Stratospheric Stratospheric PhotochemistryPhotochemistry

Recall:Recall:– E=hE=h or E=hc/ or E=hc/

h= 6.62 x 10h= 6.62 x 10-34-34 Js (Planck’s constant) Js (Planck’s constant) c= 3.0 x 10c= 3.0 x 1088 m/s (speed of light) m/s (speed of light)

The amount of energy absorbed by 1 The amount of energy absorbed by 1 mole of matter when each molecule mole of matter when each molecule in it absorbs one photon is:in it absorbs one photon is:– E = 119,627/E = 119,627/ (E is in kJ/mol) (E is in kJ/mol)

Dissociation of Molecular Dissociation of Molecular OxygenOxygen

OO22 2 O 2 O HHoo=494kJ/mol=494kJ/mol

=119,627kJ mol=119,627kJ mol-1-1 nm/494 kJ mol nm/494 kJ mol-1-1

=241 nm=241 nm

Any OAny O22 molecule that absorbs a photon molecule that absorbs a photon of light of wavelength 241 nm of light of wavelength 241 nm or or shortershorter has sufficient excess energy has sufficient excess energy to dissociateto dissociate

The Natural Ozone CycleThe Natural Ozone Cycle

Formation of OzoneFormation of Ozone

Ozone ProductionOzone Production

The reaction of oxygen atoms with The reaction of oxygen atoms with oxygen molecules produces all of the oxygen molecules produces all of the ozone in the stratosphereozone in the stratosphere

During daylight, ozone is constantly During daylight, ozone is constantly formed by this processformed by this process

Rate of formation is dependent on the Rate of formation is dependent on the amount of UV light & the concentration amount of UV light & the concentration of oxygen molecules at a given of oxygen molecules at a given altitudealtitude

The Ozone LayerThe Ozone Layer

OO22 is more abundant at the bottom is more abundant at the bottom of the stratosphere than at the top of the stratosphere than at the top (air density)(air density)

Relatively little ORelatively little O22 is dissociated is dissociated because most of the high energy UV because most of the high energy UV has been filtered out above.has been filtered out above.

Thus, the ozone layer doesn’t Thus, the ozone layer doesn’t extend below the stratosphereextend below the stratosphere

The Ozone LayerThe Ozone Layer

At top of stratosphere, UV-C is At top of stratosphere, UV-C is greater but Ogreater but O22 concentration is low concentration is low

O atoms tend to recombine before O atoms tend to recombine before they collide with an Othey collide with an O22 molecule molecule

Thus, little ozone is formed hereThus, little ozone is formed here

The Ozone LayerThe Ozone Layer

The density of ozone reaches a The density of ozone reaches a maximum where the product of maximum where the product of UV-C intensity and OUV-C intensity and O22 concentration is a maximumconcentration is a maximum

Maximum ozone densityMaximum ozone density– ~25 km over tropics~25 km over tropics– ~21 km over mid-latitudes~21 km over mid-latitudes– ~18 km over subarctic regions~18 km over subarctic regions

•Most oxygen atoms produced in the stratosphere by photochemicaldecomposition of ozone or O2 subsequently react with intact O2

molecules to reform ozone•Some oxygen atoms react with intact ozone molecules to destroy them by conversion to O2

Ozone’s Natural CycleOzone’s Natural Cycle

Destruction of the Ozone Destruction of the Ozone LayerLayer

Fact or Fiction?Fact or Fiction?

EvidenceEvidence

Evidence for the partial destruction of the Evidence for the partial destruction of the ozone layer has come from satellite ozone layer has come from satellite monitoring ozone levels over Antarcticamonitoring ozone levels over Antarctica

Natural weather patterns cause a depletion in Natural weather patterns cause a depletion in the ozone layer every spring (“ozone hole”)the ozone layer every spring (“ozone hole”)

In monitoring this process, meteorologists In monitoring this process, meteorologists have found that the hole is becoming larger have found that the hole is becoming larger and longer-livedand longer-lived

Spring 1979 Satellite Map Spring 1979 Satellite Map of the Ozone Holeof the Ozone Hole

Spring 1991 Satellite Map Spring 1991 Satellite Map of the Ozone Holeof the Ozone Hole

Ozone MeasurementsOzone Measurements

Ozone Level Over N. Ozone Level Over N. AmericaAmerica

The ozone balance can be compared to The ozone balance can be compared to a leaky bucket. As long as water is a leaky bucket. As long as water is poured into the bucket at the same rate poured into the bucket at the same rate that water leaks out, the amount of that water leaks out, the amount of water in the bucket will remain constant.water in the bucket will remain constant.

As long as ozone is being created at the As long as ozone is being created at the same rate that it is being destroyed, the same rate that it is being destroyed, the total amount of ozone will remain the total amount of ozone will remain the same.same.

Ozone ReactivityOzone Reactivity

Ozone is a highly unstable Ozone is a highly unstable molecule.molecule.

Ozone is highly reactive donating Ozone is highly reactive donating its “extra” oxygen atom to its “extra” oxygen atom to nitrogen, hydrogen and chlorine.nitrogen, hydrogen and chlorine.

Bond CleavageBond Cleavage

Homolytic Homolytic cleavage yields cleavage yields highly reactive highly reactive radicalsradicals

Heterolytic Heterolytic cleavage yields cleavage yields ionic speciesionic species

Where Does Atmospheric Where Does Atmospheric Chlorine Come From?Chlorine Come From?