CHEM-5151_S05_L7

CHEM-5151_S05_L7 - Lecture 7: Photochemistry of Important...

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1 Required Reading: FP Chapter 4 Atmospheric Chemistry CHEM-5151 / ATOC-5151 Spring 2005 Prof. Jose-Luis Jimenez Lecture 7: Photochemistry of Important Atmospheric Species Outline of Lecture • General remarks •O 2 3 • Nitrogen species • Aldehydes and ketones •CFC s We won’t cover everything in class, read the rest in the book Have to know how to find + interpret quickly
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2 Reminder from last time • Sunlight drives chemistry of trop. & strat. – “Hot” photons break bonds and create free radicals – Radicals can react with many molecules – Obj: calculate rates of photolysis & product generation ] [ ) ( ) ( ) ( ] [ ] [ A d F A J dt A d A A A × = = λ φ σ Generic reaction: A + h ν B + C J A – first order photolysis rate of A (s -1 ) σ A ( ) – wavelength dependent cross section of A (cm 2 /molec) A ( ) – wavelength dependent quantum yield for photolysis F( ) – spectral actinic flux density (#/cm 2 /s) This lecture Last lecture General Remarks • In order to photodissociate a molecule it must be excited above its dissociation energy (D 0 ). • In the lower troposphere, only molecules with D 0 corresponding to λ > 290 nm are photochemically active. Most common atmospheric molecules, including N 2 , CO, O 2 , CO 2 , CH 4 , NO, etc. are stable against photodissociation in the troposphere. • In addition, the molecule should have bright electronic transitions above D 0 . For example, HNO 3 has a low dissociation energy (D 0 = 2.15 eV) but it needs UV for its photolysis because it does not have appropriate electronic transitions in the visible. • In general, both the absorption cross sections and photodissociation quantum yields are wavelength dependent. • Photoionization processes are generally not important in the lower atmosphere (ionization potentials for most regular molecules > 9 eV). Photodissociation is the most important class of photochemical process in the atmosphere: AB + h v A + B From S. Nidkorodov
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3 O 2 Electronic Transitions • Always start in ground state X 3 Σ g - • Only transitions to triplet states are spin-allowed X 3 Σ g - A 3 Σ u + forbbiden because - → + – Occurs weakly, “Herzberg continuum”, 190-300 nm X 3 Σ g - B 3 Σ u - is allowed – Schumann-Runge system, 175-200 nm, bands due to different vib-rot states B 3 Σ u - crossed by 3 Π u repulsive state, dissociates to O( 3 P )+O( 3 P ) – Later (< 175 nm) spectrum is continuum, dissociation of B 3 Σ u - to O( 3 P )+O( 1 D ) From F-P&P Oxygen Spectrum •O 2 photolysis in the 200-240 nm range is the major source of O 3 in the stratosphere 2 can absorb nearly all radiation with λ = 10-200 nm high up in the atmosphere From Brasseur and Solomon Solve in class : Estimate the length of air column at P = 0.01 Torr and T= 200 K (characteristic of 80 km altitude) required to reduce the radiation flux at 150 nm by 10 orders of magnitude.
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This note was uploaded on 03/03/2012 for the course CHEM 5151 taught by Professor Staff during the Spring '08 term at Colorado.

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CHEM-5151_S05_L7 - Lecture 7: Photochemistry of Important...

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