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OCR for page 167
Appendix C
RECENT DEVELOPMENTS IN STRATOSPHERIC PHOTOCHEMISTRY
Steven C. Wofsy and Jennifer A. Logan
Division of Applied Sciences
Harvard University
INTRODUCTION
Studies of the stratospheric ozone layer are impeded by
two characteristics common to many environmental ques-
tions. First, it is impossible to perform meaningful,
controlled experiments to test the response of the system
to changes in environmental parameters. Second, the
chemistry of the system is very complex, involving labile
species at low concentrations subject to transport
processes that are not well understood. These
difficulties force us to rely on simulations using
theoretical models to assess possible perturbations to
stratospheric ozone. The models are inevitably too
simple to describe the complete physical system and yet
are often so complicated that it may be quite difficult
to understand the models and to draw model-independent
conclusions from the results.
This paper examines recent models of stratospheric
ozone and associated chemical species, with emphasis on
developments subsequent to the earlier NRC study on the
stratosphere (NRC 1979). The discussion relies primarily
on calculations performed using our own one-dimensional
model of the stratosphere (Logan et al. 1978, Wofsy 1978)
and on results from two recent two-dimensional models
(Miller et al. 1981; Steed et al. 1982; Ko, Sze, and
co-workers reported in Hudson et al. 1982). This choice
reflects our access to model results and our view that
these models contain most of the essential features of
other operational models.
167
OCR for page 168
168
EFFECTS OF NEW KINETIC DATA ON MODEL RESULTS
Species Concentrations
Stratospheric models in use during the previous NRC study
(NRC 1979) appeared to underestimate by a factor of
between 2 and 5 concentrations of NO and NO2 below 25
km, and to overestimate the concentration of C10 by a
factor exceeding 10 at the same altitudes. These
discrepancies may be attributed to inaccurate values for
kinetic data affecting calculation of the concentration
of the OH radical. Below 25 km, NO and NO2 are
controlled by chemical exchange with HNO3, the major
odd-nitrogen species, with the main reactions being
NO2 + OH + M ~ HNO3 + M
HNO3 + he ~ NO2 + OH
NO2 + he ~ NO + O
NO + O3 ~ NO2 + O2
Nitrogen dioxide and nitric oxide concentrations thus
vary inversely as the concentration of OH,
(1)
(2)
(3)
(4)
[NO2] ~ J2[HNO3] (5a)
kl[M] [OH]
[NO] ~ J2[HNO3] J3 1 (5b)
kl[M] k4[O3] [OH]
where [x] denotes the concentration of species x and
ki(Ji) refers to the rate coefficient (photolysis
rate) for the ith chemical reaction.
The concentration of C10 also is controlled by inter-
change with a more abundant species, HC1, but in this
case C10 increases with OH. The principal reactions are
HC1 + OH ~ H2O + C1(6)
C1 + O3 + C10 + O2
C1 + CH4, H2, H2CO ~ HC1 + CH3, H. HC(8-10)
C10 + NO ~ C1 + NO2,(11 )
which lead to the expression
OCR for page 169
169
C10 ~
~ [OH]2
k6[HCl] [OH]k7[O3]
{k8 [CH4]+kg [H2] +klo [H2CO]}kll[NO]
k6[HCl]k4k7[O3] kl[M]
{k8 [CH4] +kg [H2]+klo [H2CO]}kllJ3J2 [HNO3]
(12a)
(12b)
Hence [C10] increases as [OH] 2. McConnell and Evans
(1978) pointed out that model and observations could be
brought into agreement if it was assumed that the model
overestimated the concentrations of OH, and they noted
that such an error could strongly affect estimates quoted
in NRC (1979) for the response of ozone to enhanced
levels of stratospheric chlorine or odd nitrogen.
New laboratory measurements lend support to the
hypothesis advanced by McConnell and Evans (1978) and
others (Turco et al. 1981). Wine et al. (1981) and
Nelson et al. (1981) showed that the rate for the reaction
OH + HNO3 ~ H2O + NO3
(13)
increases at low temperature. This reaction is the major
sink for odd hydrogen below 25 km, as shown in Figure
C.1. Rates for reactions involving peroxynitric acid
(HOONO2 or HNO4) have also been revised recently as
shown in Table C.1. Rates for formation of HNO4 and
for reaction between OH and HNO4 appear to be faster
than formerly believed,
HO2 + NO2 + M ~ HNO4 + M
HNO4 + OH ~ H2O + products
(14)
(IS)
(NASA 1981, Littlejohn and Johnston 1980, see also Hudson
et al. 1982), whereas photolysis of HNO4 may be slower
than indicated by earlier studies,
HNO4 + hv ~ products
(Molina and Molina 1981). These results,
further work, indicate that reaction (15)
pathway for loss of odd hydrogen ~
(16)
if confirmed by
is a ma~or
(see F~gure C.1).
Figure C.2 shows how calculated profiles for OH,
HO2, C10, NO, and NO2 (at noon) have changed in
response to the new laboratory rate data. Model
concentrations of OH have been lowered by about a factor
of 3 at 20 km, NO and NO2 have been increased by
a
OCR for page 170
170
50 .
45
40
E
-
35
a
1 1 1 1 1 1 1 1 1- 1 -W-I 1 1 1 1 ~1 W- ~1 1 1 1 1 11 ' ~
\
OH + HNO3 V- OH + H202
HO2+0H \
-it\
\
30
25
20
15
-
\
me,, i,: OH + HNO4
/ \
/
~ / O H + H NO4
/~/
I ~t l l l ,/ /~OH+HNO3
.. 1. 1 l,__ I I_I I I I I
LL L_ .
2 103 104
RATE ( cm~3 sect )
FIGURE C.1 Rates for loss of odd hydrogen, averaged over a 24-hour period. Profiles
are shown for 30°N latitude at equinox. Results are from the Harvard one-dimensional
model (Logan et al. 1978) using kinetic data from Hudson et al. (1982~.
OCR for page 171
171
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OCR for page 172
172
55
50
45
-
~ 40
5
Hi:
30
25 _
20 1 , 1~,,l/ 1 1/1 ,
lo6
a
107
55
50
45
40
535
30
25
20 ... . 1_ ~ .~_~11 I i ~__,_, 1 ,, ~1-~1 I,,,
105 1o6 10 ~
CtO (cm~3)
C
1 1 1 1 1 1 1 11 1 1 1 1 1 1 1 11
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I r--r- I l-r-~-~---r--~ I I i l:
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81479
lo8 lo6
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b
1 1 ~1 1 1
- 1- - - 1 1 1 1 -rams ~- ~I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1o6
~\,,,1979
1981
Slow CIO+N,~ ~ ~1980
fit/
8
109
FIGURE C.2 Altitude profiles for (a) HO2, (b) OH, (c) C1O, (d) NO and NO2 at
noon. The labels 1979, 1980, and 1981 indicate rate constant sets shown in Table C.1
(Hudson and Reed 1979, NASA 1981, and Hudson et al. 1982, respectively).
lo8
OCR for page 173
173
' ' ''''4 '' ' ' '''''I ' 1
40
30
20
40
Y 30
LLJ
C)
~ 20
NO2
_
, _
-
-
1 1
- d
10
107
.~
'it
)
1979,~' /
1981
1 1 1 1 1~x
NO
1
!\
,- I
lo8
NUMBER DENSITY (cm)
FIGURE C.2 (Continued)
109
OCR for page 174
174
. . .
similar factor, and calculated C1O concentrations have
decreased by nearly a factor of 10.
It may seem surprising that relatively modest changes
in the rates for (13) through (16) should have such
dramatic effects on calculated profiles for OH. Chemical
interchange among BOX radicals is quite rapid in the
lower stratosphere, with lifetimes for HO2 and OH at
noon about 50 and 10 s, respectively. The fast reactions
establish the ratio of [OH] to [HO2], but radical
production and loss reactions control the absolute
concentrations. Recombination reactions for HOX
radicals are inefficient in the lower stratosphere, such
that the chemical lifetime for the sum of HO2, H. and
OH exceeds 500 s (see Figures C.1 and C.2). Hence slow
processes such as (13) and (15) can exert a major
influence on the composition of the stratosphere. Slow
recombination reactions are difficult to study in the
laboratory, especially for stratospheric temperatures and
pressures, and the future may well hold further chemical
surprises in this area.
The present set of reaction rate data brings calcula-
tions and observations into reasonably close agreement
below 30 km, as shown in Figures C.3, C.4, and C.5 for
OH, HNO3, NO2, NO, O. and C1O. The figures also
illustrate the relatively poor agreement obtained by
using the 1979 rate data. Unfortunately, the comparison
is not yet definitive. Data on OH and O are nonexistent
below 30 km, and few simultaneous observations are avail-
able for NO, NO2, and HNO3. The vertical gradient
for NO does not coincide very well with observations by
Ridley and co-workers (Ridley and Schiff 1981, Ridley and
Hastie 1981) (Figure C.3d) but does agree with data
obtained by Horvath and Mason (1978) (see also Hudson et
al. 1982) (Figure C.3c). The model predicts more HNO3
than is observed between 25 and 30 km. The apparent
discrepancy observed for O (Figure C.3f) at low altitude
may be attributed to differences for [O3] and local
albedo between the model and the particular observations.
The model does predict accurate values of the ratio
[O]/[O3], as shown in Figure C.3g.
Observations of C1O require special consideration.
Reported measurements are shown in Figure C.4 (Weinstock
et al. 1981, Anderson et al. 1980). Summer data (solar
declination of >0) fall in a rather narrow band, as
predicted by the model, except for anomalous results
obtained on June 15, 1979, and July 14, 1977. (The
anomalous Bastille Day profile (July 14, 1977, Anderson
OCR for page 175
175
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
70:
LLJ 60 _
~ _
SO-SO _
4Sr
_
- a
~_
40 1 , 1 , 1 1 1 1 1 J 1 1 1 1 1
6
10t
OH CONCENTRATION (cm~3)
1 1 1 1 1 1 1 1
BAllOON-BORNE IN SITU
· 12 Jan 1976 ( x2) x 80°
~40-~ 26 Apr 1977(x2)x.80° i
Y ° 14 JuI 1977,x 41°
35 :~:~
107
OH CONCENTRATION (cm~3)
I i
lo8
1
I ~
30
lo6
1 111
lo8
FIGURE C.3 Model results for OH in the (a) upper and (b) middle stratosphere;
(c) HNO3; (d) NO2; NO in the (e) lower and (I) upper stratosphere; (g) 0(3P);
and O [O] / [O3 ] compared with measurements. The measurements are presented
and discussed in Hudson et al. (1982). Calculations are appropriate for 30°N latitude
at equinox and for solar zenith angles and local times as indicated.
OCR for page 176
176
45
35
30
25:
!
50
45
40
1 1 1 1 1 1 1 11 1 ,1 1 1 1 1 111 1 1 1 1 1 i 1i
fischer (1980) HNO
_ 0 May 1979 - 31 °N 3
O Feb 19?9-31°N \ :cicu/oted MlOlATITUDES
Arno~d eto~ (1980) 0 ~ ,' NORTH
_* Nov 1977-45°N * \
Evans et al (1978) ~\
· Jul-Aug 74-76 51°N ~0 v)\
_ Harries et al. (1976) 5 ~ 0\
0 Sept 1974-45°N 3Vo \
Fontanellc etal (1975) O- ~ \O|
· Ju~ 1973-48°N 0310-
Murcray et c~. (1980) ~ ~i
j v Oct 1979-32°N O=/~|
20- L zrus and Gandrua(1974) ~ 8
r~nc 1971 0 0 ~ii
~ 0
0 SPring 1971
I O SPring 1972 0 c
15r 0 SPring 1973
~32°N
101 1 1 1 1 1 1 1 ! 1 1 1 1 1 11611 1 1 1 1 1 1 1 1 1 1 ! ! ! ! ! !
0.1 1 10
MlXiNG RATIO (ppbv)
VERTICAl COlUMN I
11 to40km
(86 ~ 4.0)x1o15cm~
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
SUNSET 32-33°N
- MURCRAY ~ COWORKERS x
° 7 DEC 1967 IR Absorption
- Murcray et al (1974)
'& 9 fEB 1977 Visible Absorption
~Goldman et al (1978)
- 35 a 10 OCT 1979 IR Absorption
Blatherwick et al ( 1980)
30- flSCHER ~ COWORKERS (1980)
~:
1
25
20
O 9 fEB 79 ~ Visi ble Absorption
-~ 5 MAY 79'
- NO?
15 1 1 1 1 1 llll 1 ,
71° 86° 94°
~/
o
~/
~ /
d
1 11 1 111 1 1 1 11 1111 1 1 1 1 1 1 11
0.1 1 10
M IXI NG RATIO ( ppbv)
FIGURE C.3 (Continued)
OCR for page 177
177
451 1 1 1 1 1 iIII 1 1 1 1 tIIiI 1 1 1 1 1 IiII 1 i i
RII)I fY ~ COWORKERS
25
l
15-
... .
40-~ 25 OCT 1977 32°N S5-75°
e 12 DEC 1977 34°S 75-533
35 _ ~ 14 DEC 1977 34°S 75-53°
v 30 OCT 1978 32°N 53-69°
I o 8 NOV 1978 32°N 55-75°
~, 301 ~12 AUG 1978 51°N 54-57°
~I
251
5 1
~ 20'
oO' -
v
;~- ~
N O 4~
..'
1 1 1 1 1 1 1 1 1 1 1 ! 1 1 1 1 1 1 1 1 1 1
101 1 1 1 1 1 1111 ~
0.1 1 10
MIXING RATIO (ppbv)
1
e
1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
60
55
45
40
35
30
25
HORVATH ~ MASON x
-° 8 APR 1975 39°N 41°
o 19 MAR 1976 39°N 49°
_ 0 14 JUl 1980 39°N 16°
16 OCT 1980 39°N 61°
NO
oo
o \~X-71°
o o \
o~ o
od o
o~^ 0
o~,o
aO
o)
o o
,`70
/
~/
o
i~1 ~ ~ ~ ~ ~ ~ ~1 ~ ~ ~ ~ ~1 ~ ~ ~ ~ l,
f
0.1 1 10
MIXING RATIO (ppbv)
FIGURE C.3 (Continued)
OCR for page 195
195
\
40
30
-
-
Cl 20
-\
, _ .
10
Oo 0.5
CH4 MIXING RATIO ( ppm)
a
Ehhalt 44°; 48°N
28/6/79 X15/11/77
16/6/79 +15/12/77
° 21/12/77
\30°N
\\
\~ ~
45°: .;
.~1
\~\
\
Bush et al 41°N
2/14/78
0 5/11/18
Farmer et al 2/76 32°N
~1
Shhalt et al. 44°N -
· 6/7/77 · 6/16/77
· 9/9/77 · 9/26/77
\ ~
W, ~-
No
N5*
~ Wo _
to j
of
taco
1 x
1.0 1.5
FIGURE C.11 Model results for (a) CH4, (b) N2 O. (c) CF2 C12, (d) CFC13, (e) CH3 C1,
and (f) C2 H6 compared with observations. The measurements are discussed in Hudson
et al. (1982), and model profiles are from the Harvard one-dimensional model with
mixing surfaces, unless otherwise indicated.
