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Unformatted text preview: Atmospheric Chemistry Module I. Introductory Remarks In t his m odule, y ou w ill l earn a bout Atmospheric Chemistry. This type of chemistry has n ow e volved i nto a m ajor b ranch o f chemistry and is considered by the National Science F oundation a s o ne o f t he G rand Challenges of the 21st century. T he 1 995 Nobel Prize in Chemistry was given to t hree a tmospheric chemists. This module will b egin w ith a n introduction to a few key p oints t hat a re The Nobel Medal important for understanding the atmosphere and climate. We will then present an atmospheric chemistry highlight in the form of a case study that every student with a basic understanding of science should know. A. Weather and Climate The most important distinction between weather and climate is that weather occurs on a short time frame from hours to weeks and is typically limited for the future by imperfect weather models. You might for example ask a friend who lives in another city "how's the weather today?". You would not ask your friend "Hey, how's the climate today?" Weather is usually limited i n a rea t o r egions ( such a s t he Chicagoland region or the San Francisco Bay). Climate, i n c ontrast, i s t he t otal o f a ll atmospheric processes and parameters (such as temperature, pressure etc) for a large area (such as the Mediterranean, the Gulf of Mexico or the Midwest) and occurs on a time scale that is longer than weather. Existing climate models such as GCM's, or General Circulation Models, can c alculate c limate c onditions b ack f or hundreds and thousands of years. However, outlooks onto climate change in the future are currently severely limited by the complexity of climate prediction models (see below). Wind patterns in the Earth's atmosphere. B. Structure of the Atmosphere The atmosphere has a layered structure, and the main layers and the regimes separating them have names: the layer located between the ground a nd a bout s everal h undred m eters altitude is called the boundary layer. Over the oceans, this layer is also called the marine boundary layer (MBL). Boundary layers are part of the troposphere, which extends up to an altitude of about 10-12 km. When you fly from coast to coast, or between continents, your plane is located at the very top of the troposphere. Cumulus clouds, the big white fluffy clouds often seen on nice sunny days, are located in the troposphere. All weather t akes p lace i n t he t roposphere. Temperatures and pressures in the troposphere decrease from around 300 K and 760 Torr at ground level with increasing altitude, falling as low as 200K and 10 Torr at the top of the troposphere. At t he t op o f t he t roposphere l ies t he tropopause, which separates the troposphere from t he a bove-lying s tratosphere. T he tropopause is a thin layer characterized by an abrupt negative pressure gradient. This negative pressure gradient was originally thought to prevent any movement of air masses from the troposphere up into the stratosphere, however, several mechanisms for lifting tropospheric air up into the stratosphere are now known. A key mechanism is based on Lee waves, which are due t o o rographic f eatures o n t he g round (mainly m ountain r anges) t hat d eflect f ast moving air masses from the lower atmosphere into the tropopause, where mixing with the stratosphere occurs, similar to mixing between two gently stirred immiscible liquids such as oil and water. The stratosphere extends from an altitude of about 1 0-12 k m u p t o a bout 2 5 k m. T he presence o f s tratospheric o zone a nd i ts photolysis reactions (see below) in this layer cause temperatures to increase as UV-radiation from the sun is converted into heat. Pressure levels in the stratosphere are low (0.1 to 1 Torr near the equator, up to 40 Torr near the poles). In general, the variation of pressure with altitude is given by the relation p(h)=p(0).exp-mgh/(RT), where h is the height, P(0) is the pressure at sea level, g is the acceleration due to gravity, m is the average mass of air (29 g/mol), T is the temperature and R is the universal gas constant. This formula is also called the barometric formula. Low pressures in the stratosphere result in small gas phase concentrations of reactive trace gases in the stratosphere. Small concentrations slow down the rates of reactions, and if reactants collide for reaction in the stratosphere, t hey b etter r eact f ast a nd efficiently before separating again. Radical reactions allow for fast and efficient chemical transformation, and thus stratospheric chemistry is d ominated b y r adical r eactions. Photochemical reactions, i.e. reactions that require sunlight, are important as well in the stratosphere since much of the sunlight entering the stratosphere from space has the right energy and intensity to drive photochemistry at this altitude. Further, the impact of surface reactions in the stratosphere is much more apparent compared t o s urface r eactions i n t he troposphere. energy radiation (even higher in energy than UV radiation) from the sun is converted into heat via ionization reactions of molecules and atoms that are located in this layer. Thus, the temperatures increase again to several hundred K (which is one of the reasons why spacecraft need heatdeflecting s hields u pon r e-entry i nto t he atmosphere from space). With i ncreasing a ltitude, t he i onosphere smoothly c hanges i nto s pace, w here t he temperature falls down to just below 3K. This is due to the cosmic background radiation, whose spectrum still carries ripples from the Big Bang, as evidenced by 1992 data from NASA's $150 million Cosmic Background Explorer COBE satellite. COBE cosmic background radiation. space-shuttle12.htm Above the stratosphere lies the stratopause, which separates the above-lying ionosphere from the stratosphere. In the ionosphere, high- II. Stratospheric Ozone Depletion: A Case Study After this brief overview, we will present one of the m ost i mportant s tories i n a tmospheric chemistry. This is the story of stratospheric ozone depletion. We are currently experiencing the highest rates of annual ozone depletion over the poles and this effect stems from human activities carried out over 20 years ago. Every spring, h uge a reas t he s ize o f t he N orth American c ontinent s how l ow l evels o f stratospheric ozone above the North and South Polar regions. This episodic and localized event is called stratospheric ozone depletion. While it is clear to most people that ozone depletion is due t o c hlorofluorocarbons ( CFC's), i t i s generally not well understood why the event is local and occurs on a regular annual basis. The goal of this case study is to explain these two features of stratospheric ozone depletion. Arctic ozone hole in March 1997 was only about the size of the Antarctic ozone hole in 1985. But because of the fast growth of the Antarctic ozone hole since the mid 1980s, one of the main questions in atmospheric sciences is whether the growth rate of the northern ozone UV-A and similar hole will beUV-B to Visible the growth rate ofIRhe t southern o zone h ole a nd w hether s imilar increases in mutagenic UV-B radiation at the planet's surface will occur. If this is the case, then, in the very near future, the human population living in North America, Europe and the northern parts of Asia will have to b ear t he c onsequences o f t he l argest anthropogenic influence on naturally occurring processes known to date. Increases in UV-B radiation h ave a lready b een f ound o ver Scandinavia, t he B alkans, S iberia, N orth America, S outh A merica, N ew Z ealand, Australia and southern Africa. The Antarctic ozone hole in 1998. A. Stratospheric Ozone Protects Life on Earth Life on planet Earth's surface is protected from mutagenic UV-B radiation originating from the Sun by a thin layer of stratospheric ozone. The annual appearance of the ozone hole over Antarctica since the 1980's and also over the Arctic since the 1990's has led to an increase of UV-B radiation over the north and south polar regions. UV-A UV-B VIS IR The electromagnetic spectrum from the UV to the IR. The size of the ozone hole over Antarctica now matches t he s ize o f t he N orth A merican continent, and thus far, the impact of increased UV-B radiation on the human population has been relatively small. This is due to the fact that most of the densely populated off-equatorial land m asses a re l ocated o n t he n orthern hemisphere of the planet, where the size of the and The discovery of increased UV-B radiation over densely populated areas is of immediate concern to the continuation of our existence on this planet: it is now known that a decrease in the amount of stratospheric ozone has many direct and indirect consequences for humans. Among direct c onsequences o f i ncreased U V-B radiation f or h umans a re p hotochemically induced c ataract a nd s kin c ancer. O ther consequences of increased UV-B radiation that go along with a thinner ozone layer include a decreased d ensity o f p hytoplankton i n t he oceans. Phytoplankton constitute a chief sink of atmospheric carbon dioxide and is a major food source f or m arine a nimals. A s l ess phytoplankton become available, shrimp, crab and fish populations decrease. Many countries with coastal ranges depend on fishing, and their economies are immediately affected by the decreases in phytoplankton. mg_id=4729 Phytoplankton in the Gulf of Baja California. B. Chemistry in the Stratosphere As m entioned a bove, a k ey p oint a bout stratospheric chemistry is that it occurs in a region of the atmosphere that is relatively low in total pressure. Thus, the collision frequency, which is related to the chance of molecules hitting o ne a nother s o t hey c an u ndergo chemical reactions, is much lower than in the troposphere. Therefore, if reactants finally do meet in the stratosphere, they better react rapidly and efficiently. Thus, chemistry in the stratosphere is dominated by radical reactions. Important stratospheric reagents include NO2, O, OH and O3. The first three of these species are radicals and highly reactive, since they do not fulfill the Lewis octet rule. Measurements that determine the chemical composition of the polar stratosphere are carried out u sing b oth s pace b orne a nd a irborne instruments. The concentrations of important species in the stratosphere can be downloaded directly from NASA websites and you will actually d o t his t o f ind o ut w hat t he stratospheric ozone concentrations were above your hometown over the course of the past year. The most important species in the stratosphere is probably ozone, O3, which protects life at the earth's surface from DNA-damaging UV-B radiation. Its chemistry is discussed extensively in section B. Space-borne instruments include for example the platform of the NASA's Upper Atmosphere R esearch S atellite ( UARS). Besides o zone, t his satellite measures the concentration of atmospheric t race gases a nd a erosols between l atitudes o f 80º N to 80º S. The instruments measuring chemical composition Public/Analysis/UARS/urap/ in t he s tratosphere home.html include the Cryogenic Limb Array Etalon Spectrometer (CLAES), the Halogen Occultation Experiment (HALOE), and the Microwave Limb Sounder (MLS). CLAES measures ClONO2, H2O, HNO3, N2O5, and aerosol surface areas per unit volume. HALOE measures H2O, HCl, and aerosols. The MLS measures ClO, H2O, and HNO3. Air-borne measurements in the stratosphere are performed during missions of NASA's ER-2 plane. This plane is a redesigned U2 spy plane and is capable of 6 hour long stratospheric flights at altitudes around 20 km (or shorter flights up to 24 km altitude), corresponding to ambient pressures around 40 Torr. Missions include t he A irborne A rctic S tratospheric Expedition I a nd I I f rom B angor, M aine, Fairbanks, Alaska, and Stavanger, Norway to l atitudes a round 6 5º N , a nd t he A irborne Southern Hemisphere Ozone Experiment/Measurement for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) from Christchurch, New Zealand towards latitudes around 70º S. These missions approach the edge of the polar vortex and measure NOy (NOy is odd nitrogen, i.e. the sum of HNO3, HONO, HO2NO2, and N2O5), OCl, HCl, H2O and aerosol surface areas per unit volume, as well as temperature and total pressure. volume (ppbv), and ClONO2 and HCl mixing ratios are around 1 ppbv. Table I shows the partial pressures and number densities for H2O, HNO3, ClONO2 and HCl that are obtained at a typical ambient pressure around 40 Torr for the measured mixing ratios. It can be seen that the partial pressures of the trace gases ClONO2, H Cl, a nd H NO3 a re between 10-8 to 10-7 Torr. Table I Volume mixing ratios (VMR) and partial pressures (px) of stratospheric trace gases measured during the polar winters of 1992 and 1994. All partial pressures in Torr, all the number densities, or concentrations, are reported in molecules/cm3. Boarding the ER-2 Research Plane. The chemical composition determined during the s pace b orne a nd a irborne m issions i s reported in terms of volume mixing ratios (VMR's), which correspond to partial molar volumes. T ypical s tratospheric c onditions during the polar winter are around 180 to 200 K, with ambient pressures around 40 Torr. The amount of H2O is typically one part per million by volume (ppmv), also reported in the form of so-called mixing ratios. HNO3 mixing ratios are found to be around 10 parts per billion by Some trace gases cannot be measured using the instruments on the ER-2 or UARS. These immeasurable trace gases include HOCl, and the measured trace gas concentrations of ClO and NOy have to be subjected to model calculations in order to infer the contribution of HOCl to the observed o verall t race g as c oncentration. Typical uncertainties for HOCl determined in this way are around 70 to 80%. C. Ozone Depletion Summary In the 1950's, the Dupont Company introduced chloro-fluorocarbons a s r efrigerants a nd propellants. Since they are chemically inert and have physical properties that lead to cheap refrigeration a nd air c onditioning, they were believed to be ideal molecules with no HCFCl2 harmful side effects. H owever, because of their chemical inertness in the absence of UV-radiation, CFC's can reach the stratosphere on a global scale. Because of the slow mixing time through the tropopause, a CFC molecule emitted at ground level can take up to 20 years to reach the stratosphere. In the stratosphere, CFC's are broken a part b y sunlight via photochemical reactions that result in the formation of chlorine atoms. Chlorine atoms go on to destroy stratospheric ozone, with ClO radicals formed as intermediates. NO2, another important gas phase species in the stratosphere, can r eact w ith t he i ntermediate C lO. T he resulting product is ClONO2, or chlorine nitrate, which i s r elatively s table a nd n ot v ery photolabile ( i.e. d oes n ot u ndergo f ast photochemical reactions). In each ClONO2 molecule, the chlorine originates from a CFC molecule. Thus, ClONO2 formation should slow d own o zone d epletion s ince o zone destroying chlorine radicals are scavenged. low as 185K. Under the low-pressure conditions prevalent in the stratosphere, water vapor will condense to form ice crystals only once these extremely low temperatures are reached. Susan Solomon, a research scientist at the National Oceanic a nd A tmospheric A dministration (NOAA), proposed that the surfaces of these ice crystals could act as a catalyzing medium for reactions that would convert chlorine reservoirs such as ClONO2 molecules originating from CFC's into photolabile species such as Cl2. 10May_crystal.third.gif Ice crystals collected in the upper troposphere. Susan Solomon wins the National Medal of Science. However, e ven r elatively s table " chlorine reservoir species" such as chlorine nitrate, ClONO2, can lead to ozone depletion: With the advent o f t he p olar w inter, s tratospheric temperatures over the poles drop to levels as It is now well known that these photolabile species leave the ice surfaces once stratospheric temperatures s tart t o r ise a gain w ith t he beginning of the polar spring. As soon as the sun appears over the horizon, the photolabile species are broken apart by sunlight, resulting in chlorine atoms, which then go on to destroy ozone. D. Discovery of the Ozone Hole 1. 1974: The Prediction Chlorofluorocarbons (CFC's) h ave b een widely u sed i n refrigerators, a ir conditioning u nits and as propellants in spray cans since the 1950's. CFC's have low melting points, low v iscosities, small surface / tensions and high densities, which make them perfect for the use as refrigerants and aerosols. They are non toxic, non flammable, chemically inert and thermally stable. In the 1970's, James Lovelock et al. d etermined w ith s ensitive measuring equipment that CFC's are widely distributed throughout the atmosphere. This is largely attributed to their long atmospheric lifetimes which are for instance 60 years for CCl3F and 120 years for CCl2F2. Already in the mid 1970's, scientists proposed that reaction schemes involving chlorine could be responsible for stratospheric ozone depletion. io2.gif Sherry Rowland and Mario Molina. Using Lovelock's observations, Mario Molina and Sherwood Rowland proposed that chlorine, formed by photo-dissociation of CFC's in the stratosphere, could undergo radical reactions with ozone, thereby catalytically destroying stratospheric ozone. Shortly before that, Richard Stolarski and Ralph Cicerone had proposed that chlorine from space shuttle e xhaust a s w ell a s f rom v olcanic emissions could photochemically react with ozone in catalytic pathways. Rich Stolarski and Ralph Cicerone. 2. 1984: The Measurements Total o zone, i . e . t he atmospheric c olumn ozone a bundances, i s measured i n D obson units (DU), and a normal value of total ozone is about 300 DU (1 DU ≈ 2.69 x 1 016 O3 -2), molecules cm corresponding to a 3 mm thick o zone c olumn under s tandard t emperature a nd p ressure conditions. Ozone abundances can be measured using space borne instrumentation such as the total ozone measurement spectrometer (TOMS) or the solar backscatter ultraviolet spectrometer (SBUV) aboard the Nimbus 7 satellite, or the microwave limb sounder (MLS) on the upper atmosphere research satellite (UARS). Airborne measurement techniques include equipment aboard NASA's ER2 plane allowing for in situ sampling of the lower stratosphere during the flight as was done during the Airborne Southern Hemisphere Ozone Expedition/Measurements for A ssessing t he E ffects o f S tratospheric Aircraft (ASHOE/MAESA). Balloon sondes (for e xample d uring t he E uropean A rctic Stratospheric Ozone Experiment EASOE) can be e mployed a s w ell: t hey a llow f or m easurement o f c hemical c omposition, temperature, and pressure while following cloud parcel trajectories. Ground based light detection and ranging (LIDAR) techniques are used for stationary measurements. Layer, which was signed in September 1987. It is important to note that these legislative actions occurred at an unprecedented pace and were processed at a rate many times faster than conventional legislative activities. British research station in Antarctica. Decreases in stratospheric ozone have been observed above the polar regions for the past ten to 15 years. In the early 1980s, Joe Farman and his team from the British Antarctic Survey (BAS) a nalyzed r outine m easurement d ata which were taken over Haley Bay, Antarctica, using ground based equipment. It was found that during every October since about 1975, total ozone dropped down to around 100 DU, with recovery to 300 DU following in December. The BAS data was supported by space borne measurements performed by NASA. The initial discovery of the appearance of the ozone hole raised immediate concern. Intense research i nvolving f ield s tudies, c omputer modeling and laboratory experiments focused on the causes of the appearance of the ozone hole. It is now well known that the annual appearance of the ozone hole over Antarctica and the Arctic with the advent of the polar spring is caused by the onset of photochemical reactions b etween o xygen a nd c hlorine introduced by CFC's. Using parametrization programs, it was possible to predict fast rates of stratospheric ozone depletion with negligible impact from chlorine sources other than CFC's. Numerous f ield s tudies s howed t hat t he reactions p roposed i nitially b y M olina, Rowland, Stolarski and Cicerone do indeed occur in the stratosphere. Testimonies of scientists in front of the US. Congress underlined the importance of ozone depletion, and international conventions led to the Vienna Convention for the Protection of the Ozone Layer in March of 1985 and the Montreal Protocol on Substances That Deplete the Ozone The Vienna Convention c alled f or a n intense research program o n t he processes that lead to ozone depletion. The M ontreal P rotocol, modified i n r egular follow-up m eetings on the basis of new experimental findings on o zone d epletion, formally bans CFC's from p roduction i n order to halt stratospheric o zone depletion, but the long lifetimes o f C FC's a re t he r eason f or t he continued decrease in stratospheric ozone over the poles. Under the current CFC phase-out schedules, global UV levels over the poles are predicted to peak around the turn of the century in association with a peak loading of chlorine atoms in the stratosphere. After adaptation of the V ienna C onvention a nd t he M ontreal Protocol in the United States, the Clean Air Act Amendments of 1990 and the consequent ban of nonessential CFC containing products by the Environmental Protection Agency in January 1994 represented a more aggressive political approach. E. Reactions Involved in Ozone Depletion We will now turn to a more detailed look at the physical a nd c hemical processes w hich a re involved in stratospheric ozone d epletion. In nature, s tratospheric ozone is constantly being formed a nd d estroyed, and i t w as t he B ritish scientist Sidney Chapman who r ecognized a nd formulated this dynamic equilibrium of stratospheric ozone. The socalled Chapman cycle is shown below. It describes the conversion of UV-radiation below 310 n m t o h eat. T he n aturally o ccurring reactions formulated in the Chapman cycle lead to an ozone density profile, which peaks in the stratosphere, the region of the atmosphere between 10 and 50 km altitude. The Chapman cycle generates about 300 million tons of ozone per day. At mid-latitudes and an altitude o f a bout 2 0 k m, t he m aximum concentration of stratospheric ozone is 1014 ozone molecules per cm3, forming the so-called ozone layer. The depletion of the ozone layer results from a fundamental d istortion o f t he n aturally occurring C hapman c ycle b y c hlorine originating from CFC's. CFC's are brought into the s tratosphere b y c onvection. T hey a re photodissociated by sunlight, yielding chlorine atoms (Cl), which can react with ozone to form ClO radicals. This chief gas phase reaction system is shown again in reactions (1.1) to (1.3): Net: Cl + O3 fi ClO + O2 ClO + O fi Cl + O2 O3 + O fi O2 + O2 (1.1) (1.2) (1.3) schemes of computer parametrization programs and yielded better results than computations using the previous models. ClO can react with oxygen atoms (reaction (1.2)) or nitrous oxide (NO), reforming Cl and molecular oxygen (O2) or nitrogen dioxide (NO2). ClO can also react either with abundant HO2 r adicals, y ielding h ypochlorous a cid (HOCl) and O2, or with NO2 v ia a collision partner, to form chlorine nitrate (ClONO2). Because of their stable chlorine-oxygen bonds, HCl, HOCl and ClONO2 were initially thought to be "safe" chlorine containing species, and they are referred to as chlorine reservoir species. It was assumed that their formation corresponds to t he s equestering o f c hlorine a toms a nd thereby the conversion of active chlorine to inactive chlorine. This would effectively slow down ozone depletion rates. However, the ozone loss rates predicted when the formation of chlorine r eservoirs w as i ncluded i n parametrization programs did not nearly match the observed ozone losses. It w as t he m ismatch o f t he o bserved a nd predicted ozone depletion rates that prompted the search for additional pathways of chlorine activation. This search led to the hypothesis that chlorine reservoir species such as ClONO2, HCl and HOCl could undergo heterogeneous reactions with H2O and HCl on ice crystal surfaces, yielding dichlorine (Cl2), which would desorb (i.e. leave the surface) immediately into the g as p hase a nd b e p hotodissociated b y sunlight into Cl radicals with the advent of the polar spring. This type of recycling of active chlorine w as i ncorporated i n t he r eaction Three of the key reactions leading to ozone depletion are the heterogeneous reactions of ClONO2 with HCl, ClONO2 with H2O, and HOCl with HCl. These processes are shown in reactions (1.4) to (1.6). Chlorine is known to desorb immediately into the gas phase, and the nitric acid (HNO3) formed in reactions (1.4) and (1.5) is highly soluble in ice crystals. surface ClONO2 + HCl fi HNO3 (ads) + Cl2 (g) (1.4) surface ClONO2 + H2O fi HNO3 (ads) + HOCl (1.5) surface HOCl + HCl fi H2O + Cl2 (g) (1.6) The surfaced- processing reactions d+ shown in reactions (1.4)-(1.6) ultimately result in the formation of active chlorine. As a result, ozone is effectively depleted by radical Proposed mechanism for the T he ClONO2 hydrolysis on ice surfaces. reactions. sequestering of water and nitrogen in the form of H NO3 c orresponds t o d ehydration a nd denitrification in the stratosphere. Since it is the nitrogen species NO2 that can scavenge chlorine radicals efficiently to form the chlorine reservoir compounds, stratospheric denitrification inhibits the conversion of reactive ClO to unreactive ("safe") C lONO2, l eading t o e ven m ore enhanced ozone depletion. H2 O ClO NO2/M ClONO2 50 HOCl O NO hn HCl Cl Cl2 hn CH4 HO2 Cl2 Altitude [km] HCl O3 hn HO2 of PSC particles are known: type I PSC particles consist of nitric acid hydrates (NAH, primarily the mono-, di-, and the trihydrates NAM, NAD, and NAT, the exact ratio of HNO3 to H2O is not known) and type II PSC particles consist of water ice. The stratosphere is dry, and it is surprising that clouds form at all in the stratosphere. Despite the low vapor pressure of water in the stratosphere (10-4 - 10-6 Torr), PSC particles form at very low temperatures, and it is only in the polar stratosphere where the temperature drops down to 195 K resulting in type I PSC formation or 188 K resulting in type II PSC formation. OH HCl CFC's Up 10 Down x SP 0N Polar Stratospheric Clouds over Kiruna, Sweden. 90 E 180 S 90 W Summary of reactions involved in stratospheric ozone depletion. Blue arrows indicate ice-catalyzed reactions. F. Polar Stratospheric Clouds In the hypothesis that stratospheric chlorine recycling can take place at a significant rate via heterogeneous surface processing reactions, it was p roposed t hat t he r eactions o ccur o n substrates that are present in the form of socalled polar stratospheric cloud (PSC) particles. PSC/atmospheric gas volumetric ratios seem insignificant (10-12 - 1 0-11), b ut e fficient heterogeneous processes on the PSC particle surfaces make PSC's responsible for chlorine activation and recycling as well as stratospheric denitrification. Two naturally occurring types 1. The Polar Vortex How are the extremely cold temperatures that are necessary for PSC formation reached? Heat transfer in the stratosphere is largely radiative in nature. Thus, with the advent of the polar night, stratospheric temperatures drop rapidly over the poles, and it is the resulting pressure drop combined w ith t he p lanetary m otion t hat produces extremely strong westerly winds, reaching speeds up to 100 m/s (200 mph!). This episodic event leads to the formation of the polar night jet or polar vortex. The circumpolar wind p atterns a re d ifferent i n t he t wo hemispheres: the Antarctic vortex is much more pronounced and stronger than the Arctic vortex. This is due in part to the different geographic patterns of the two hemispheres: large features such as the Rocky Mountains or the Urals are found in the northern hemisphere, whereas in the southern hemisphere, only the Andes are found at higher latitudes. The more prevalent orographic features in the northern hemisphere can lead to the build up of planetary or Rossby waves that result in efficient, large-scale mixing of t he t roposphere a nd t he s tratosphere. mountain waves. Mountain or Lee waves are rapid streams of air that are localized near geographic elevations. Rapid movement of the air upward to high altitudes results in quick cooling of the stratosphere. 2. PSC Measurements PSC's are often studied using ground based, airborne o r s pace b orne L IDAR ( Light Detection And Ranging). LIDAR is similar to re.jpg The Northern Circumpolar Jet, or Polar Vortex. The consequent exchange of heat, called sudden stratospheric warming, results in warmer Arctic temperatures associated with elevated pressures, and thus the vortex over the North Polar region breaks up much more often than the Antarctic vortex. In accordance, field measurements show that the low temperatures necessary for PSC events are reached more often inside the Antarctic vortex than inside the Arctic vortex. A ground-based LIDAR system. Mountain o r l ee W aves m ix t he l ower a nd u pper atmosphere. Nevertheless, recent measurements show an increase in the number of Arctic PSC events, which m ay b e l inked t o c ooling o f t he stratosphere c aused b y g lobal w arming o r RADAR (radio detection and ranging) but is based on the use of short laser pulses. LIDAR can be used for the detection of micron-size particles such as dust and ice crystals and also for molecular species. LIDAR studies yield particle location or chemical composition as a function of altitude, and the temperature of the surrounding. McCormick et al. observed that PSC's form at altitudes of 20 km during the Antarctic winter. The spatial extent of polar stratospheric clouds can be huge: their size can reach 15º in latitude (1700 km) and 5 km in altitude! Due to their small size (1-10 mm) and because they grow slowly and under near equilibrium conditions, PSC particles are thought to be most likely single crystalline. Single crystals of ice growing i n n ature h ave t heir b asal p lane exposed, with crystal growth occurring by addition of water molecules to the prism plane. Thus, the physical and chemical events relevant to heterogeneous chlorine activation on PSC particles occur most likely on the basal ice plane. PSC particle lifetimes are on the order of minutes to hours with Type I PSC particles living about 10 times longer than type II PSC particles: due to their smaller size, the former have slower fallout rates. The shortest PSC lifetimes are found during rapid Lee wave events. In general, Type I PSC's are most common, and they are observed during 80 to 90% of all cloud sightings. References Farman, J. C.; Gardiner, B. G.; Shanklin, J. D. Nature 1985, 315, 207. Stolarski, R. S. Nature 1986, 322, 308. Lovelock, J. E.; Maggs, R. J.; Wade, R. J. Nature 1973, 241, 194. Molina, M. J.; Rowland, F. S. Nature 1974, 249, 810. Anderson, J. G.; Toohey, D. W. Science 1991, 251, 39. Solomon, S. Nature 1990, 347, 347. Crutzen, P. J.; Arnold, F. N ature 1 986 , 3 24 , 651. Molina, M. J.; Tso, T.-L.; Molina, L. T.; Wang, F. C.-Y. Science 1987, 238, 1253. Cicerone, R. Science 1987, 237, 35. Solomon, S.; Garcia, R. R.; Rowland, F. S.; Wuebbles, D. J. Nature 1986, 321, 755. Crutzen, P. J.; Arnold, F. N ature 1 987 , 3 24 , 651. Salawitch, R. K.; Gobbi, G. P.; Wofsy, S. C.; McElroy, M. B. Nature 1989, 339, 525. Hearing before the Subcommittee on Science, Technology and Space of the Committee on Commerce, Science, and Transportation, United States Senate. One Hundred First Congress, First Session on Arctic and Antarctic Ozone Depletion, chaired by Hon. A. Gore, Jr. 1989. Scientific Assessment of Stratospheric Ozone, World Meteorological Organization, 1989. ...
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