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Earth’s Atmosphere
Atmosphere
Envelope of gases that surround the EarthEnvelope of gases that surround the Earth Protects the Earth Protects the Earth Provides materials necessary to support all Provides materials necessary to support all
forms of lifeforms of life
4 Regions based on Temperature
TroposphereTroposphere StratosphereStratosphere MesosphereMesosphere ThermosphereThermosphere
Troposphere
0-12 km0-12 km Temperature Temperature decreases decreases with altitudewith altitude
Minimum of 215 KMinimum of 215 K WeatherWeather Upper limit is the tropopauseUpper limit is the tropopause
Stratosphere
10-50 km10-50 km Temperature Temperature increasesincreases with altitude with altitude
Maximum of 275 KMaximum of 275 K Upper limit is the stratopause Upper limit is the stratopause
Mesosphere
50-85 km50-85 km Temperature Temperature decreases decreases with altitudewith altitude
Minimum of 190 KMinimum of 190 K Upper limit is the mesopauseUpper limit is the mesopause
Thermosphere
Above 85 kmAbove 85 km Temperature Temperature increases increases with altitudewith altitude
Pressure
Decreases in a regular way with increasing Decreases in a regular way with increasing elevationelevation
Troposphere and stratosphere account for Troposphere and stratosphere account for 99.9% of the mass of the atmosphere99.9% of the mass of the atmosphere
Earth’s Composition
NN22 and O and O22 make up 99% of atmosphere make up 99% of atmosphere
COCO22
Noble gasesNoble gases TABLE 18.1TABLE 18.1
Parts per Million
Unit of concentrationUnit of concentration One part by One part by volumevolume in 1 million volume in 1 million volume
units of the wholeunits of the whole Volume is proportionate to moleVolume is proportionate to mole
Volume fraction = mole fractionVolume fraction = mole fraction
N2 versus O2
NN2 2 has a triple bond and Ohas a triple bond and O2 2 has a double bondhas a double bond
OO2 2 much more reactive and has lower bond much more reactive and has lower bond
energy than Nenergy than N2 2
Outer Regions of the Atmosphere
Beyond stratosphereBeyond stratosphere
Outer defense against radiation and high-Outer defense against radiation and high-energy particlesenergy particles
Photodissociation
Shorter-wavelength/ higher-energy radiations Shorter-wavelength/ higher-energy radiations in the ultraviolet range of spectrum cause in the ultraviolet range of spectrum cause chemical changeschemical changes
For radiation to fall on Earth’s atmosphere:For radiation to fall on Earth’s atmosphere: Photons with sufficient energyPhotons with sufficient energy Molecules absorb photonsMolecules absorb photons
Photodissociation
Rupture of a chemical bond resulting from Rupture of a chemical bond resulting from absorption of a photon by a moleculeabsorption of a photon by a molecule
No ions formedNo ions formed ½ of electrons stay with one of the atoms & ½ of electrons stay with one of the atoms &
½ stay with the other½ stay with the other 2 neutral particles2 neutral particles
Photodissociation of O2
Bond energy = 495 kJ/molBond energy = 495 kJ/mol
+ hv +
Photoionization
Occurs when a molecule absorbs radiation Occurs when a molecule absorbs radiation (a photon) and the absorbed energy causes (a photon) and the absorbed energy causes an electron to be ejected from the moleculean electron to be ejected from the molecule Becomes positively charged ionBecomes positively charged ion
Ozone in the Upper Atmosphere
OO33 is the key absorber of photons having is the key absorber of photons having
wavelengths from wavelengths from 240-310 nm240-310 nm Below altitude of 90 km, most short-Below altitude of 90 km, most short-
wavelengths (< 240 nm) have been wavelengths (< 240 nm) have been absorbed by Nabsorbed by N22, O, O22, and atomic O, and atomic O
Continued
30-90 km:30-90 km:
+ O2 O3*
*excess energy (releases 105 kJ/mol)
Continued
OO33 collides with other atoms or molecules, collides with other atoms or molecules,
M (usually NM (usually N22 or O or O22), & transfers energy), & transfers energy
O + OO + O22 O O33**
OO33* + M * + M O O33 + M* + M*
O + OO + O22 + M + M O O33 + M* + M*
Effects on Rate of O3 Formation
1.1. Presence of O atoms (favored at higher Presence of O atoms (favored at higher altitudes)altitudes)
2.2. Molecular collisions (favored at lower Molecular collisions (favored at lower altitudes)altitudes)
Continued
Highest rate of OHighest rate of O3 3 formation occurs in a formation occurs in a
band at 50 km altitude band at 50 km altitude
90% of O90% of O33 is found in the stratosphere is found in the stratosphere
After Formation
OO33 does not last long does not last long
It absorbs solar radiation and decomposes It absorbs solar radiation and decomposes back into O and Oback into O and O2 2
Cyclic Process
1.1. OO22 + + hvhv O + O O + O
2.2. O + OO + O22 + M + M O O33 + M* (heat released) + M* (heat released)
3.3. OO3 3 + + hvhv O O22 + O + O
4.4. O + O + M O + O + M O O22 + M* (heat released) + M* (heat released)
1 & 3: photochemical (initiated by a solar photon)1 & 3: photochemical (initiated by a solar photon)
2 & 4: exothermic chemical reactions2 & 4: exothermic chemical reactions
Net resultNet result: solar radiant energy converts to thermal energy: solar radiant energy converts to thermal energy
Depletion of O3 Layer
1970s: CFCs depleting ozone1970s: CFCs depleting ozone CF ClCF Cl3 3 and CF and CF22ClCl22
Used in refrigerators, propellants, foaming agentsUsed in refrigerators, propellants, foaming agents CFCs diffuse in stratosphereCFCs diffuse in stratosphere
Exposed to radiationExposed to radiationPhotodissociation occursPhotodissociation occurs
Photodissociation of CFCs
C-Cl bond is weaker than C-F bondC-Cl bond is weaker than C-F bond Free Cl atoms are formed when Free Cl atoms are formed when = 190-225 = 190-225
nmnm Greatest at altitude of 30 kmGreatest at altitude of 30 km
CFCF22ClCl2 2 + + hvhv CF CF22Cl + ClCl + Cl
Continued
Cl reacts with ozoneCl reacts with ozone
Cl + OCl + O33 ClO + O ClO + O22
Sometimes ClO regenerates free Cl atoms Sometimes ClO regenerates free Cl atoms (photodissociation)(photodissociation)
ClO + ClO + hvhv Cl + O Cl + O
Cl-catalyzed decomposition of O3 to O2
2Cl + 2O2Cl + 2O33 2ClO + 2O 2ClO + 2O22
2ClO + 2ClO + hvhv 2Cl + 2O 2Cl + 2O
O + O O + O O O22
2Cl + 2O2Cl + 2O3 3 + 2ClO + 2O + 2ClO + 2O 2ClO + 2Cl +3O 2ClO + 2Cl +3O22 + 2O = + 2O =
2O2O3 3 3O 3O22
Limiting use of CFCs
1987 Montreal Protocol on Substances that 1987 Montreal Protocol on Substances that Deplete the Ozone LayerDeplete the Ozone Layer
1992: 100 nations agreed to ban CFC 1992: 100 nations agreed to ban CFC production by 1996production by 1996
Replacing CFCs
HydrofluorocarbonsHydrofluorocarbons C-H bond replaces that of C-ClC-H bond replaces that of C-Cl
ex: CHex: CH22FCFFCF33 (HFC-134a) (HFC-134a)
Natural Depletion
Natural sources that contribute Cl and Br to Natural sources that contribute Cl and Br to atmosphere (the methyl's) atmosphere (the methyl's) CHCH33Cl and CHCl and CH33BrBr
1/3 of depletion (2/3 human activities)1/3 of depletion (2/3 human activities)
Homework
Page 797Page 797 15-21 odd only15-21 odd only
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