JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. D9, PAGES 11,617-11,623, MAY 16, 2000
Potential importance of the reaction CO q- HNO3
D. J. Lary 1
Department of Geophysics and Planetary Sciences, Tel Aviv University, Tel Aviv, Israel
D. E. Shallcross 2
Centre for Atmospheric Science, Cambridge University, Cambridge, England.
Abstract. CO has a strong thermodynamic potential for reducing HNO3 to
HONO. If the reaction of HNO3 with CO does proceed via heterogeneous catalysis
on sulfuric acid aerosols in our atmosphere, then this data assimilation study shows
that the model is better able to reproduce the observed N()x/HN()3 ratio even with
a 3' value as low as lx10 -4. This is particularly true in the upper troposphere and
lower stratosphere. We would like to highlight the possibility that elements such as
iron deposited in the lower stratosphere by meteorites may be catalyzing this and
other reactions within sulfate aerosols.
1. Introduction
Over just the last 5 years many studies have high-
lighted the fact that models do not reproduce the ob-
served nitrogen partitioning well, and in particular the
NOx/HNO3 ratio [Osterman et al., 1999, Singh et al.,
1998, Jae.qld et ai., 1998, Sen et al., 1998, Wang and
Jacob, 1998, Kotamarthi et al., 1997, Kondo et al.. 1997
, Hauglustaine et al., 1996. Jacob et al., 1996, Folkins
et al., 1995]. The recent World Meteorological Orga-
nization (WMO) report [World Meteorological Oqqani-
zation, 1998] concludes that below 30 km the observed
NO.2/NO and NO.2/HNO3 ratios are generally greater
than usually calculated by models. Below 30 km is
where we have appreciable amounts of sulfate and other
types of aerosol. It is therefore well worth investigating
possible candidates for surface reactions not currently
considered by models.
The inability of the models to reproduce the observed
NO.,/HNO3 ratio is due to either an overestimate in
the rate of HNO3 production, an underestimate of the
rate of HNO3 destruction. or the complete omission of
an HNO3 destruction process such as an unrecognized
heterogeneous reaction. A recent study [Fairbrother et
ai., 1997] noted that CH4 and CO have exceedingly
large thermodynamic potentials for reducing HNO3 to
HONO. So it is thermodynamically feasible that the re-
action of HNO3 with CO will proceed via heterogeneous
catalysis in our atmosphere [Fairbrother et, al., 1997].
 Also at the Centre for Atmospheric Science, Cambridge Uni-
versity, Cambridge, England.
 Now at the School of Chemistry, University of Bristol, Bristol,
England.
Copyright 2000 by the American Geophysical Union.
Paper number 1999JD900790.
0148-0227/00/1999JD900790509.00
This paper examines the likely impact of this reaction
on the chemistry of the upper troposphere and strato-
sphere. If it does take place, then this reaction is likely
to be important, as CO and HNO3 are both species
which are relatively abundant.
Even though thermodynamically feasible, it may well
be that the CO + HNO: reaction requires a catalyst to
proceed. This could be supplied by the large range of
chemical elements. including iron. which are deposited
in the atmosphere by ablating meteorites. Recently
[MurthV el, ai., 1998] have reported in situ measure-
ments of meteoritic material. mercury, and other ele-
ments in aerosols in the height range 5 to 19 kin.
Recent measurements [Murphy et al., 1998] reveal that
althogh stratost)heric aerosols primarily consisted of
sulfuri(' acid and water, many also contained meteoritic
material. More than half of the spectra taken indicated
that iron was present in the sulfat, e aerosols [Mrphy et
al.. 1998]. Just above the tropopause, small amounts
of mercury were found in over half of the aerosol parti-
cles that were analyzed. Overall, there was tremendous
variety in aerosol composition. One measure of this
diversity is that at least 45 elements were detected in
aerosol particles.
We would like to highlight the possibility that ele-
ments such as iron deposited in the lower stratosphere
by meteorites may be catalyzing reactions within sulfate
aerosols.
2. Calculations
We used the technique of four-dimensional variational
(4D-VAa) data assimilation [Fisher and Lary, 1995]
on data from the Atmospheric Trace Molecule Spec-
troscopy (ATMOS) Experiment to determine an opti-
mum set of initial conditions for our numerical model
calculations. This was done so that we could simul-
taneously use all the observations made by ATMOS.
11,617

11,618 LARY AND SHALLCROSS: POTENTIAL IMPORTANCE OF THE REACTION CO + HNOa
together with our numerical model, to give us the best
fit model simulation to these ATMOS data. Data as-
similation has been used extensively in meteorology and
more recently for atmospheric hemistry [Menke, 1984,
Courtier and Talagrand, 1987, Cobh, 1997, Courtier et
al., 1993, Khattatov etal., 1999, Larl, 1999, Lard and
Shallcross, 1999]
In this study the 4D-VAR data assimilation was per-
formed using a set of 29 stacked, independent, boxes.
The boxes were stacked in the equivalent potential vor-
ticity (PV) latitude - theta flow tracking coordinate sys-
tem [Lart etal., 1995] at an equivalent PV latitude of
40øS on 29 isdntropic surfaces between 300 and 1800 K.
The equivalent PV latitude of 40øS was chosen as we
used data from the STS45/ATLAS 1 mission which was
launched on March 24, 1992, from the Kennedy Space
Center. During its 8 days of operation, the ATMOS
instrument made observations spanning a substantial
portion of the globe. The 53 measurements taken at
orbital sunrise covered the midlatitude and equatorial
regions of the Earth from 30øS to 30øN. The 41 sun-
set observations were made at 25øS to 55øS. For the
duration of ATLAS 1 the equivalent PV latitude for
which the vertical profiles covered the largest range of
altitudes, and fox' which the largest number of species
was observed: was centered on about 40øS. The assim-
ilation window used was one day so that we can have 1
complete diurnal cycle.
The numerical model used is the extensively validated
AutoChem model [Fisher and Lary, 1995]. The model is
explicit and uses the adaptive timestep, error monitor-
ing [Steer and Bulirsch, 1980], time integration scheme
[Press etal., 1992] for stiff systems of equations. Pho-
relysis rates are calculated using full spherical geometry
and multiple scattering [Lart and Ptle, 1991 Lart and
P.vle, 1991b, Meier etal., 1982, Nicelet etal., 1982] with
a treatment of spherical geometry [Anderson, 1983].
In this study the model described a total of 59 species.
There were 54 integrated species are integrated, namely;
O(1D), O(3p), 03, N, .NO, NO2, NO3, N2Os, HONO,
HNOa, HO.2-NO2, CN. N(O. HCN, C1, C12, C10, (1OO.
OC10, C120.2, C1NO:, C10.XO.2. HC1, HOC1, CH3OC1,
Br. Br2, BrO, BrONO2, BrONO, HBr, HOBr. BrC1.
H2, H, OH, HO.2, H20.2, CH3, CH30, CH302, CH3OH,
CH3OOH. CH3ONO2. CH302NO2, HCO, HCHO, CH4,
CH3Br, CF2C12, CO, N20, CO.2. and H20. The model
contains a total of 366 reactions. 241 bimolecular re-
.
actions, 31 trimolecular reactions, 48 photolysis reac-
tions, 46 heterogeneous reactions based on standard ki-
netic reference data [DeMote etal., 1997, Atkinson et
al., 19971, with some very recent updates for NO2 and
HNOu kinetics [Donahue etal., 1997, Fulle etal.. 1998,
Brown etal., 1999a, Brown etal., 1999b].
2.1. ATMOS Case Study
Here we use 4D-¾R data assimilation to look at a case
study using data from the Atmospheric Trace Molecule
Spectroscopy (ATMOS) experiment to examine the ef-
fect of the postulated heterogeneous reaction CO +
HNO. ATMOS [Binsland etal., 1996, Abrams et
al., 1996a, Abrams etal., 1996b, Abrams etal., 1996c,
Abrams etal., 1996d, Abrams etal., 1996e, Abbas etal.,
1996b, Binsland etal., 1998a, Binsland etal., 1998b]
is an infrared Fourier transform interferometer which
has on four occasions flown in the payload bay of the
space shuttle and measures the concentrations of gases
present in the atmosphere at altitudes between 10 and
150 km. As the shuttle's orbit carries it into and out
of the Earth's shadow, the ATMOS instrument views
the Sun as it sets or rises through the atmosphere. The
spectrometer measures changes in the infrared compo-
nent of sunlight as the Sun's rays pass through the
atmosphere. Trace gases absorb very specific wave-
lengths which allows the determination of which gases
are present, their concentrations, and at what altitudes.
ATMOS has flown four times and ATMOS. (More in-
formation on ATMOS can be found from the web site
http://rein u s.j pl. nasa. gov/. )
2.2. NO/HNO3 Ratio
The solid curve in Figure 1 shows the NO/HNO3 ratio
produced by performing 4D-¾54R on the ATMOS data
when the reaction of CO with HNO3 on sulfate aerosols
was not included in the model. The dashed line denotes
0 01 0.1 1 10 100
........ I ........ I ........ I , ,
1250 ø /F1250
Normal Model # o., ß [
........ With CO+HN03 / J.
1050  Observations f 1050 850 850
ß ß/! -
ß  [ 50' I/i r4 450 ).....,,%,.... 250 ........  ........ I ........  ........ 1 250
0.01 0.1 1 10 100
NOx/HNO 3
Figure 1. A comparison bem'een the observed
NO./HNO3 ratio (diamonds) and that produced by
performing 4D-VAR on the ATMOS data. The solid
line denotes the case when the reaction of CO with
HN(): on sulfate aerosols was not included in the model,
the dashed line denotes the case when it was included
in the model with ?=2x10 -4. The ATMOS data used
simultaneously by our 4D-VAR analysis were 03, NO,
N()2, N20, HONO2, HO2NO2, HCN, C1ONO2. HC1,
CH4, CO, NO, CO2, and H20. The vertical profile of
sulfate aerosol surface area used came from the Strato-
spheric Aerosol and Gas Experiment 2 (SAGE 2). The
observations were made by the space-shuttle-born At-
roespheric Trace Molecule Spectroscopy (ATMOS) in-
strument on March 29, 1992.

LARY AND SHALLCROSS: POTENTIAL IMPORTANCE OF THE REACTION CO + HNOa 11,619
1,800
Overall rate of HNO3
(b)
1,800
Overall rate of HNO3
1,600
1,4oo
1,200
1,0;00
 1,200J
e 1 000.
0 3 6 9 12 15 18 21 24
Local Solar Time (hours)
(c)
% Production of HNO3 due to OH+NO2
1,800! ........ \" I: ....... r '
i ': ' 1,600 '¾.i . ''i
......
:. :'::i. .. .... t.' ..:. ß . .....,:
:/.: : ......
6
0 3 6 9 12 15 18 21 24
LoI Solar  (hour)
% Pruion of HNO3 due to H20+N205
20,000
10,000
0
-lO,OOO
-20,000
100
;' '.11 ß
.::. .
1,200- :::.. ,Z:½ 60
1,000 '. /:. 40
,..:, : .:. ... .. [.;:...:.'
..
600' ::' \"  0 400
0 a 6 9 a2 s  21 24
1,600-
1,4oo
1,200
1,ooo
......
800 ....... - ' o .........
6004 ....
...:..: . ..... ....,...: . :.......:½----::::-, ....
4OO
0 3 6 9 12 15 18 21 24
Local Solar Time (hours)
% Production of HNO3 due to HO2+NO3
1,800: ........i ':'---',, .... ß , ',':!iJ': ..... 1 600 :';'/'/'/\" ß
1,200 i
8ooi
6oo.!
4001
0 3 6 9 12 15 18 21 24
Local Solar Time (hours)
(o
% Production of HN03 due to H20+BrON02
0 3 6 9 12 15 18 21 24
Local Solar Time (hours) Local Solar Time (hours)
Figure 2. (a and b) Calculated net rate of change for HNO3, in units of molecules cm -3 s -1.
Figure 2a is the calculation made without the postulated reaction of HNO3 with CO on sulfate
aerosols. Figure 2b is the calculation made with the postulated reaction with -),=2x10 -4. (c-f)
Calculations made without the postulated reaction of HNO3 with CO on sulfate aerosols. Figure
2c shows the calculated percentage of HNO3 production due to the gas phase reaction of OH
with NO2. Figure 2d shows the calculated percentage of HNO3 production due to the gas phase
reaction of HO2 with NO3. Figure 2e shows the calculated percentage of HNO3 production due
to the hydrolysis of N2Os on sulfate aerosols. Figure 2f shows the calculated percentage of HNO3
production due to the hydrolysis of BrONO2 on sulfate aerosols. In each case the x axis is local
solar time in hours, and the y axis is altitude shown as a potential temperature (Kelvin).
,q
I20,000
ß
10,000
-10,000
-20,000
100
80
ß
'\" 60
40
:.:::
lOO
80
.... 60
.... 20 ,:...,:. 
Do

11,620 LARY AND SHALLCROSS: POTENTIAL IMPORTANCE OF THE REACTION CO + HNOs
(b)
% loss of HNO3 due to OH+HNO3
0 3 :6
1,800
1,600;
1,4001i
'i
1,000-
sooi
600
400i.
0
Local Solar Time (hours)
% loss of HNO3 due to hv(b)+HNO3
,%]o
60
.:.,'>'=: 40
Lq o
% loss of HNO3 due to hv(a)+HNO3
(d)
3 6 9 12 15 18 21 24 3
Local Solar Time (hours)
Local Solar Time (hours)
% loss of HNO3 due to CO+HNO3
1,600 .:..i! :i:i '
1,400. ::: ......
:i '
 ,::..i;:
1,000; ':i:, -\":.:::'
8oo ...... \"'\"
6 ': ..................... :'' ::t' '-
'..
40O-
o 6 9 12 15 18 21 24
Local Solar Time (hours)
Figure 3. (a to d) are all for calculations made with the postulated reaction of HN03 with CO
on sulfate aerosols with ?:2x10 -4. (a) Calculated percentage of HN03 destruction due to the
gas phase reaction of OH with HN03. (b) Calculated percentage of HNO3 destruction due to
photolysis yielding OH + NO2. (c) Calculated percentage of HN03 destruction due to photolysis
yielding O(3p) + HONO. (d) Calculated percentage of HN03 destruction due to the postulated
heterogeneous reaction of CO with HN03 with ?=2x10 -4. In each case the x axis is local solar
time in hours, and the y axis is altitude shown as a potential temperature (Kelvin).
100
0
lOO
60
4O
20
.
o
the case when it was included in the data assimilation
with 7=2x10 -4
Several values were used and ?-2x10 -4 seemed to
give the best agreement. The ATMOS data, used simul-
taneously by our 4D-VAP analysis were O, NO,
NO, HNOu, HObNOb, HCN, C1ONO, HC1, CH4,
CO, NO, CO, and HO. The vertical profile of sul-
fate aerosol surface area used came from the Strato-
spheric Aerosol and Gas Experiment 2 (SAGE 2). The
diamonds are the observations of the NOz/HNOu ra-
tio made by the space shuttle born Atmospheric Trace
Molecule Spectroscopy (ATMOS) instrument for March
29, 1992.
We can see in Figure I that even with such low val-
ues for ? the model calculations agree pretty well with
the observations made by ATMOS in the lower strato-
sphere. Let us now examine the reason for this by look-
ing at the chemical budget of HNO3 calculated by the
model.
2.3. Production of HNO3
Let us look at the main production processes for HNO3,
examine their relative roles, and see how this is al-
tered by including the postulated heterogeneous reac-
tion CO+HNO3 with '7=2x10 -4.
The most rapid HNO3 production is in the night-
time lower stratosphere/upper troposphere. This is
due to heterogeneous reactions, primarily the hydrol-
ysis of N2Os but also of BrON02 and C1ONO2. During
the day there is a net loss of HNO3 due to photoly-
sis. When we add an additional loss of HNO3, that is
with the postulated reaction, the net production rate of
HNO. during the night decreases from approximately
3.5x104 molecules cm -3 s -1 to 2.5x104 molecules cm -3

LARY AND SHALLCROSS: POTENTIAL IMPORTANCE OF THE REACTION CO + HNO3 11,621
s -1. Figures 2a and 2b show the calculated net rates of
change for HNO3 in units of molecules cm -3 s -1 with-
out and with the postulated reaction.
If we consider the relative contribution of the four
major HNO3 production reactions shown in Figure 2c-
2f, we see that during the day the reaction of OH with
NO2 is the major source of HNO3, particularly where
there is less sulfate aerosol. During the night in the
upper stratosphere the reaction of HO2 with NO3 may
also play a role if there is a channel producing HNO3.
The reactions of NO3 with CH3OH and HCHO also play
a small part in the nighttime upper stratosphere.
In the upper troposphere and lower stratosphere the
main production of HNO3 is due to heterogeneous reac-
tions on sulfate aerosols (Figure 2e and 2f). The most
important of these is the hydrolysis of N2Os. The hy-
drolysis of N20. makes the largest relative contribution
to HNO3 production during the night when no photoiy-
sis is occurring, and during the early morning when the
N205 concentration is still relatively large (Figure 2e).
But it is also interesting to see how important the hy-
drolysis of BrONO2 is (Figure 2f), particularly as the
total atmospheric loading of bromine is so much less
than that of nitrogen and chlorine. The hydrolysis of
BrONO2 makes the largest contribution to HNO3 pro-
duction just after sunset. However, during the day be-
tween approximately 20% and 40% of the HNO3 pro-
duction is due to the hydrolysis of BrONO2. The hy-
drolysis of C1ONO2 plays a minor role, peaking at about
6% just after sunset in the lower stratosphere.
2.4. Destruction of HNO3
For most of the sunlit atmosphere the major loss is the
photolysis channel which produces OH + NO2. There is
also a minor channel which produces O(3p) + HONO.
However, in the lower stratosphere and upper tropo-
sphere he reaction with OH is important. Its largest
relative contribution is close to sunrise and sunset Fig-
ure 3a-3c.
The contribution due to the postulated rem'tion of
HNO3 with CO is most important in the upper tropo-
sphere and lower stratosphere where we have the most
aerosol (Figure 3d). It is also the only major nighttime
loss.
2.5. Recent Gas Phase Kinetics Update
Since the last atmospheric chemistry kinetic reviews
[DeMove et al., 1997, Atkinson et al., 1997], there have
been new kinetic measurements of key reactions relevant
to HNO. destruction and production [Donahue e al.,
199Z _Full½ et al., 1998. Brown et al., 1999a, Portmann et
al., 1999, Brown et al., 1999b 1. The effects of this new
kinetic data has been evaluated [Lary and Shallcross,
1999] who found that as the partitioning of OH and
HO2 is a strong function of the amoun of NO present
increasing the NO/HNO3 ratio shifts the OH/HO2 ra-
tio in favor of OH. This increases the OH concentration
by up to 40% below in the lowermost stratosphere and
upper troposphere. Corresponding to this there is also
a 5% to 10% increase in HC1 due to the reaction of
OH with C10. The the technique of 4D-Var was used to
show objectively that the new kinetic measurements re-
suit in an improvement of the model simulations [Lary
and Shallcross, 1999]. Ve used these data here and
recommend their use in atmospheric modeling studies.
2.6. H202 -[- HNO3
It has also been noted that the reaction of H202 +
HNO3 is thermodynamically favorable [Fairbrother et
al., 1997]. We included this reaction in some model
calculations and found that it was not likely to be a
major loss of HNO3 even with 7 values of up to 2x10 -3.
3. Summary
I nas been oted that CO has an exceedingly srong
thermodynamic potential for reducing HN03 to HONO
Lra,'o'rvther et al., I * inc reaction of - , 3 with
CO does proceed via heterogeneous catalysis in our at-
toosphere, then it is capable of reducing the calculated
NO/HNO3 ratio bringing the calculated HNO3 pro-
file into closer agreement with observations. Clearly
measurements of this process are warranted, since a de-
crease of the NO/HNO ratio leads to an increase in
the oxidizing capacity of the atmosphere.
Acknowledgments. It is a pleasure to acknowledge
the following: The government of Israel for an Alon Fellow-
ship: the Royal Society for a Royal Society University Re-
search Fellowship; the NERC and EU for research support;
and Simon Hall of Cambridge University who has provided
such excellent computational support. Anyone interested in
performing data assimilation is welcome to contact David
Lary.
References
Abbas, 5I.M., et al., The hydrogen budget of the s;rato-
sphere inferred from ATMOS measurements of water and
methane eophy. tcs. Left., 29(17), 2405-2408. 1996.
Abbas, M.M., et al.. Seasonal variations of water vapor
in the lower stratosphere inferred from ATMOS/ATLAS-
3 measurements of water and methane, Geophys. Res.
Lctt., 23(17). 2401-2404. 1996.
Abrarob. [.C.. et al., On the assessment and uncertainty
of atmospheric trace gas burden measurements with high
resolution infrared solar occultation spectra from space
by the ATMOS experiment, Geophys. Res. Lett., 25(17),
2337-2340, 1996.
Abrams. M.C.. A. Goldman, M.R. Gunson, C.P. Rinsland,
and R. Zander, Observations of the infrared solar spec-
trum from space by the ATMOS experiment, Applied
Optzcs, 3'5(16), 2747 2751, 1996.
Abrams. M.C., M.R. Gunson, A.Y. Chang, C.P. Rinsland,
and R. Zander, Remote sensing of the ©'s atmosphere
from space with highresolution Fourier-transform spec-
troscopy: Development and methodology of data process-
ing for the ATMOS experiment, Applied Optics, 35(16).
2774-2786 1996.
Abrams. 5I.C. et al., ATMOS/ATLAS-3 observations of
long-lived tracers and descent in the antarctic vortex in
november 1994, Gcophys. Res. Left., 23(17), 2341-2344,
1996.
Anderson. D.E., The troposphere-stratosphere radiation-
field at twilight- A spherical model, Planet. Space Sci..
31(12), 1517-1523, 1983.
Atkinson, R., D.L. Baulch, R.A. Cox. R.F. Hampson, J.A.

11,622 LARY AND SHALLCROSS: POTENTIAL IMPORTANCE OF THE REACTION CO + HNOa
Kerr, M.J. Rossi, and J. Troe, Evaluated kinetic and pho-
tochemical data for atmospheric chemistry: supplement
vi. iupac subcommittee on gas kinetic data evaluation for
atmospheric chemistry, J. Phys. ahem. Ref. Data, 26,
1329-1499 1997.
Brown, S.S., R.K. Talukdar, and A.R. Ravishankara, Rate
constants for the reaction OH+NO2HNOa+M under
atmospheric onditions, Chem. Phys. Lett., 299(3-4), 277-
284, 1999a.
Brown, S.S., R.K. Talukdar, and A.R. Ravishankara, Recon-
sideration of the rate constant of the reaction of hydroxyl
radicals with nitric acid, J. Phys. ahem., 103(16), 3031-
3037, 1999b.
Cohn, S.E., An introduction to estimation theory, J. Met.
Soc. Japan, 75(1B), 257-288, 1997.
Courtier, P. and O. Talagrand, Variational assimilation
of meteorological observations with the adjoint vorticity
equation. 2. Numerical results, ). J. R. Meteorol. Soc.,
113(478), 1329-1347, 1987.
Courtier, P., J. Derber, R. Errico, J.F. Louis, and T. Vukice-
vic, Important literature on the use of adjoint, variational-
methods and the Kalman filter in meteorology, Tellus A.,
d 5(5), 342-357, 1993.
DeMore. W. B., C. J. Howard, S. P. Sander, A. R. Ravis-
hankara, D. M. Golden, C. E. Kolb, M. J. Hampson, R.
F. Molina, and M. J. Kurylo, Chemical kinetics and pho-
tochemical data for use in stratospheric modeling, JPL
Publ. 97-4 12, Jet Propul. Lab., Calif. Inst. of Technol.,
Pasadena, 1997.
Donahue, N.M., M.K. Dubey, R. Mohrschladt, K.L. Demer-
jian, and J.G. Anderson, High-pressure flow study of the
reactions OH+NOx= HNOa: Errors in the falloff region,
J. Ceophys. Res., 102, 6159-6168, 1997.
Fairbrother, D.H., D.J.D. Sullivan, and H.S. Johnston,
Global thermodynamic atmospheric modeling: Search for
new heterogeneous reactions, J. Phys. Chem. A, 101(40),
7350-7358 1997.
Fisher, M. and D.J. Lary, Lagrangian 4-dimensional vari-
ational data assimilation of chemical-species, Q. J. R.
Meteo7vl. Sot., 121(527 Part A), 1681-1704. 1995.
Folkins, I.A., A.J. Weinheimer, B.A. Ridley, J.G. Walega,
B. Anderson, J.E. Collins, G. Sachse, R.F. Pueschel, and
D.R. Blake, Oa, NO v, and NOx/NOy in the upper tro-
posphere of the equatorial pacific, J. Gcophys. Res.,
100(DlO), 20,913-20,926, 1995.
Fulle, D., H.F. Hamann, H. Hippler, and J. Troe, Tempera-
ture and pressure dependence of the addition reactions of
OH to NO and to NO2. IV Saturated laser-induced fluo-
rescence measurements up to 1400 bar, J. Phys. Chem.
Ref. Data, 108, 5391 5397, 1998.
Hauglustaine, D.A., B.A. Ridley, S. Solomon, P.G. Hess,
and S. Madronich, HNO3/NOx ratio in the remote tro-
posphere during MLOPEX 2: Evidence for nitric acid re-
duction on carbonaceous aerosols'?, Geophys. Res. Left.,
23(19), 2609-2612, 1996.
Jacob, D.J., B.G. Heikes, S.M. Fan J.A. Logan, D.L.
Mauzerall, J.D. Bradshaw, H.B. Singh, G.L. Gregory,
R.W. Talbot, D.R. Blake, and G.W. Sachse, Origin of
ozone and nox in the tropical troposphere- A photochemi-
cal analysis of aircraft observations over the south atlantic
basin, J. Geophys. Res., 101(D19), 24235-24250, 1996.
Jaegld, L., D.J. Jacob, Y. ¾Vang, A.J. ¾Veinheimer, B.A. Rid-
icy, T.L. Campos, G.W. Sachse, and D.E. Hagen, Sources
and chemistry of NO in the upper troposphere over the
United States, Geophys. Res. Left., 25(10), 1705-1708,
1998.
Khattatov, B. V., J. C. Gille, L.V. Lyjak, G. P. Brasseur,
V. L. Dvortsov, A. E. Roche, and J. W. Waters, Assimi-
lation of photochemically active species and a case anal-
ysis of UARS data, J. Geophys. Res., 10J(D15), 18,715-
18,737, 1999.
Kondo, Y., S. Kawakami, M. Koike. D.B r. Fahey, H. Naka-
jima, Y. Zhao, N. Toriyama, M. Kanada, G.W. Sachse,
and G.L. Gregory, Performance of an aircraft instrument
for the measurement of NOv, J. Geophys. Res., 102(D23),
28663-28671, 1997.
Kotamarthi, V.R., et al., Evidence of heterogeneous chem-
istry on sulfate aerosols in stratospherically influenced air
masses sampled during PEMWest B, J. Geophys. Res.,
102(D23), 28425-28436, 1997.
Lary, D.J., Data assimilation: A powerhfi tool for atmo-
spheric chemistry, Philos. Trans. R. Soc. Set. A, in press
1999.
Law, D.J. and J.A. Pyle, Diffuse-radiation, twilight, and
photochemistry: 1., J. Atmos. Chem., 13(4), 373-392,
1991a.
Lary, D.J. and J.A. Pyle, Diffuse-radiation, twilight, and
photochemistry: 2., J. Atmos. Chem., 13(4), 393 406,
1991b.
Lary, D.J. and D.E. Shallcross, 4d-var data assimilation
and the role of new rate coefficients in controlling the
HNO3/NO ratio, J. Atmos. Chem., in press 1999.
Lary, D.J., M.P. Chipperfield, J.A. Pyle, W.A. Norton, and
L.P. R,iishojgaard, 3-dimensional tracer initialization and
gcncral diagnostics using cquivalcnt pv latitude-potential-
temperature coordinates, Q. j. R. Meteorol. Soc., 121(521
PtA), 187-210, 1995.
Meier, R.R., D.E. Anderson, and M. Nicolet, Radiation-
field in the troposphere and stratosphere from 240-1000
nm 1. General-analysis, Planet. Space Sci., 30(9), 923-
933, 1982.
Menke, W., Geophysical Data Analysis: Discrete Inverse
Theory, Academic, San Diego, Calif., 1984.
Murphy, D.M., D.S. Thomson, and T.M.J. Mahoney, In
situ measurements of organics, meteoritic material, mer-
cury, and other elcmcnts in acrosols at 5 to 19 kilometers,
Science, 282(5394), 1664-1669, 1998.
Nicolet, M., R.R. Meier, and D.E. Anderson, Radiation-
field in the troposphere and stratosphere .2. Numerical-
analysis, Planet. Space Sci., 30(9), 935-983, 1982.
Osterman, G.B., B. Sen, G.C. Toon, R.J. Salawitch, J.J.
Margitan, J.F. Blavier, D.W. Fahey, and R.S. Gao, Parti-
tioning of NOv species in the summer arctic stratosphere,
Ceophys. Res. Left., 26(8), 1157-1160, 1999.
Portmann, R.W., S.S. Brown, T. Gierczak, , R.K. Talukdar,
J.B. Burkholder, and A.R. Ravishankara, Role of nitrogen
oxides in the lower stratosphere: A reevaluation based on
laboratory studies, Geophys. Res. Left., 26, 2387-2390,
1999.
Press, W.H., S.A. Teukolsky, W.T. Vetterling, and B.P.
Flannery, Numerical Recipes in Forgran - The Art of Sci-
entific Computing, Cambridge Univ. Press, New York,
2nd ed., 1992.
Rinsland, C.P., et al., Trends of OCS, HCN, SFe, CHC1F2
in thc lower stratospherc from 1985 and 1994 ATMOS
experiment measurements near 30øN latitude, Geophys.
Res. Left., 23(17), 2349-2352, 1996.
Rinsland, C.P., et al., ATMOS/ATLAS-3 infrared profile
measurements of trace gases in the november 1994 tropical
and subtropical upper troposphere, J. Quant. Spec. Rad.
Trans., 60(5), 891 901, 1998.
Rinsland, C.P., e al., ATMOS/ATLAS-3 infrared profile
measurements of clouds in the tropical and subtropical
upper troposphere, J. Quant. Spec. Rad. Trans., 60(5),
903-919 1998.
Sen, B., G.C. Toon, G.B. Osterman, J.F. Blavier, J.J. Mar-
gitan, R.J. Salawitch, and G.K. 5:m, Measurements of
reactive nitrogen in the stratosphere, J. Geophys. Res.,
103(D3), 3571-3585, 1998.
Singh, H.B., et al., Latitudinal distribution of reactive nitro-
gen in the free troposphere over the pacific ocean in late
winter early spring, J. Geophys. Res., 103(D21), 28237-
28246, 1998.

LARY AND SHALLCROSS: POTENTIAL IMPORTANCE OF THE REACTION CO + HNOa 11,623
Stocr, J. and R. Bulirsch, Introduction to Numerical Anal-
ysis, Springer-Verlag, New York, 1980.
Wang, Y.H. and D.J. Jacob, Anthropogenic forcing on tro-
pospheric ozone and OH since preindustrial times, J. Gco-
phys. Res., 103(D23), 31123-31135, 1998.
WMO, Scientific assessment of ozone depletion: 1998, Tech-
nical Report 44, WMO Global Ozone Res. and Monitor.
Proj., Geneva, Switzerland, 1998.
D. J. Lary, Department of Geophysics and Planetary
Sciences, Tel Aviv University, 69978, Tel Aviv, Israel.
( d avid. lary @ at m. ch. cam. ac. uk)
D. E. Shallcross, School of Chemistry, University of Bris-
tol, Bristol, England.
(Received February 15, 1999; revised July 22, 1999;
accepted July 27, 1999.)