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    Ozone Layer Solid Research Essay

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    Subject: TABLE OF CONTENTSHow to get this FAQCopyright StatementGeneral remarksCaveats, Disclaimers, and Contact InformationTABLE OF CONTENTS1. THE STRATOSPHERE1. 1) What is the stratosphere?1.

    2) How is the composition of air described? 1. 3) How does the composition of the atmosphere change with2. THE OZONE LAYER2. 1) How is ozone created?2. 2) How much ozone is in the layer, and what is a2.

    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 Variation2. 5. b) Year-to-year variations. 2.

    6) What are CFC’s?2. 7) How do CFC’s destroy ozone?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?3. REFERENCES FOR PART IIntroductory ReadingBooks and Review ArticlesMore Specialized ReferencesInternet Resources—————————–Subject: 1.

    THE STRATOSPHERE—————————–Subject: 1. 1) What is the stratosphere?The stratosphere extends from about 15 km to 50 km. In thestratosphere temperature _increases_ with altitude, due to theabsorption of UV light by oxygen and ozone. This creates a globalinversion layer which impedes vertical motion into and within the stratosphere – since warmer air lies above colder air, convectionis inhibited. The word stratosphere is related to the wordstratification or layering.

    The stratosphere is often compared to the troposphere, which isthe atmosphere below about 15 km. The boundary – called the tropopause – between these regions is quite sharp, but itsprecise location varies between ~9 and ~18 km, depending upon latitude and season. The prefix tropo refers to change: the troposphere is the part of the atmosphere in which weather occurs. This results in rapid mixing of tropospheric air.

    [Wayne] [Wallace and Hobbs]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 increaseswith altitude again, and the exosphere, beyond ~400 km, whichfades into the background of interplanetary space. In the uppermesosphere and thermosphere electrons and ions are abundant, sothese regions are also referred to as the ionosphere. In technicalliterature the term lower atmosphere is synonymous with thetroposphere, middle atmosphere refers to the stratosphereand mesosphere, while upper atmosphere is usually reserved for thethermosphere and exosphere. This usage is not universal, however,and one occasionally sees the term upper atmosphere used todescribe everything above the troposphere (for example, in NASA’sUpper 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, andin a complicated way because the temperature also varies withaltitude.

    It is therefore awkward to report concentrations ofatmospheric species in units like g/cc or molecules/cc. Instead,it is convenient to report the mole fraction, the relativenumber of molecules of a given type in an air sample. Atmosphericscientists usually call a mole fraction a mixing ratio. Typicalunits for mixing ratios are parts-per-million, billion, ortrillion by volume, designated as ppmv, ppbv, and pptvrespectively. (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. ) Thuswhen someone says the mixing ratio of hydrogen chloride at 3 kmis 0.

    1 ppbv, he means that 1 out of every 10 billion molecules inan air sample collected at that altitude will be an HCl molecule. [Wayne] [Graedel and Crutzen]—————————–Subject: 1. 3) How does the composition of the atmosphere change withaltitude? (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_ chemicalspecies are well-mixed – their mixing ratios are independent ofaltitude. If a species’ mixing ratio changes with altitude, somekind 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 100 km. Why is this? Vertical transport in the troposphere takes place byconvection and turbulent mixing. In the stratosphere and in themesosphere, 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 thussaid to be well mixed. [Chamberlain and Hunten] [Wayne] [Wallace and Hobbs]Experimental measurements of the fluorocarbon CF4 demonstrate thishomogeneous mixing. CF4 has an extremely long lifetime in thestratosphere – probably many thousands of years. The mixing ratioof CF4 in the stratosphere was found to be 0.

    056-0. 060 ppbv from 10-50 km, with no overall trend. [Zander et al. 1992]An important trace gas that is *not* well-mixed is water vapor. Thelower troposphere contains a great deal of water – as much as 30,000ppmv in humid tropical latitudes.

    High in the troposphere, however,the water condenses and falls to the earth as rain or snow, so thatthe stratosphere is extremely dry, typical mixing ratios being about5 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 actuallyproduced _in situ_ by the oxidation of stratospheric methane. [SAGE II]Sometimes that part of the atmosphere in which the chemicalcomposition of stable species does not change with altitude iscalled the homosphere. The homosphere includes the troposphere,stratosphere, and mesosphere. The upper regions of the atmosphere – the thermosphere and the exosphere – are then referred to as the heterosphere. [Wayne] [Wallace and Hobbs]—————————–Subject: 2.

    THE OZONE LAYER—————————– Subject: 2. 1) How is ozone created?Ozone is formed naturally in the upper stratosphere by shortwavelength ultraviolet radiation. Wavelengths less than ~240nanometers are absorbed by oxygen molecules (O2), which dissociate togive O atoms. The O atoms combine with other oxygen molecules to make ozone: O2 + hv -* O + O (wavelength * 240 nm)O + O2 -* O3—————————–Subject: 2. 2) How much ozone is in the layer, and what is aDobson Unit ?A Dobson Unit (DU) is a convenient scale for measuring the totalamount of ozone occupying a column overhead.

    If the ozone layerover the US were compressed to 0 degrees Celsius and 1 atmospherepressure, 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 thicknessof 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 designedthe standard instrument used to measure ozone from the ground.

    TheDobson spectrophotometer measures the intensity solar UV radiation atfour wavelengths, two of which are absorbed by ozone and two ofwhich are not [Dobson 1968b]. These instruments are still in usein 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 the1920’s (see http://www. umnw.

    ethz. ch/LAPETH/doc/totozon. html)and some other stations have records that go back nearly aslong, 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 tonearly 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 3×10^15 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 byozone, which then falls apart to give an O atom and an O2 molecule.

    The O atom soon encounters another O2 molecule, however (at all times,the concentration of O2 far exceeds that of O3), and recreates O3:O3 + hv -* O2 + OO + O2 -* O3Thus _ozone absorbs UV radiation without itself being consumed_; the net result is to convert UV light into heat. Indeed, this iswhat causes the temperature of the stratosphere to increase withaltitude, giving rise to the inversion layer that traps molecules inthe 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 O3 molecule meet:O + O3 -* 2 O2 (recombination). This reaction is slow, however, and if it were the only mechanism for ozone loss, the ozone layer would be about twice as thickas it is.

    Certain trace species, such as the oxides of Nitrogen (NOand NO2), Hydrogen (H, OH, and HO2) and chlorine (Cl, ClO and ClO2)can catalyze the recombination. The present ozone layer is aresult of a competition between photolysis and recombination;increasing the recombination rate, by increasing theconcentration of catalysts, results in a thinner ozone layer. Putting the pieces together, we have the set of reactions proposedin the 1930’s by Sidney Chapman:O2 + hv -* O + O (wavelength * 240 nm) : creation of oxygen atomsO + O2 -* O3 : formation of ozoneO3 + hv -* O2 + O (wavelength * 320 nm) : absorption of UV by ozoneO + O3 -* 2 O2 : recombination . Since the photolysis of O2 requires UV radiation whilerecombination does not, one might guess that ozone should increaseduring the day and decrease at night. This has led some people tosuggest that the antarctic ozone hole is merely a result of thelong antarctic winter nights.

    This inference is incorrect, becausethe recombination reaction requires oxygen atoms which are alsoproduced by photolysis. Throughout the stratosphere the concentration of O atoms is orders of magnitude smaller than the concentration of O3 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 tooperate 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 fromseason to season. There are smaller variations on time scales ofyears and more. [Wayne] [Rowland 1991] We discuss these in turn. —————————–Subject: 2. 5.

    a) Regional and Seasonal VariationSince solar radiation makes ozone, one expects to see thethickness of the ozone layer vary during the year. This is so,although the details do not depend simply upon the amount of solarradiation received at a given latitude and season – one must alsotake atmospheric motions into account. (Remember thatboth production and destruction of ozone require solar radiation. )The ozone layer is thinnest in the tropics, about 260 DU, almostindependent of season. Away from the tropics seasonal variationsbecome important. For example:Location Column thickness, Dobson UnitsJan Apr Jul OctHuancayo, Peru (12 degrees S) : 255 255 260 260Aspendale, Australia (38 deg.

    S): 300 280 335 360Arosa, Switzerland (47 deg. N): 335 375 320 280St. Petersburg, Russia (60 deg. N): 360 425 345 300These are monthly averages.

    Interannual standard deviations amountto ~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 highlatitudes). Notice that the highest ozone levels are found in the _spring_, not, as one might guess, in summer, and the lowest in thefall, not winter.

    Indeed, at high latitudes in the Northern Hemispherethere 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 1968a] [Garcia] [Salby and Garcia] [Brasseur and Solomon]The antarctic ozone hole, discussed in detail in Part III, fallsfar outside this range of natural variation. Mean October ozoneat Halley Bay on the Antarctic coast was 117 DU in 1993, down from 321 DU in 1956. —————————–Subject: 2.

    5. b) Year-to-year variations. Since ozone is created by solar UV radiation, one expects to seesome correlation with the 11-year solar sunspot cycle. Highersunspot activity corresponds to more solar UV and hence more rapidozone production.

    This correlation has been verified, althoughits effect is small, about 2% from peak to trough averaged over theearth, about 4% in polar regions. [Stolarski et al. ]Another natural cycle is connected with the quasibiennial oscillation, in which tropical winds in the lower stratosphereswitch 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 entire globe. Episodes of unusual solar activity (solar proton events) can alsoinfluence ozone levels, by producing nitrogen oxides in the upperstratosphere and mesosphere. This can have a marked, thoughshort-lived, effect on ozone _concentrations_ at very high altitudes,but the effect on total column ozone is usually small since most ofthe ozone is found in the lower and middle stratosphere.

    Ozone canalso be depleted by a major volcanic eruption, such as El Chichon 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 theradiation balance in the stratosphere by scattering light, and whichconvert inactive chlorine compounds to active, ozone-destroying forms. [McCormick et al. 1995]. This too is a transient effect, lasting 2-3 years.

    —————————–Subject: 2. 6) What are CFC’s?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 unreactive, 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 bya factor of 4.

    [Solomon]The most important CFC’s for ozone depletion are:Trichlorofluoromethane, CFCl3 (usually called CFC-11 or R-11);Dichlorodifluoromethane, CF2Cl2 (CFC-12 or R-12); and1,1,2 Trichlorotrifluoroethane, CF2ClCFCl2 (CFC-113 or R-113). R stands for refrigerant. One occasionally sees CFC-12 referred to as F-12, and so forth; theF stands for Freon, DuPont’s tradename for these compounds. In discussing ozone depletion, CFC is occasionally used to describe a somewhat broader class of chlorine-containing organiccompounds that have similar properties – unreactive in thetroposphere, but readily photolyzed in the stratosphere.

    These include:HydroChloroFluoroCarbons such as CHClF2 (HCFC-22, R-22);Carbon Tetrachloride (tetrachloromethane), CCl4;Methyl Chloroform (1,1,1 trichloroethane), CH3CCl3 (R-140a);and Methyl Chloride (chloromethane), CH3Cl. (The more careful publications always use phrases like CFC’s andrelated 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 CCl4 are nearly inert in the troposphere, and havelifetimes of 50-200+ years. Their major sink is photolysis by UVradiation.

    [Rowland 1989, 1991] The hydrogen-containing halocarbonsare more reactive, and are removed in the troposphere by reactionswith OH radicals. This process is slow, however, and they live longenough (1-20 years) for a substantia fraction to reach the stratosphere. Most of Part II is devoted to stratospheric chlorine chemistry;look there for more detail. —————————–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, ClONO2. 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. The simplest is:Cl + O3 -* ClO + O2 ClO + O -* Cl + O2 Net effect: O3 + O -* 2 O2 Note that the Cl atom is a _catalyst_ – it is not consumed by the reaction.

    Each Cl atom introduced into the stratosphere candestroy thousands of ozone molecules before it is removed. The process is even more dramatic for Bromine – it has no stablereservoirs, 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 inthe stratosphere are very small in absolute terms. The mixing ratioof chlorine from all sources in the stratosphere is about 3 partsper billion, (most of which is in the form of CFC’s that have notyet fully decomposed) whereas ozone mixing ratios are measured inparts per million. Bromine concentrations are about 100 timessmaller still.

    (See Part II. )The complete chemistry is very complicated – more than 100distinct species are involved. The rate of ozone destruction at any given time and place depends strongly upon how much Cl is presentas Cl or ClO, and thus upon the rate at which Cl is released fromits reservoirs. This makes quantitative _predictions_ of futureozone depletion difficult. [Rowland 1989, 1991] [Wayne]The catalytic destruction of ozone by Cl-containing radicals was firstsuggested 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’sprovided such a source. [Molina and Rowland 1974][Rowland and Molina 1975]For this and for their many subsequent contributions to stratosphericozone chemistry Rowland and Molina shared the 1995 NobelPrize in Chemistry, together with Paul Crutzen, discoverer of the NOx cycle. (The official announcement from the Swedish Academy can be foundon the web at http://www.

    nobel. se/announcement95-chemistry. html . )—————————–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 compoundsare calculated with respect to this reference point. Thus a compound withan 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 ofthe total amount of ozone destroyed by a fixed amount of compound x tothe amount of ozone destroyed by the same mass of CFC-11:Global loss of Ozone due to xODP(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 tranport models. Since the ODP is a relative measure, it is fairly robust, not overlysensitive to changes in the input data or to the details of the modelcalculations.

    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 nature of the halogen (bromine-containing halocarbons usually have much higher ODPs than chlorocarbons, 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 (CH3CCl3 has a lower ODP than CFC-11, becausemuch of the CH3CCl3 is destroyed in the troposphere. )The ODP as defined above is a steady-state or long-term property. Assuch it can be misleading when one considers the possible effects of CFCreplacements. Many of the proposed replacements have short atmosphericlifetimes, which in general is good; however, if a compound has a short_stratospheric_ lifetime, it will release its chlorine or bromine atomsmore quickly than a compound with a longer stratospheric lifetime. Thusthe short term effect of such a compound on the ozone layer is largerthan would be predicted from the ODP alone (and the long-term effectcorrespondingly smaller. )(The ideal combination would be a shorttropospheric lifetime, since those molecules which are destroyed in thetroposphere don’t get a chance to destroy any stratospheric ozone,combined with a long stratospheric lifetime.

    ) To get around this, theconcept of a Time-Dependent Ozone Depletion Potential has beenintroduced [Solomon and Albritton] [WMO 1991]:Loss of ozone due to X over time period TODP(x,T) == ———————————————-Loss of ozone due to CFC-11 over time period TAs T-*infinity, this converges to the steady-state ODP defined previously. The following table lists time-dependent and steady-state ODP’s fora few halocarbons [Solomon and Albritton] [WMO 1991]Compound Formula Ozone Depletion Potential 10 yr 30 yr 100 yr Steady StateCFC-113 CF2ClCFCl2 0. 56 0. 62 0.

    78 1. 10carbon tetrachloride CCl4 1. 25 1. 22 1. 14 1.

    08methyl chloroform CH3CCl3 0. 75 0. 32 0. 15 0. 12HCFC-22 CHF2Cl 0. 17 0.

    12 0. 07 0. 05Halon – 1301 CF3Br 10. 4 10. 7 11. 5 12.

    5—————————–Subject: 2. 9) What about HCFC’s and HFC’s? Do they destroy ozone?HCFC’s (hydrochlorofluorocarbons) differ from CFC’s in that onlysome, rather than all, of the hydrogen in the parent hydrocarbonhas been replaced by chlorine or fluorine. The most familiarexample is CHClF2, 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 thehydroxyl (OH) radical, so a large fraction of the HCFC’s aredestroyed before they reach the stratosphere. Molecule formolecule, 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 thatduring its lifetime a typical HCFC will have destroyed 1-10% asmuch 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 thereforelarger than one would predict from the steady-state ozone depletionpotential. When evaluating substitutes for CFC’s, the time-dependentozone depletion potential, discussed in the preceding section,is more useful than the steady-state ODP.

    [Solomon and Albritton]HFC’s, hydrofluorocarbons, contain no chlorine at all, and hencehave an ozone depletion potential of zero. (In 1993 there weretentative reports that the fluorocarbon radicals produced by photolysis of HFC’s could catalyze ozone loss, but this has nowbeen shown to be negligible [Ravishankara et al. 1994]) A familiarexample is CF3CH2F, known as HFC-134a, which is being used in someautomobile air conditioners and refrigerators. HFC-134a is moreexpensive and more difficult to work with than CFC’s, and while ithas no effect on stratospheric ozone it is a greenhouse gas (thoughsomewhat less potent than the CFC’s). Some engineers have argued that non-CFC fluids, such as propane-isobutane mixtures, are better substitutes for CFC-12 in auto air conditioners than HFC-134a.

    —————————–Subject: 2. 10) *IS* the ozone layer getting thinner?There is no question that the ozone layer over antarctica has thinneddramatically 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 theeffect are small. After carefully accounting for all of the known natural variations, a net decrease of about 3% per decade for the period 1978-1991was found. This is a global average over latitudes from 66 degreesS to 66 degrees N (i.

    e. the arctic and antarctic are excluded incalculating the average). The depletion increases with latitude,and is somewhat larger in the Southern Hemisphere. Over the US, Europeand Australia 4% per decade is typical; on the other hand there wasno significant ozone loss in the tropics during this period. (See,however, [Hofmann et al.

    1996] for more recent trends which appear toshow a decline in some tropical stations. ) The depletion is larger in the winter months, smaller in the summer. [Stolarski et al. ] [WMO 1994]The following table, extracted from a much more detailed one in[Herman et al. 1993], illustrates the seasonal and regional trends in_percent per decade_ for the period 1979-1990:Latitude Jan Apr Jul Oct Example65 N -3.

    0 -6. 6 -3. 8 -5. 6 Iceland55 N -4. 6 -6.

    7 -3. 1 -4. 4 Moscow, Russia45 N -7. 0 -6.

    8 -2. 4 -3. 1 Minneapolis, USA35 N -7. 3 -4. 7 -1.

    9 -1. 6 Tokyo25 N -4. 2 -2. 9 -1.

    0 -0. 8 Miami, FL, USA5 N -0. 1 +1. 0 -0.

    1 +1. 3 Somalia5 S +0. 2 +1. 0 -0. 2 +1.

    3 New Guinea25 S -2. 1 -1. 6 -1. 6 -1. 1 Pretoria, S. Africa35 S -3.

    6 -3. 2 -4. 5 -2. 6 Buenos Aires 45 S -4. 8 -4. 2 -7.

    7 -4. 4 New Zealand55 S -6. 1 -5. 6 -9. 8 -9.

    7 Tierra del Fuego65 S -6. 0 -8. 6 -13. 1 -19. 5 Palmer Peninsula (These are longitudinally averaged satellite data, not individualmeasurements at the places listed in the right-hand column. Thereare longitudinal trends as well.

    A recent reanalysis of theTOMS data yields trends that differ in detail from the above,being somewhat smaller at the highest latitudes. [McPeterset al. 1996]. )It should be noted that one high-latitude ground station (Tromsoin Norway) has found no long-term change in total ozone changebetween 1939 and 1989. [Larsen and Henriksen][Henriksen et al.

    1992]The reason for the discrepancy is not known. [WMO 1994]Between 1991 and 1993 these trends accelerated. Satellite andground-based measurements showed a remarkable decline for 1992 andearly 1993, a full 4% below the average value for the preceding twelveyears and 2-3% below the _lowest_ values observed in the earlierperiod. In Canada the spring ozone levels were 11-17% below normal[Kerr et al. ]. By February 1994 ozone over the United States hadrecovered to levels similar to 1991, [Hofmann et al.

    1994b] and in thespring of 1995 they were down again, to levels lower than any previousyear other than 1993. [Bojkov et al. 1995] Sulfate aerosols from theJuly 1991 eruption of Mt. Pinatubo are the most likely cause of theexceptionally low ozone in 1993; these aerosols can convert inactivereservoir chlorine into active ozone-destroying forms, and can alsointerfere with the production and transport of ozone by changing thesolar radiation balance in the stratosphere.

    [Brasseur and Granier][Hofmann and Solomon] [Hofmann et al. 1994a] [McCormick et al. 1995]Another cause may be the unusually strong arctic polar vortex in1992-93, which made the arctic stratosphere more like the antarcticthan is usually the case. [Gleason et al.

    ] [Waters et al. ] In anyevent, the rapid ozone loss in 1992 and 1993 was a transient phenomenon,superimposed upon the slower downward trend identified before 1991. —————————–Subject: 2. 11) Is the middle-latitude ozone loss due to CFC emissions?That’s the majority opinion, although it’s not a universal opinion.

    The present trends are too small and the atmospheric chemistry and dynamics too complicated to allow a watertight case to be made (as _has_ been made for the far larger, but localized, depletion in the Antarctic Ozone hole; see Part III. ). Other possible causesare being investigated. To quote from the 1991 Scientific Assessmentpublished by the World Meteorological Organization, p.

    4. 1 [WMO 1991]:The primary cause of the Antarctic ozone hole is firmlyestablished to be halogen chemistry. . . .

    There is not a fullaccounting of the observed downward trend in _global ozone_. Plausible mechanisms include heterogeneous chemistry on sulfateaerosols [which convert reservoir chlorine to active chlorine -R. P. ] and the transport of chemically perturbed polar air to middlelatitudes. Although other mechanisms cannot be ruled out, thoseinvolving the catalytic destruction of ozone by chlorine andbromine appear to be largely responsible for the ozone loss and _are the only ones for which direct evidence exists_.

    (emphases mine – RP)The Executive Summary of the subsequent 1994 scientific assessment (available on the Web at http://www. al. noaa. gov/WWWHD/pubdocs/WMOUNEP94. html)states:Direct in-situ meaurements of radical species in the lowerstratosphere, coupled with model calculations, have quantitatively shownthat the in-situ photochemical loss of ozone due to (largely natural)reactive nitrogen (NOx) compounds is smaller than that predicted fromgas-phase chemistry, while that due to (largely natural) HOx compoundsand (largely anthropogenic) chlorine and bromine compounds is largerthan that predicted by gas-phase chemistry. This confirms the key roleof chemical reactions on sulfate aerosols in controlling the chemicalbalance of the lower stratosphere.

    These and other recent scientificfindings strengthen the conclusion of the previous assessment that theweight of scientific evidence suggests that the observed middle- andhigh-latitude ozone losses are largely due to anthropogenic chlorine andbromine compounds. [WMO 1994] For a contrasting view, see [Henriksen and Roldugin]. A legal analogy might be useful here – the connection between_antarctic_ ozone depletion and CFC emissions has been proved beyonda reasonable doubt, while at _middle latitudes_ there is only probable cause for such a connection. One must remember that there is a natural 10-20 year time lagbetween CFC emissions and ozone depletion. Ozone depletion today is(probably) due to CFC emissions in the 1970’s.

    Presentcontrols on CFC emissions are designed to avoid possibly largeamounts of ozone depletion 30 years from now, not to repair thedepletion that has taken place up to now. [Prather et al. 1996]—————————–Subject: 2. 12) If the ozone is lost, won’t the UV light just penetrate deeper into the atmosphere and make more ozone?This does happen to some extent – it’s called self-healing – andhas the effect of moving ozone from the upper to the lowerstratosphere.

    Recall that ozone is _created_ by UV with wavelengthsless than 240 nm, but functions by _absorbing_ UV with wavelengthsgreater than 240 nm. The peak of the ozone absorption band is at ~250nm, and the cross-section falls off at shorter wavelengths. The O2and O3 absorption bands do overlap, though, and UV radiation between200 and 240 nm has a good chance of being absorbed by _either_ O2 orO3. [Rowland and Molina 1975] (Below 200 nm the O2 absorptioncross-section increases dramatically, and O3 absorption isinsignificant in comparison.

    ) Since there is some overlap, a decreasein ozone does lead to a small increase in absorption by O2. This is aweak feedback, however, and it does not compensate for the ozonedestroyed. Negative feedback need not imply stability, just aspositive feedback need not imply instability. Numerical calculations of ozone depletion take the self-healing phenomenon into account, by letting the perturbed ozone layer comeinto equilibrium with the exciting radiation.

    —————————–Subject: 2. 13) Do Space Shuttle launches damage the ozone layer?Very little. In the early 1970’s, when little was known aboutthe role of chlorine radicals in ozone depletion, it was suggestedthat HCl from solid rocket motors might have a significant effectupon the ozone layer – if not globally, perhaps in the immediatevicinity of the launch. It was immediately shown that the effectwas negligible, and this has been repeatedly demonstrated since. Each shuttle launch produces about 200 metric tons of chlorine asHCl, of which about one-third, or 68 tons, is injected into the stratosphere.

    Its residence time there is about three years. Afull year’s schedule of shuttle and solid rocket launches injects 725 tons of chlorine into the stratosphere. This is negligible comparedto chlorine emissions in the form of CFC’s and related compounds (~1 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.

    1990] [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 directorypub/usenet/news.

    answers/space/controversy, and on the world-wide web at:http://www. cis. ohio-state. edu/hypertext/faq/usenet/space/controversy/faq.

    html—————————–Subject: 2. 14) Will commercial supersonic aircraft damage the ozone layer?Short answer: Probably not. This problem is very complicated,and a definitive answer will not be available for several years, but present model calculations indicate that a fleet of high-speed civil transports would deplete the ozone layer by * 2%. [WMO 1991, 1994]Long answer (this is a tough one):Supersonic aircraft fly in the stratosphere.

    Since vertical transport in the stratosphere is slow, the exhaust gases from a supersonic jet can stay there for two years or more. The most important exhaust gases are the nitrogen oxides, NO and NO2, collectively referred to as NOx. NOx is produced from ordinary nitrogen and oxygen by electrical discharges (e. g.

    lightning) and by high-temperature combustion (e. g. in automobile and aircraft engines). The relationship between NOx and ozone is complicated.

    In the troposphere, NOx _makes_ ozone, a phenomenon well known to residents of Los Angeles and other cities beset by photochemical smog. At high altitudes in the troposphere, similar chemical reactions produce ozone as a byproduct of the oxidation of methane; for this reason ordinary subsonic aircraft actually increase the thickness of the ozone layer by a very small amount. Things are very different in the stratosphere. Here the principalsource of NOx is nitrous oxide, N2O (laughing gas).

    Most of theN2O in the atmosphere comes from bacteriological decomposition oforganic matter – reduction of nitrate ions or oxidation of ammoniumions. (It was once assumed that anthropogenic sources were negligiblein comparison, but this is now known to be false. The total anthropogenic contribution is estimated at 8 Tg (teragrams)/yr,compared to a natural source of 18 Tg/yr. [Khalil and Rasmussen]. )N2O, unlike NOx, is very unreactive – it has an atmospheric lifetime of more than 150 years – so it reaches the stratosphere, where most ofit is converted to nitrogen and oxygen by UV photolysis. However, a small fraction of the N2O that reaches the stratosphere reacts instead with oxygen atoms (to be precise, with the very rare electronicallyexcited singlet-D oxygen atoms), and this is the major natural source of NOx in the stratosphere; about 1.

    2 million tons are produced each year in this way. This source strength would be matched by 500 of the SST’s designed by Boeing in the late 1960’s, each spending 5 hours per day in the stratosphere. (Boeing was intending to sell 800 of these aircraft. ) The Concorde, a slower plane, produces less than half asmuch NOx and flies at a lower altitude; since the Concorde fleet issmall, its contribution to stratospheric NOx is not significant.

    Before sending large fleets of high-speed aircraft into the stratosphere, however, one should certainly consider the possible effects of increasing the rate of production of an important stratospheric trace gas by as much as a factor of two. [CIC 1975]In 1969, Paul Crutzen discovered that NOx could be an efficient catalyst for the destruction of stratospheric ozone: [Crutzen 1970]NO + O3 -* NO2 + O2NO2 + O -* NO + O2——————————-net: O3 + O -* 2 O2(For this and other contributions to ozone research, Crutzen,together with Rowland and Molina, was awarded the 1995 Nobel Prizein Chemistry. The official announcement from the Swedish Academy is available at http://www. nobel.

    se/announcement95-chemistry. html . )Two years later, Harold S. Johnston made the connection to SSTemissions. Until then it had been thought that the radicals H, OH,and HO2 (referred to collectively as HOx) were the principalcatalysts for ozone loss; thus, investigations of the impact ofaircraft exhaust on stratospheric ozone had focussed on emissions ofwater vapor, a possible source for these radicals.

    (The importance ofchlorine radicals, Cl, ClO, and ClO2, referred to as – you guessed it- ClOx, was not discovered until 1973. ) It had been argued -correctly, as it turns out – that water vapor injection wasunimportant for determining the ozone balance. The discovery ofthe NOx cycle threw the question open again. Beginning in 1972, the U. S. National Academies of Science andEngineering and the Department of Transportation sponsored anintensive program of stratospheric research.

    [CIC 1975] It soonbecame clear that the relationship between NOx emissions and theozone layer was very complicated. The stratospheric lifetime ofNOx is comparable to the timescale for transport from North toSouth, so its concentration depends strongly upon latitude. Muchof the NOx is injected near the tropopause, a region wherequantitative modelling is very difficult, and the results ofcalculations depend sensitively upon how troposphere-stratosphereexchange is treated. Stratospheric NOx chemistry is _extremely_complicated, much worse than chlorine chemistry. Among otherthings, NO2 reacts rapidly with ClO, forming the inactive chlorinereservoir ClONO2 – so while on the one hand increasing NOx leadsdirectly to ozone loss, on the other it suppresses the actionof the more potent chlorine catalyst. And on top of all of this, theSST’s always spend part of their time in the troposphere, where NOxemissions cause ozone increases.

    Estimates of long-term ozonechanges due to large-scale NOx emissions varied markedly from yearto year, going from -10% in 1974, to +2% (i. e. a net ozone _gain_)in 1979, to -8% in 1982. (In contrast, while the estimates of theeffects of CFC emissions on ozone also varied a great deal in theseearly years, they always gave a net loss of ozone.

    ) [Wayne]The discovery of the Antarctic ozone hole added a new piece to thepuzzle. As described in Part III, the ozone hole is caused byheterogeneous chemistry on the surfaces of stratospheric cloudparticles. While these clouds are only found in polar regions,similar chemical reactions take place on sulfate aerosols which arefound throughout the lower stratosphere. The most important of theaerosol reactions is the conversion of N2O5 to nitric acid:N2O5 + H2O -* 2 HNO3 (catalyzed by aerosol surfaces)N2O5 is in equilibrium with NOx, so removal of N2O5 by thisreaction lowers the NOx concentration.

    The result is that in thelower stratosphere the NOx catalytic cycle contributes much less tooverall ozone loss than the HOx and ClOx cycles. Ironically, thesame processes that makes chlorine-catalyzed ozone depletion somuch more important than was believed 10 years ago, also makeNOx-catalyzed ozone loss less important. In the meantime, there has been a great deal of progress in developingjet engines that will produce much less NOx – up to a factor of 10 -than the old Boeing SST. The most recent model calculations indicatethat a fleet of the new high-speed civil transports would depletethe ozone layer by 0.

    3-1. 8%. Caution is still required, since theexperiment has not been done – we have not yet tried adding largeamounts of NOx to the stratosphere. The forecasts, however, aregood. [WMO 1991, Ch. 10] [WMO 1994] Very recently, a new complicationhas appeared: _in situ_ measurements in the exhaust plume of aConcorde aircraft flying at supersonic speeds indicate that theground-based estimates of NOx emissions are accurate, but that theexhaust also contains large amounts of sulfate-based particulates[Fahey et al.

    1995]. Since reactions on sulfate aerosols are believedto play an important role in halogen-catalyzed ozone depletion, it maybe advisable to concentrate on reducing the sulfur content of thefuels that are to be used in new generations of supersonic aircraft,rather than further reducing NOx emissions. . . . .

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    . . _Aside_: One sometimes hears that the US government killed the SSTproject in 1971 because of concerns raised by H. S.

    Johnston’s work on NOx. This is not true. The US House of Representatives had alreadyvoted to cut off Federal funding for the SST when Johnston beganhis calculations. The House debate had centered around economics and the effects of noise, especially sonic booms, although there were some vague concerns about pollution and one physicist had testifiedabout the possible effects of water vapor on ozone. About 6 weeks after both houses had voted to cancel the SST, its supporters succeeded in reviving the project in the House.

    In the meantime, Johnston had sent a preliminary report to several professional colleagues and submitted a paper to _Science_. A preprint of Johnston’s report leaked to a small California newspaper which published a highly sensationalized account. The story hit the press a few days before the Senate voted, 58-37, not to revive the SST. (The previous Senate vote had been 51-46 to cancel the project. The reason for the larger majority in the second vote was probably the statement by Boeing’s chairman that at least $500 million more would be needed to revive the program.

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    . . . .. . . .. —————————–Subject: 2. 15) What is being done about ozone depletion?The 1987 Montreal Protocol (full text available on the world-wide web athttp://www. unep. org/unep/secretar/ozone/treaties.htm) specified thatCFC emissions should be reduced by 50% by the year 2000 (they had been _increasing_ by 3% per year. ) This agreement was amended inLondon in 1990, to state that production of CFC’s, CCl4, and halons should cease entirely by the year 2000. Restrictions were also appliedapplied to other Cl sources such as methylchloroform. (The details ofthe protocols are complicated, involving different schedules for differentcompounds, delays for developing nations, etc. ) The phase-out schedulewas accelerated by four years by the 1992 Copenhagen agreements. A great deal of effort has been devoted to recovering and recycling CFC’s that arecurrently being used in closed-cycle systems. For more information about legal and policy issues, see the books by[Bene*censored*] and [Litvin], and the following web sites:http://www. unep. org/unep/secretar/ozone/home. htm http://www.unep. ch/ozone/ (European mirror site for above)http://www. epa. gov/docs/ozone/index. htmlhttp://www.ciesin. org/TG/OZ/ozpolic. htmlRecent NOAA measurements [Elkins et al. 1993] [Montzka et al. 1996] show that the _rate of increase_ of halocarbon concentrations in the atmosphere has decreased markedly since 1987. It appears that theProtocols are being observed. Under these conditions total stratospheric chlorine is predicted to peak at 3. 8 ppbv in the year 1998, 0. 2 ppbv above 1994 levels, and to slowly decline thereafter. [WMO 1994] Extrapolation of current trends suggests that the maximum ozone losses will be [WMO 1994]:Northern Mid-latitudes in winter/Spring: 12-13% below late 1960’s levels, ~2. 5% below current levels. Northern mid-latitudes in summer/fall: 6-7% below late 1960’s levels,~1. 5% below current levels. Southern mid-latitudes, year-round: ~ 11% below late 1960’s levels,~2.5% below current levels. Very little depletion has been seen in the tropics and little isexpected there. After the year 2000, the ozone layer will slowly recover over a period of 50 years or so. The antarctic ozone hole is expected to last until about 2045. [WMO 1991,1994] Some scientists are investigating ways to replenish stratosphericozone, either by removing CFC’s from the troposphere or by tying upthe chlorine in inactive compounds. This is discussed in Part III. —————————–Subject: 3. REFERENCES FOR PART IA remark on references: they are neither representative norcomprehensive. There are _hundreds_ of people working on theseproblems. Where possible I have limited myself to articles thatare (1) available outside of University libraries (e. g. _Science_ or _Nature_ rather than archival journals such as _J. Geophys. Res._) and (2) directly related to the frequently asked questions. I have not listed papers whose importance is primarily historical. (I make an exception for the Nobel-Prize winning work of Crutzen,Molina and Rowland. ) Readers who want to see who did what shouldconsult the review articles listed below, or, if they can get them,the WMO reports which are extensively documented. —————————–Subject: Introductory Reading[Garcia] R. R. Garcia, Causes of Ozone Depletion, _Physics World_April 1994 pp 49-55. [Graedel and Crutzen] T. E. Graedel and P. J. Crutzen, _Atmospheric Change: an Earth System Perspective_, Freeman, NY 1993.[Rowland 1989] F. S. Rowland, Chlorofluorocarbons and the depletionof stratospheric ozone, _American Scientist_ _77_, 36, 1989. [Rowland and Molina 1994] F. S. Rowland and M. J. Molina, Ozone depletion: 20 years after the alarm, _Chemical and Engineering News_, 15 Aug. 1994, pp. 8-13.[Zurer] P. S. Zurer, Ozone Depletion’s Recurring SurprisesChallenge Atmospheric Scientists, _Chemical and Engineering News_,24 May 1993, pp. 9-18. —————————–Subject: Books and Review Articles[Bene*censored*] R. Bene*censored*, _Ozone Diplomacy_, Harvard, 1991.[Brasseur and Solomon] G. Brasseur and S. Solomon, _Aeronomy of the Middle Atmosphere_, 2nd. Edition, D. Reidel, 1986[Chamberlain and Hunten] J. W. Chamberlain and D. M. Hunten,_Theory of Planetary Atmospheres_, 2nd Edition, Academic Press, 1987[Dobson 1968a] G. M. B. Dobson, _Exploring the Atmosphere_, 2nd Edition, Oxford, 1968. [Dobson 1968b] G. M. B. Dobson, Forty Years’ research on atmosphericozone at Oxford, _Applied Optics_, _7_, 387, 1968. [CIC 1975] Climate Impact Committee, National Research Council,_Environmental Impact of Stratospheric Flight_, National Academy of Sciences, 1975. [Johnston 1992] H. S. Johnston, Atmospheric Ozone, _Annu. Rev. Phys. Chem. _ _43_, 1, 1992.[Ko et al. ] M. K. W. Ko, N.-D. Sze, and M. J. Prather, BetterProtection of the Ozone Layer, _Nature_ _367_, 505, 1994. [Litvin] K. T. Litvin, _Ozone Discourses_, Columbia 1994. [McElroy and Salawich] M. McElroy and R. Salawich, Changing Composition of the Global Stratosphere, _Science_ _243, 763, 1989. [Rowland and Molina 1975] F. S. Rowland and M. J. Molina,Chlorofluoromethanes in the Environment, Rev. Geophys. & Space Phys. _13_, 1, 1975.[Rowland 1991] F. S. Rowland, Stratospheric Ozone Depletion, _Ann. Rev. Phys. Chem._ _42_, 731, 1991.[Salby and Garcia] M. L. Salby and R. R. Garcia, Dynamical Perturbationsto the Ozone Layer, _Physics Today_ _43_, 38, March 1990.[Solomon] S. Solomon, Progress towards a quantitative understandingof Antarctic ozone depletion, _Nature_ _347_, 347, 1990.[Wallace and Hobbs] J. M. Wallace and P. V. Hobbs,_Atmospheric Science: an Introductory Survey_, Academic Press, 1977.[Wayne] R. P. Wayne, _Chemistry of Atmospheres_, 2nd. Ed., Oxford, 1991.[WMO 1988] World Meteorological Organization, _Report of the International Ozone Trends Panel_, Global Ozone Research and Monitoring Project – Report #18.[WMO 1989] World Meteorological Organization, _Scientific Assessment of Stratospheric Ozone: 1991_Global Ozone Research and Monitoring Project – Report #20.[WMO 1991] World Meteorological Organization, _Scientific Assessment of Ozone Depletion: 1991_Global Ozone Research and Monitoring Project – Report #25.[WMO 1994] World Meteorological Organization, _Scientific Assessment of Ozone Depletion: 1994_Global Ozone Research and Monitoring Project – Report #37.The Executive Summary of this report is available on theWorld-Wide Web at—————————–Subject: More Specialized References[Bojkov et al. 1995] R. D. Bojkov, V. E. Fioletov, D. S. Balis,C. S. Zerefos, T. V. Kadygrova, and A. M. Shalamjansky, Further ozone decline during the northern hemisphere winter-springof 1994-95 and the new record low ozone over Siberia,Geophys. Res. Lett. _22_, 2729, 1995.[Brasseur and Granier] G. Brasseur and C. Granier, Mt. Pinatuboaerosols, chlorofluorocarbons, and ozone depletion, _Science__257_, 1239, 1992.[Crutzen 1970] P. J. Crutzen, The influence of nitrogen oxides on theatmospheric ozone content, _Quart. J. R. Met. Soc._ _90_, 320, 1970.[Elkins et al. 1993] J. W. Elkins, T. M. Thompson, T. H. Swanson,J. H. Butler, B. D. Hall, S. O. Cummings, D. A. Fisher, and A. G. Raffo, Decrease in Growth Rates of Atmospheric Chlorofluorocarbons 11 and 12, _Nature_ _364_, 780, 1993.[Fahey et al. 1995] D. W. Fahey, E. R. Keim, K. A. Boering,C. A. Brock, J. C. Wilson, H. H. Jonsson, S. Anthony, T. F. Hanisco,P. O. Wennberg, R. C. Miake-Lye, R. J. Salawich, N. Louisnard, E. L. Woodbridge, R. S. Gao, S. G. Donnelly, R. C. Wamsley,L. A. Del Negro, S. Solomon, B. C. Daube, S. C. Wofsy, C. R. Webster,R. D. May, K. K. Kelly, M. Loewenstein, J. R. Podolske, and K. R. Chan,Emission Measurements of the Concorde Supersonic Aircraft in theLower Stratosphere, _Science_ _270_, 70, 1995.[Gleason et al.] J. Gleason, P. Bhatia, J. Herman, R. McPeters, P.Newman, R. Stolarski, L. Flynn, G. Labow, D. Larko, C. Seftor, C.Wellemeyer, W. Komhyr, A. Miller, and W. Planet, Record Low GlobalOzone in 1992, _Science_ _260_, 523, 1993.[Henriksen and Roldugin] K. Henriksen and V. Roldugin, Total ozonevariations in Middle Asia and dynamics meteorological processesin the atmosphere, _Geophys. Res. Lett._ _22_, 3219, 1995.[Henriksen et al. 1992] K. Henriksen, T. Svenoe, and S. H. H. Larsen,On the stability of the ozone layer at Tromso, J. Atmos. Terr. Phys._55_, 1113, 1992.[Herman et al.] J. R. Herman, R. McPeters, and D. Larko,Ozone depletion at northern and southern latitudes derivedfrom January 1979 to December 1991 TOMS data,J. Geophys. Res. _98_, 12783, 1993.[Hofmann and Solomon] D. J. Hofmann and S. Solomon, Ozone destruction through heterogeneous chemistry following the eruption of El Chichon, J. Geophys. Res. _94_, 5029, 1989.[Hofmann et al. 1994a] D. J. Hofmann, S. J. Oltmans, W. D. Komhyr, J. M. Harris, J. A. Lathrop, A. O. Langford, T. Deshler, B. J. Johnson, A. Torres, and W. A. Matthews,Ozone Loss in the lower stratosphere over the United States in1992-1993: Evidence for heterogeneous chemistry on the Pinatuboaerosol, Geophys. Res. Lett. _21_, 65, 1994.[Hofmann et al. 1994b] D. J. Hofmann, S. J. Oltmans, J. M. Harris,J. A. Lathrop, G. L. Koenig, W. D. Komhyr, R. D. Evans, D. M. Quincy,T. Deshler, and B. J. Johnson,Recovery of stratospheric ozone over the United States in the winterof 1993-94, Geophys. Res. Lett. _21_, 1779, 1994.[Hofmann et al. 1996] D. J. Hofmann, S. J. Oltmans, G. L. Koenig,B. A. Bodhaine, J. M. Harris, J. A. Lathrop, R. C. Schnell, J. Barnes,J. Chin, D. Kuniyuki, S. Ryan, R. Uchida, A. Yoshinaga, P. J. Neale,D. R. Hayes, Jr., V. R. Goodrich, W. D. Komhyr, R. D. Evans, B. J. Johnson,D. M. Quincy, and M. Clark, Record low ozone at Mauna Loa Observatoryduring winter 1994-95: A consequence of chemical and dynamicalsynergism?, Geophys. Res. Lett. _23_, 1533, 1996.[Kerr et al.] J. B. Kerr, D. I. Wardle, and P. W. Towsick,Record low ozone values over Canada in early 1993,Geophys. Res. Lett. _20_, 1979, 1993.[Khalil and Rasmussen] M. A. K. Khalil and R. Rasmussen, The GlobalSources of Nitrous Oxide, _J. Geophys. Res._ _97_, 14651, 1992.[Larsen and Henriksen] S. H. H. Larsen and T. Henriksen, Persistent Arctic ozone layer, _Nature_ _343_, 134, 1990.[McCormick et al. 1995] M. P. McCormick, L. W. Thomason, and C. R. Trepte, Atmospheric effects of the Mt Pinatubo eruption,_Nature_ _373_, 399, 1995.[McPeters et al. 1996] R. D. McPeters, S. M. Hollandsworth, andC. J. Seftor, Long-term ozone trends derived from the 16-year combinedNimbus 7/Meteor 3 TOMS Version 7 record, Geophys. Res. Lett. _23_,3699, 1996.[Molina and Rowland 1974] M. J. Molina and F. S. Rowland,Stratospheric sink for chlorofluoromethanes: chlorineatom-catalyzed destruction of ozone, _Nature_ _249_, 810, 1974.[Montzka et al. 1996] S. A. Montzka, J. H. Butler, R. C. Myers,T. M. Thompson, T. H. Swanson, A. D. Clarke, L. T. Lock, and J. W. Elkins, Decline in the Tropospheric Abundance of Halogenfrom Halocarbons: Implications for Stratospheric Ozone Depletion,_Science_ _272_, 1318, 1996.[Prather et al. 1990] M. J. Prather, M.M. Garcia, A.R. Douglass, C.H.Jackman, M.K.W. Ko, and N.D. Sze, The Space Shuttle’s impact onthe stratosphere, J. Geophys. Res. _95_, 18583, 1990.[Prather et al. 1996] M. J. Prather, P. Midgley, F. S. Rowland,and R. Stolarski, The ozone layer: the road not taken,_Nature_ _381_, 551, 1996.[Ravishankara et al. 1994] A. R. Ravishankara, A. A. Turnipseed,N. R. Jensen, S. Barone, M. Mills, C. J. Howard, and S. Solomon,Do Hydrofluorocarbons Destroy Stratospheric Ozone?,_Science_ _263_, 71, 1994.[SAGE II] Special Section on the Stratospheric Aerosol and Gas Experiment II, _J. Geophys. Res._ _98_, 4835-4897, 1993.[Solomon and Albritton] S. Solomon and D.L. Albritton,Time-dependent ozone depletion potentials for short- and long-termforecasts, _Nature_ _357_, 33, 1992.[Stolarski et al.] R. Stolarski, R. Bojkov, L. Bishop, C. Zerefos,J. Staehelin, and J. Zawodny, Measured Trends in StratosphericOzone, Science _256_, 342 (17 April 1992)[Waters et al.] J. Waters, L. Froidevaux, W. Read, G. Manney, L.Elson, D. Flower, R. Jarnot, and R. Harwood, Stratospheric ClO andozone from the Microwave Limb Sounder on the Upper AtmosphereResearch Satellite, _Nature_ _362_, 597, 1993.[Zander et al. 1992] R. Zander, M. R. Gunson, C. B. Farmer, C. P.Rinsland, F. W. Irion, and E. Mahieu, The 1985 chlorine andfluorine inventories in the stratosphere based on ATMOSobservations at 30 degrees North latitude, J. Atmos. Chem. _15_,171, 1992.—————————–Subject: Internet ResourcesThis list is preliminary and by no means comprehensive; it includes a few sites that I have found particularly useful and which providegood starting points for further exploration.Probably the most extensive collection of online resources is that providedby the Consortium for International Earth Science Information Network: includes links to many other documents, including on-line versionsof some of the original research papers. At the present time portionsof the site are very much under construction. Lenticular Press publishes a multimedia CD-ROM (for Apple Macintosh)containing ozone data and images, as well as a hypertext document similarto this FAQ. For sample images and information about ordering the CD,see Note that these samples are copyrightedand may not be further distributed.The NOAA Aeronomy Lab: , has the text of the Executive Summary of the 1994 WMO ScientificAssessment, United Nations Environmental Program (UNEP) Ozone Secretariat:Main page (Nairobi, Kenya).Mirror site (Geneva, Switzerland).The US Environmental Protection Agency has an ozone page that includeslinks to both science and policy resources: of the more interesting scientific web pages include:The Centre for Antarctic Information and Research (ICAIR) in New Zealand: Environment Canada: TOMS home page: EASOE home page: UARS Project Definition page: HALOE home page: British Antarctic Survey: ETH Zuerich Institute for Atmospheric Science Institute for Meteorology at the Free University of Berlin: Climate Prediction Center’s TOVS Total Ozone Analysis page: USDA UV-B Radiation Monitoring Program Climate Network,[ By Archive-name | By Author | By Category | By Newsgroup ] [ Home | Latest Updates | Archive Stats | Search | Usenet References | Help ] Send corrections/additions to the FAQ Maintainer: [emailprotected] Last Update September 28 2000 @ 04:24 AM Ozone Depletion FAQ Part IV: UV Radiation and its EffectsFrom: [emailprotected] (Robert Parson)Newsgroups: sci.environment,sci.answers,news.answersSubject: Ozone Depletion FAQ Part IV: UV Radiation and its EffectsFollowup-To: posterDate: 24 Dec 1997 20:51:43 GMTOrganization: University of Colorado, BoulderExpires: Sun, 1 Jan 1998 00:00:00 GMTMessage-ID: *[emailprotected]*Reply-To: [emailprotected]Summary: This is the fourth of four files dealing with stratosphericozone depletion. It describes the properties of solar UVradiation and some of its biological effects.Keywords: ozone layer depletion UVB UVA skin cancer phytoplanktonArchive-name: ozone-depletion/uvLast-modified: 16 Dec 1997Version: 5.9—————————–Subject: How to get this FAQThese files are posted to the newsgroups sci.environment, sci.answers,and news.answers. They are also archived at a variety of sites. Thesearchives work by automatically downloading the faqs from the newsgroupsand reformatting them in site-specific ways. They usually update tothe latest version within a few days of its being posted, although inthe past there have been some lapses; if the Last-Modified date inthe FAQ seems old, you may want to see if there is a more recent versionin a different archive. Many individuals have archived copies on their own servers, but theseare 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-lineresources cited in the faqs, can be found at: versions can be found at:—-B. Anonymous ftpTo, in the directory /pub/usenet/news.answers/ozone-depletionTo, in the directory /usenet/news.answers/ozone-depletionLook for the four files named intro, stratcl, antarctic, and uv.—-C. Regular emailSend the following messages to [emailprotected]:send usenet/news.answers/ozone-depletion/introsend usenet/news.answers/ozone-depletion/stratclsend usenet/news.answers/ozone-depletion/antarcticsend usenet/news.answers/ozone-depletion/uvLeave the subject line blank.If you want to find out more about the mail server, send amessage to it containing the word help. —————————– Subject: Copyright Notice************************************************************************ 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 RemarksThis file deals with the physical properties of ultravioletradiation and its biological consequences, emphasizing thepossible effects of stratospheric ozone depletion. It frequentlyrefers back to Part I, where the basic properties of the ozonelayer are described; the reader should look over that file first.The overall approach I take is conservative. I concentrate on whatis known and on most probable, rather than worst-case, scenarios.For example, I have relatively little to say about theeffects of UV radiation on plants – this does not mean that theeffects are small, it means that they are as yet not wellquantified (and moreover, I am not well qualified to interpret theliterature.) Policy decisions must take into account not only themost probable scenario, but also a range of less probable ones.will probably do, but also the worst that he could possibly do.There have been surprises, mostly unpleasant, in this field in thepast, and there are sure to be more in the future. In general,_much_ less is known about biological effects of UV-B than aboutthe physics and chemistry of the ozone layer.—————————–Subject: Caveats, Disclaimers, and Contact Information| _Caveat_: I am not a specialist. In fact, I am not an atmospheric| scientist at all – I am a physical chemist studying gas-phase| reactions who talks to atmospheric scientists. In this part

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