METO 621

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METO 621

• Lesson 24

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The Troposphere

• In the Stratosphere we had high energy photons so
  that oxygen atoms and ozone dominated the
  chemistry.
• In the troposphere we have lower energy photons,
  and the chemistry is dominated by the OH and NO3
  radicals.
• OH is generated photochemically (i.e. only during
  the day), NO3 is rapidly photolyzed during the
  day, so it can only survive at night.
• NO3 is generally less reactive then OH, its peak
  concentration is higher.
• OH provides an efficient scavenging mechanism for
  both natural and anthropogenic trace constituents

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Dry and Wet Deposition

• Dry deposition removal of gases and particles
  by a direct transfer from the atmosphere to the
  surface.
• Wet deposition removal of gases and particles
  carried to the surface in water rain, snow, fog
  etc.
• Dry deposition is known for SO2, O3, CO2, and
  SO3.
• Wet deposition of gaseous species requires that
  they be water soluble. Terms used are rainout, or
  washout.
• Acid rain is an example of the rainout of
  sulfurous and nitric acids, produced in polluted
  atmospheres.

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Dry and Wet Deposition
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Oxidation and Transformation

• Let us assume that no methane has been oxidized.
• Then OH is produced by the following reactions
• O3 hn ? O(1D) O2(1Dg)
• O(1D) H2O ? OH OH
• It should be noted that the O(1D) does not stay
  around for long, and is quenched to the ground
  state. The ground state then quickly combines
  with molecular oxygen to reform ozone.
• The OH formed reacts mainly with CO and CH4
• OH CO ? H CO2
• OH CH4 ? CH3 H2O

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Oxidation and Transformation

• These compounds then react with molecular oxygen
• H O2 M ? HO2 M
• CH3 O2 M ? CH3O2 M
• If the concentration of NO is very low then
  further reactions convert the peroxy radicals to
  water vapor and carbon dioxide.
• However if the nitrogen oxides are present then
  we get
• HO2 NO ? OH NO2
• CH3O2 NO ? CH3O NO2
• This then followed by
• NO2 hn ? NO O
• O O2 M ? O3 M

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Oxidation and Transformation
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Oxidation and Transformation

• Analogous reactions can be written for the higher
  hydrocarbons, e.g. C8H18 octane.
• If we assign the formula RH to these hydrocarbons
  then we get
• RH OH ? R H2O
• R O2 M ? RO2 M
• RO2 NO ? RO NO2
• This is the basis of photochemical smog.
• The photolysis of the resultant NO2 is the only
  known way of producing ozone in the troposphere.
• The RO is further reduced to aldehydes and other
  organic compounds by OH, all of which can
  eventually produce ozone.

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Oxidation and Transformation
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Oxidation and Transformation
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The nitrate radical

• The nitrate radical NO3 plays a significant role
  in the troposphere.
• It is formed by the reaction
• NO2 O3 ? NO3 O2
• During the day it is rapidly photolyzed
• NO3 hn ? NO2 O or NO O2
• However at night the NO3 is stable and can react
  with hydrocarbons
• NO3 RH ? HNO3 R
• R can now react with molecular oxygen and begin
  the oxidation process

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The nitrate radical
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The nitrate radical
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Chemical lifetimes wrt OH and O3
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Schematic of biogenic emissions