Advances in the Chemistry of Atmosphere

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Advances in the Chemistry of Atmosphere
Welcome to

• CHEM-ATOC 419/619

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COURSE OUTLINE

• Introduction Earths atmosphere, chemical
  composition and its vertical structure
• Radiation balance of atmosphere green house
  gases, absorption and photochemistry
• Oxidation potential of the atmosphere
  atmospheric oxidants and homogeneous chemistry
• Aerosols and heterogeneous chemistry
• Selected topics Chemistry of ozone hole and
  air pollution
• Formation process of cloud chemical reactions
  in and on cloud particles
• State-of-the-art field measurement techniques in
  atmospheric chemistry
• Atmospheric modeling 0, 1-D, 2-D and 3-D
  modeling
• Chemistry of the climate change
• Your research topics!

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• Summary of last time

• Kinetics
• Introduction to photochemistry
• Todays Lecture
• Photochemistry of ozone and some other
  atmospheric oxidants

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Photochemistryin Atmosphere
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Photochemistry

• Hartley band
•  O3 h? (? ? 320 nm)? O2 (1?g) O(1D)
• I.O(1D) H2O ? 2 HO
• (k298 2.2 ? 10-10 cm3 molecule-1 s-1)
• -d? O(1D)?/dt k5?H2O??O(1D)?
•  
• II. O(1D) M ? O(3P) M
• (k298 2.9 ? 10-11 cm3 molecule-1 s-1)
• -d? O(1D)?/dt k6?M??O(1D)?
• k5?H2O??O(1D)?/ k6?M??O(1D)? k5?H2O?/ k6?M?

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Photochemistry

• E.g.
• For ?H2O? 3.9 ? 1017 molecule cm-3
• T 298K, RH 50, the above ratio is about
  0.12
•  
• O3 h? (? ? 410 nm)? O2 (1?g-) O(1D)
• Spin forbidden!
• Ravishankara 2001 showed that it can be
  significant!!

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A short paper to read
Classical survival probability for ozone
photodissociation in the Hartley band Gérard
Parlant Laboratoire de Structure et Dynamique
des Systèmes Moléculaires et Solides (UMR 5636),
CC 014, Université Montpellier II, 34095
Montpellier Cedex 5, France (Received 14
February 2000 accepted 28 February
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A reference to read
Absolute cross sections of ozone at atmospheric
temperatures for the Wulf and the Chappuis
bands Z. El Helou, S. Churassy, G. Wannous, R.
Bacis, and E. Boursey Laboratoire de
Spectrométrie Ionique et Moléculaire, Centre
National de la Recherche Scientifique, UMR 5579,
Université Claude Bernard Lyon 1, Bâtiment Alfred
KASTLER, 10 rue AMPERE, Domaine Universitaire de
la DOUA, 69622 Villeurbanne Cedex France
(Received 25 January 2005 accepted 27 April
2005 published online 28 June 2005)
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Photochemistry

• Huggins bands at ? ? 320 nm - spin forbidden! 
• O3 h? ? O2 (1?g or1?g ) O(3P)
• Chappius region (440-850 nm, the products are to
  appear to be the ground state species 
• O3 h? ? O2 (X3?g-) O(3P)
• In both regions greater than 320 nm, the
  absorption coefficient is one or two orders of
  magnitude less than that at 300 nm.
• They produce O(3P) rather than O(1D)
• If NO2 is present, production of ground state
  oxygen is more due to NO2 rather than ozone
  photolysis, and as the photolysis of ozone also
  lead to regeneration of ozone in this region.
• At ? gt 320 nm, this high wavelength ozone
  absorption does not play a significant role in
  urban chemistry, however, it can play important
  roles in RURAL areas.

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Chappuis bands
Comparison of previous measurements of the ozone
absolute absorption cross-section in the Chappuis
bands at ambient temperature
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The ozone absorption cross section in the
Chappuis bands recorded at 293 and 202  K
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Photochemistry

• Calculation of Photolysis Rates
• Ia(?) ?(?) J(?) ?X? Photons/(cm3 s)
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• X h? ? X ? X h?'
• (M) ? X
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• The total rate of photolysis of X at wave length
  ? is
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• Total RP(?) ? ?(?) ?(?) J(?) d? ?X (molecule
  cm-3 s-1)
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Photochemistry

• We sum the absorption occur in a discrete manner
  at wave length intervals, we can write
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• Total RP of X ? (?290 nm, ?i) ?(?) ?(?) J(?)?X?
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• The total photolysis rate is also equal to an
  effective first-order rate constant, times the
  concentration
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• Total photolyis of X kp ?X? (molecule cm-3
  s-1)
• kp (s-1) ? (?290 nm, ?i) ?(?) ?(?) J(?)

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Photochemistry

• Photolysis of Aldehydes
• Formaldehyde (methonal) HCHO
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• HCHO h? ( ? lt 370 nm)? H HCO (a)
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• ? H2 CO (b)
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• In presence of oxygen, H forms HO2 which in
  presence of NO forms HO radical.
•  

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Photochemistry

• The sum of the primary yields are equal to unity
  for up to about 330 nm and then falls off at
  longer wavelengths.
•  
• ? 2b (1-exp (112.8 -0.347?))/(1(p/760)(
  ?-329)(364-?)
•  
• CH3CHO h? ? CH3 HCO (a)
• ? CH4 CO (b)
• ? H CH3CO (c)
• ? H2 CH2CO (d)
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• Reference Lee, E. K. C. and R. S. Lewis,
  photochemistry of simple aldehydes and ketones in
  the gas phase, adv. Photochem., 12, 1 (1980)

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Photochemistry

• Ketones and Dicarbonyls
• Simple ketones absorb light in the actinic UV out
  to about 330 nm.
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• Simple ketones undergo ?-cleavage
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• CH3C(O)CH3 h? ? CH3 CH3CO
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• Larger ketones may two bond cleavage pathways
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• CH3C(O)C2H5 h? ? CH3CO C2H5
•       ? CH3 C2H5CO

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Photochemistry

• For biacetyl, photolysis rate predominate over
  reactions with HO and ozone in the removal of
  these species from atmosphere.
• (CHO)2 h?? 2 CO H2 (a)
• ? HCHO CO (b)
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• The yield of formaldehyde formation is about 13
  proceeding channel b.
• Dicarboxylic acids formed from oxidation of the
  corresponding dicarbonyl compounds. 
• (COOH)2 h? ( ?? 313 nm)? CO2 HCOOH (a)
• ? CO2 CO H2O (b)
• Ratio of quantum yields of a and b are 2.6

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Photochemistry

• HONO and Organic nitrogenated compounds
• NO NO2 H2O ? 2 HONO
• Keq (atm-1) exp (-15.56 4.73 ? 10 3/T)
•  (Stockwell and Calvert, 1978)
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• Photolysis of HONO is a major HO source in
  polluted urban areas
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• HONO h? ( ? lt 400 nm) ? HO NO (quantum yield
  almost unity)
•  
• Similarly, RONO ? RO NO

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DOAS
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DOAS
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Photochemistry

• HNO3 h? (200 lt ? lt 320 nm) ? HO NO2
• Quantum yield of unity.
• HNO3 h? ? O HNO2 (? 0.03)
• ? H NO3 (? lt 0.002)
• Organic nitrates have an absoption band that
  tails in to the actinic UC ( ?? 290 nm). The
  absorption coefficient above 290 nm is fairly
  small.
• RCH2ONO2 h? ? RCH2O NO2
• ? RC(O)H HONO
• ? RCHO2ONO O 
• The first pathway is believed to be the major
  channel.

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Summary

• Quantum yield
• Branching
• Allowed and forbidden bands of ozone
• Some important atmospheric species that undergo
  photolysis