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Earth’s Atmosphere

Earth’s Atmosphere. Atmosphere. Envelope of gases that surround the Earth Protects the Earth Provides materials necessary to support all forms of life. 4 Regions based on Temperature. Troposphere Stratosphere Mesosphere Thermosphere. Troposphere. 0-12 km

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Earth’s Atmosphere

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  1. Earth’s Atmosphere

  2. Atmosphere • Envelope of gases that surround the Earth • Protects the Earth • Provides materials necessary to support all forms of life

  3. 4 Regions based on Temperature • Troposphere • Stratosphere • Mesosphere • Thermosphere

  4. Troposphere • 0-12 km • Temperature decreases with altitude • Minimum of 215 K • Weather • Upper limit is the tropopause

  5. Stratosphere • 10-50 km • Temperature increases with altitude • Maximum of 275 K • Upper limit is the stratopause

  6. Mesosphere • 50-85 km • Temperature decreases with altitude • Minimum of 190 K • Upper limit is the mesopause

  7. Thermosphere • Above 85 km • Temperature increases with altitude

  8. Pressure • Decreases in a regular way with increasing elevation • Troposphere and stratosphere account for 99.9% of the mass of the atmosphere

  9. Earth’s Composition • N2 and O2 make up 99% of atmosphere • CO2 • Noble gases • TABLE 18.1

  10. Parts per Million • Unit of concentration • One part by volume in 1 million volume units of the whole • Volume is proportionate to mole • Volume fraction = mole fraction

  11. N2 versus O2 • N2 has a triple bond and O2 has a double bond • O2 much more reactive and has lower bond energy than N2

  12. Outer Regions of the Atmosphere • Beyond stratosphere • Outer defense against radiation and high-energy particles

  13. Photodissociation • Shorter-wavelength/ higher-energy radiations in the ultraviolet range of spectrum cause chemical changes • For radiation to fall on Earth’s atmosphere: • Photons with sufficient energy • Molecules absorb photons

  14. Photodissociation • Rupture of a chemical bond resulting from absorption of a photon by a molecule • No ions formed • ½ of electrons stay with one of the atoms & ½ stay with the other • 2 neutral particles

  15. Photodissociation of O2 • Bond energy = 495 kJ/mol + hv  +

  16. Photoionization • Occurs when a molecule absorbs radiation (a photon) and the absorbed energy causes an electron to be ejected from the molecule • Becomes positively charged ion

  17. Ozone in the Upper Atmosphere • O3 is the key absorber of photons having wavelengths from 240-310 nm • Below altitude of 90 km, most short-wavelengths (< 240 nm) have been absorbed by N2, O2, and atomic O

  18. Continued • 30-90 km: + O2  O3* *excess energy (releases 105 kJ/mol)

  19. Continued • O3 collides with other atoms or molecules, M (usually N2 or O2), & transfers energy O + O2 O3* O3* + M  O3 + M* O + O2 + M  O3 + M*

  20. Effects on Rate of O3 Formation • Presence of O atoms (favored at higher altitudes) • Molecular collisions (favored at lower altitudes)

  21. Continued • Highest rate of O3 formation occurs in a band at 50 km altitude • 90% of O3 is found in the stratosphere

  22. After Formation • O3 does not last long • It absorbs solar radiation and decomposes back into O and O2

  23. Cyclic Process • O2 + hv O + O • O + O2 + M  O3 + M* (heat released) • O3 + hv  O2 + O • O + O + M  O2 + M* (heat released) 1 & 3: photochemical (initiated by a solar photon) 2 & 4: exothermic chemical reactions Net result: solar radiant energy converts to thermal energy

  24. Depletion of O3 Layer • 1970s: CFCs depleting ozone • CF Cl3 and CF2Cl2 • Used in refrigerators, propellants, foaming agents • CFCs diffuse in stratosphere • Exposed to radiation • Photodissociation occurs

  25. Photodissociation of CFCs • C-Cl bond is weaker than C-F bond • Free Cl atoms are formed when  = 190-225 nm • Greatest at altitude of 30 km CF2Cl2 + hv CF2Cl + Cl

  26. Continued • Cl reacts with ozone Cl + O3 ClO + O2 • Sometimes ClO regenerates free Cl atoms (photodissociation) ClO + hv Cl + O

  27. Cl-catalyzed decomposition of O3 to O2 2Cl + 2O3 2ClO + 2O2 2ClO + hv 2Cl + 2O O + O  O2 2Cl + 2O3 + 2ClO + 2O  2ClO + 2Cl +3O2 + 2O = 2O3  3O2

  28. Limiting use of CFCs • 1987 Montreal Protocol on Substances that Deplete the Ozone Layer • 1992: 100 nations agreed to ban CFC production by 1996

  29. Replacing CFCs • Hydrofluorocarbons • C-H bond replaces that of C-Cl • ex: CH2FCF3 (HFC-134a)

  30. Natural Depletion • Natural sources that contribute Cl and Br to atmosphere (the methyl's) • CH3Cl and CH3Br • 1/3 of depletion (2/3 human activities)

  31. Homework • Page 797 • 15-21 odd only

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