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) 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
The Stratosphere
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.
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 oddoxygen, while reaction 4 is the ‘sink’.
• The next figure shows a plot of ozone and atomic oxygen
versus altitude.
Ozone-only chemistry
Altitude, km
Ratio of atomic oxygen to ozone
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.
d[odd  oxygen]
from equation 1
 2P1[O2 ]
dt
d[odd  oxygen]
from equation 4
 2k4 [O][O3 ]
dt
and in steady state
P 1[O2 ]  k 4 [O][O3 ]
•
Now consider equations 2 and 3
Ozone-only chemistry
from equation 2
from equation 3
in steady state
which yields
d [O3 ]
  P3 [O3 ]
dt
d [O3 ]
 k 2 [O][O2 ][ M ]
dt
P3 [O3 ]  k 2 [O ][O2 ][ M ]
P3
[O]

[O 3 ] k 2 [O2 ][ M ]
if we now substitute the relation between atomic oxygen
and ozone obtained above, we get
P1k 2 [ M ]
[O3 ]  [O2 ]
k 4 P3
Ozone-only chemistry
Zonally averaged ozone
concentration vs altitude
Zonally averaged rate of ozone
formation from O2 photolysis
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.
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.
Catalytic Cycles
• The rate coefficient for the first step of the catalytic
cycle are usually much faster than the reaction
O+O3→O2+O2 and the catalytic cycle is favored.
• The cycles are then said to involve HOx, NOx, ClOx
species, and we refer to families.
Catalytic Cycles
Catalytic Cycles
• Other catalytic cycles which do not fit into the O+XO 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.
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.
The leaky bucket model
Fraction of the odd-oxygen loss rate
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.
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 + hν → NO + O
Net
O3 + hν → O2 + O
• Known as a null cycle
Natural Sources and Sinks
• The catalytic families HOx, NOx, ClOx, and BrOx, appear to be
present in the natural ‘unpolluted’ atmosphere. In today’s
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’.
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.