Unit4 Lecture2

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Published on February 20, 2008

Author: Vincenza

Source: authorstream.com

ES 1111:  ES 1111 Lecture 2 - The Ozone Layer & Ozone Hole (Page 222 of first text, but mostly from book “Earth Under Siege”) The Beginning of Ozone:  The Beginning of Ozone It is the ozone layer that allowed life to evolve onto land Prior to the ozone layer, organisms lived in the murky waters or in the deep ocean The ozone layer wasn’t always there because oxygen itself was rare at the start Photosynthesis by aquatic plants enriched the atmosphere with oxygen The Ozone Layer:  The Ozone Layer The ozone layer is at its maximum between 20 and 30 kilometers above the surface, and is located in the stratosphere The ozone layer is vital for life because it shields the surface from high-energy ultraviolet radiation Currently, concerns about the thinning of the ozone layer are being raised by the world’s nations Ozone is three atoms of oxygen bonded together as a molecule (O3) Creation of Stratospheric Ozone:  Creation of Stratospheric Ozone Stratospheric ozone is created differently than the ozone found near the surface Photodissociation – when a photon of sunlight strikes a molecule with enough energy to break the molecular bonds apart Diatomic oxygen is photodissociated by ultraviolet sunlight (wavelengths 0.25 micrometers or less) The reactions that create ozone are: O2 + UV light —> O + O 2 ( O + O2 + M —> O3 + M ) Net Reaction: 3O2 —> 2O3 Where M is a mediator molecule to allow the reaction Destruction of Stratospheric Ozone:  Destruction of Stratospheric Ozone The same photochemical process that creates ozone also destroys it (a good thing because all oxygen would be converted to ozone eventually without it!) The photochemical destruction of ozone is: O3 + UV —> O + O O + O3 —> O2 + O2 The total amount of oxygen in all forms is conserved “Odd oxygen” always wants to pair up evenly Budget of Stratospheric Ozone:  Budget of Stratospheric Ozone At any given moment, the stratosphere holds about 3 Gigatons of ozone The creation of ozone by the photolysis of diatomic oxygen by UV light is 1 Gigaton per year The destruction of ozone by UV light also equals 1 Gigaton per year If all the ozone were to disappear suddenly, it would take 3 years to replenish it Vertical Ozone Layer Distribution:  Vertical Ozone Layer Distribution Figure showing the vertical distribution of ozone in the atmosphere. Peak values are found between 20 and 30 kilometers, with rapidly decreasing amounts above and below that level Measurement of Ozone:  Measurement of Ozone Ozone column – the number of ozone molecules contained in a column of air 1 square centimeter in area that extends vertically from the surface up through the atmosphere 1 Dobson Unit = 2.7 x 1016 molecules in the ozone column The average amount in the atmosphere is 300 Dobson Units If you took all the ozone and brought it down to the surface, the ozone layer would be about 3 mm thick Variations in Ozone Distribution:  Variations in Ozone Distribution Every point on the globe has at least 250 DU of ozone overhead Ozone distribution is zonally symmetric (parallel contours of DU with latitude circles) Ozone increases as you go from the tropics to the poles (destruction stronger at tropics) Seasonal variations are higher over high latitudes and small over tropics, with the most ozone found in late-winter/early-spring Latitudinal Ozone Distribution:  Latitudinal Ozone Distribution Figure showing the global distribution of total ozone in the Earth’s atmosphere The Ozone Hole:  The Ozone Hole A loss of half the typical amount of ozone has been observed over Antarctica The culprit of the ozone hole is believed to be CFC’s: chlorofluorocarbons (contains chlorine, fluorine, and carbon) A series of meteorological circumstances results in the ozone hole over Antarctica Chlorofluorocarbons:  Chlorofluorocarbons Man-made compounds created to act as propellants and refrigerants In the old days, refrigerators used hazardous materials like Ammonia, and leaky refrigerators could result in death! Chlorofluorocarbons are stable compounds, do not break down easily, and are nontoxic Because of their stability, they remain in the atmosphere for a long time (allowing their long travel to stratosphere) Eventually, they leak into the stratosphere (by overshooting tops in thunderstorms) where they do break down in the presence of ice and ultraviolet light Chlorine in particular is the problem that creates the hole Chlorine’s Attack on Ozone:  Chlorine’s Attack on Ozone First, CFC’s are broken down by light CFCl3 + UV —> CFCl2 + Cl The free chlorine then reacts with ozone: O3 + Cl —> O2 + ClO ClO + O —> Cl + O2 Note that ozone is destroyed into diatomic oxygen Also note that the chlorine that started it all ends up being released at the end Catalytic Reaction – a repetitive reaction sequence where chlorine is produced at the end, able to react yet again As a result, one chlorine atom can destroy hundreds of thousands of ozone molecules Chlorine’s Lifetime:  Chlorine’s Lifetime Chlorine is locked into the CFC molecule, which is very stable It takes about 50 years for a CFC molecule to make its way up to the stratosphere Once in the stratosphere, high energy UV radiation breaks it apart Chlorine can remain in the stratosphere for 100 years (all the while destroying ozone molecules) To remove the stray chlorine atom, it must react with hydrogen to produce hydrochloric acid (HCl) or combine to form ClONO2 Meteorology of the Ozone Hole:  Meteorology of the Ozone Hole The reason why the ozone hole has been observed over Antarctica and nowhere else is due to meteorological circumstances: Fast winds (polar vortex) develops in the Stratosphere around Antarctica in the winter, isolating the stratospheric air Extremely cold temperatures allowing the formation of clouds in the stratosphere (ice allows the CFCs to become unstable) With the first rays of Sun in the spring, UV light breaks up the CFCs and releases the chlorine Ozone hole develops as catalytic reaction takes over in the Antarctic spring Once the polar vortex weakens, mixing of stratospheric air from lower latitudes heals the ozone hole Ozone Layer Anomalies:  Ozone Layer Anomalies Figure showing stratospheric ozone concentrations globally, with the ozone hole evident over Antarctica. An ozone “dent” appears over the North Pole. Reductions of 15% are found over the North Pole, but the same meteorological conditions that are found over Antarctica are not found there. Thinning Ozone = More UV:  Thinning Ozone = More UV The ozone hole is alarming because higher ultraviolet radiation amounts are a serious threat to almost all living organisms Ultraviolet light comes in four forms: Far UV: Wavelengths < 250 nm, entirely absorbed by oxygen in the upper atmosphere Ultraviolet C: Wavelengths of 250-290 nm, extremely hazardous and currently is completely absorbed by the ozone layer (organisms have no natural protection) Ultraviolet B: Wavelengths of 290-320 nm, biologically hazardous, organisms produce melanin (skin pigments) that can absorb UV-B and prevent damage to deeper tissues. Can cause skin cancer and cataracts over time. A small amount of UV-B reaches the surface Ultraviolet A: Wavelengths >320 nm, tanning beds use this type, suspected of having health consequences. 100% reaches the ground For every 1% decrease in total ozone, the UV-B radiation dose increases 2-4% Penetration of UV in the Atmosphere:  Penetration of UV in the Atmosphere Figure showing the depth of penetration for far UV, UV-C, B, and A in our atmosphere. Skin Cancers and Latitude:  Skin Cancers and Latitude Figure showing that skin cancers decrease as you go to higher latitudes (because sunlight is weaker). In addition, recall that the ozone layer is weaker in the tropics, so tropical latitudes get a double-whammy (high insolation and a weak ozone layer) Other Ways to Destroy Ozone:  Other Ways to Destroy Ozone Jets flying in the stratosphere release NO, which also has a catalytic reaction to destroy ozone: O3 + NO —> NO2 + O2 NO2 + O —> NO + O2 Net reaction: O3 + O —> 2O2 Other sources of chlorine that can later reach the stratosphere are methyl chloride (CH3Cl), released by biological activity, and hydrochloric acid (HCl) released by volcanic eruptions Can We Repair the Hole?:  Can We Repair the Hole? Planes flying in the troposphere can create ozone by emitting NO and going through the photochemical reactions of smog. However, ozone doesn’t last long for this to fix the hole Lasers may allow us to break down the CFCs before they reach the stratosphere, but such lasers would fry birds that flew in the beam and pose risks to aircraft Another idea involved charging chlorine by having it obtain extra electrons and have these charged atoms accelerated out of the atmosphere by Earth’s normal electrical field. Many problems exist with this idea, including the fact that an airplane would need to fly up there to emit electrons In general, the problem is so large that any attempt to engineer a solution would be feeble The Montreal Protocol:  The Montreal Protocol The only real solution today is to limit the production and emission of CFCs The Montreal Protocol (1987) and later amendments phased out CFC use Nearly 150 countries have signed the treaty New compounds to perform the tasks of CFCs are being designed that are safe Third-world countries require assistance in technology to phase out their CFCs Future Outlook:  Future Outlook Because of the long residence times, the ozone hole will continue getting worse before it gets better Levels of chlorine in the atmosphere have leveled off in response to the protocols Projections in chlorine amount have us returning to pre-1960s levels of 2 ppbv by 2060-2080 Reduction of ozone would have doubled between 1990 and 2030 without the Montreal Protocol Chlorine Levels in the Future:  Chlorine Levels in the Future Figure showing the forecasted chlorine concentration in our atmosphere given the Montreal Protocol and the subsequent London and Copenhagen amendments. The Montreal Protocol would not have reduced chlorine levels, which is why we had the subsequent amendments. Other Increases in Radiation Dosage:  Other Increases in Radiation Dosage Earth’s magnetic field keeps us protected from many charged particles that would be hazardous to our health and our atmosphere The magnetic field is created by hot, liquid iron flowing in our outer core Our magnetic field has weakened and reversed polarity in the past, and is forecast to do so again (we are overdue) Weakening of the field during reversal would increase our radiation dosage Without our magnetic field, the atmosphere would be stripped from our planet (like Mars, which currently doesn’t have a magnetic field)

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