ ---- B. Anonymous ftp To rtfm.mit.edu, in the directory /pub/usenet/news.answers/ozone-depletion To ftp.uu.net, in the directory /usenet/news.answers/ozone-depletion Look for the four files named intro, stratcl, antarctic, and uv. ---- C. Regular email Send the following messages to mail-server@rtfm.mit.edu: send usenet/news.answers/ozone-depletion/intro send usenet/news.answers/ozone-depletion/stratcl send usenet/news.answers/ozone-depletion/antarctic send usenet/news.answers/ozone-depletion/uv Leave the subject line blank. If you want to find out more about the mail server, send a message to it containing the word "help". ----------------------------- Subject: Copyright Statement *********************************************************************** * Copyright 1997 Robert Parson * * * * This file may be distributed, copied, and archived. All such * * copies must include this notice and the paragraph below entitled * * "Caveat". Reproduction and distribution for personal profit is * * not permitted. If this document is transmitted to other networks or * * stored on an electronic archive, I ask that you inform me. I also * * ask you to keep your archive up to date; in the case of world-wide * * web pages, this is most easily done by linking to the master at the * * ohio-state http URL instead of storing local copies. Finally, I * * request that you inform me before including any of this information * * in any publications of your own. Students should note that this * * is _not_ a peer-reviewed publication and may not be acceptable as * * a reference for school projects; it should instead be used as a * * pointer to the published literature. In particular, all scientific * * data, numerical estimates, etc. should be accompanied by a citation * * to the original published source, not to this document. * *********************************************************************** ----------------------------- Subject: General Information 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. Instead of relying upon qualitative statements about relative lifetimes, solubilities, and so forth, I have tried to give a sense of the actual magnitudes involved. Fundamentally, this Part of the FAQ is about measurements, and I have therefore included some tables to illustrate trends; the data that I reproduce is in every case a small fraction of what has actually been published. In the first section I state the present assessment of stratospheric chlorine sources and trends, and then in the next section I discuss the evidence that leads to those conclusions. After a brief discussion of Bromine and Iodine in section 3, I answer the most familiar challenges that have been raised in section 4. Only these last are actually "Frequently Asked Questions"; however I have found the Question/Answer format to be useful in clarifying the issues in my mind even when the questions are rhetorical, so I have kept to it. ----------------------------- Subject: Caveats, Disclaimers, and Contact Information | 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, especially errors. | On the other hand, if you find this document in an online archive | somewhere, I am not responsible for any *other* information that may | happen to reside in that archive. This file should not be cited as | a reference in publications off the net; rather, it should 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) rparson@spot.colorado.edu Robert.Parson@colorado.edu ----------------------------- Subject: TABLE OF CONTENTS How to get this FAQ Copyright Notice General Information Caveats, Disclaimers, and Contact Information TABLE OF CONTENTS 1. CHLORINE IN THE STRATOSPHERE - OVERVIEW 1.1) Where does the Chlorine in the stratosphere come from? 1.2) How has stratospheric chlorine changed with time? 1.3) How will stratospheric chlorine change in the future? 2. THE CHLORINE CYCLE 2.1) What are the sources of chlorine in the troposphere? 2.2) In what molecules is _stratospheric_ chlorine found? 2.3) What happens to organic chlorine in the stratosphere? 2.4) How do we know that CFC's are photolyzed in the stratosphere? 2.5) How is chlorine removed from the stratosphere? 2.6) How is chlorine distributed in the stratosphere? 2.7) What happens to the Fluorine from the CFC's? 2.8) Summary of the Evidence 3. BROMINE AND IODINE 3.1) Does Bromine contribute to ozone depletion? 3.2) How does bromine affect ozone? 3.3) Where does the bromine come from? 3.4) How about Iodine? 4. COMMONLY ENCOUNTERED OBJECTIONS 4.1) CFC's are 4-8 times heavier than air, so how can they 4.2) CFCs are produced in the Northern Hemisphere, so how do they get down to the Antarctic? 4.3) Sea salt puts more chlorine into the atmosphere than CFC's. 4.4) Volcanoes put more chlorine into the stratosphere than CFC's. 4.5) Space shuttles put a lot of chlorine into the stratosphere. 4.6) Most CFC's are decomposed by soil bacteria and other terrestrial mechanisms. 5. REFERENCES FOR PART II Introductory Reading Books and Review Articles More specialized references ----------------------------- Subject: 1. CHLORINE IN THE STRATOSPHERE - OVERVIEW ----------------------------- Subject: 1.1) Where does the Chlorine in the stratosphere come from? ~80% from CFC's and related manmade organic chlorine compounds, such as carbon tetrachloride and methyl chloroform ~15-20% from methyl chloride (CH3Cl), most of which is natural. A few % from inorganic sources, such as volcanic eruptions. [Russell et al. 1996] [WMO 1991, 1994] [Solomon] [AASE] [Rowland 1989,1991] [Wayne] These estimates are based upon >20 years' worth of measurements of organic and inorganic chlorine-containing compounds in the earth's troposphere and stratosphere. Particularly informative is the dependence of these compounds' concentrations on altitude and their increase with time. The evidence is summarized in section 2 of this FAQ. ----------------------------- Subject: 1.2) How has stratospheric chlorine changed with time? 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 major increases. On the other hand, emissions of CFC's and related manmade compounds have increased dramatically, reaching a peak in 1987. Extrapolating back, one infers that total stratospheric chlorine has increased by a factor of 4 since 1950. ----------------------------- Subject: 1.3) How will stratospheric chlorine change in the future? Since the 1987 Montreal Protocol (see Part I) production of CFC's and related compounds has been decreasing rapidly, and in consequence their rate of growth in the atmosphere has fallen dramatically [Elkins et al. 1993] [Prinn et al. 1995] [Montzka et al. 1996] The data below show that CFC-12 concentrations have nearly stabilized while CFC-11 has actually begun to decrease. Growth Rate, pptv/yr Year CFC-12 CFC-11 1977-84 17 9 [Elkins et al. 1993] 1985-88 19.5 11 " 1993 10.5 2.7 " 1995 5.9 -0.6 [Montzka et al. 1996] Methyl chloroform and carbon tetrachloride are also decreasing, while CFC-113 has stabilized. Overall, tropospheric chlorine from halocarbons peaked in 1995 and has begun to decline. The time scale for mixing tropospheric and lower stratospheric air is about 3-5 years, so _stratospheric_ chlorine is expected to peak in about 1998 and then to decline slowly, on a time scale of about 50 years. [WMO 1994] [Montzka et al. 1996] ----------------------------- Subject: 2. THE CHLORINE CYCLE ----------------------------- Subject: 2.1) What are the sources of chlorine in the troposphere? Let us divide the chlorine-containing compounds found in the atmosphere into two groups, "organic chlorine" and "inorganic chlorine". The most important inorganic chlorine compound in the troposphere is hydrogen chloride, HCl. Its principal source 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 ppbv, depending strongly upon location (e.g. smaller values over land.) However, HCl dissolves very readily in water (giving hydrochloric acid), and condensation of water vapor efficiently removes HCl from the _upper_ troposphere. Measurements show that the HCl mixing ratio is less than 0.1 ppbv at elevations above 7 km, and less than 0.04 ppbv at 13.7 km. [Vierkorn-Rudolf et al.] [Harris et al.] There are many volatile organic compounds containing chlorine, but most of them are quickly decomposed by the natural oxidants in the troposphere, and the chlorine atoms that were in these compounds eventually find their way into HCl or other soluble species and are rained out. The most important exceptions are: ChloroFluoroCarbons, of which the most important are CF2Cl2 (CFC-12), CFCl3 (CFC-11), and 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. As a result of this enormous difference in atmospheric lifetimes, there is more chlorine present in the lower atmosphere in halocarbons than in HCl, even though HCl is produced in much larger quantities. Total tropospheric organic chlorine amounted to ~3.8 ppbv in 1989 [WMO 1991], and this mixing ratio is very nearly independent of altitude throughout the troposphere. Methyl Chloride, the only ozone-depleting chlorocarbon with a major natural source, makes up 0.6 ppbv of this total. Compare this to the tropospheric HCl mixing ratios given above: < 0.5 ppbv at sea level, < 0.1 ppbv at 3 km, and < 0.04 ppbv at 10 km. ----------------------------- Subject: 2.2) In what molecules is _stratospheric_ chlorine found? The halocarbons described above are all found in the stratosphere, and in the lower stratosphere they are the dominant form of chlorine. At higher altitudes inorganic chlorine is abundant, most of it in the form of HCl or of _chlorine nitrate_, ClONO2. These are called "chlorine reservoirs"; they do not themselves react with ozone, but they generate a small amount of chlorine-containing radicals - Cl, ClO, ClO2, and related species, referred to collecively as the "ClOx family" - which do. An increase in the concentration of chlorine reservoirs leads to an increase in the concentration of the ozone-destroying radicals. ----------------------------- Subject: 2.3) What happens to organic chlorine 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 to give ClO: Cl + O3 -> ClO + O2 which can go on to react with O to release Cl again, closing a catalytic cycle: ClO + O -> Cl + O2 or can react with nitrogen dioxide to form the metastable compound chlorine nitrate: 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 leads to HCl and ClONO2. Most of the inorganic chlorine in the stratosphere therefore 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] ----------------------------- Subject: 2.4) How do we know that CFC's are photolyzed in the stratosphere? The UV photodissociation cross-sections for the halocarbons have been measured in the laboratory; these tell us how rapidly they will dissociate when exposed to light of a given wavelength and intensity. We can combine this with the measured intensity of radiation in the stratosphere and deduce the way in which the mixing ratio of a given halocarbon should depend upon altitude. Since there is almost no <230 nm radiation in the troposphere or in the lowest parts of the stratosphere, the mixing ratio should be independent of altitude there. In the middle stratosphere the mixing ratio should drop off quickly, at a rate which is determined by the photodissociation cross-section. Thus each halocarbon has a characteristic signature in its mixing ratio profile, which can be calculated. Such calculations (first carried out in the mid 1970's) agree well with the distributions presented in the next section. There is direct evidence as well. Photolysis removes a chlorine atom, leaving behind a reactive halocarbon radical. The most likely fate of this radical is reaction with oxygen, which starts a long chain of reactions that eventually remove all the chlorine and fluorine. Most of the intermediates are reactive free radicals, but two of them, COF2 and COFCl, are fairly stable and live long enough to be detected - and have been. [Zander et al. 1992, 1994]. ----------------------------- Subject: 2.5) How is chlorine removed from the stratosphere? Since the stratosphere is very dry, water-soluble compounds are not quickly washed out as they are in the troposphere. The stratospheric lifetime of HCl is about 2 years; the principal sink is transport back down to the troposphere. ----------------------------- Subject: 2.6) 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. From all this work we now have a clear and consistent picture of the processes that carry chlorine through the stratosphere. Let us begin by asking where inorganic chlorine is found. In the troposphere, the HCl mixing ratio decreased markedly with increasing altitude. In the stratosphere, on the other hand, it _increases_ with altitude, rapidly up to about 35 km, and then more slowly up to 55km and beyond. This was noticed as early as 1976 [Farmer et al.] [Eyre and Roscoe] and has been confirmed repeatedly since. Chlorine Nitrate (ClONO2), the other important inorganic chlorine compound in the stratosphere, also increases rapidly in the lower stratosphere, and then falls off at higher altitudes. These results strongly suggest that HCl in the stratosphere is being _produced_ there, not drifting up from below. Let us now look at the organic source gases. Here, the data show 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 turnover in organic chlorine correlates nicely with 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. At the bottom of the stratosphere almost all of the chlorine is organic, and at the top it is all inorganic. [Fabian et al. ] [Zander et al. 1987, 1992, 1996] [Penkett et al.] Finally, there are the stable reaction intermediates, COF2 and COFCl. These have been found in the lower and middle stratosphere, exactly where one expects to find them if they are produced from organic source gases and eventually react to give inorganic chlorine. 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 and Intermediates, Mixing ratios in ppbv Alt., CH3Cl CCl4 CCl2F2 CCl3F CHClF2 CH3CCl3 C2F3Cl3 || COFCl km 12.5 .580 .100 .310 .205 .066 .096 .021 || .004 15 .515 .085 .313 .190 .066 .084 .019 || .010 20 .350 .035 .300 .137 .061 .047 .013 || .035 25 .120 - .175 .028 .053 .002 .004 || .077 30 - - .030 - .042 - - || .029 40 - - - - - - - || - Inorganic Chlorine and Totals, Mixing ratios in ppbv 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 25 1.027 .849 .028 .032 || 1.936 0.702 2.64 30 1.452 1.016 .107 .077 || 2.652 0.131 2.78 40 2.213 0.010 .234 .142 || 2.607 - 2.61 I have included the intermediate COFCl in the Total Organic column. It should be noted that COFCl was not measured directly in this experiment, although the related intermediate COF2 was. This is just an excerpt. The original tables give results every 2.5km from 12.5 to 55km, together with a similar inventory for Fluorine. Standard errors on total Cl were estimated to be 0.02-0.04 ppbv. [Zander et al. 1996] provide a similar inventory for the year 1994; once again the total chlorine at any altitude is approximately constant, but at ~3.5 ppbv instead of ~2.6 ppbv, indicative of the increase in anthropogenic halocarbons between 1985 and 1994. Notice that the _total_ chlorine at any altitude is nearly constant at ~2.5-2.8 ppbv. This is what we would expect if the sequence of reactions that leads from organic sources to inorganic reservoirs was fast compared to vertical transport. Our picture, then, would be of a swarm of organic chlorine molecules slowly spreading upwards through the stratosphere, being converted into inorganic reservoir molecules as they climb. In fact this oversimplifies things - photolysis pops off a single Cl atom which does reach its final destination quickly, but the remaining Cl atoms are removed by a sequence of slower reactions. Some of these reactions involve compounds, such as NOx, which are not well-mixed; moreover, "horizontal" transport does not really take place along surfaces of constant altitude, so chemistry and atmospheric dynamics are in fact coupled together in a complicated way. These are the sorts of issues that are addressed in atmospheric models. Nevertheless, this simple picture helps us to understand the qualitative trends, and quantitative treatments confirm the conclusions [McElroy and Salawich] [Russell et al. 1996]. 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. ----------------------------- Subject: 2.7) What happens to the Fluorine from the CFC's? Most of it ends up as Hydrogen Fluoride, HF. The total amount of HF in the stratosphere increased by a factor of 3-4 between 1978 and 1989 [Zander et al., 1990] [Rinsland et al.]; the relative increase is larger for HF than for HCl (a factor of 2.2 over the same period) because the natural source, and hence the baseline concentration, is much smaller. For the same reason, the _ratio_ of HF to HCl has increased, from 0.14 in 1977 to 0.23 in 1990. As discussed above, the decomposition of CFC's in the stratosphere produces reaction intermediates such as COF2 and COFCl which have been detected in the stratosphere. COF2 in particular is relatively stable and makes a significant contribution to the total fluorine; the total amount of COF2 in the stratosphere increased by 60% between 1985 and 1992 [Zander et al. 1994] The total Fluorine budget, as a function of altitude, adds up in much the same way as the chlorine budget. [Zander et al. 1992, 1994] [Luo et al.] The most comprehensive measurements of stratospheric HF are those made by the Halogen Occultation Experiment (HALOE) on the UARS satellite [Luo et al.] [Russell et al. 1996] Information about HALOE is available on the World-Wide-Web at http://haloedata.larc.nasa.gov/home.html . ----------------------------- Subject: 2.8) Summary of the Evidence a. Inorganic chlorine, primarily of natural origin, is efficiently removed from the troposphere; organic chlorine, primarily anthropogenic, is not, and in the upper troposphere organic chlorine dominates overwhelmingly. b. In the stratosphere, organic chlorine decreases with altitude, since at higher altitudes there is more short-wave UV available to photolyze it. Inorganic chlorine _increases_ with altitude. At the bottom of the stratosphere essentially all of the chlorine is organic, at the top it is all inorganic, and reaction intermediates such as COF2 are found at intermediate altitudes. c. Both HCl and HF in the stratosphere have been increasing steadily, in a correlated fashion, since they were first measured in the 1970's. Reaction intermediates such as COF2 are also increasing. ----------------------------- Subject: 3. BROMINE ----------------------------- Subject: 3.1) Does Bromine contribute to ozone depletion? 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. In the antarctic chlorine is more important than Bromine, but at middle latitudes their effects are comparable. ----------------------------- Subject: 3.2) How does bromine affect ozone? 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, one principal pathway by which Br destroys ozone also involves Cl: BrO + ClO -> BrCl + O2 BrCl + hv -> Br + Cl 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. Another important mechanism combines Br with HOx radicals: BrO + HO2 -> HOBr HOBr + hv -> Br + OH Br + O3 -> BrO + O2 OH + O3 -> HO2 + O2 ----------------------- Net: 2 O3 -> 3 O2 ----------------------------- Subject: 3.3) Where does the bromine come from? a.) Methyl Bromide The largest source of stratospheric Bromine is methyl bromide, CH3Br. It is also the most poorly characterized source. Much of it is naturally produced in the oceans, but a significant portion (30-60%, according to [Khalil et al.) is manmade; it is widely used as a fumigant. Methyl bromide is also produced during biomass burning, which can be either natural or anthropogenic [Mano and Andreae]. The 1994 assessment from the World Meteorological Organization [WMO 1994] estimates the major sources as: Oceans: 60-160 ktons/yr Fumigation: 20-60 ktons/yr Biomass burning: 10-50 ktons/yr . This assessment estimates the atmospheric lifetime of methyl bromide to be 0.8-1.7 years (best estimate 1.3 years) and its ozone depletion potential to be about 0.6 . However, recent laboratory and field experiments [Shorter et al.] indicate that large amounts of methyl bromide are consumed by soil bacteria. This would push the atmospheric lifetime down to the lower limit of 0.8 years, and reduce the ozone depletion potential to 0.4; it may also require adjustments in the estimated sources. Methyl bromide is also produced in the combustion of leaded gasolines, which use ethylene dibromide as a scavenger. One estimate for the methyl bromide emissions from this source gave 9-22 ktons/yr, but another estimate gave only 0.5-1.5 ktons/yr. b.) Halons Another important Bromine source is the family of "halons", widely used in fire extinguishers. Like CFC's these compounds have long atmospheric lifetimes (65 years for CF3Br) and very little is lost in the troposphere. [WMO 1994]. Halons are scheduled for phase-out under the Montreal Protocol, and their rate of increase in the atmosphere has slowed by a factor of three since 1989. (Before then halon concentrations were increasing by 15-20% _per year_.) ----------------------------- Subject: 3.4) And how about about Iodine? Since Chlorine and Bromine radicals both enter into ozone-destroying catalytic cycles, it comes as no surprise that Iodine can do so as well. One possible mechanism is: ClO + IO -> Cl + I + O2 Cl + O3 -> ClO + O2 I + O3 -> IO + O2 _______________________ Net: 2 O3 -> 3 O2 Note that this is precisely analogous to the Bromine/Chlorine cycle given in section 3.2; the Iodine acts in concert with Chlorine. There are also cycles in which Iodine and Bromine, and Iodine and OH, act together. At present it is not known whether there is enough Iodine in the stratosphere to make these reactions important for the overall ozone balance. The principle source of atmospheric iodine is methyl iodide, produced in large quantities by marine biota. Methyl iodide, like methyl chloride and bromide, is insoluble in water and is thus not "frozen out" at the tropopause; however it has a much shorter atmospheric lifetime so only a small fraction survives long enough to reach the stratosphere. It has recently been suggested [Solomon et al. 1994a,b] that this small fraction may nevertheless be large enough to influence ozone depletion in the lowest part of the stratosphere. (Current models using only chlorine and bromine chemistry predict significantly less ozone loss in these regions than has been observed.) More measurements will be needed to resolve this issue. Anthropogenic sources of stratospheric iodine are negligible. Trifluoromethyliodide, CF3I, has been suggested as a substitute for halons, since unlike halons, CF3I has a short atmospheric lifetime. [Solomon et al. 1994b] estimate its ozone depletion potential (ODP) to be less than 0.008 and probably less than 0.0001; CF3Br, in contrast, has an ODP of 7.8. Iodine may be accelerating the rate at which (mostly) anthropogenic chlorine and (partly) anthropogenic bromine destroy ozone, but iodine in itself is not an anthropogenic influence. ----------------------------- Subject: 4. COMMONLY ENCOUNTERED OBJECTIONS ----------------------------- Subject: 4.1) CFC's are 4-8 times heavier than air, so how can they reach the stratosphere? This is answered in Part I of this FAQ, section 1.3. Briefly, atmospheric gases do not segragate by weight in the troposphere and the stratosphere, because the mixing mechanisms (convection, "eddy diffusion") do not distinguish molecular masses. ----------------------------- Subject: 4.2) CFCs are produced in the Northern Hemisphere, so how do they get down 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, in both troposphere and stratosphere, 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 almost uniformly as a function of latitude, with a gradient of ~10% from Northern to Southern Hemispheres. [Singh et al. 1979] [Elkins et al. 1993] ----------------------------- Subject: 4.3) Sea salt puts more chlorine into the atmosphere than CFC's. True, but not relevant because this chlorine is in a form (HCl) that is rapidly removed from the troposphere. Even at sea level there is more chlorine present in organic compounds than in HCl, and in the upper troposphere and lower stratosphere organic chlorine dominates overwhelmingly. See section 2.1 above. ----------------------------- Subject: 4.4) Volcanoes put more chlorine into the stratosphere than CFC's. Short Reply: False. Volcanoes 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 area. As is so often the case, there is a seed 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 and other natural sources of HCl. (In fact tropospheric HCl from volcanoes is neglible compared to HCl from sea salt.) However, we cannot use this argument to dismiss MAJOR volcanic eruptions, which can in principle inject HCl directly into the middle 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 30 Krakatau, 1883 6 50 (estimated) Tambora, 1815 7 80-200 (estimated) [Smithsonian] [Symonds et al.] [Sigurdsson] [Pinatubo] [WMO 1988] [Bluth et al.] [McCormick et al. 1995] 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. (Note that St. Helens produced much less aerosol than El Chichon - St. Helens blew out sideways, dumping a large ash cloud over eastern Washington, rather than ejecting its gases into the stratosphere.) Passively degassing volcanoes such as Kilauea and Erebus are far too weak to penetrate the stratosphere, but explosive eruptions like El Chichon and Pinatubo need to be considered in detail. 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. [Cadle] [Johnston] [Sigurdsson] [Symonds et al.] 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 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 of volcanic 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 that a _single_ recent eruption produced ~500 times as much chlorine as a year's worth of CFC production. This wildly inaccurate number appears to have originated as an editorial mistake in a scientific encyclopedia.) It is very difficult to reconcile the higher estimates 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 of the HCl has an inorganic origin, where is all that _organic_ stratospheric 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 and Coffey]. The much bigger eruption of Pinatubo produced less [Mankin, Coffey and Goldman] [Wallace and Livingston 1992], - 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 is 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 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, these estimates raised an important scientific question that needed to be resolved by _direct_ measurements in the stratosphere. * Direct measurements on El Chichon, the largest eruption of the 1980's, and on Pinatubo, the largest since 1912, 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 since 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. Mt. Erebus has in fact been simmering quietly for over a century [ARS] but the estimate of 1000 tons/day of HCl only applied to an especially active period between 1976 and 1983 [Kyle et al. 1990]. Moreover, that estimate has been since been reduced to 167 tons/day (0.0609 Mt/year). 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 19 tons/day (0.007 Mt/year) in 1986, and 36 tons/day (0.013 Mt/year) in 1991. [Zreda-Gostynska et al.] 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. ----------------------------- Subject: 4.5) Space shuttles put a lot of chlorine into the stratosphere. Simply false. 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 200 metric tons of chlorine as HCl, of which about one-third, or 68 tons, is injected into the stratosphere. Its residence time there is about three years. A full year's US schedule of shuttle and solid rocket launches injects 725 tons of chlorine into the stratosphere. The European Space Agency's Ariane rocket makes a similar contribution, with 57 tons of HCl deposited in the stratosphere for each launch. These inputs are negligible compared to chlorine emissions in the form of CFC's and related compounds (~ 1.0 million tons/yr in the 1980's, of which ~0.3 Mt reach the stratosphere each year). It is also small in comparison to natural sources of stratospheric chlorine, which amount to about 75,000 tons per year. [Prather et al.] [WMO 1991] [Ko et al.] See also the sci.space FAQ, Part 10, "Controversial Questions", available by anonymous ftp from rtfm.mit.edu in the directory pub/usenet/news.answers/space/controversy, or on the world-wide web at: http://www.cis.ohio-state.edu/hypertext/faq/usenet/space/controversy/faq.html Subject: 4.6) Most CFC's are decomposed by soil bacteria and other terrestrial mechanisms. This argument is based upon a misinterpretation of measurements made by Khalil and Rasmussen. These scientists did show that some CFC's such as CFC-11 and CFC-12 (but not CFC-113) were taken up by soils in Australia [Khalil and Rasmussen 1989] and by rice paddies in China [Khalil et al. 1990]. However, the amounts that are disposed of in this way are small compared to the amounts that end up in the stratosphere. A recent summary [Khalil and Rasmussen 1993] concludes that out of a total of 9152 Gigagrams (Gg) of CFC-11 produced, only 1 Gg has been removed by soils and 33 Gg reside in the oceans; in contrast, 1709 Gg have been photolyzed in the stratosphere, 741 Gg are presently in the stratosphere, and 5360 Gg are in the troposphere. Most of the remainder is still trapped in foams, aerosols, etc. and has not yet been released to the atmosphere. (In contrast, soil bacteria do appear to consume large quantities of methyl bromide, CH3Br. [Shorter et al.]) ----------------------------- Subject: 5. 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. ----------------------------- Subject: Introductory Reading [Graedel and Crutzen] T. E. Graedel and P. J. Crutzen, _Atmospheric Change: an Earth System Perspective_, Freeman, 1993. [Rowland 1989] F. S. 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