Earth’s Atmosphere

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

4 Regions based on Temperature Troposphere Troposphere Stratosphere Stratosphere Mesosphere Mesosphere Thermosphere Thermosphere

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

Stratosphere km km Temperature increases with altitude Temperature increases with altitude  Maximum of 275 K Upper limit is the stratopause Upper limit is the stratopause

Mesosphere km km Temperature decreases with altitude Temperature decreases with altitude  Minimum of 190 K Upper limit is the mesopause Upper limit is the mesopause

Thermosphere Above 85 km Above 85 km Temperature increases with altitude Temperature increases with altitude

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

Earth’s Composition N 2 and O 2 make up 99% of atmosphere N 2 and O 2 make up 99% of atmosphere CO 2 CO 2 Noble gases Noble gases TABLE 18.1 TABLE 18.1

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

N 2 versus O 2 N 2 has a triple bond and O 2 has a double bond N 2 has a triple bond and O 2 has a double bond  O 2 much more reactive and has lower bond energy than N 2

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

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

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

Photodissociation of O 2 Bond energy = 495 kJ/mol Bond energy = 495 kJ/mol + hv  +

Photoionization Occurs when a molecule absorbs radiation (a photon) and the absorbed energy causes an electron to be ejected from the molecule 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

Ozone in the Upper Atmosphere O 3 is the key absorber of photons having wavelengths from nm O 3 is the key absorber of photons having wavelengths from nm  Below altitude of 90 km, most short- wavelengths (< 240 nm) have been absorbed by N 2, O 2, and atomic O

Continued km: km: + O 2  O 3 * *excess energy (releases 105 kJ/mol)

Continued O 3 collides with other atoms or molecules, M (usually N 2 or O 2 ), & transfers energy O 3 collides with other atoms or molecules, M (usually N 2 or O 2 ), & transfers energy O + O 2  O 3 * O 3 * + M  O 3 + M* O + O 2 + M  O 3 + M*

Effects on Rate of O 3 Formation 1. Presence of O atoms (favored at higher altitudes) 2. Molecular collisions (favored at lower altitudes)

Continued Highest rate of O 3 formation occurs in a band at 50 km altitude Highest rate of O 3 formation occurs in a band at 50 km altitude 90% of O 3 is found in the stratosphere 90% of O 3 is found in the stratosphere

After Formation O 3 does not last long O 3 does not last long It absorbs solar radiation and decomposes back into O and O 2 It absorbs solar radiation and decomposes back into O and O 2

Cyclic Process 1. O 2 + hv  O + O 2. O + O 2 + M  O 3 + M* (heat released) 3. O 3 + hv  O 2 + O 4. O + O + M  O 2 + 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

Depletion of O 3 Layer 1970s: CFCs depleting ozone 1970s: CFCs depleting ozone  CF Cl 3 and CF 2 Cl 2  Used in refrigerators, propellants, foaming agents CFCs diffuse in stratosphere CFCs diffuse in stratosphere  Exposed to radiation  Photodissociation occurs

Photodissociation of CFCs C-Cl bond is weaker than C-F bond C-Cl bond is weaker than C-F bond Free Cl atoms are formed when = nm Free Cl atoms are formed when = nm  Greatest at altitude of 30 km CF 2 Cl 2 + hv  CF 2 Cl + Cl

Continued Cl reacts with ozone Cl reacts with ozone Cl + O 3  ClO + O 2 Sometimes ClO regenerates free Cl atoms (photodissociation) Sometimes ClO regenerates free Cl atoms (photodissociation) ClO + hv  Cl + O

Cl-catalyzed decomposition of O 3 to O 2 2Cl + 2O 3  2ClO + 2O 2 2ClO + hv  2Cl + 2O O + O  O 2 2Cl + 2O 3 + 2ClO + 2O  2ClO + 2Cl +3O 2 + 2O = 2O 3  3O 2

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

Replacing CFCs Hydrofluorocarbons Hydrofluorocarbons  C-H bond replaces that of C-Cl  ex: CH 2 FCF 3 (HFC-134a)

Natural Depletion Natural sources that contribute Cl and Br to atmosphere (the methyl's) Natural sources that contribute Cl and Br to atmosphere (the methyl's)  CH 3 Cl and CH 3 Br 1/3 of depletion (2/3 human activities) 1/3 of depletion (2/3 human activities)

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