STRATOSPHERIC OZONE LECTURE
BACKGROUND
HISTORY of Hole
- Joseph Farman (British): October (Antarctic spring) measurement in the last decade (late seventies) show 50 % depletion. Later, the overall depletion is estimated to be 70 % (3 % on global basis).
- Stokarski and Krueger (NASA): Satellite observation - demonstrated the geographic distribution of hole. Thick as Mt. Everest, area of U.S.
- Hoffman (Wyoming): depletion in range of 12-30 km
CAUSES (EARLY THOUGHTS):
(1) Antarctic Vortex (upward moving wind replace ozone-rich stratosphere displaced by ozone-poor troposphere (physical mech.). Testable: if true, then other gas components rich in the troposphere should be detected in the hole as well -- WRONG! Not enough flux to do it. Net flux in ozone layer is down from higher altitudes.
(2) NO (nitrous oxide) is responsible. It is known that in the troposphere, this is one mechanism of destroying ozone: NO + O3 = NO2 + O2 (reverse of net ozone formation rxn where NO2 = NO + O and the latter react with O2). This theory holds that solar activity (prod. NO at high altitudes) and atm. circulation combined to produce NO which subsequently destroy ozone. PROBLEM: NO is depleted, not enriched in the ozone hole !! Impt for stratospheric depletion of ozone in general but not the ozone hole.
WHY THE ANSWER IS CHEMICAL, YET NOT OBVIOUS INITIALLY:
The immobilization of Cl and Nox takes place readily in the stratosphere via:
ClO + NO2 = ClONO2 (chlorine nitrate) and
Cl + CH4 = HCl + CH3 (radical)
This is why early modelers thought that CFCs are not going to be a very significant problem.
If immobilization readily takes place, why is ozone depleted in ozone hole? The secret is in the clouds (PSCs).
Polar Stratospheric Clouds (PSCs): "Most beautiful clouds on Earth"
(1) nacreous clouds (iridescence) - water
(2) non-iridescent water clouds (larger nuclei)
(3) HNO3 cloud (nitric acid trihydrate - NAT)
Chemical Reactions Within PSCs free Cl from the "non-reactive" reservoirs.
(1) cloud particles provide a substrate which enable "liberation" of Cl2 from the two non-reactive reservoirs, i.e., HCl + ClONO2 = Cl2 + HNO3. This "liberation" needs sunlight so the ozone hole develops during the spring break (October in Antarctic). Cl2 destroys two ozone molecules to produce ClO w/c recombines as a Cl2O2 dimer which is readily decomposed by sunlight into Cl2 and O2.
(2) HNO3 is taken out by cloud droplets (nitric acid trihydrate) enhancing Cl2 production and reducing the formation of chlorine nitrate (by using up NO2 forming HNO3, by reaction with OH radicals, and again taking HNO3 out).
(3) In addition to PSCs, volcanic clouds/sulphate aerosols also help destroy ozone by liberating Cl from non-reactive reservoirs.
HOW ARE PSCs FORMED ??
What you need to do is cool the ozone layer or lower stratosphere. Displace it with colder tropospheric air (happens readily, for example, as a consequence of tropospheric circulation over mountains). -70 deg celsius, HNO3 clouds form by nucleating on sulphate aerosols; - 65 deg celsius, water clouds form by nucleating on HNO3 clouds or to sulphate aerosols. Whether you form nacreous clouds or non-iridescent clouds depend on rapidity of stratospheric cooling (Slow - non-iridiscent).
ANTARCTIC HOLE SUMMARY
In summary, coldest part of year (Aug) to ppt the clouds (June to September), then first sunlight (Sept-October) to initiate the process (peak depletion in October). By November, Vortex disintegrate spreading ozone depleted air in S. hemisphere. This also explains why not severe in the arctic (not cold enough to form PSCs). The hole forms because the stable Antarctic Vortex concentrate the clouds in an area until sunlight arrives in spring. This contrast with Arctic where the vortex will have disintegrated by the time the sun peaked.
Some feedback effects also. Less ozone less warming in the ozone layer. This means colder and more PSCs (also more stable vortex).
High concentrations of ClO is nevertheless observed in the Arctic ozone layer. Suggesting that if PSCs can form there (colder), an ozone hole could develop (true for many other latitudes).
Furthermore, after the break-up of the vortex, the ozone-depleted Antarctic ozone layer spreads into the troposphere-stratosphere boundary promoting lowere ozone levels globally. Some suggest that even during polar consumption, ozone-rich air enters the vortex.
Volcanic and other aerosols are also contributors to lower latitudes, which together with increased Cl levels in stratosphere can create ozone depletions in lower latitude regions.
SOLUTIONS:
(1) transport ozone to Antarctic vortex (expensive and not necessarily good)
(2) stop CFC emission (done - Montreal Protocol; Subsequent agreement complete phase out by end of century).