Ozone Depletion
O 3 in troposphere Photo-chem rxns produce O 3 from NOx, HC and O 2 O 3 is a strong oxidant.
O 3 in troposphere Causes eye & lung damage to mammals Damages or kills leaves Weakens or kills trees
O 3 in stratosphere O 3 forms when sunlight strikes O 2 About 90% of Earth ’ s O 3 is in stratosphere
UV
UV-A UV-A is nm (longest of the UV) Least energetic of the UV, causes some cell damage All UV-A reaches Earth ’ s surface O 3 doesn ’ t absorb UV-A
UV-B UV-B is most harmful to cells Most UV-B absorbed by O 3, some reaches Earth ’ s surface O 3 depletion has increased UV-B at Earth ’ s surface
UV-C UV-C is nm (shortest of the UV) Most energetic of the UV ’ s All is absorbed by O 3 in stratosphere
Measuring O 3 Ground-based monitors & satellites 1 ppb of ozone = 1 Dobson unit (DU)
Ozone should be added to the stratosphere constantly But…..for the little problem of the CFCs
Chloro-fluoro-carbon rxns Cl 3 CF + UV Cl 2 + Cl Cl + O 3 ClO + O 2 ClO + O Cl + O 2
Thus, CFCs stop the production of ozone.
CFCs Are used during industrial processes & for refrigeration. Are non-reactive, thus can drift for years, eventually in stratosphere. The chlorine is a catalyst that can be used over & over, as many as 100,000 O3 O2
Antarctic O 3 depletion
mid-May (winter) Dark & cold (< - 80º C) Cold air descending (high pressure) Coriolis effect sets up a strong westerly wind (counter-clockwise) = a vortex Vortex traps Antarctic air
mid-May continued Clouds of ice crystals form in stratosphere, providing surface area for CFC-O 3 rxns Clouds & winds trapped within vortex
Oct – Nov (springtime) Increasing sunlight including UV CFCs - O 3 rxns increase > 50% of stratospheric O 3 is destroyed over Antarctica
mid-December (end of spring) Warming temps cause vortex to break up. Ozone-rich air from the north floods into Antarctica While ozone-depleted air flows northward into S. America & New Zealand
Ozone hole
Ozone “ hole ” Not really a hole; more of a thinning. Defined as concentrations of O3 < 200 ppb Occurs during Antarctic spring (Sept-Nov)
At its “ peak ” in September, the ozone hole was 27.2 million km 2 in 1998 (3 rd ) 29.5 mkm 2 in 2000 (largest) 28.7 mkm 2 in 2003 (2 nd largest) 24.3 mkm 2 in mkm 2 in 2005
Why the declines? Declines may be linked to warmer winter Antarctic temperatures. Global warming?
Ozone trends (1) Depletion probably began mid 1970s (2) Sulfate aerosol emissions from the 1991 Mt Pinatubo eruptions probably caused greater depletion
Ozone trends continued (3) At polar latitudes, sun is lower in sky and light passes through more atmosphere. Thus, less UV reaches surface at polar latitudes.
(4) Industrials areas have more ozone at surface levels. Ozone absorbs UV and may be why scientists can ’ t find increased levels of O 3 in Northern Hemisphere
Ozone trends continued (5) The energy of > UV-B at Earth ’ s surface causes > formation of surface O 3.
Why no ozone “ hole ” at North Pole? Warmer temps compared to S. Pole Jet stream tends to meander rather than creating vortex However, recent measurements show 5% O3 depletion over North Pole.
Effects of O 3 depletion
Marine food chains > UV causes decline in productivity of polar phytoplankton Equatorial phytoplankton have adaptations for UV, no surprise
UV Damage to Humans (1) Clouding of eye ’ s cornea cataracts
(2) Basal carcinoma
Basal carcinoma
(3) Melanoma
(4) Changes to DNA
Is a suntan really a good idea? (5) Wrinkles
As long as O 3 depletion continues, It would be a good idea to add a few things to your self-survival kit.
The end