Ozone depletion. yesterday introduction and ozone chemistry. today Antarctic ozone hole

Ozone depletion yesterday – introduction and ozone chemistry today – Antarctic ozone hole Yesterday: • most of the O3 (90 %) is in stratosphere (15...
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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 atmospherecentimeters 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 early October (southern hemisphere Spring) • eventually disappears by early December

Ozone hole – Antarctic ozone hole • the ozone hole was first noticed in1970s by the British Antarctic Survey and finally reported in 1985 – it came as a big 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

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 Antarctic stratosphere from the rest of the globe (because of fast spinning) • it is particularly strong over Antarctic because of landocean distribution – not a case in northern hemisphere • air inside the vortex is slowly sinking

Ozone hole – Antarctic ozone hole • at those extremely low temperatures (below -80C) inside the vortex, polar stratospheric cloud (PSC) can form • cloud particles are mixture of frozen water and nitrous acid • stratosphere is very “dry”, but if cold enough, water will condense • 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 in relatively inert (harmless) form – chlorine nitrate (ClONO2) 2) PSC particles provide surface on which heterogeneous chemical reactions occur, converting chlorine from ClONO2 to molecular form - Cl2

Ozone hole – Antarctic ozone hole 1) remove nitrogen needed for storing chlorine in relatively inert (harmless) form – chlorine nitrate (ClONO2) • under normal conditions nitrous oxide (NO2) is abundant in 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 heterogeneous chemical reactions occur, converting chlorine from ClONO2 to molecular form - Cl2 • heterogeneous chemical reactions – occur on solid surfaces, e.g. PSC particles

• molecular chlorine Cl2 as such is not harmful to the O3, but it can be converted to reactive chlorine radicals Cl, ClO • sinking air in the vortex carries cloud particles down and permanently removes NO2 from stratosphere • no new O3 or other molecules can be brought into the polar vortex

Ozone hole – Antarctic ozone hole • during SH Winter stratospheric temperatures drop significantly and PSC form • in southern hemisphere Spring (late August – early September) sunlight reaches stratospheric air over Antarctic • molecular chlorine is photolyzed to atomic (reactive) chlorine

• ozone levels drop significantly in late September – early October • by the end of October polar vortex breaks down (due to stratospheric 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 unique combination of conditions (chemistry, atmospheric circulation and availability of sunlight) • during SH Winter (May – August) PSCs form and help convert inert forms of chlorine (chlorine nitrate) to molecular chlorine Cl2 • at the beginning of SH Spring (late August) sunlight releases chlorine atoms (Cl) • Cl destroys O3 in chlorine catalytic cycle and it’s concentration drops significantly during October • by the end of October polar vortex breaks down and the hole disappears

Ozone hole – Antarctic ozone hole Ozone hole 2004 – smaller than usual

Ozone hole – Antarctic ozone hole • corresponding ozone hole over Arctic is rarely observed and much weaker • Arctic polar vortex is less developed because of different land – ocean distribution (big mountain ranges stir atmosphere up) • stratospheric winter temperature is rarely low enough for PSC creation • also, Arctic polar vortex breaks up earlier in Spring

Mid-latitude ozone depletion Is there O3 depletion in mid-latitudes (where most people live)? • 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 stratospheric circulation) • 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 shows significant (6 %) decrease of O3 concentrations in midlatitudes (tropics not affected). However, atmospheric chemistry discussed so far can explain only 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 treaties 2. develop ozone-friendly substitutes

Mechanisms for stopping O3 depletion

1. Montreal Protocol (1987) and Montreal Accord (1997) pose strict limits on freon and halon emissions • goal to decrease stratospheric Cl to levels as before 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

HCFC

HFC

• add H atom to CFC to make • replace Cl with fluorine F in it more reactive CFCs • 95 % of HCFCs will be • poses no threat to O3 destroyed in troposphere • fluorine is extremely and never reach reactive radical but it forms stratosphere hydrofluoric acid HF which is stable and settles down to troposphere

Questions Will the ozone hole get bigger? No. • it has been increasing steadily but will probably not get deeper than in 1993 • area will not increase (limited by the vortex circulation) • duration has increased too, but is limited by weakening 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 global warming – more PSCs