Unit IX. The Ozone Hole(s) A.Discovery: Antarctic Ozone Hole Satellites have been monitoring the total amount of ozone (mostly in the stratosphere) since the late 1970s. Satellites accurately recorded the sudden disappearance of at least half of all the ozone, but the computer rejected the results for many years.

B. What is Ozone? 1.Ozone: A gaseous molecule that contains three oxygen atoms (O 3 ), rather than the normal two atoms (O 2 ). This is NASTY stuff: Extremely reactive Strongly oxidizing Formation and destruction of ozone is going on all the time in a natural cycle. This natural cycle is also being impacted by human activity.

Unit IX. The Ozone Hole(s) A. Discovery: Antarctic Ozone Hole B. What is Ozone? 1. Ozone: A gaseous molecule (O 3 ). 2. Where does it come from? O 2 + UV photon O + O O + O 2 + M O 3 + M O 3 + photon O 2 + O O + O 3 2 O 2 slow fast slow

O 2 + UV photon O + O O + O 2 + M O 3 + M O 3 + photon O 2 + O O+ O 3 2 O 2 slow fast slow Production Destruction

O 2 + UV photon O + O O + O 2 + M O 3 + M O 3 + photon O 2 + O O+ O 3 2 O 2 slow fast slow Recycling Destruction

O 2 + UV photon O + O O + O 2 + M O 3 + M O 3 + photon O 2 + O O+ O 3 2 O 2 slow fast slow Production Destruction Recycling

Photodissociation of O 3 both heats the stratosphere, and consumes (blocks) most of the shorter UV energy from the Sun: hence, the “ozone shield” Most O 3 production is in the tropical stratosphere, because the solar flux is highest there (perpendicular to Sun’s energy). Stratospheric winds move O 3 to higher latitudes.

3. Why is it mostly in the stratosphere? The small amount of very short wave energy (<200 nm) required to split O 2 molecules is consumed in the stratosphere and is not found below 20 km. C. UV Radiation UV radiation is between 200 and 400 nm We divide the UV portion into 3 segments: UVA: longer  ans you UVB: middle  Burns you; skin cancers BCC (most will get), melanoma, squamous UVC: shortest Destroys DNA

Tropopause 90% of O 3 is in the stratosphere

We can repair minor (UVB) damage; not UVC DNA

O 3 absorbs very efficiently at UVC, and moderately well at UVB, but not at UVA. Consequently, at Earth surface, UVA and UVB More ozone in the stratosphere means less UVB at Earth’s surface.

D. Measuring ozone concentrations Dobson Units (DU): 1 DU = layer of pure ozone cm thick at 1 atmosphere. Typical concentration is 300 DU Tropospheric ozone: 10% of all ozone is in the troposphere from both natural and industrial sources. Its reactive nature cleans the troposphere of pollutants, but at ground level it is a major eye and lung irritant and kills plants. LA and Denver photochemical smog….

Unit IX. The Ozone Hole(s) E. Other ozone depletion mechanisms. 1. Total amount of ozone is always 30% less than predicted if photodissociation is the only loss. There are other trace gases in the stratosphere…. Can these interact with ozone?

2. Chlorine catalytic cycle Chlorine as a catalyst. Catalyst: A substance that increases the rate of a reaction, but is itself unchanged by the reaction. Cl + O 3 ClO +O 2 ClO + OCl + O 2 fast Cl + O 3 ClO +O 2 fast As a catalyst, Chlorine is now free to repeat the cycle of ozone destruction

Unit IX. The Ozone Hole(s) E. Other ozone depletion mechanisms % less ozone than predicted 2. Chlorine catalytic cycle 3. Other catalytic radicals Nitrogen (NO + O 3 = NO 2 + O 2 ) (NO 2 + O = NO + O 2 ) N 2 O + O = 2 NO

Unit IX. The Ozone Hole(s) E. Other ozone depletion mechanisms % less ozone than predicted 2. Chlorine catalytic cycle 3. Other catalytic radicals Nitrogen (NO + O 3 = NO 2 + O 2 ) (NO 2 + O = NO + O 2 ) Sources of N 2 O in the stratosphere… Natural from microbial activity in soils Soil microbials enhanced by fertilizers Also from jet exhaust (NO)……particularly if they fly in the stratosphere (SST)

Unit IX. The Ozone Hole(s) E. Other ozone depletion mechanisms. 4. CFCs and the chlorine catalytic cycle What are CFCs? Chloro-Fluoro-Carbon compounds: compounds containing chlorine, fluorine and carbon. A simple CFC: CCl 3 F Carbon tetrachloride: CCl 4

Unit IX. The Ozone Hole(s) E. Other ozone depletion mechanisms. 4. CFCs and the chlorine catalytic cycle a) sources: leaky fridges/AC/sprays b) fate of CFCs. Because they are an inert gas, CFCs have a long residence time in the troposphere. But how do they then get through the tropopause into the stratosphere? Through rare large convective storms in the tropics. Mixed slowly (1 to 2 yr) by stratospheric winds.

Unit IX. The Ozone Hole(s) 4. CFCs and the chlorine catalytic cycle a) sources: leaky fridges/AC/sprays b) fate of CFCs. c) solar radiation and CFCs. Short-wave UV radiation breaks CFC bonds, releasing atomic Cl, which is then free to participate in the chlorine catalytic cycle, where each Cl atom destroys tens of thousands of O 3 molecules. The chlorine cycle is eventually broken when Cl combines with CH 4 or H 2 to form HCl, which is scrubbed out by rain.

solar radiation Destruction of CFCs in the stratosphere UVC

d) Natural sources of Cl in the stratosphere: HCl (from mild volcanic eruptions) Methyl chloride (CH 3 Cl) from marine biota. But CFCs supply a larger portion of stratospheric Cl.

Unit IX. The Ozone Hole(s) F. The Antarctic Ozone Hole 1. Only occurs in the Antarctic spring (September, October [strongest]). 2. Was not present in the early 1970s…. only after ~1976.

Global ozone distribution in October

Unit IX. The Ozone Hole(s) F. The Antarctic Ozone Hole 1. Only occurs in the Antarctic spring (September, October [strongest]). 2. Was not present in the early 1970s…. only after ~ In 2003, the Antarctic ozone hole was the second largest on record.

Unit IX. The Ozone Hole(s) F. The Antarctic Ozone Hole 1. Only occurs in the Antarctic spring (September, October [strongest]). 2. Was not present in the early 1970s…. only after ~ In 2003, the Antarctic ozone hole was the second largest on record. 4. Is ozone depletion restricted to the Antarctic?

Unit IX. The Ozone Hole(s) F. The Antarctic Ozone Hole 1. Only occurs in the Antarctic spring (September, October [strongest]). 2. Was not present in the early 1970s…. only after ~ In 2003, the Antarctic ozone hole was the second largest on record. 4. Is ozone depletion restricted to the Antarctic? No: also small global decreases 5. Is there an Arctic Ozone Hole? Yes, but much less severe and more variable.

Unit IX. The Ozone Hole(s) F. The Antarctic Ozone Hole G. Mitigation

Unit IX. The Ozone Hole(s) F. The Antarctic Ozone Hole G. Mitigation: Montreal Protocols and ammendments have nearly eliminated CFC and related production. Long residence times of CFCs means it will take abot 50 years for the ozone to fully recover. But it will recover.