METO 637

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METO 637

• Lesson 14

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Photochemical chain initiation

• In the troposphere several species are present
  that absorb solar ultraviolet radiation and can
  initiate radical-chain reactions.
• Ozone is phololyzed at wavelengths less than 310
  nm and the following reactions occur
• O3 h? ? O1(D) O2(1?g)
• O1(D) H2O ? OH OH
• Since H2O is a minor constituent in the
  atmosphere, and O1(D) is quenched by collisions
  with O2 and N2 the OH production rate is not
  high.
• Note that the O3P that results from the quenching
  will quickly be converted back to ozone.

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Photochemical chain initiation

• Note that the rate of production of OH depends
  uniquely on the quantum yield of O1(D) as a
  function of wavelength.
• Another source of atomic oxygen in the
  troposphere comes from the dissociation of
  nitrogen dioxide at wavelengths less than 400 nm
• NO2 h? ? NO O
• The NO molecule can be oxidized back to NO2, and
  so the NO2/NO cycle can be catalytic. However, in
  contrast to the stratosphere, the oxidant is not
  O.

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Oxidation steps

• The OH radical reacts mainly with CO and CH4
• OH CO ? H CO2
• OH CH4 ? CH3 H2O
• In the unpolluted atmosphere about 70 of the OH
  reacts with CO, and 30 with CH4.
• These reactions are followed by
• H O2 M ? HO2 M
• CH3 O2 M ? CH3O2 M
• Both products are peroxy radicals (they oxidize)
• Where the NO concentrations are low we get
• HO2 HO2 ? H2O2 O2
• CH3O2 HO2 ? CH3OOH O2

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Oxidation steps high NO

• If NO is high then the hyperoxides react rapidly
• HO2 NO ? OH NO2
• CH3O2 NO ? CH3O NO2
• The methoxy radical, CH3O, can then react with O2
  
• CH3O O2 ? HCHO HO2
• HCHO, formaldehyde, is dissociated ijn the
  atmosphere to produce H and CHO
• Followed by HCO O2 ? CO HO2
• Hence CH4 has been oxidized to CO and HOX

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Chemistry of the troposphere
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Higher hydrocarbons

• Higher hydrocarbons have similar oxidation steps
  to methane.
• In this case we represent the hydrocarbon as RH,
  (for methane R CH3).
• For ethane and propane R is formed by subtraction
  of the hydrogen atom. Next RO2 is formed which
  oxidizes NO to NO2
• RO2 NO ? RO NO2
• RO ? R RCHO
• RO O2 ? RRCO HO2
• Followed by
• NO2 h? ? NO O
• O O2 M? O3 M

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Tropospheric ozone production

• Note that what the complete cycle has done is to
  dissociate molecular oxygen using two photons.
  One at about 310 nm (4 eV) and one at 400 nm (3
  eV).
• NO2 h? ? NO O
• O O2 M? O3 M
• OH CO ? H CO2
• H O2 M ? HO2 M
• HO2 NO ? OH NO2
• CO 2O2 h? ? CO2 O3
• Similar chain reactions can be written for RO2

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Tropospheric ozone production

• In the free troposphere, and with a small amount
  of NO, the loss mechanism for ozone is
• HO2 O3 ? OH 2O2
• OH CO ? H CO2
• H O2 M ? HO2 M
• CO O3 ? CO2 O2
• This loss mechanism is large for low NO
  concentrations provides a photochemical sink
  for ozone.

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Tropospheric ozone oxidation
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Importance of NOX

• As we noted before NO is crucial to the
  production of ozone, especially in polluted
  atmospheres.
• Two reservoirs for NOX have been identified
  nitric acid and peroxyacetylnitrate (PAN). Both
  of these gases dissociate in the daytime, but are
  quite significant at night.
• There are, in fact, a whole range of
  peroxynitrates that exist in the atmosphere.

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Oxidation of a VOC - daytime
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The nitrate radical

• The nitrate radical was first observed in 1881.
  It is formed by the reaction
• NO2 O3 ? NO3 O2
• It can be stored as Nitrogen Pentoxide N2O5
• During the day the NO3 radical is rapidly
  photolyzed, the product being either NO or NO2.
• Although the OH radical is usually the main agent
  of attack on the hydrocarbons in daylight, NO3
  can be the most important agent at night
• NO3 RH ? HNO3 R

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Concentrations of NO3 and peroxy radicals
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Oxidation of VOCs at night