today – antarctic ozone hole · ozone depletion yesterday – introduction and ozone chemistry...

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


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

• apparently didn’t 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


• 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


• 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


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


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


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


• 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


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 it’sconcentration drops significantly during October

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


Ozone hole 2004 – smaller than usual


• 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


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


2. Freon substitutes

• replace Cl with fluorine F inCFCs

• poses no threat to O3

• fluorine is extremelyreactive 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

today – antarctic ozone hole · ozone depletion yesterday – introduction and ozone chemistry...

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