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Stratospheric Photochemistry

Stratospheric Photochemistry. Recall: E=h  or E=hc/ h= 6.62 x 10 -34 Js (Planck’s constant) c= 3.0 x 10 8 m/s (speed of light) The amount of energy absorbed by 1 mole of matter when each molecule in it absorbs one photon is: E = 119,627/  (E is in kJ/mol).

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Stratospheric Photochemistry

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  1. Stratospheric Photochemistry • Recall: • E=h or E=hc/ • h= 6.62 x 10-34 Js (Planck’s constant) • c= 3.0 x 108 m/s (speed of light) • The amount of energy absorbed by 1 mole of matter when each molecule in it absorbs one photon is: • E = 119,627/ (E is in kJ/mol)

  2. Dissociation of Molecular Oxygen O2 2 O Ho=494kJ/mol =119,627kJ mol-1 nm/494 kJ mol-1 =241 nm Any O2 molecule that absorbs a photon of light of wavelength 241 nm or shorter has sufficient excess energy to dissociate

  3. The Natural Ozone Cycle

  4. Formation of Ozone

  5. Ozone Production • The reaction of oxygen atoms with oxygen molecules produces all of the ozone in the stratosphere • During daylight, ozone is constantly formed by this process • Rate of formation is dependent on the amount of UV light & the concentration of oxygen molecules at a given altitude

  6. The Ozone Layer • O2 is more abundant at the bottom of the stratosphere than at the top (air density) • Relatively little O2 is dissociated because most of the high energy UV has been filtered out above. • Thus, the ozone layer doesn’t extend below the stratosphere

  7. The Ozone Layer • At top of stratosphere, UV-C is greater but O2 concentration is low • O atoms tend to recombine before they collide with an O2 molecule • Thus, little ozone is formed here

  8. The Ozone Layer • The density of ozone reaches a maximum where the product of UV-C intensity and O2 concentration is a maximum • Maximum ozone density • ~25 km over tropics • ~21 km over mid-latitudes • ~18 km over subarctic regions

  9. Most oxygen atoms produced in the stratosphere by photochemical • decomposition 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

  10. Ozone’s Natural Cycle

  11. Destruction of the Ozone Layer Fact or Fiction?

  12. Evidence • Evidence for the partial destruction of the ozone layer has come from satellite monitoring ozone levels over Antarctica • Natural weather patterns cause a depletion in the ozone layer every spring (“ozone hole”) • In monitoring this process, meteorologists have found that the hole is becoming larger and longer-lived

  13. Spring 1979 Satellite Map of the Ozone Hole

  14. Spring 1991 Satellite Map of the Ozone Hole

  15. Ozone Measurements

  16. Ozone Level Over N. America

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

  18. Ozone Reactivity • Ozone is a highly unstable molecule. • Ozone is highly reactive donating its “extra” oxygen atom to nitrogen, hydrogen and chlorine.

  19. Bond Cleavage • Homolytic cleavage yields highly reactive radicals • Heterolytic cleavage yields ionic species

  20. Where Does Atmospheric Chlorine Come From?

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