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

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