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  • Ozone depletion

    yesterday introduction and ozone chemistry

    today Antarctic ozone hole

  • Yesterday:

    most of the O3 (90 %) is in stratosphere (15 30 km) it is constantly being produced (and destroyed) by

    absorption of UV solar radiation acts as a UV filter the amount of O3 present is measured in column

    depth which can be expressed in atmosphere-centimeters or Dobson units (1 DU = 1000 atm-cm)

    average total column depth is about 300 DU O3 can be destroyed additionally by various radicals

    (e.g. Cl, NO, Br,...) in catalytic processes ozone-destroying radicals are mostly of

    anthropogenic origin (freons, halons, nitrous oxide,...)

  • Ozone hole Antarctic ozone hole

    the Antarctic ozone hole is a region of extreme ozone loss(up to 60%) that has been appearing since the 1970s.

    hole begins to develop each August and culminates by earlyOctober (southern hemisphere Spring)

    eventually disappears by early December

  • Ozone hole Antarctic ozone hole

    the ozone hole was first noticed in1970s by the BritishAntarctic Survey and finally reported in 1985 it came as abig surprise to scientists

    apparently didnt exist before 1975 overlooked by NASA Nimbus-7 satellite for 6 years can be as big as 1.5 times larger than the US similar hole observed over Arctic in March much smaller

  • Ozone hole Antarctic ozone hole

    the process starts in southern hemisphere (SH) Winter (May September)

    temperature drops to -80C to -100C (-112F to -148F) strong vortex (whirlpool) forms in stratosphere over Antarctic

    vortex isolates the Antarcticstratosphere from the rest ofthe globe (because of fastspinning)

    it is particularly strong overAntarctic because of land-ocean distribution not acase in northern hemisphere

    air inside the vortex is slowlysinking

  • Ozone hole Antarctic ozone hole

    at those extremely low temperatures (below -80C) inside thevortex, polar stratospheric cloud (PSC) can form

    cloud particles are mixture of frozen water and nitrous acid

    stratosphere is very dry, butif cold enough, water willcondense

    occur at high altitudes (10 24 km)

    have specific iridescent color important for O3 chemistry rare over Artic not cold

    enough

  • Ozone hole Antarctic ozone hole

    The effect of PSC on O3 chemistry is twofold:

    1) remove nitrogen needed for storing chlorine inrelatively inert (harmless) form chlorine nitrate(ClONO2)

    2) PSC particles provide surface on whichheterogeneous chemical reactions occur, convertingchlorine from ClONO2 to molecular form - Cl2

  • Ozone hole Antarctic ozone hole

    1) remove nitrogen needed for storing chlorine in relativelyinert (harmless) form chlorine nitrate (ClONO2)

    under normal conditions nitrous oxide (NO2) is abundantin stratosphere and reacts with chlorine radicals (Cl, ClO)to create inert chlorine nitrate ClONO2

    chlorine nitrate is safe storage reservoir of chlorine in PSCs most of available NO2 converts to nitric acid

    HNO3 and is incorporated in cloud particles this results in higher concentrations of reactive chlorine

    (Cl, ClO) in stratosphere when PSCs are present

  • Ozone hole Antarctic ozone hole

    2) PSC particles provide surface on which heterogeneouschemical reactions occur, converting chlorine fromClONO2 to molecular form - Cl2

    heterogeneous chemical reactions occur on solidsurfaces, e.g. PSC particles

    molecular chlorine Cl2 as such is not harmful to the O3, butit can be converted to reactive chlorine radicals Cl, ClO

    sinking air in the vortex carries cloud particles down andpermanently removes NO2 from stratosphere

    no new O3 or other molecules can be brought into the polarvortex

  • Ozone hole Antarctic ozone hole

    during SH Winter stratospheric temperatures dropsignificantly and PSC form

    in southern hemisphere Spring (late August earlySeptember) sunlight reaches stratospheric air over Antarctic

    molecular chlorine is photolyzed to atomic (reactive) chlorine

    ozone levels drop significantly in late September earlyOctober

    by the end of October polar vortex breaks down (due tostratospheric warming)

    fresh O3 and NO2 is brought from lower latitudes ozone hole collapses until the following October

  • Ozone hole Antarctic ozone hole

  • Ozone hole Antarctic ozone hole

    Summary:

    ozone hole occurs over Antarctic because of uniquecombination of conditions (chemistry, atmosphericcirculation and availability of sunlight)

    during SH Winter (May August) PSCs form and helpconvert inert forms of chlorine (chlorine nitrate) tomolecular chlorine Cl2

    at the beginning of SH Spring (late August) sunlightreleases chlorine atoms (Cl)

    Cl destroys O3 in chlorine catalytic cycle and itsconcentration drops significantly during October

    by the end of October polar vortex breaks down and thehole disappears

  • Ozone hole Antarctic ozone hole

    Ozone hole 2004 smaller than usual

  • Ozone hole Antarctic ozone hole

    corresponding ozone hole over Arctic is rarelyobserved and much weaker

    Arctic polar vortex is less developed because ofdifferent land ocean distribution (big mountainranges stir atmosphere up)

    stratospheric winter temperature is rarely low enoughfor PSC creation

    also, Arctic polar vortex breaks up earlier in Spring

  • Mid-latitude ozone depletion

    Is there O3 depletion in mid-latitudes (where most peoplelive)?

    difficult to detect O3 changes in mid-latitudes no ozone hole lot of natural variability

  • Mid-latitude ozone depletion

    Identified causes for natural mid-latitude O3 variability:

    strong annual cycle ( 75 DU due to stratosphericcirculation)

    quasi biennial oscillation (27 months) sunspot cycle (11 years)

    more O3 production during solar maxima explains only 2 3 % of variability

    When all known natural variability is removed, data showssignificant (6 %) decrease of O3 concentrations in mid-latitudes (tropics not affected).

    However, atmospheric chemistry discussed so far can explainonly one third of this decrease!

  • Mechanisms for stopping O3 depletion

    Can the O3 depletion be stopped and ozone hole mended?

    The only way is to stop emitting ozone-damaging chemicals(CFCs) then the O3 layer will mend itself

    ozone depletion was early recognized as a global problem

    two categories of solutions:1. reduce freon and halon emissions through international

    treaties2. develop ozone-friendly substitutes

  • Mechanisms for stopping O3 depletion

    1. Montreal Protocol (1987) and Montreal Accord(1997) pose strict limits on freon and halonemissions

    goal to decrease stratospheric Cl to levels asbefore ozone hole (2 ppb) by 2060

    decrease chlorine concentrations to natural level,0.6 ppb within a century

  • Mechanisms for stopping O3 depletion

    2. Freon substitutes

    replace Cl with fluorine F inCFCs

    poses no threat to O3 fluorine is extremely

    reactive radical but it formshydrofluoric acid HF whichis stable and settles downto troposphere

    add H atom to CFC to makeit more reactive

    95 % of HCFCs will bedestroyed in troposphereand never reachstratosphere

    HFCHCFC

  • Questions

    Will the ozone hole get bigger?

    No.

    it has been increasing steadily but will probablynot get deeper than in 1993

    area will not increase (limited by the vortexcirculation)

    duration has increased too, but is limited byweakening of the vortex in SH Spring

  • Questions

    Does greenhouse effect cause the ozone hole?

    No.

    but there is a link between the two phenomena:

    CFCs are greenhouse gasses

    stratosphere is actually cooling due to globalwarming more PSCs