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| Lecture 34 - Ozone Depletion Ozone in the stratosphere absorbs UV short-wave radiation. This absorption capacity is what has created the stratosphere, as the Ozone heats up the upper atmosphere when it absorbs UV radiation. Remember that the Stratosphere is marked by Temperatures INCREASING with altitude. UV Radiation in the wavelength range fro 200 – 400 can be subdivided into 3 ranges: The longer wavelengths are UVA, middle UVB and the shortest are UVC. Solar flux increases with increasing wavelength so that there are more UVA photons available at the top of the atmosphere than UVB or UVC. Most of the UVA photons make it to the surface, as Ozone doesn’t absorb UVA very efficiently. Wavelength Range (nm) Name Biological Effect ______________________________________________ 320-400 UVA Relatively harmless; causes tanning but not burning 290-320 UVB Harmful; causes sunburn, skin cancer and other disorders 200-290 UVC Extremely harmful but almost completely absorbed by ozone The solar flux of the most dangerous UVC wavelengths is low, and ozone absorption is very efficient so very few UVC photons hit the earth’s surface. Current concern is with UVB radiation. The solar flux is relatively high, and ozone absorption efficiency is relatively low at this wavelength. Over exposure to UVB radiation can lead to skin cancer and is harmful to the eyes. Other animals are sensitive to UVB and plants exhibit slower growth and smaller leaves. Comparison of solar UVB radiation: Flux at ground level and ozone column depth over Melbourne, Australia shows the relationship between ozone depletion and UVB is highly correlated. Ozone and Atmospheric layers http://see.gsfc.nasa.gov/edu/SEES/strat/class/Chap_3/3_Js/3-04.jpg Ozone exists in the troposphere as well as the stratosphere. 10% of earth’s ozone is in the Troposhpere, playing an important role in cleaning the lower atmosphere of pollutants such as CO and SO2 (as O3 is a source of oxidizing radicals, which are highly reactive molecules). The radicals are split by shortwave UV radiation. So a certain amount of tropospheric ozone is good, but too much at ground level is bad. Ozone in high concentrations is toxic to plants and is a key component of the photochemical smog over LA and Denver. Ozone concentrates at about 25 km altitude. Ozone is not distributed evenly across all altitudes. It tends to accumulate more in the Higher latitudes. Ozone is measured in Dobson Units (DU). Dobson units (DU) a unit of measurement equivalent to a layer of pure ozone that is .001 cm thick at 1 atm pressure. This would be equal to 1000 DU, so typical Midlatitude ozone column depth is about 300 DU. Chemistry of Ozone Production and Destruction: The Chapman Mechanism O2 + UV photon -> O + O (1) O + O2 + M -> O3 + M (2) O3 + photon -> O2 + O (3) O + O3 -> 2 O2 (4) O2 + UV photon -> O + O (1) First is the production of two atomic oxygen atoms when a UV photon hits a molecule of oxygen. This is a slow process. O + O2 + M -> O3 + M (2) The next reaction involves a third molecule (M - can be N2, O2, 40Ar, or any of the other molecules in Earth’s atmosphere). This reaction is fast and is analogous to billiards shot. If you shoot at a single, isolated ball, chances are that the cue ball and target ball will roll away in different directions. If a third ball is in contact with the target ball, however, and the cue hits the pear dead on, the cue ball and the target ball will remain together, but the third ball will roll away. O3 + photon -> O2 + O (3) This reaction can occur with photons in the visible light range and is fast. Because many more visible photons than UV photons are available, O3 is photolyzed much faster than O2. And, O3 can be photolyzed at ground level, while O2 can only be photolyzed above about 20 km. This is where all the short-wavelength UV radiation is that is required to split O2. O + O3 -> 2 O2 (4) This is a slow reaction, but is key to understanding ozone photochemistry. When Ozone reacts with atomic oxygen, the ozone is destroyed permanently, whereas when ozone is photolyzed as in reaction 3, the resulting O atom is free to combine with another O2 molecule, reforming O3. Odd Oxygen [Ox] Ox includes all pure, oxygen-containing atoms or molecules that have an odd number of oxygen (e.g., O3) Atoms. [Ox] = [O] + [O3] Molecular oxygen (O2) contains an even number of O atoms so it is not counted as Ox. With the concept of Odd Oxygen, we can analyze the Chapman mechanism in more detail. Reaction 1 produces two O atoms from one O2, so the change in Ox is +2. Reaction 4 destroys both an O and an O3, so the change in Ox is -2. The significance is that the 2 slow reactions are the ones that count in terms of the abundance of odd oxygen. It is the destruction of odd oxygen that really matters in trace element chemistry. The Chapman mechanism included in a computer model of the stratosphere predicted about 30% more ozone than actually is present. So other processes must be destroying ozone, or destroying odd oxygen. Chapman mechanism ignored the effect of atmospheric trace constituents like nitrous oxide, water vapor and freons. These gases can be photolyzed, producing highly reactive radicals that keep ozone abundances lower than they would be otherwise. One of these radicals is nitric oxide (NO). The reactions are shown: NO + O3 -> NO2 + O2 FAST (1) NO2 + O -> NO + O2 FAST (2) NET: O3 + O --> 2 O2 FAST In the first reaction, nitric oxide reacts with ozone, forming NO2 (nitrogen dioxide) and molecular oxygen. NO2 is a major component of photchemical smog. In the second reaction, NO2 reacts with atomic oxygen, reforming nitric oxide and producing a second O2 molecule The radical acts as a catalyst – speeds up the reaction without itself being altered or lost. Another radical that has a catalytic cycle involves Chlorine (Cl). Cl + O3 --> ClO + O2 FAST (1) ClO + O --> Cl + O2 FAST (2) NET: O3 + O --> 2 O2 FAST These radicals are not lost in the reactions, so they can go on to destroy 100s or 1000s of odd oxygens. Ozone depleting Compounds NO and NO2 are odd nitrogen Produced by: N2O + O* -> 2 NO Chlorine has some natural sources, but most natural Cl remains in troposphere. The largest sources of stratospheric chlorine are from chlorofluorocarbons, or freons (CFCs) The Antarctic Ozone Hole: The ozone hole was overlooked for 6 years in the Nimbus-7 satellite measurements because the observed column depths were considered too low to be real. It was discovered in 1985 and didn’t exist prior to 1976. When looking at the ozone levels over Antarctica, typically the month of October is the most severe in terms of depletion. This is Spring for the southern hemisphere. The chemistry discussed earlier are all homogeneous reactions - reactions between molecules that are in the gas phase. Chemistry that causes ozone hole, however, involves heterogeneous reactions, such reactions that occur on solid surfaces, such as particles. Particles involved in the formation of the Antarctic ozone hole are collections of droplets called Polar Stratospheric clouds (PSC). Polar Stratospheric clouds (PSC) – These clouds were first discovered by high flying spy planes. The stratosphere is generally too dry for clouds to form. In winter, the polar stratosphere is so cold (-80C), however, that certain trace atmospheric constituents can condense. Odd nitrogen is taken up in the form of HNO3 and incorporated into cloud droplets as PSCs. This removal of NO2 from the atmosphere allows reactive chlorine concentrations to increase, because less chlorine is bound up as chlorine nitrate. The PSC particles also help convert unreactive forms of chlorine into reactive chlorine by providing surfaces on which heterogeneous reactions can occur. E.g. ClONO2 + HCl -> Cl2 + HNO3; Molecular chlorine, Cl2, doesn’t react directly with ozone, but is readily photolyzed to atomic chlorine. Cl2 + photon -> Cl + Cl. |
| Lecture 35: Ozone Depletion cont’d This Lecture will be in part an in depth review of what we covered last time, but also new material. The web-links are very important, so please follow them. This should take you about 1 hour to move through completely. Ozone: What is it: Ozone (O3) is a relatively unstable molecule made up of three atoms of oxygen (O). Although it represents only a tiny fraction of the atmosphere, ozone is crucial for life on Earth. In the stratosphere, ozone is created primarily by ultraviolet radiation. When high-energy ultraviolet rays strike ordinary oxygen molecules (O2), they split the molecule into two single oxygen atoms, known as atomic oxygen. A freed oxygen atom then combines with another oxygen molecule to form a molecule of ozone. There is so much oxygen in our atmosphere, that these high-energy ultraviolet rays are completely absorbed in the stratosphere. Formation of ozone movie: http://earthobservatory.nasa.gov/Library/Ozone/Anim/ozone_creation_final.mov History of the Ozone Hole and its Discovery The British Antarctic Survey (BAS, website: http://www.antarctica.ac.uk) has been measuring Ozone levels for 35 years above two research stations on Antarctica, the Halley and Vernadsky Stations). Using a Dobson spectrophotometer, they measured the intensities of two wavelengths of UV light, one of which is strongly absorbed by Ozone. When the machine is properly calibrated, the ratio of the intensities of these 2 wavelengths gives a measure of Ozone in the stratosphere in Dobson Units. Up to the mid 1970s, the ozone above Halley was typically around 300 DU through the Antarctic winter, until late October (spring). Then the Ozone typically increased to about 400 DU by the beginning of December (Summer) and began to decline, reaching around 300 DU by March (late Fall). This is the natural seasonal variation in Ozone due to the solar input from the sun (remember that Ozone needs photons to form). However, since the 1980s, the Spring values of Ozone began dropping, and a "hole" (really a thinning) appeared over the Antarctic research stations, with ozone values dropping below 100 DU by October. This "hole" was first discovered by Joe Farman, Brian Gardiner and Jonathan Shanklin, who published a paper in Nature in 1985. Both research stations show the similar pattern. International Efforts In the southern hemisphere spring of 1987 an international study of the Antarctic ozone hole was conducted. This included high-altitude flights, laser radar and microwave experiments from stations on Antarctica all measuring the values of Ozone and Chlorine in the stratosphere. The measurements showed conclusively that Cl plays a major role in Ozone destruction. A similar study was conducted over the Northern hemisphere Arctic and while there is greater day to day variability in the ozone levels in the Northern hemisphere, the thinning is not as great as that in the Southern hemisphere. Long-term monitoring of the Ozone levels above the Antarctic have been continuing and university researchers have worked out theoretical models of the structure and dynamics of the Ozone hole. The Montreal Protocol This was an international agreement in 1987 aimed at halving the use of CFCs by 1999. Subsequent reviews of the Protocol in 1990 and 1992 imposed more stringent requirements that all production of CFCs, CCl4 and halons (other Ozone destroying chemicals produced by human activities) end by the year 2000. Because they are so stable, despite the ban on CFC production, the levels of CFCs continued to increase for a while after their production ended. Destruction of Ozone by CFCs Insert Images of Cl destroying Ozone and atomic O (both forms of Odd Oxygen), both from the Earth Observatory (NASA) website.  Cl+O3-ClO+O2 ClO+O-Cl+O2Starting in the early 1970's, scientists found theoretical evidence that human production of chlorine-containing chemicals such as chlorofluorocarbons (CFCs) destroys ozone. CFCs are compounds made up of chlorine, fluorine and carbon bound together. Because they are extremely stable molecules, CFCs do not react easily with other chemicals in the lower atmosphere. One of the few forces that can break up CFC molecules is ultraviolet radiation. In the lower atmosphere, CFCs are protected from ultraviolet radiation by the ozone layer itself. CFC molecules thus are able to migrate intact up into the stratosphere. Although the CFC molecules are heavier than air, the air currents and mixing processes of the atmosphere carry them into the stratosphere. Once in the stratosphere, the CFC molecules are no longer shielded from ultraviolet radiation by the ozone layer. Bombarded by the sun’s ultraviolet energy, CFC molecules break up and release chlorine atoms. Free chlorine atoms then react with ozone molecules, taking one oxygen atom to form chlorine monoxide and leaving an ordinary oxygen molecule. See movie clips showing formation and destruction of ozone http://earthobservatory.nasa.gov/Library/Ozone/Anim/ozone_creation_final.mov (Creation) http://earthobservatory.nasa.gov/Library/Ozone/Anim/ozone_destruction_final.mov (destruction) When a chlorine monoxide molecule encounters a free atom of oxygen, the oxygen atom breaks up the chlorine monoxide, stealing the oxygen atom and releasing the chlorine atom back into the stratosphere to destroy more ozone. This reaction happens over and over again, allowing a single atom of chlorine to act as a catalyst, destroying many molecules of ozone. Fortunately, chlorine atoms do not remain in the stratosphere forever. When a free chlorine atom reacts with gases such as methane (CH4), it is bound up into a molecule of hydrogen chloride (HCl), which can be carried downward from the stratosphere into the troposphere, and washed away by rain. Therefore, if humans stop putting CFCs and other ozone-destroying chemicals into the stratosphere, the ozone layer eventually may repair itself. Statospheric clouds – Nacreas Clouds and PSCs  NacCloudDuring the Antarctic winter, a strong westerly circulation form in the atmosphere above the continent, called the Circumpolar vortex. This allows the interior to cool, and temperatures fall below –80C in the Stratosphere. Clouds forming at these very low temperatures are made up of ice particles – frozen Nitric Acid (HNO3), providing a surface for the chemical reactions between Cl and Ozone to occur. These are heterogeneous reactions, because they require a solid surface. Later, in the So. Hemisphere summer, the vortex breaks up, the stratosphere warms and the clouds no longer contain ice particles for the reactions to occur. The ozone hole disappears and ozone is gradually reformed. This is very well-explained in the Ozone Hole Tour, Section III. http://www.atm.ch.cam.ac.uk/tour/part3.html Satellite Data TOMS – Total Ozone Mapping Spectrometer (a NASA project) has been mapping the ozone levels since late 1978. The TOMS has been aboard several satellites during its tenure and provides up to the day data on the ozone levels. Its latest incarnation was launched in 1996 onboard an Earth Probe Satellite Create you own animation with TOMS (Satellite) dataset. This satellite dataset covers the period 1978 – 2002 You can select a starting and ending month and year to at this website: http://earthobservatory.nasa.gov/Observatory/Datasets/ozone.toms.html For a tour of the Ozone Hole, go to the following website and select the tour. The Centre for Atmospheric Science website: http://www.atm.ch.cam.ac.uk/tour/ For a Refresher on Atmospheric layers, go to the following movie clip produced by the Earth Observatory at NASA: http://earthobservatory.nasa.gov/Library/Ozone/Anim/atmosphere_labels_30.mov Some thought questions: These you should be able to answer by going to the websites I have provided. Detailed understanding of the chemistry is not required, just that you know what Ozone is, and what destroys it. Why should we care about Ozone levels in the Stratosphere? How is the Ozone hole related to Global Warming (or is it)? Why doesn’t a similar hole form over the Arctic? Can the International response to the Ozone hole be a good model for Global Warming? |
