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METO 621

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METO 621 Lesson 21 The Stratosphere We will now consider the chemistry of the troposphere and stratosphere. There are two reasons why we can separate these regions (1 ... – PowerPoint PPT presentation

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Title: 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.

6
Ozone-only chemistry
Altitude, km
7
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

9
Ozone-only chemistry
10
Ozone-only chemistry
Zonally averaged ozone concentration vs altitude
Zonally averaged rate of ozone formation from O2
photolysis
11
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.

12
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.

13
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.

14
Catalytic Cycles
15
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.

16
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.

17
The leaky bucket model
18
Fraction of the odd-oxygen loss rate
19
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.

20
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

21
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.

22
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.
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