Yung Y.L., DeMore W.B. Photochemistry of Planetary Atmospheres
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396
Photochemistry
of
Planetary
Atmospheres
10.4.3
Odd
Nitrogen Chemistry
The
NO
r
that
is
derived
from
anthropogenic pollutants
has a
short
lifetime
in the
troposphere
and
does
not get
transported
to the
stratosphere.
The
principal source
of
odd
nitrogen
in the
stratosphere
is
derived
from
the
long-ranged carrier N2O, which
is
biologically
produced (see section 10.2.1).
In the
stratosphere
N2O
reacts
with
O('D)
to
form
nitric
oxide:
There
is a
minor source
of
NO
f
in the
lower stratosphere
arising
from
the
absorption
of
galactic cosmic rays. This source
is
only about
10% of the N2O
source,
but it is
strongly
modulated
by the
solar cycle.
NO
readily reacts with
Oj:
Whether
this reaction results
in a net
destruction
of 03
depends
on the
fate
of
nitrogen
dioxide.
If
NO
2
photolyzes
via
(10.15),
then
the net
result
of
(10.45)
and
(10.15)
is
the
conversion
of one
O
x
(03)
into
another
O
A
(O). There
is no net
destruction
of odd
oxygen.
However,
if
NO
2
reacts
with
O,
there will
be a net
loss
of
O*:
There
are a
number
of
more complex
odd
nitrogen
compounds
that
can be
formed
from
NO
2
:
nitrogen
trioxide
(NOs),
and
dinitrogen pentoxide
(N
2
Os)
and
nitric
acid
(HNO
3
)
(10.13)
and
pernitric acid
(HO
2
NO
2
)
(10.41). These com-
pounds
are
chemically less reactive
than
NO and
NO
2
and
serve
as
reservoir species
for
odd
nitrogen. They
are
removed
by
photolysis
and
thermal decomposition,
and
by
reactions
with
OH,
(10.42)
and
(10.43).
The
heterogeneous conversion
of
N
2
Oj
to
HNO.1
is
discussed
in
section 10.4.7
on the
Antarctic ozone hole.
The
major chemical
sink
of odd
nitrogen
in the
stratosphere
is the
reaction
where
the N
atom
is
derived
from
This
sink
is
important
in the
upper stratosphere where
NO
photolysis
can
occur.
The
bulk
of odd
nitrogen
in the
lower stratosphere
has no
sink
via gas
phase chemical
Earth:
Human
Impact
397
reactions.
The
major removal mechanism
for odd
nitrogen
is via
transport
to the
tro-
posphere. Heterogeneous removal
on the
surface
of
aerosols
and the
polar
stratospheric
clouds
(PSCs)
may
also
contribute
to its
loss.
The
major chemical pathways determining
the
interconversion between
odd ni-
trogen compounds
are
summarized
in the
schematic diagram
in
figure
10.28a;
the
concentrations
of odd
nitrogen
species
are
shown
in figure
10.28b.
10.4.4
Chlorine
Chemistry
The
major carriers
of
chlorine
to the
stratosphere
are
CFljCl
and the
CFCs,
discussed
in
sections
10.2.1
and
10.2.2.
The
chemistry
of the
halocarbons following their break-
down
(by
photolysis
or
reaction
with
reactive radicals such
as OH) in the
stratosphere
is
similar
to
that
for CH4 in the
troposphere
described
in
section
10.3.2.
Here
we
examine
the
breakdown
of
CF
2
Cl2
as an
example.
In the
stratosphere
the
molecule
undergoes photolysis, releasing
a
Cl
atom:
The
radical CF2CI
can now be
readily destroyed
in the
following sequence
of
reactions:
Note that
the
destruction
of
CF
2
C1
2
results
in the
production
of Cl and
CIO.
The
ultimate
fate
of
COF
2
is
photolysis, followed
by the
formation
of HF, a
terminal
product
in the
stratosphere.
The
release
of the
reactive form
of
chlorine
as Cl and
CIO
leads
to the
catalytic destruction
of
ozone
by
The
efficiency
of
chlorine
for
destroying
O
3
is
mitigated somewhat
by its
reaction
with
NO:
Note that
the
cycle
converts
one
O
v
into
another
O
v
and is a
null
cycle
for the
destruction
of odd
oxygen.
The
active forms
of
chlorine
may be
converted
into
chemically less active reservoir
species
by
(a)
(b)
Figure
10.28
(a)
Schematic diagram summarizing
the odd ni-
trogen species
and
major chemical pathways.
The
dashed line
indicates
conversion
of tyOs by
heterogeneous chemstry.
Af-
ter
Logan,
J. A. et
al.,
1978,
"Atmospheric
Chemistry:
Re-
sponse
to
Human Influence." Phil. Trans. Roy. Soc.
Land.
290,
187.
(b)
Partitioning
of odd
nitrogen species
in the
strato-
sphere. Symbols represent measurements made
by the
Atmo-
spheric
Trace
Molecule Spectroscopy (ATMOS) instrument dur-
ing
the
ATLAS-3 Space Shuttle mission
at
midlatitudes
on
4-7
November 1994.
After
Michelsen,
H. A. et
al.,
1996,
"Strato-
spheric
Chlorine Partitioning: Constraints From Shuttle-Borne
Measurements
of
HC1,
C1NO
3
,
and
CIO."
Geophys.
Res. Lett.
23,2361.
398
Earth:
Human
Impact
399
The
reservoir species
do not
react with
03.
They
can be
converted back
to the
reactive
radical species
by the
reactions
A
schematic diagram showing
the
interconversion between
the
inorganic chlorine
species
is
shown
in figure
10.29a.
The
concentrations
of the
principal inorganic chlo-
rine
species
are
shown
in figure
10.29b.
10.4.5
Bromine Chemistry
The
major carrier
of
bromine
to the
stratosphere
is
CH
3
Br.
The
sources
of
CH
3
Br
are
production
in the
oceans
and
anthropogenic emissions.
It is
mainly destroyed
by
OH in the
troposphere,
but
enough
CH
3
Br
survives
to
provide
a
significant
source
of
bromine
to the
stratosphere.
The
breakdown
of
CH
3
Br
is
believed
to be
similar
to
that
of
CH4
(see section
10.3.2),
and the
details
are not
shown here. With
the
release
of the
reactive radicals,
Br and
BrO, ozone
may be
destroyed
by the
following synergistic
coupling
between bromine
and
chlorine:
Note that
this
catalytic scheme involves reactions
with
O
3
and not
with
O. For
rea-
sons discussed
in
section
10.4.7,
this
makes
the
scheme more important
in the
lower
atmosphere.
The
photochemistry
of
bromine
is
similar
to
that
of
chlorine. Much
of it is
poorly
known
and is
inferred
by
analogy
with
the
chemistry
of
chlorine.
The
reservoir species,
HBr, HOBr,
and
BrONO
2
,
may be
formed
by
The
reservoir species
are
much less stable
than
their
chlorine counterparts
and are
readily
destroyed
by
(a)
Figure
10.29
(a)
Schematic diagram summarizing
the
major chlorine
species
and
chemical
pathways.
The
conversion
of
CIONO2
and HC1 to
C^
proceeds
via a
hetergeneous
(het)
reaction.
After
Jaegle,
L.,
1996,
Stratospheric
Chlorine
and
Nitrogen Chemistry, Ph.D.
Thesis,
California
Institute
of
Technology,
(b)
Partitioning
of
inorganic chlorine
species
in
the
stratosphere.
The
triangles represent
the sum of
HCl(sunset)
+
ClONC>2(sunset)
+
ClO(midmorning).
Lines labeled
"Cly"
represent
the
abundance
of
total inorganic chlorine
necessary
to
match
the
measured
sum of
these
gases.
Model
C
(solid lines) uses kinetics
rate
coefficients
that
are
different
from
the
recommendations
of
JPL'94.
The
measurements
are
taken
by the
Atmospheric
Trace
Molecule
Spectroscopy
(ATMOS)
instrument. After
Michelsen,
H. A. et
al.,
1996,
"Stratospheric
Chlorine Partitioning: Constraints From
Shuttle-Borne Measurements
of
HC1,
C1NO
3
,
and
CIO."
Geophys.
Res.
Lett.
23,
2361.
400
Earth:
Human
Impact
401
(b)
Figure
10.30
(a)
Schematic diagram
summarizing
the
major bromine
species
and
chemical
pathways,
(b)
Altitude
profiles
for
major inorganic
bromine
species
in the
stratosphere
computed with total bromine equal
to
20
parts
per
trillion
by
volume. After
Yung,
Y. L. et
al.,
1980,
"Atmospheric
Bromine
and
Ozone
Perturbations
in
the
Lower
Stratosphere."
J.
Atmos.
Sci.
37,
339.
Figure
10.30a
presents
a
schematic
diagram
illustrating
the
interconnections
between
the
major
bromine
species.
The
concentrations
of the
major
bromine
species
are
shown
in
figure
10.30b.
402
Photochemistry
of
Planetary Atmospheres
Figure
10.31
Altitude
profiles
for
major
source
species
in the
upper
atmosphere.
The
con-
centrations
of
H
2
O,
CH
4
,
N
2
O,
CH
3
Cl,
CFClj,
and
CF
2
CI
2
are
taken
from
measurements
by
the
Atmospheric
Trace
Molecule
Spectroscopy
(ATMOS)
instrument.
The
CH3Br
con-
centrations
were
taken
over
India
(18°
N, 77° W) on 9
April
1990.
After
Lai,
S. et
al.,
1994
"Vertical
Distribution
of
Methyl
Bromide
over
Hyderabad,
India."
Tellus
46B, 373.
10.4.6
Overview
of
Catalytic
Chemistry
The
altitude profiles
of the
most important source molecules
in the
stratosphere,
H
2
O,
CH
4
,
N
2
O,
CH
3
C1,
CFC1
3
,
CF
2
C1
2
,
and
CH
3
Br,
are
shown
in figure
10.31.
The
molecules
are the
"parent
molecules"
of
HO
r
,
NO.
r
,
C1O
V
,
and
BrO,
radicals
in
the
stratosphere. Note that
all
these molecules have higher mixing ratios
in the
troposphere near
the
source
region
at the
surface
of the
planet. Their mixing ratios
fall
with
altitude
in the
stratosphere.
The
destruction
of the
source molecules
releases
the
active radicals, which
can
then react
with
ozone. With
the
exception
of
H
2
O,
all
source molecules
are
irreversibly destroyed
in the
stratosphere,
and
they must
be
replenished
by
transport
from
the
lower atmosphere.
Figures 10.32a
and
10.32b show
the
concentrations
of
O
3
and O in the
stratosphere.
The
rates
of the
rate-limiting reactions
for the
destruction
of odd
oxygen
by the
Chap-
man
reaction,
HO
V
,
odd
nitrogen, chlorine,
and
bromine
are
shown
in figure
10.32c.
Destruction
by
NO
V
is
most
important
in the
midst
of the
ozone
layer.
The
Chapman
reaction
and
destruction
by
chlorine become more important above this level. Destruc-
tion
by
HO
V
is
more important
in the
upper stratosphere
and the
lower stratosphere.
Loss
via
bromine chemistry
is
relatively
more important
in the
lower stratosphere.
Having
reviewed
the
catalytic destruction
of
ozone
by
HO,,
NO
t
,
and
halogen
compounds,
we
recognize
a
simple pattern
in the
nature
of
this catalytic chemistry.
There
are
four
integral components:
the
long-lived carrier,
the
reactive radical species
Earth:
Human
Impact
403
(b)
Figure
10.32
(a)
Vertical
distribution
of
ozone
at
midlatitude.
The
solid
line
is the
result
of a
model,
and the
data
points
are
measurements.
After
Logan,
J. A. et
al.,
1978,
"Atmospheric
Chemistry:
Response
to
Human
Influence."
Phil.
Trans. Roy.
Soc.
Land.
290,
187.
(b)
Altitude profile
of O for
30° N in
winter with
solar
zenith
angle
of
53°.
The
data
are
from
Anderson,
J. G.,
1975, "The
Measurement
of
Atomic
Oxygen
and
Hydroxyl
in the
Stratosphere,"
in
Proceedings
of
the
Fourth
Conference
on the
Climatic
Impact
Assessment
Program
(4—7
February;
U.S.
Dept.
of
Transportation
Report
no.
DOT-TSC-OST-75-38),
p.
458.
that
destroy
03, the
reservoir species that sequester
the
reactive radicals,
and the
removal
mechanism. Note
that
there
is a
wide range
in the
lifetimes
of the
reservoir
species.
For
example,
ClONOi
is
photolyzed
in a
day,
but HC1 has a
lifetime
in
excess
of a
month
in the
lower stratosphere.
We
may now
also
understand
why
there
is
relatively little direct impact
on
O
3
by
the
chemistry
of
fluorine
and
sulfur.
In the
case
of fluorine, the
major reservoir
species
is HF, an
extremely stable molecule
that
does
not
dissociate
or
react
with
OH in the
stratosphere. Once formed,
HF is the
terminal product
of
stratospheric
fluorine
and
there
is no
further
chemical
reactivity
between
HF and
other stratospheric
species.
In the
case
of
sulfur,
the
bulk
of
SO
2
and DMS
produced
in the
biosphere
is
destroyed
in the
troposphere.
The
only
long-lived
carriers
of
sulfur
in the
absence
of
volcanic
eruptions
are COS and
CS
2
.
But the
most stable reservoir species
of
sulfur
404
Photochemistry
of
Planetary
Atmospheres
Figure
10.32
(c)
Production
and
loss rates
of
odd
oxygen
in the
stratosphere. Error
bars represent
1-cr
total accuracy based
on
uncertainties
associated
with
measurements
of
radicals.
After
Jucks,
K.
W. et
al.,
1996,
"Ozone
Production
and
Loss
Rates
Measurements
in the
Middle
Stratosphere."
J.
Geophys.
Res. 101,
28785
in
the
stratosphere
is
H2SO4 aerosol, which will
not
undergo further reaction with
stratospheric
species.
The
aerosol surfaces
do
provide sites
for
condensation
of
water
in
the
formation
of
PSCs
and for
heterogeneous reactions. Thus, there
may be an
indirect
impact
on
ozone
(see
section
10.4.8).
10.4.7
Ozone
Hole
The
possible destruction
of
ozone
associated with human activities
was first
suggested
in
the
early
1970s.
The
early theories focused
on
catalytic cycles
of the
form
where
X =
H,
OH, NO, and
Cl
[chemical
schemes
(Ilia),
(Illb),
(IV)
and
(V)].
From
figure
10.32c
we
expect that most
of the
ozone destruction would occur
in
the
upper stratosphere, where
O
3
is
chemically controlled.
We
therefore expect that
most
of the
adverse anthropogenic impact would
be
confined
to the
photochemically
active
region
in the
upper
stratosphere.
The
bulk
of
ozone
that resides
in the
lower
stratosphere
(including
the
polar stratosphere)
is
dynamically controlled
and
would
not
be
directly affected
by
chemical destruction.
The
inadequacy
of
this
"classical"
view
was
clearly revealed
by its
failure
to
predict,
and
subsequently
to
explain,
the
ozone hole phenomenon discovered
in
1985. Figure
10.33a
shows
the
decadal decline
of
ozone column abundance
in the
Austral spring
in
Antarctica. Later balloon
data,
shown
in
figure
10.33b,
revealed that
the
bulk
of the
ozone
loss
occurred
in the
lower
stratosphere.
In
some
places
90% of the
initial
O
3
was
removed.
Earth:
Human
Impact
405
(a)
(b)
Figure
10.33
(a)
Lowest daily
value
of
total
ozone observed over Halley
Bay
(76°
S, 27°
W) in
October
for the
years between
1956
and
1994.
After
Jones,
A. E., and
Shanklin,
J. D.,
1995,
"Continued
Decline
of
Total Ozone over
Halley,
Antarctica, since
1985."
Nature 376,
409.
(b)
Comparison
of the
South
Pole
vertical
ozone
profiles:
predepletion (solid
line)
and
postdepletion
(dashed line). Adapted from
Hofmann,
D. J. et
al.,
1994,
"Record
Low
Ozone
at the
South
Pole
in the
Spring
of
1993."
Geophys.
Res.
Lett.
21
421.
There
followed
a
surge
of
activity
concerning
the
crucial
question
of
what
key
factors
control
stratospheric
ozone.
New
theoretical
ideas
as
well
as
laboratory
re-
sults
were
advanced.
A
series
of
polar
campaigns,
AAOE
(Airborne
Antarctic
Ozone
Expedition;
1987),
AASE
1 and 2
(1989,
1991-1992),
and
SPADE
(Stratospheric
Photochemistry,
Aerosols,
and
Dynamics
Expedition;
1992-1993),
were
organized
to
probe
the
chemistry
and
dynamics
of the
polar
stratosphere
and the
lower
stratosphere.
The new
results
can
briefly
be
summarized
as
follows:
1.
The
major loss
of
ozone
does
not
occur
in the
photochemically
active region,
but
rather
in the
lower stratosphere where
03 is
supposed
to be
dynamically controlled
(in
the gas
phase model);
2.
heterogeneous chemistry plays
a
fundamental role
in
repartitioning
the odd
nitrogen
(NO*)
and
active chlorine species
(Cl
v
)
such
that
the
destructive power
of
halogen
is
greatly
enhanced;
and