Gases and the atmosphere Chemical reactions in the atmosphere Ozone cycle Ozone depletion Greenhouse effect Global Warming Pollution in the troposphere Mary J Bojan Chem 110 1 Gases and the atmosphere Review Composition of the atmosphere Use mole fraction and ppm Pressure profile of atmosphere Low pressure means few molecular collisions Chemical reactions occur less frequently Temperature profile of atmosphere High temperature means avg KE of molecules is high Mary J Bojan Chem 110 2 Composition of the atmosphere mole fraction xi moles of i total moles N2 and O2 represent 99 of atm Mary J Bojan Chem 110 3 Composition of the atmosphere For other components use parts per million ppm ppm xi 106 Eg Neon xNe 0 00001818 Ne concentration 18 18 ppm If you know the barometric pressure you can determine the partial pressure PBAR 0 987 atm PTOT PNe xNe PTOT 18 18 10 6 0 987 atm 17 94 10 6 atm Mary J Bojan Chem 110 4 Pressure decreases exponentially with altitude WHY Pressure at any altitude depends on weight of gas above it What is pressure Mary J Bojan Chem 110 5 Pressure and altitude Altitude km 0 50 100 200 P atm collision frequency sec 1 1 1010 10 3 10 6 10 13 107 103 1 Low pressure means fewer collisions When collision frequency is low chemical reactions occur less frequently Mary J Bojan Chem 110 6 Regions of Atmosphere Based on Temperature Profile Thermosphere High energy radiation is absorbed Ions formed Mesosphere Density of gases is small Stratosphere Warming caused by ozone cycle Ultraviolet light absorbed Troposphere Life where we live weather planes Present life forms could not survive above ground if all solar radiation reaches earth Mary J Bojan Chem 110 7 Distribution of Radiation Mary J Bojan Chem 110 8 Penetration of Solar Radiation Mary J Bojan Chem 110 9 Reactions in the atmosphere Photochemical Reactions reactions that involve the absorption or emission of a photon Electronic excitation of molecules NO2 g h NO2 g Photodissociation bond broken by absorption of a photon light O2 h O O E h Photon needs sufficient energy to break bond Bond energy for O2 495kJ mol c 242 nm Photoionization removal of an outer valence electron from a molecule by absorption of a photon N2 h N2 eMary J Bojan Chem 110 10 Thermosphere Ionosphere Mary J Bojan Chem 110 11 Warming of the Stratosphere Ozone concentration peaks in stratosphere 25km concentration 10ppm Presence of ozone causes temperature increase in stratosphere Mary J Bojan Chem 110 12 Ozone O3 Structure 2 resonance forms bent bond angle 117 bond length 1 28 O2 is 1 21 in agreement with VSEPR predictions Properties Light blue gas Pungent odor smell near electrical discharges Hf 142kJ mol reactive less stable than O2 Mary J Bojan Chem 110 13 Why is ozone so important Ozone cycle formation of O3 O2 h 2O 242nm O O2 O3 uv blocking by O3 O3 h O2 O 320nm Solar radiation ultraviolet is converted to heat The small amount of O3 in stratosphere reflects the delicate balance between creation and destruction Mary J Bojan Chem 110 14 Problem Ozone depletion What causes it Chlorofluorocarbons CFCs Properties Examples CFCl3 freon 11 CF2Cl2 freon 12 relatively inert easily liquefied Non combustible volatile evaporate and diffuse upward easily Uses aerosol propellants coolants for refrig solvents for cleaning polymer manufacturing Mary J Bojan Chem 110 15 CFC destruction by ozone CF2Cl2 h CF2Cl Cl 240nm Cl O3 ClO O2 ClO O Cl O2 NET O3 O 2O2 Cl atom from CFC catalyzes O3 destruction speeds up reaction but is not used up 1 Cl atom destroys 2 million O3 molecules Global concentration of O3 has been on the decline since 1980 Mary J Bojan Chem 110 16 Greenhouse Effect Energy comes to earth from the sun Some is reflected by atmosphere Some is absorbed Most of uv and visible reaches the earth s surface causes warming Earth radiates heat back out mainly infrared energy Mary J Bojan Chem 110 17 The earth s energy balance Distribution of Radiation Solar radiation warms the earth Earth radiates infrared light back Mary J Bojan Chem 110 18 Greenhouse effect H2O and CO2 thermally regulate the atmosphere Absorption of infrared radiation Activates stretching and bending modes keeps heat from being released back into space Without the greenhouse effect Earth would be 33 colder water ice Mary J Bojan Chem 110 19 What is Global Warming Over the past century atmospheric CO2 has increased 25 Mean global temperature has increased 1 Increased quantities of CO2 may be causing global warming Mary J Bojan Chem 110 20 How serious is the problem Need to know about Carbon cycle interconversion of C in the environment Mary J Bojan Chem 110 21 The Carbon Cycle Sources of atmospheric CO2 Natural volcanic eruption sources decaying organic materials respiration Man made sources fuel combustion deforestation cement manufacturing Sinks for CO2 Formation of rock Dissolves in oceans Photosynthesis Mary J Bojan Chem 110 22 Summary 200 billion metric tons bmt of C in the form of CO2 is removed each year from the atmosphere 110 bmt removed by photosynthesis 90 bmt removed by absorption in ocean seashells coral limestone 203 bmt of C are released into the atmosphere respiration decomposition deforestation 3 5 of CO2 released fossil fuel use Mary J Bojan Chem 110 23 Summary Projections CO2 levels are expected to double in 25 years 1 5 4 5 change in global T 1 5m rise in ocean levels but then more CO2 dissolves shift in climate and crop production See National Geographic s Sept 2004 Edition Mary J Bojan Chem 110 24
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