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Major and minor sources of stratospheric chlorine

Stratospheric chlorine is at present the major catalyst of ozone depletion. Since this area is considered a little more contriversial I have referenced it more heavily. Personally I have found very little disagreement among atmospheric researchers on major causes of Ozone depletion, particularly the role of man-made chlorofluorocarbons in the depletion.

Major sources of stratospheric chlorine

At this time human activity accounts for 75-85% of the chlorine in the stratosphere. The remaning 15-20% comes almost totally from Methyl chloride, most of that from natural sources and burning of biomass. Large, explosive volcanoes contribute an additional couple of percent.(1-5)

Notice that care must be taken to specify major sources of stratospheric chlorine since sources of tropospheric chlorine are quite different (sea-spray, volcanoes, volatile organic compounds) (6,7) Most of the tropospheric chlorine compounds never make it to the stratosphere; they are quickly decomposed by natural oxidants and the chlorine converted to water-soluble species, such as HCl, which get rained out of the atmosphere. Chlorofluorocarbons are very non-reactive in the troposphere, with life-times ranging from 50-200+ years, and so they eventually make it up to the stratosphere.

Summarized evidence on the sources of stratospheric chlorine:

  1. By analyzing the elevation at which the concentrations of certain compounds are found scientist get a good idea of how stable compounds are, and also some of the mechanisms by which compounds are degraded. (R.Parson. FAQ II) In repeated observations scientist have seen the stratospheric concentration of organic chlorine compounds decrease with altitude while the concentration of inorganic chlorine increases proportionatly; concentrations of both have been increasing proportionatly since the first systematic measurements in 1977.(8-12)
  2. Field observations are confirmed by experimenting with gases and gas reaction-rates in the laboratory, research which has been going on for over twenty years.
  3. The amount of fluoride released natually as organic compounds is very small, almost all of the flourine in the stratosphere comes from man-made compounds such as CFC's. Since CFC's should deposit chlorine and fluorine at approxiamately the same rate, measurements of stratospheric fluorine should indicate what proportion of the stratospheric chlorine is from CFC's.

Minor sources of stratospheric chlorine

Volcanoes: Volcanoes differ greatly in the amount of HCl they put out. But since, on the average, over 90% of a volcanic plume is water-vapor most of the HCl produced gets precipitated out of the atmosphere in 1-7days. Large amounts of sulfates also present in plumes cause water droplets to form quickly; the chemistry that can take place on these droplets is complicated and not fully understood. Within the last 200 years our planet has had six volcanoes with enough explosive power to project material into the stratosphere. The most recent two: El Chichon in 1982 and Mt.Pinatubo in 1991, were very closely studied by atmospheric scientist. Using balloons, satellites and even planes which flew through through the volcanic plumes, and large masses of data were collected and analyzed. Modeling and later observations have shown that more than 99% of the volcanic HCl is removed by absorbtion on to water droplets or ice crystals without ever becoming catalytically active in the stratosphere.(15,17 ). Somewhat of a stir in the popular press was created by active eruptions of an antarctic volcano, Mount Erebus, from 1976 through 1983, but its impact on stratospheric chlorine levels (16,17) was shown to be minimal.

Space shuttle and other high altitude flights: In the early 1970's it was suggested than chlorine from solid rocket boosters might have a significant effect on the ozone layer. A study by Cicerone and Steadman,1974, found the resulting contribution of chlorine to be relitively small as have many studies since then. The most current article on the issue calculates that the shuttle program has relatively little effect on the stratospheric chlorine levels, although it does raise some concern about the particles of aluminum that the solid-booster engine sprays into the atmosphere. Each shuttle launch produces about 68 tons of HCl, most of that released in the troposphere. Ten launches per year would amount to less than 0.06% of the yearly chlorofluorcarbons released which was 1.2 million tons per year in the 1980's.(18)

These references I obtained from R.Parsons FAQ II and I read as many of them as I could get a hold of. To paraphrase Professor Parson's comments on this list: it is neither representitive nor comprehensive of the large and continuing flow of research articles in this area. Two good summary articles:

  • F.S.Rowland, "Chlorofluorcarbons and the depletion of stratospheric ozone", _American Scientist_, 77, 36._1989.
  • P.S.Zurer, "Ozone depletion's reoccuring suprises challenge atmospheric scientists" , _Chemical and Engineering News_, 24 May 1993, pg 9-18

Specific references:

  1. World Meterological Organization(WMO). _Scientific Assessment of Ozone Depletion: 1991_,Report#25
  2. S.Solomon, "Progess towards qualitative understanding of Ozone depletion"' _Nature_,347: 347, _1990
  3. End of Mission Statement, second airborne artic stratospheric expedition, NASA;_April 30th. 1992
  4. F.S. Rowland, "Stratospheric Ozone Depletion",_Annual Reviews in Physics and Chemistry_, 42: 731_. 1991
  5. R.P.Wayne,_Chemistry of Atmospheres_, 2nd edition._, 1991
  6. G.W.Harris, D.Klemp, and T.Zenker, ''An Upper limit on the HCl near-surface mixing ratio over the Atlantic", _ Journal of Atmospheric Chemistry_. 15: 327_, 1992
  7. B.Vierkorn-Rudolf et al. , "Vertical Profile of HCl in the Troposphere", _Journal of Atom. Chem._, 2: 47_ 1984
  8. Fabian et.al. , "Simultaniously measured vertical profile of H2,CH4, CO, N2O, CFCl3, and CF2Cl2 in the midlatitude stratosphere and the troposphere.", _Journal of Geophysical Research_, 84: 3149_. 1979
  9. P. Fabian et al., "Halocarbons in the Stratosphere", _Nature_, 294: 733_. 1981
  10. R.Zander et al., "Infrared Spectroscopic measurements of halogenated source gases in the stratosphere with the ATMOS instrument",. _Journal.Geophys.Res._, 92: 9836_, 1987
  11. R.Zander et al.., "Stratospheric CLONO2, HCl, and HF concentration profiles derived from ATMOS/Spacelab 3 observations- an update.", _Journal.Geophys.Res._, 95: 20519_, 1990
  12. S.A.Penkett et al., "Methyl Chloride in the Stratosphere",._Nature_, 283: 58_. 1980
  13. C.P. Rinsland et.al., "Infrared measurement of HF and HCl total column abundances above Kitt Peak, 1977-1990", . _Journal.Geophys.Res._, 96: 15523.__, 1991
  14. Special Mt.Pinatubo issue, . _Geophysical Research Letters._, 19_, #2.,.1992_. ( Many great research articles!)
  15. P.R.Kyle et. al., "Emission rates of sulfur dioxide, trace gases and and metals from Mount Erebus, Antarctica". _Geophysical Research Letters._, 17: 2125_.1990
  16. G.Zreda-Gostynska et. al.,"Chlorine. Fluorine, and Sulfur Emissions from Mount Erebus, Antarctica and estimated contributions to the anarctic atmosphere", _Geophysical Research Letters._, 20: 1959_.1993
  17. A.Tabazadeh and R.Turco, "Stratospheric Chlorine Injection by Volcanic Eruptions: HCl scavenging and Implications for Ozone.", _Science_, 260: 1082., _1993
  18. M.J.Prather et al.,"The Spaceshuttle's impact on the Stratosphere"._Journal.Geophys.Res._, 965: 18583.__, 1991

    Author: Brien Sparling

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Curator: Jill Dunbar
Last Update: May 30, 2001
NASA Official: Walt Brooks