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Lecture 5

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GEOG 221
Nancy Ross

GEOG 221 Lecture 5 Ozone: can’t live with it, can’t live without it  Ozone: naturally occurring gas, trace gas  Most ozone is found in a layer between 15 and 35 km above the surface (stratosphere)  Ozone layer here is beneficial to life on earth (absorbs harmful UV radiation from the sun)  Photochemical ozone (created near the surface) is a secondary pollutant  High energy UV (far) is absorbed high in the atmosphere, near UV is transformed into 3 different UV types (A, B, C)  Higher energy = shorter wavelength  Ozone: UV radiation connection o All UV-C (highest energy – most destructive) is absorbed by O3 layer (destroys DNA) o Most UV-B is absorbed – but holes are causing increased exposure and health risks (skin cancer, etc)  Cumulative, happens over time – a lifetime of exposure leads to problems later in life o Nearly all UV-A reaches the surface (mostly benign)  Plays a role in some health effects such as immune system, etc  Parts of sunlight have enough energy to do damage to your system  Ozone: photochemistry of creation o O3 rapidly created in a 3-body reaction: where M is a third molecule which carries off the excess energy during the 3-way collision but remains unaffected (M is typically oxygen or nitrogen) o If M is not there, too much energy to stick together o If nothing else happened, all oxygen would be converted to 03 – not a great thing! o Ozone chemistry:  Oxygen molecules are photolyzed, yielding 2 oxygen atoms  Ozone and oxygen atoms are continuously being interconverted as solar UV breaks ozone and the oxygen atom reacts with another oxygen molecule  This interconversion process converts UV radiation into thermal energy, heating the atmosphere  Ozone is lost by a reaction of the oxygen atom or the ozone molecule with each other, or some other trace gas such as chlorine o There are cycles happening in the atmosphere o Oxygen converted to ozone, needs to go the other way as well! o Energy coming from the sun in form of UV splits up O2 o A competing process reduces 03: 03 is photodissociated by UV radiation into atomic and molecular oxygen o The natural creation-destruction cycle repeats o O3 dissociation is sped up by the presence of catalytic agents  Ozone: catalytic destruction o Catalytic destruction destroy O3 in the presence of molecule X which reacts and removes O3 while being regenerated in the process o In the stratosphere X is=  nitric oxide (NO) (XO = NO2)  hydroxyl (OH) = HO2  chlorine (Cl) = ClO  bromine (Br) = BrO o these catalytic agents cause a reduction of the ozone layer in the stratosphere: BAD because we need the ozone layer (stratosphere ozone is GOOD) o UV radiation strikes a CFC molecule, causes a chlorine atom to break away. It collides with an ozone molecule, steals an oxygen atom to form chloride monoxide (ClO) and leave a molecule of ordinary oxygen. When a free atom of oxygen collides with the chloride monoxide, the two oxygen atoms form a molecule of oxygen. The chlorine atom is released and free to destroy more ozone.  The CFC connection o Chlorofluorocarbons are a family of chemically stable compounds with low toxicity that are purely artificially created  Safely used as refrigerants and propellants o So stable that there appears to be no way in which the CFC is removed  Do not react with scavenging agents such as OH  Not water soluble so not washed out by precipitation or dissolved into oceans  Each molecule has a typical lifespan of 100 years o The only sink appears to be transport into the stratosphere  Here UV radiation is more intense and can dissociate CFC’s  BUT: this releases chlorine atoms which act as catalyst in O3 destruction  Fortunately, HCl is transferred downwards from stratosphere to troposphere at the same rate as CFC’s are transferred upwards o CFCs are not naturally occurring: before their creation there were none, as soon as they were released the ozone layer started thinning  How much ozone is there? o Ozone layer thickness expressed in terms of “Dobson units”  Measure of physical thickness of layer if compressed in the earth’s atmosphere o It is very thin (around 2 pennies)  1 DU is equivalent to 2.7 x 10^16 ozone molecule cm^-2 of surface o Ozone depletion is most severe over the south pole where a large “hole” opens each year during Antarctic spring (Northern hemisphere’s fall)  Hole is defined as regions over which ozone layer is < 220 DU in thickness  Evidence for Stratospheric Ozone Depletion o Hole discovered by British scientists in 1985 who, while working in the Antarctic noticed that each October, the DU dropped  Speculated a link to increase of CFC’s o By studying earlier records it was determined that the hole had formed since 1970’s o Hole has continued to grow and deepen reaching record low of 100 DU in 1999 (<1/3 1960’s) o 1998 it covered 26 million km^2 and lasted for 3 months, depletions of 70% o It now reaches beyond the tip of South America (also touches Australia) o Less DU means less thickness of ozone, less stopping of UV radiation  Cause of Antarctic Ozone Hole o Combination of right meteorological conditions and wrong human activity o The ‘polar vortex’ acts as a containment vessel  strong subsidence isolates the southern polar atmosphere o Stratospheric ice clouds form over the southern polar region o Chlorine and bromine (CFC’s) from human activities are converted to active chemical forms by UV-C  Vigorously interact chemically with ozone as a catalytic agent
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