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 O3 rather than the normal two atoms O2 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 O3 2 Where does it come from O2 UV photon O O O O2 M O3 M O3 photon O O3 O2 O 2 O2 slow fast fast slow Production O2 UV photon O O O O2 M O3 M O3 photon O2 O O O3 Destruction 2 O2 slow fast fast slow O2 UV photon O O2 M O3 photon O O3 Recycling Destruction O O O3 M O2 O 2 O2 slow fast fast slow Production O2 UV photon O O O O2 M ORecycling 3 M O3 photon O2 O O O3 Destruction 2 O2 slow fast fast slow Photodissociation of O3 both heats the stratosphere and consumes blocks most of the shorter UV energy from the Sun hence the ozone shield Most O3 production is in the tropical stratosphere because the solar flux is highest there perpendicular to Sun s energy Stratospheric winds move O3 to higher latitudes 3 Why is it mostly in the stratosphere The small amount of very short wave energy 200 nm required to split O2 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 O3 is in the stratosphere DNA We can repair minor UVB damage not UVC O3 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 0 001 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 O3 ClO O2 ClO O Cl O2 Cl O3 ClO O2 fast fast 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 1 30 less ozone than predicted 2 Chlorine catalytic cycle 3 Other catalytic radicals Nitrogen NO O3 NO2 O2 NO2 O NO O2 N2O O 2 NO Unit IX The Ozone Hole s E Other ozone depletion mechanisms 1 30 less ozone than predicted 2 Chlorine catalytic cycle 3 Other catalytic radicals Nitrogen NO O3 NO2 O2 NO2 O NO O2 Sources of N2O 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 CCl3F Carbon tetrachloride CCl4 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 O3 molecules The chlorine cycle is eventually broken when Cl combines with CH4 or H2 to form HCl which is scrubbed out by rain solar radiation UVC Destruction of CFCs in the stratosphere d Natural sources of Cl in the stratosphere HCl from mild volcanic eruptions Methyl chloride CH3Cl 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 1976 3 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 1976 3 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 1976 3 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