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From: (Robert Parson) Newsgroups: sci. environment, sci. answers, news. answers Subject: Ozone Depletion FAQ Part I: Introduction to the Ozone Layer Organization: University of Colorado, Boulder Expires: Sun, 1 Feb 1998 00: 00: 00 GMT Message-ID: 67 rss$ Summary: This is the first of four files dealing with stratospheric ozone depletion.
It provides scientific background for the more detailed questions in the other three parts. Keywords: ozone layer cfc stratosphere depletion 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is a more recent version Many individuals have archived copies on their own servers, but these are often seriously out of date and in general are not recommended. (Limited) hypertext versions, with embedded links to some of the on-line resources cited in the faqs, can be found at: web web web web ftp: //rtfm. mit. edu / pub /usenet / news . answers / ozone -d election/ ftp: //ftp. uu.
net / usenet /news. answers / ozone -delete on/ 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, stretch, antarctic, and uv. Send the following messages to mail-: send usenet / news . answers / ozone -depletion / intro send usenet / news . answers / ozone -depletion / stretch send usenet / news .
answers / ozone -depletion / antarctic send usenet / news . answers / ozone -depletion / uv If you want to find out more about the mail server, send a message to it containing the word "help." Copyright 1997 Robert Parson This file may be distributed, copied, and archived. All such copies must include this notice, the preceding instructions on how to obtain a current version, and the paragraph below entitled "Caveat. " 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 one of the archives listed above 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. This is the first of four FAQ files dealing with stratospheric ozone depletion. This part deals with basic scientific questions about the ozone layer, and serves as an introduction to the remaining parts which are more specialized. Part II deals with sources of stratospheric chlorine and bromine, part III with the Antarctic Ozone Hole, and Part IV with the properties and effects of ultraviolet radiation.
The later parts are mostly independent of each other, but they all refer back. to Part I. I emphasize physical and chemical mechanisms rather than biological effects, although I make a few remarks about the latter in part IV. I have little to say about legal and policy issues other than a very brief summary at the end of part I. The overall approach I take is conservative. I concentrate on what is known and on most probable, rather than worst-case, scenarios.
For example, I have relatively little to say about the effects of UV radiation on terrestrial plants - this does not mean that the effects are small, it means that they are as yet not well quantified (and moreover, I am not well qualified to interpret the literature. ) Policy decisions must take into account not only the most probable scenario, but also a range of less probable ones. There have been surprises, mostly unpleasant, in this field in the past, and there are sure to be more in the future. 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 | reactions 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 all 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 document should not be | cited in publications off the net; rather, it should be used as a | pointer to the published literature. Corrections and comments are welcomed. Department of Chemistry and Biochemistry University of Colorado (for which I do not speak) This FAQ is dedicated to the memory of Carl J. Ly censored , who was one of the first people to read it through carefully and who helped me to clarify my presentation. Carl was not a scientist, but he had a profound understanding of and love for science and an outstanding talent for presenting scientific results in non-technical language.
Caveats, Disclaimers, and Contact Information 1. 2) How is the composition of air described? 1. 3) How does the composition of the atmosphere change with 2. 2) How much ozone is in the layer, and what is a 2. 3) How is ozone distributed in the stratosphere? 2. 4) How does the ozone layer work? 2. 5) What sorts of natural variations does the ozone layer show? 2. 5. a) Regional and Seasonal Variation 2. 8) What is an "Ozone Depletion Potential?" 2. 9) What about HCFC's and HFC's? Do they destroy ozone? 2. 10) IS the ozone layer getting thinner? 2. 11) Is the middle-latitude ozone loss due to CFC emissions? 2. 12) If the ozone is lost, won't the UV light just penetrate 2. 13) Do Space Shuttle launches damage the ozone layer? 2. 14) Will commercial supersonic aircraft damage the ozone layer? 2. 15) What is being done about ozone depletion? Subject: 1. 1) What is the stratosphere?
The stratosphere extends from about 15 km to 50 km. In the stratosphere temperature increases with altitude, due to the absorption of UV light by oxygen and ozone. This creates a global "inversion layer" which impedes vertical motion into and within the stratosphere - since warmer air lies above colder air, convection is inhibited. The word "stratosphere" is related to the word The stratosphere is often compared to the "troposphere", which is the atmosphere below about 15 km. The boundary - called the "tropopause" - between these regions is quite sharp, but its precise location varies between ~ 9 and ~ 18 km, depending upon latitude and season.
The prefix "top" refers to change: the troposphere is the part of the atmosphere in which weather occurs. This results in rapid mixing of tropospheric air. Above the stratosphere lie the "mesosphere", ranging from ~ 50 to ~ 100 km, in which temperature decreases with altitude; the "thermosphere", ~ 100 - 400 km, in which temperature increases with altitude again, and the "exosphere", beyond ~ 400 km, which fades into the background of interplanetary space. In the upper mesosphere and thermosphere electrons and ions are abundant, so these regions are also referred to as the "ionosphere." In technical literature the term "lower atmosphere" is synonymous with the troposphere, "middle atmosphere" refers to the stratosphere and mesosphere, while "upper atmosphere" is usually reserved for the thermosphere and exosphere. This usage is not universal, however, and one occasionally sees the term "upper atmosphere" used to describe everything above the troposphere (for example, in NASA's Upper Atmosphere Research Satellite, UARS. ) Subject: 1. 2) How is the composition of air described? (Or, 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 "mole fraction", the relative number of molecules of a given type in an air sample. Atmospheric scientists usually call a mole fraction a "mixing ratio." Typical units for mixing ratios are parts-per-million, billion, or trillion by volume, designated as "ppmv", "prev", and "put" respectively. (The expression "by volume" reflects Avogadro's Law - for an ideal gas mixture, equal volumes contain equal numbers of molecules - and serves to distinguish mixing ratios from "mass fractions" which are given as parts-per-million by weight. ) Thus when someone says the mixing ratio of hydrogen chloride at 3 km is 0. 1 prev, he means that 1 out of every 10 billion molecules in an air sample collected at that altitude will be an HCl molecule. Subject: 1. 3) How does the composition of the atmosphere change with altitude? (Or, how can CFC's get up to the stratosphere when they are heavier than air? ) 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 Why is this? Vertical transport in the troposphere takes place by convection and turbulent mixing. In the stratosphere and in the mesosphere, it takes place by "eddy diffusion" - the gradual mechanical mixing of gas by motions on small scales.
These mechanisms do not distinguish molecular masses. Only at much higher altitudes do mean free paths become so large that molecular diffusion dominates and gravity is able to separate the different species, bringing hydrogen and helium atoms to the top. The lower and middle atmosphere are thus [Chamberlain and Hunter] [Wayne] [Wallace and Hobbs] Experimental measurements of the fluorocarbon CF 4 demonstrate this homogeneous mixing. CF 4 has an extremely long lifetime in the stratosphere - probably many thousands of years. The mixing ratio of CF 4 in the stratosphere was found to be 0. 056 - 0. 060 prev from 10 - 50 km, with no overall trend. [Zander et al. 1992 ] An important trace gas that is not well-mixed is water vapor. The lower troposphere contains a great deal of water - as much as 30, 000 ppmv in humid tropical latitudes.
High in the troposphere, however, the water condenses and falls to the earth as rain or snow, so that the stratosphere is extremely dry, typical mixing ratios being about 5 ppmv. Indeed, the transport of water vapor from troposphere to stratosphere is even less efficient than this would suggest, since much of the small amount of water in the stratosphere is actually produced in situ by the oxidation of stratospheric methane. [SAGE II] Sometimes that part of the atmosphere in which the chemical composition of stable species does not change with altitude is called the "homo sphere." The homo sphere includes the troposphere, stratosphere, and mesosphere. The upper regions of the atmosphere - the "thermosphere" and the "exosphere" - are then referred to as the "hetero sphere." [Wayne] [Wallace and Hobbs] Ozone is formed naturally in the upper stratosphere by short wavelength ultraviolet radiation. Wavelengths less than ~ 240 nanometers are absorbed by oxygen molecules (O 2), which dissociate to give O atoms. The O atoms combine with other oxygen molecules to O 2 + hv - O + O (wavelength 240 nm) Subject: 2. 2) How much ozone is in the layer, and what is a A Dobson Unit (DU) is a convenient scale for measuring the total amount of ozone occupying a column overhead.
If the ozone layer over the US were compressed to 0 degrees Celsius and 1 atmosphere pressure, it would be about 3 mm thick. So, 0. 01 mm thickness at 0 C and 1 at is defined to be 1 DU; this makes the average thickness of the ozone layer over the US come out to be about 300 DU. In absolute terms, 1 DU is about 2. 7 x 10 ^ 16 molecules / cm ^ 2. The unit is named after G. M. B.
Dobson, who carried out pioneering studies of atmospheric ozone between ~ 1920 - 1960. Dobson designed the standard instrument used to measure ozone from the ground. The Dobson spectrophotometer measures the intensity solar UV radiation at four wavelengths, two of which are absorbed by ozone and two of which are not [Dobson 1968 b]. These instruments are still in use in many places, although they are gradually being replaced by the more elaborate Brewer spectrophotometers. Today ozone is measured in many ways, from aircraft, balloons, satellites, and space shuttle missions, but the worldwide Dobson network is the only source of long-term data. A station at Arosa in Switzerland has been measuring ozone since the 1920 's (see web) and some other stations have records that go back nearly as long, although many were interrupted during World War II.
The present worldwide network went into operation in 1956 - 57. Subject: 2. 3) How is ozone distributed in the stratosphere? In absolute terms: about 10 ^ 12 molecules / cm ^ 3 at 15 km, rising to nearly 10 ^ 13 at 25 km, then falling to 10 ^ 11 at 45 km. In relative terms: ~ 0. 5 parts per million by volume (ppmv) at 15 km, rising to ~ 8 ppmv at ~ 35 km, falling to ~ 3 ppmv at 45 km. Even in the thickest part of the layer, ozone is a trace gas. In all, there are about 3 billion metric tons, or 3 x 10 ^ 15 grams, of ozone in the earth's atmosphere; about 90 % of this is in the stratosphere.
Subject: 2. 4) How does the ozone layer work? UV light with wavelengths between 240 and 320 nm is absorbed by ozone, which then falls apart to give an O atom and an O 2 molecule. The O atom soon encounters another O 2 molecule, however (at all times, the concentration of O 2 far exceeds that of O 3), and recreates O 3: Thus ozone absorbs UV radiation without itself being consumed; the net result is to convert UV light into heat. Indeed, this is what causes the temperature of the stratosphere to increase with altitude, giving rise to the inversion layer that traps molecules in the troposphere. The ozone layer isn't just in the stratosphere; the ozone layer actually determines the form of the stratosphere. Ozone is destroyed if an O atom and an O 3 molecule meet: This reaction is slow, however, and if it were the only mechanism for ozone loss, the ozone layer would be about twice as thick as it is.
Certain trace species, such as the oxides of Nitrogen (NO and NO 2), Hydrogen (H, OH, and HO 2) and chlorine (Cl, ClO and ClO 2) can catalyze the recombination. The present ozone layer is a result of a competition between photolysis and recombination; increasing the recombination rate, by increasing the concentration of catalysts, results in a thinner ozone layer. Putting the pieces together, we have the set of reactions proposed O 2 + hv - O + O (wavelength 240 nm): creation of oxygen atoms O + O 2 - O 3: formation of ozone O 3 + hv - O 2 + O (wavelength 320 nm): absorption of UV by ozone O + O 3 - 2 O 2: recombination. Since the photolysis of O 2 requires UV radiation while recombination does not, one might guess that ozone should increase during the day and decrease at night.
This has led some people to suggest that the "antarctic ozone hole" is merely a result of the long antarctic winter nights. This inference is incorrect, because the recombination reaction requires oxygen atoms which are also produced by photolysis. Throughout the stratosphere the concentration of O atoms is orders of magnitude smaller than the concentration of O 3 molecules, so both the production and the destruction of ozone by the above mechanisms shut down at night. In fact, the thickness of the ozone layer varies very little from day to night, and above 70 km ozone concentrations actually increase at night. (The unusual catalytic cycles that operate in the antarctic ozone hole do not require O atoms; however, they still require light to operate because they also include photolytic steps. See Part III. ) Subject: 2. 5) What sorts of natural variations does the ozone layer show? There are substantial variations from place to place, and from season to season.
There are smaller variations on time scales of years and more. [Wayne] [Rowland 1991 ] We discuss these in turn. Subject: 2. 5. a) Regional and Seasonal Variation Since solar radiation makes ozone, one expects to see the thickness of the ozone layer vary during the year. This is so, although the details do not depend simply upon the amount of solar radiation received at a given latitude and season - one must also take atmospheric motions into account. (Remember that both production and destruction of ozone require solar radiation. ) The ozone layer is thinnest in the tropics, about 260 DU, almost independent of season. Away from the tropics seasonal variations Location Column thickness, Dobson Units Jan Apr Jul Oct Huancayo, Peru (12 degrees S): 255 255 260 260 Aspen dale, Australia (38 deg. S): 300 280 335 360 Arosa, Switzerland (47 deg.
N): 335 375 320 280 St. Petersburg, Russia (60 deg. N): 360 425 345 300 These are monthly averages. Interannual standard deviations amount to ~ 5 DU for Huancayo, 25 DU for St. Petersburg. [Rowland 1991 ]. Day-to-day fluctuations can be quite large (as much as 60 DU at high latitudes).
Notice that the highest ozone levels are found in the spring, not, as one might guess, in summer, and the lowest in the fall, not winter. Indeed, at high latitudes in the Northern Hemisphere there is more ozone in January than in July! Most of the ozone is created over the tropics, and then is carried to higher latitudes by prevailing winds (the general circulation of the stratosphere. ) [Dobson 1968 a] [Garcia] [Sale and Garcia] [Brasseur and Solomon] The antarctic ozone hole, discussed in detail in Part III, falls far outside this range of natural variation. Mean October ozone at Halley Bay on the Antarctic coast was 117 DU in 1993, down Subject: 2. 5. b) Year-to-year variations.
Since ozone is created by solar UV radiation, one expects to see some correlation with the 11 -year solar sunspot cycle. Higher sunspot activity corresponds to more solar UV and hence more rapid ozone production. This correlation has been verified, although its effect is small, about 2 % from peak to trough averaged over the earth, about 4 % in polar regions. [Stolarski et al. ] Another natural cycle is connected with the "quasi biennial oscillation", in which tropical winds in the lower stratosphere switch from easterly to westerly with a period of about two years. This leads to variations of the order of 3 % at a given latitude, although the effect tends to cancel when one averages over the Episodes of unusual solar activity ("solar proton events") can also influence ozone levels, by producing nitrogen oxides in the upper stratosphere and mesosphere. This can have a marked, though short-lived, effect on ozone concentrations at very high altitudes, but the effect on total column ozone is usually small since most of the ozone is found in the lower and middle stratosphere.
Ozone can also be depleted by a major volcanic eruption, such as El Chicken in 1982 or Pinatubo in 1991. The principal mechanism for this is not injection of chlorine into the stratosphere, as discussed in Part II, but rather the injection of sulfate aerosols which change the radiation balance in the stratosphere by scattering light, and which convert inactive chlorine compounds to active, ozone-destroying forms. [McCormick et al. 1995 ]. This too is a transient effect, lasting 2 - 3 years. CFC's - ChloroFluoroCarbons - are a class of volatile organic compounds that have been used as refrigerants, aerosol propellants, foam blowing agents, and as solvents in the electronic industry. They are chemically very un reactive, and hence safe to work with. In fact, they are so inert that the natural reagents that remove most atmospheric pollutants do not react with them, so after many years they drift up to the stratosphere where short-wave UV light dissociates them.
CFC's were invented in 1928, but only came into large-scale production after ~ 1950. Since that year, the total amount of chlorine in the stratosphere has increased by The most important CFC's for ozone depletion are: Trichlorofluoromethane, Cfc 3 (usually called CFC- 11 or R- 11); Dichlorodifluoromethane, CF 2 Cl 2 (CFC- 12 or R- 12); and 1, 1, 2 Trichlorotrifluoroethane, CF 2 Clcfcl 2 (CFC- 113 or R- 113). "R" stands for "refrigerant." One occasionally sees CFC- 12 referred to as "F- 12 ", and so forth; the"F" stands for "Freon", DuPont's trade In discussing ozone depletion, "CFC" is occasionally used to describe a somewhat broader class of chlorine-containing organic compounds that have similar properties - un reactive in the troposphere, but readily photolyzed in the stratosphere. These include: Hydro ChloroFluoroCarbons such as Chclf 2 (HCFC- 22, R- 22); Carbon Tetrachloride (tetrachloromethane), CCl 4; Methyl Chloroform (1, 1, 1 trichloroethane), CH 3 CCl 3 (R- 140 a); and Methyl Chloride (chloro methane), CH 3 Cl. (The more careful publications always use phrases like "CFC's and related compounds", but this gets tedious. ) Only methyl chloride has a large natural source; it is produced biologically in the oceans and chemically from biomass burning. The CFC's and CCl 4 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 substantia fraction to reach the stratosphere.
Most of Part II is devoted to stratospheric chlorine chemistry; Subject: 2. 7) How do CFC's destroy ozone? CFC's themselves do not destroy ozone; certain of their decay products do. After CFC's are photolyzed, most of the chlorine eventually ends up as Hydrogen Chloride, HCl, or Chlorine Nitrate, Cl ONO 2. These are called "reservoir species" - they do not themselves react with ozone. However, they do decompose to some extent, giving, among other things, a small amount of atomic chlorine, Cl, and Chlorine Monoxide, ClO, which can catalyze the destruction of ozone by a number of mechanisms. Note that the Cl atom is a catalyst - it is not consumed by the reaction.
Each Cl atom introduced into the stratosphere can destroy thousands of ozone molecules before it is removed. The process is even more dramatic for Bromine - it has no stable "reservoirs", so the Br atom is always available to destroy ozone. On a per-atom basis, Br is 10 - 100 times as destructive as Cl. On the other hand, chlorine and bromine concentrations in the stratosphere are very small in absolute terms. The mixing ratio of chlorine from all sources in the stratosphere is about 3 parts per billion, (most of which is in the form of CFC's that have not yet fully decomposed) whereas ozone mixing ratios are measured in parts per million. Bromine concentrations are about 100 times The complete chemistry is very complicated - more than 100 distinct species are involved.
The rate of ozone destruction at any given time and place depends strongly upon how much Cl is present as Cl or ClO, and thus upon the rate at which Cl is released from its reservoirs. This makes quantitative predictions of future ozone depletion difficult. [Rowland 1989, 1991 ] [Wayne] The catalytic destruction of ozone by Cl-containing radicals was first suggested by Richard Stolarski and Ralph Cicerone in 1973. However, they were not aware of any large sources of stratospheric chlorine. In 1974 F.
Sherwood Rowland and Mario Molina realized that CFC's provided such a source. [Molina and Rowland 1974 ][Rowland and Molina 1975 ] For this and for their many subsequent contributions to stratospheric ozone chemistry Rowland and Molina shared the 1995 Nobel Prize in Chemistry, together with Paul Crutzen, discoverer of the NOx cycle. (The official announcement from the Swedish Academy can be found on the web at web. ) Subject: 2. 8) What is an "Ozone Depletion Potential?" The ozone depletion potential (ODP) of a compound is a simple measure of its ability to destroy stratospheric ozone. It is a relative measure: the ODP of CFC- 11 is defined to be 1. 0, and the ODP's of other compounds are calculated with respect to this reference point. Thus a compound with an ODP of 0. 2 is, roughly speaking, one-fifth as "bad" as CFC- 11. More precisely, the ODP of a compound "x" is defined as the ratio of the total amount of ozone destroyed by a fixed amount of compound x to the amount of ozone destroyed by the same mass of CFC- 11: Global loss of Ozone due to x ODP (x) = = Global loss of ozone due to CFC- 11.
Thus the ODP of CFC- 11 is 1. 0 by definition. The right-hand side of the equation is calculated by combining information from laboratory and field measurements with atmospheric chemistry and transport models. Since the ODP is a relative measure, it is fairly "robust", not overly sensitive to changes in the input data or to the details of the model calculations. That is, there are many uncertainties in calculating the numerator or the denominator of the expression, but most of these cancel out when the ratio is calculated. The ODP of a compound will be affected by: The nature of the halogen (bromine-containing halocarbons usually have much higher Odp's than chloro carbons, because atom for atom Br is a more effective ozone-destruction catalyst than Cl. ) The number of chlorine or bromine atoms in a molecule. Molecular Mass (since ODP is defined by comparing equal masses rather than equal numbers of moles. ) Atmospheric lifetime (CH 3 CCl 3 has a lower ODP than CFC- 11, because much of the CH 3 CCl 3 is destroyed in the troposphere. ) The ODP as defined above is a steady-state or long-term property.
As such it can be misleading when one considers the possible effects of CFC replacements. Many of the proposed replacements have short atmospheric lifetimes, which in general is good; however, if a compound has a short stratospheric lifetime, it will release its chlorine or bromine atoms more quickly than a compound with a longer stratospheric lifetime. Thus the short term effect of such a compound on the ozone layer is larger than would be predicted from the ODP alone (and the long-term effect correspondingly smaller. ) (The ideal combination would be a short tropospheric lifetime, since those molecules which are destroyed in the troposphere don't get a chance to destroy any stratospheric ozone, combined with a long stratospheric lifetime. ) To get around this, the concept of a Time-Dependent Ozone Depletion Potential has been introduced [Solomon and Albritton] [WMO 1991 ]: Loss of ozone due to X over time period T ODP (x, T) = = Loss of ozone due to CFC- 11 over time period T As T- infinity, this converges to the steady-state ODP defined previously. The following table lists time-dependent and steady-state ODP's for a few halocarbons [Solomon and Albritton] [WMO 1991 ] Compound Formula Ozone Depletion Potential 10 yr 30 yr 100 yr Steady State CFC- 113 CF 2 Clcfcl 2 0. 56 0. 62 0. 78 1. 10 carbon tetrachloride CCl 4 1. 25 1. 22 1. 14 1. 08 methyl chloroform CH 3 CCl 3 0. 75 0. 32 0. 15 0. 12 HCFC- 22 CHF 2 Cl 0. 17 0. 12 0. 07 0. 05 Halon - 1301 CF 3 Br 10. 4 10. 7 11. 5 12. 5 Subject: 2. 9) What about HCFC's and HFC's? Do they destroy ozone? HCFC's (hydro chlorofluorocarbons) differ from CFC's in that only some, rather than all, of the hydrogen in the parent hydrocarbon has been replaced by chlorine or fluorine.
The most familiar example is Chclf 2, known as "HCFC- 22 ", used as a refrigerant and in many home air conditioners (auto air conditioners use CFC- 12). The hydrogen atom makes the molecule susceptible to attack by the hydroxyl (OH) radical, so a large fraction of the HCFC's are destroyed before they reach the stratosphere. Molecule for molecule, then, HCFC's destroy much less ozone than CFC's, and they were suggested as CFC substitutes as long ago as 1976. Most HCFC's have ozone depletion potentials around 0. 01 - 0. 1, so that during its lifetime a typical HCFC will have destroyed 1 - 10 % as much ozone as the same amount of CFC- 12. Since the HCFC's are more reactive in the troposphere, fewer of them reach the stratosphere.
However, they are also more reactive in the stratosphere, so they release chlorine more quickly. The short-term effects are therefore larger than one would predict from the steady-state ozone depletion potential. When evaluating substitutes for CFC's, the "time-dependent ozone depletion potential", discussed in the preceding section, is more useful than the steady-state ODP. [Solomon and Albritton] HFC's, hydro fluorocarbons, contain no chlorine at all, and hence have an ozone depletion potential of zero. (In 1993 there were tentative reports that the fluorocarbon radicals produced by photolysis of HFC's could catalyze ozone loss, but this has now been shown to be negligible [Ravishankara et al. 1994 ]) A familiar example is CF 3 CH 2 F, known as HFC- 134 a, which is being used in some automobile air conditioners and refrigerators. HFC- 134 a is more expensive and more difficult to work with than CFC's, and while it has no effect on stratospheric ozone it is a greenhouse gas (though somewhat less potent than the CFC's).
Some engineers have argued that non-CFC fluids, such as propane-iso butane mixtures, are better substitutes for CFC- 12 in auto air conditioners than HFC- 134 a. Subject: 2. 10) IS the ozone layer getting thinner? There is no question that the ozone layer over antarctica has thinned dramatically over the past 15 years (see part III). However, most of us are more interested in whether this is also taking place at middle latitudes. The answer seems to be yes, although so far the After carefully accounting for all of the known natural variations, a net decrease of about 3 % per decade for the period 1978 - 1991 was found. This is a global average over latitudes from 66 degrees S to 66 degrees N (i.
e. the arctic and antarctic are excluded in calculating the average). The depletion increases with latitude, and is somewhat...
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