On
the
stratospheric
chemistry of hydrogen cyanide
Armin Kleinbo
̈hl,
1
Geoffrey C. Toon,
1
Bhaswar Sen,
1
Jean-Franc
̧ois L. Blavier,
1
Debra K. Weisenstein,
2
Rafal S. Strekowski,
3
J. Michael Nicovich,
4
Paul H. Wine,
4,5
and Paul O. Wennberg
6
Received 9 February 2006; revised 30 March 2006; accepted 24 April 2006; published 3 June 2006.
[
1
] HCN profiles measured by solar occultation
spectrometry during 10 balloon flights of the JPL MkIV
instrument are presented. The HCN profiles reveal a
compact correlation with stratospheric tracers. Calculations
with a 2D-model using established rate coefficients for the
reactions of HCN with OH and O(
1
D) severely
underestimate the measured HCN in the middle and upper
stratosphere. The use of newly available rate coefficients for
these reactions gives reasonable agreement of measured and
modeled HCN. An HCN yield of
30% from the reaction of
CH
3
CN with OH is consistent with the measurements.
Citation:
Kleinbo
̈ hl, A., G. C. Toon, B. Sen, J.-F. L. Blavier,
D. K. Weisenstein, R. S. Strekowski, J. M. Nicovich, P. H.
Wine, and P. O. Wennberg (2006), On the stratospheric
chemistry of hydrogen cyanide,
Geophys. Res. Lett.
,
33
,
L11806, doi:10.1029/2006GL026015.
1. Introduction
[
2
] Hydrogen cyanide (HCN) was first detected in the
Earth’s stratosphere by means of airborne infrared absorp-
tion spectrometry [
Coffey et al.
, 1981], and shortly later by
microwave techniques [
Carli et al.
, 1982]. Profile informa-
tion on HCN in the stratosphere was obtained by several
remote measurements [
Abbas et al.
, 1987;
Zander et al.
,
1988;
Jaramillo et al.
, 1988, 1989]. Tropospheric HCN was
first detected by
Rinsland et al.
[1982] using ground-based
infrared absorption spectrometry. This work was followed
by several other studies based on infrared absorption
measurements, most recently by
Kasai et al.
[2005] and
Rinsland et al.
[2005]. In-situ detections of HCN have been
achieved by mass spectrometric methods in the stratosphere
[
Spreng and Arnold
, 1994;
Schneider et al.
, 1997] and more
recently gas chromatographic methods in the troposphere
[
Singh et al.
, 2003].
[
3
] HCN is produced in the troposphere mainly by
biomass burning and to a lesser extent by coal burning
[
Li et al.
, 2003]. Uptake into the ocean has been suggested
to be the major tropospheric loss mechanism [
Li et al.
,
2003]. Removal also occurs by the reaction with OH
[
Cicerone and Zellner
, 1983]. Chemical loss of HCN in
the stratosphere is primarily caused by the reactions with
OH and O(
1
D), and by photolysis. Modeling of strato-
spheric HCN was undertaken by
Cicerone and Zellner
[1983] and
Brasseur et al.
[1985]. However, measurements
of HCN exceeded their 1D-model calculations at altitudes
above
20–25 km. In addition, a connection between
atmospheric HCN and CH
3
CN had been proposed [
Murad
et al.
, 1984].
Brasseur et al.
[1985] investigated HCN as
a progenitor of CH
3
CN but concluded that this was
unlikely.
[
4
] Here we present a new set of atmospheric HCN
volume mixing ratio (VMR) profiles from balloon-borne
infrared solar occultation measurements by the Jet Propul-
sion Laboratory (JPL) MkIV interferometer [
Toon
, 1991]
between 1994 and 2004. We compare measured HCN-tracer
correlations with 2D-model calculations that apply (1) rate
coefficients for HCN destruction by OH based on the JPL
recommendation [
Sander et al.
, 2003] and by O(
1
D) based
on the assumption by
Cicerone and Zellner
[1983], and (2)
newly available rate coefficients for these reactions recently
determined from laboratory measurements [
Strekowski
,
2001]. We show that the new rate coefficients significantly
improve the agreement between measurements and model,
and investigate the role of CH
3
CN as a possible source of
HCN.
2. HCN Solar Occultation Measurements
[
5
] Simultaneous measurements of VMR profiles of
HCN and more than 30 other stratospheric gases were taken
by the JPL MkIV Fourier transform interferometer. The
MkIV covers a spectral range between 650 and 5650 cm
1
with a spectral resolution of
0.01 cm
1
. During balloon-
borne operation, the MkIV views the sun through the
atmospheric limb at sunset or sunrise, providing a high
sensitivity to trace gases due to the long paths through the
atmosphere. The measured sunset and sunrise spectra are
ratioed against an exo-atmospheric spectrum derived from
low-airmass measurements from float altitude.
[
6
] The retrieval of the VMR of atmospheric trace gases
is performed in two stages: First, spectral fitting is per-
formed using a nonlinear least squares algorithm to deter-
mine the slant column abundances of each target gas in each
spectrum. Second, a vertical profile is retrieved by solving
the matrix equation that relates the measured slant columns
to the calculated geometrical slant paths on a vertical grid of
2 km spacing. For this, a linear equation solver is used
together with a smoothing constraint.
[
7
] For the HCN retrieval 23 microwindows in a fre-
quency band between 3254 and 3362 cm
1
are analyzed
GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L11806, doi:10.1029/2006GL026015, 2006
1
Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, California, USA.
2
Atmospheric and Environmental Research, Inc., Lexington, Massa-
chusetts, USA.
3
School of Earth and Atmospheric Science, Georgia Institute of
Technology, Atlanta, Georgia, USA.
4
School of Chemistry and Biochemistry, Georgia Institute of Technol-
ogy, Atlanta, Georgia, USA.
5
Also at School of Earth and Atmospheric Science, Georgia Institute of
Technology, Atlanta, Georgia, USA.
6
California Institute of Technology, Pasadena, California, USA.
Copyright 2006 by the American Geophysical Union.
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L11806
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(see auxiliary material
1
). The HCN line parameters have
been subject of a major revision in the latest version of
HITRAN [
Rothman et al.
, 2005]. The use of these line
parameters give better consistency between microwindows
compared to previous versions of HITRAN, and lead to
HCN VMRs that are about 10% higher. The determined
slant columns for each spectrum are a weighted average of
the slant columns retrieved in all microwindows.
[
8
] The present analysis comprises 10 balloon flights,
which were performed between 1994 and 2004. Four flights
were launched from Ft. Sumner, NM (34.5
N, 104.2
W),
two from Fairbanks, AK (64.8
N, 147.7
W), and four from
Esrange, Sweden (67.9
N, 21.1
E).
[
9
] Figure 1 shows the retrieved HCN profiles from the
10 balloon flights. The precision of the HCN retrieval,
derived from the residual of the spectral fits, is about 5–
30 ppt. The largest contribution to the systematic errors are
the spectroscopic parameters of the HCN lines, which cause
a systematic uncertainty of 10% [
Rothman et al.
, 2005]. A
high variability in HCN is observed in the troposphere.
Above the tropopause (
12–14 km at mid-latitudes,
8–
10 km at polar latitudes) the HCN VMRs are more constant,
around 220 ppt. In the stratosphere HCN decreases with
altitude. The decrease is considerably faster in the polar
profiles than in the mid-latitude profiles. This is typical for a
trace gas with a source in the troposphere and a sink in the
upper stratosphere, and is caused by the atmospheric large
scale circulation. The fastest HCN decrease is observed
inside the polar vortex, where diabatic descent of the
airmasses had occurred (Dec. 1999, Mar. 2000, Dec.
2002). We note that both mid-latitude and polar data sets
agree well with historic measurements from literature
sources.
[
10
] To further assess the tracer characteristics of HCN
we next consider the HCN profiles on isopleths of the
long-lived stratospheric tracers CH
4
(Figure 2) and N
2
O
(see auxiliary material). This should remove the variability
induced by the large scale circulation [
Plumb and Ko
,
1992]. Both tracers were measured simultaneously with
HCN in the same airmass by the MkIV instrument. CH
4
has its source in the troposphere and destruction in the
stratosphere occurs mainly by the reaction with OH. N
2
O
is also produced in the troposphere, in the stratosphere it is
destroyed by photolysis and by the reaction with O(
1
D).
The correlations are very compact, confirming the long
lifetime of stratospheric HCN, and that latitudinal differ-
ences are mainly caused by large scale circulation.
3. Model Comparisons
[
11
] We compare the retrieved HCN VMRs and tracer
correlations with HCN-tracer correlations calculated by a
2-dimensional model [
Weisenstein et al.
, 2004]. The model
runs were performed with a resolution of 9.5
in latitude
and 1.2 km in altitude. The 2-D residual circulation and
eddy diffusion coefficients were calculated from observed
climatological values of temperature, H
2
O, zonal wind,
and ozone [
Fleming et al.
, 1999]. Rate coefficients for
chemical reactions were taken from
Sander et al.
[2003],
photolysis cross-sections from
Sander et al.
[2000]. The
model was initialized with a uniform VMR of 220 ppt
HCN at the surface, leading to VMRs slightly lower at the
tropopause.
[
12
] In total, 7 model runs were performed in which
different parameters were used for the HCN chemistry.
The main considerations were the different recommenda-
tions for the reaction of HCN with OH, the destruction of
HCN by photolysis, and the plausibility of HCN as a
product of the reaction of CH
3
CN with OH. As no measured
UV absorption cross sections are available at wavelengths
greater than 160 nm, photolysis was considered by using the
absorption cross sections of HCl as had been done previ-
ously [
Cicerone and Zellner
, 1983;
Brasseur et al.
, 1985].
The reactions and their rate coefficients used in the different
model runs are summarized in Table 1.
[
13
] Figure 2 (middle) shows the results of the
model runs that used the recommendations for the HCN
Figure 1.
Retrieved vertical profiles of HCN from 10 balloon flights between 1994 and 2004 vs. altitude measured by the
MkIV instrument at mid-latitudes (left) and at polar latitudes (right). The error bars (only given for one profile in each plot
for clarity) give the precisions. Also included are profile measurements from literature sources (spaceborne infrared solar
occultation measurements [
Zander et al.
, 1988], airborne gas chromatographic measurements [
Singh et al.
, 2003], balloon-
borne far-infrared measurements [
Abbas et al.
, 1987], ground-based microwave measurements [
Jaramillo et al.
, 1988,
1989], balloon-borne mass spectrometric measurements [
Spreng and Arnold
, 1994], airborne mass spectrometric
measurements [
Schneider et al.
, 1997]).
1
Auxiliary material is available at ftp://ftp.agu.org/apend/gl/
2006gl026015.
L11806
KLEINBO
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HL ET AL.: STRATOSPHERIC HCN CHEMISTRY
L11806
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destruction by OH of
Sander et al.
[2003] and for the
destruction of O(
1
D) of
Cicerone and Zellner
[1983]. Run
‘‘a’’ also includes photolysis. For comparison with the
measurements, the model output at 35
N and at 70
N was
taken and compared to the average profile of the MkIV
measurements on isopleths of CH
4
. Run ‘‘a’’ substantially
underestimates the amount of HCN in the stratosphere.
The modeled HCN VMR is below 30 ppt already at a CH
4
level of 1 ppm, whereas the measured HCN VMR is still
greater than 160 ppt. Ignoring the destruction of HCN by
OH as done in run ‘‘b’’ we obtain modeled profiles that
are slightly closer to the measurements but with the HCN
amount in the stratosphere still severely underestimated.
This suggests that the HCl photolysis cross sections are
not a good surrogate for HCN. Run ‘‘c’’ shows the
modeled HCN VMR if photolysis is neglected and only
destruction by OH and O(
1
D) is taken into account. The
result is nearly identical to run ‘‘a’’, showing that the loss
of HCN is dominated by the fast destruction by reaction
with OH.
[
14
] New rate coefficients for the reactions of HCN with
OH and O(
1
D) have recently been measured [
Strekowski
,
2001]. The experimental methods used for the rate coef-
ficient measurements are described in
Hynes and Wine
[1996] and
Strekowski et al.
[2000]. Details of the OH +
HCN and O(
1
D) + HCN studies will be published else-
where. Runs ‘‘d’’–‘‘g’’ in Figure 2 (right) show the results
of model calculations applying these new rate coefficients.
Run ‘‘d’’ also includes photolysis based on HCl cross
sections. We see a modeled HCN profile similar to runs
‘‘a’’ and ‘‘b’’, indicating that also in this run the destruc-
tion by photolysis seems unrealistically high. Run ‘‘e’’
neglects photolysis and applies only the destruction mech-
anisms by OH and O(
1
D). This model run gives a more
reasonable representation with the measured HCN profile,
with agreement within the error bars in the lowermost
stratosphere (1.5–1.7 ppm CH
4
) and in the upper strato-
sphere (
0.3 ppm CH
4
). In the middle stratosphere a
slight underestimate of the measured HCN VMRs by up
to
20 ppt remains. The change in the modeled HCN
profile in run ‘‘e’’ compared to run ‘‘c’’ is dominated by
the change in the rate of the reaction with OH because the
rate of the reaction with O(
1
D) differs only by 10–30%
from the estimate by
Cicerone and Zellner
[1983].
[
15
] The dotted lines in Figure 2 show the resulting HCN
profile if the rate of the reaction with OH is increased/
decreased by 25%, a conservative estimate of the accuracy
of the rate coefficient. Considering this uncertainty, we
Figure 2.
(left) Retrieved vertical profiles of HCN vs. CH
4
, measured simultaneously by the MkIV instrument. The black
line is an average of all measurements created by averaging the HCN VMRs in bins of 0.15 ppm CH
4
with the gray shaded
area giving the standard deviation. (middle) MkIV average vs. CH
4
compared to model runs using old HCN reaction rates
from
Sander et al.
[2003] and
Cicerone and Zellner
[1983]. (right) MkIV average vs. CH
4
compared to model runs using
the new HCN chemistry. For a detailed description of the individual model runs see Table 1.
Table 1.
Reaction Rates Used for the Different Model Runs
Reaction
Rate Coefficients
Used in Model Run
Reference
HCN + OH
!
products
a
A
= 1.2
10
13
,
E
a
R
= 400
a, c
Sander et al.
[2003]
HCN + O(
1
D)
!
products
b
k
=1
10
10
a, b, c
Cicerone and Zellner
[1983]
HCN + h
n
!
products
absorption cross sections of HCl
a, b, d
Sander et al.
[2000]
HCN + OH
!
products
c
k
0
= 4.28
10
33
,d,e,f,g
Strekowski
[2001]
k
1
= 4.25
10
13
e
1150
T
F
c
= 0.8
HCN + O(
1
D)
!
products
a,d
A
= 7.7
10
11
,
E
a
R
=
100
d, e, f, g
Strekowski
[2001]
CH
3
CN + OH
!
products
a,e
A
= 7.8
10
13
,
E
a
R
= 1050
f, g
Sander et al.
[2003]
a
Rate coefficient is expressed by
k
=
A
e
Ea
RT
ðÞ
, where A is given in
cm
3
molec
:
s
and
T
is the temperature in K.
b
Rate coefficient (given in
cm
3
molec
:
s
) is assumed constant throughout the atmosphere.
c
Termolecular reaction parameterized as
k
=
k
0
M
½
k
1
k
0
M
½ þ
k
1
F
c
1
þ
lg k
0
M
½
=
k
1
ðÞ
ðÞ
2
ðÞ
1
, with
k
0
in
cm
6
molec
:
2
s
and
k
1
in
cm
3
molec
:
s
,[
M
] is the molecular air
density.
d
Rate coefficient given is for forming products other than HCN + O(
3
P).
e
An HCN yield of 100% and 30% was assumed for the runs f and g, respectively.
L11806
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L11806
3of5
achieve reasonable agreement between the measured HCN
profile and the one modeled in run ‘‘e’’.
[
16
] In early studies on the HCN chemistry a link
between HCN and CH
3
CN had been suggested [
Murad et
al.
, 1984;
Brasseur et al.
, 1985]. More recently
Tyndall et
al.
[2001] studied the products of Cl- and OH-initiated
oxidation of CH
3
CN in the laboratory. They report that Cl-
initiated oxidation of CH
3
CN leads to the formation of
HC(O)CN, the fate of which is to produce HCN with a yield
of 50%. In the OH-initiated oxidation they report an
HC(O)CN yield of (40 ± 20)%, however, no other carbon
bearing products could be unambiguously identified. We
therefore consider the influence of a potential yield of
HCN from the destruction of CH
3
CN by the reaction with
OH (see Table 1), which is the dominant loss reaction of
CH
3
CN in the stratosphere. CH
3
CN has been initialized in
the model with a tropospheric value of 150 ppt. The
CH
3
CN chemistry in the model is the same as in
Kleinbo
̈hl
et al.
[2005], which has shown to give excellent agreement
with recent infrared measurements of stratospheric
CH
3
CN.
[
17
] Run ‘‘f’’ in Figure 2 shows the modeled HCN profile
in the case of 100% production of HCN from the reaction
CH
3
CN + OH. It can be seen that this produces a significant
overestimation of the HCN VMR in the stratosphere.
However, if a yield of 30% of HCN is assumed (run ‘‘g’’
in Figure 2) one obtains a model profile that is closer to the
measurements than the profile of run ‘‘e’’ in which no HCN
production had been assumed.
4. Summary and Implications
[
18
] Measurements of the stratospheric distribution of
HCN by infrared solar occultation spectrometry provide
new constraints on its photochemical lifetime. HCN is
found to be highly correlated with longlived tracers such
as CH
4
and N
2
O, suggesting its stratospheric lifetime is
much longer than calculated using accepted chemistry
[
Sander et al.
, 2003].
[
19
] Using a 2D-model, we have investigated the chem-
istry of HCN and find that recently reported measurements
of the rate coefficient for the main sink, the reaction with
OH, give reasonable agreement between the calculated and
measured profiles. This result is achieved assuming that
the loss of HCN via photolysis is negligible. We estimate
that the photochemical lifetime of HCN is
^
10 years in
the stratosphere below 30 km and
^
5 years in the upper
troposphere - about an order of magnitude longer than
estimated using the currently accepted chemistry (see
auxiliary material). We note that the tropospheric HCN
lifetime against oxidation is considerably larger than
against uptake into the ocean (
6 months, [
Li et al.
,
2003]). Finally, these observations do not rule out a
substantial yield (
30%) of HCN from the reaction of
CH
3
CN with OH.
[
20
] Further improvement in HCN modeling at upper
stratospheric and mesospheric altitudes can be expected
once measurements of absorption cross-sections and
quantum yields for the photolysis of HCN become
available.
[
21
]
Acknowledgments.
We would like to thank the various launch
crews for conducting the balloon flights, and D. Petterson and J. Landeros
of JPL for their excellent support prior and during the measurement
campaigns. Work at AER was supported by the NASA ACMAP program.
Work at Georgia Tech was supported by the NASA UARP program. Work
at Jet Propulsion Laboratory, California Institute of Technology, was
performed under a contract with the National Aeronautics and Space
Administration.
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