METO 621

1
METO 621

• Lesson 21

2
The Stratosphere

• We will now consider the chemistry of the
  troposphere and stratosphere. There are two
  reasons why we can separate these regions
• (1) The stratosphere absorbs most of the
  shortwave radiation from the sun, hence the
  stratosphere has high energy photons to induce
  photochemistry. The troposphere must make do with
  lower energy photons.
• (2) The temperature decreases with altitude in
  the troposphere, implying a basically unstable
  atmosphere with considerable vertical mixing. In
  the stratosphere the temperature increases with
  altitude. This implies a stable atmosphere with
  little vertical mixing.
• Substances injected into the stratosphere take a
  long time to reach the troposphere, and can build
  up to significant levels

3
The Stratosphere
4
Ozone-only chemistry

• First approach to the theoretical explanation of
  the ozone layer was by Chapman, 1930, who
  proposed a static pure oxygen photochemical
  stratosphere.
• The reactions were Dodd-oxygen
• O2 hn ? O O2 2 1
• O O2 M ? O3 M 0 2
• O3 hn ? O O2 0 3
• O O3 ? O2 O2 -2 4
• O O M ? O2 M -2 5
• Reaction 5 can be ignored in the stratosphere.
  Reactions 1 and 3 give excited atoms, but these
  are quickly quenched to the ground state. No
  excited state chemistry is assumed.

5
Ozone-only chemistry

• Reactions 2 and 3 interconvert O3 and O rapidly
  in the stratosphere. Reaction 2 has a half-life
  of as little as 100 sec. Ozone has a similar
  lifetime during the day. Hence we can consider O
  O3 as a species known as odd-oxygen.
• Hence reactions 2 and 3 do nothing as far as
  odd-oxygen is concerned.
• Ignoring reaction 5, then reaction 1 is the
  source of odd-oxygen, while reaction 4 is the
  sink.
• The next figure shows a plot of ozone and atomic
  oxygen versus altitude.

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Ozone-only chemistry
Altitude, km
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Ratio of atomic oxygen to ozone
8
Ozone-only chemistry

• Let the rate of production of odd-oxygen for
  reaction 1 be P1, and that for reaction 3 be P3.
• In steady state the amount of odd-oxygen produced
  in reaction 1 must equal the number destroyed in
  reaction 4.

• Now consider equations 2 and 3

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Ozone-only chemistry
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Ozone-only chemistry
Zonally averaged ozone concentration vs altitude
Zonally averaged rate of ozone formation from O2
photolysis
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Ozone-only chemistry

• The previous figure shows (1) ozone
  concentrations, (2) the rate of formation of
  ozone from the photolysis of molecular oxygen,
  both as a function of altitude and latitude
• At the equator the ozone layer is centered at 25
  km, where the production rate is negligible,
  while the production rate of atomic oxygen
  reached a maximum at 40 km.
• The lack of a correspondence between ozone
  concentration and (P1)1/2 indicates an inadequacy
  in the Chapman model
• The first clue as to what was happening was put
  forward by Bates and Nicolet in 1950 to explain
  ozone concentrations in the mesosphere.

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Catalytic Cycles

• Bates and Nicolet suggested the following set of
  reactions
• OH O3 ? HO2 O2
• HO2 O ? OH O2
• net reaction O O3 ? O2 O2
• This is called a catalytic cycle. In this case
  the OH radical is the catalyst, in that it
  destroys odd oxygen but is not consumed itself.
  This cycle can be generalized to be
• X O3 ? XO O2
• XO O ? X O2
• net reaction O O3 ? O2 O2
• There are many species that fill the role of X.
  The most important are H, OH, NO, Cl, Br, and
  possibility I.

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Catalytic Cycles

• The rate coefficient for the first step of the
  catalytic cycle are usually much faster than the
  reaction OO3?O2O2 and the catalytic
  cycle is favored.
• The cycles are then said to involve HOx, NOx,
  ClOx species, and we refer to families.

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Catalytic Cycles
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Catalytic Cycles

• Other catalytic cycles which do not fit into the
  OXO mold have been identified
• OH O ?O2 H
• H O2 M ?HO2 M
• HO2 O ? OH O2
• Net O O M ? O2 M
• OH O3 ? HO2 O2
• HO2 O3 ? OH O2
• Net O3 O3 ? 3O2
• Cycle does not need atomic oxygen, can be
  effective at low altitudes where the
  concentration of atomic oxygen is low.

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Catalytic Cycles

• Another cycle of interest is the following
• OH O3 ? HO2 O2
• HO2 O3 ? OH O2
• Net O3 O3 ? 3O2
• This cycle does not need atomic oxygen, and can
  be effective at low altitudes where the
  concentration of atomic oxygen is low.

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The leaky bucket model
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Fraction of the odd-oxygen loss rate
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Reservoir Species

• So far we have treated the catalytic cycles as
  independent of one another. We refer to the
  species within a cycle as a family, e.g. the
  nitrogen family.
• However, the species in one family can also
  interact with those of another family, e.g.
• ClO HO2 ? HOCl O2 (Hypochlorous acid)
• HO2 NO2 M ? HO2NO2 M (pernitric acid)
• ClO NO2 M ? ClONO2 M (chlorine nitrate)
• OH NO2 M ? HNO3 M (nitric acid)
• NO3 NO2 M ? N2O5 M (nitrogen pentoxide)
• Although these compounds can be dissociated back
  to their parent molecules, stratospheric
  circulation moves them to the poles, where the
  solar radiation is weak, and dissociation
  unlikely. They are called reservoir species.

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Reaction between cycles

• Consider the following reactions
• HO2 NO ? OH NO2
• ClO NO ? Cl NO2
• Both of these reactions short circuit the
  catalytic cycles, and hence reduce their
  efficiency.
• The full reaction cycle for the second reaction
  is
• Cl O3 ClO O2
• ClO NO ? Cl NO2
• NO2 h? ? NO O
• Net O3 h? ? O2 O
• Known as a null cycle

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Natural Sources and Sinks

• The catalytic families HOx, NOx, ClOx, and BrOx,
  appear to be present in the natural unpolluted
  atmosphere. In todays atmosphere the levels of
  ClOx and BrOx have been increased by
  anthropogenic sources.
• Most of the stratospheric NOx originates from
  tropospheric N2O, which is of biogenic origin
  (e.g. soils). This reacts with the O(1D) to start
  the NOx chemistry
• O(1D) N2O ? NO NO
• The main sources of the OH radical are
• O(1D) H2O ? OH OH
• O(1D) CH4 ? OH CH3
• The CH3 radical reacts to produce other hydrogen
  species including water vapor. Most stratospheric
  water vapor comes from methane oxidation.

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Natural Sources and Sinks

• The most abundant natural source of ClO is methyl
  chloride.
• The major contributors are the oceans. Much comes
  from the decay of organic matter. In wet
  conditions on land we get methane (CH4), in the
  sea we get CH3Cl.
• The chlorine is released by reactions with the OH
  radical, and by photodissociation above 30 km.
• Natural bromine enters the stratosphere
  principally as methyl bromide, CH3Br, which is
  produced by algae in the oceans.