of 5
Tropospheric methane retrieved from ground-based near-IR solar
absorption spectra
R. A. Washenfelder and P. O. Wennberg
California Institute of Technology, Pasadena, California, USA
G. C. Toon
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
Received 14 June 2003; revised 2 October 2003; accepted 29 October 2003; published 13 December 2003.
[
1
] High-resolution near-infrared solar absorption spectra
recorded between 1977 and 1995 at the Kitt Peak National
Solar Observatory are analyzed to retrieve column
abundances of methane (CH
4
), hydrogen fluoride (HF),
and oxygen (O
2
). Employing a stratospheric ‘‘slope
equilibrium’’ relationship between CH
4
and HF, the
varying contribution of stratospheric CH
4
to the total
column is inferred. Variations in the CH
4
column due to
changes in surface pressure are determined from the O
2
column abundances. By this technique, CH
4
tropospheric
volume mixing ratios are determined with a precision of

0.5%. These display behavior similar to Mauna Loa in
situ surface measurements, with a seasonal peak-to-peak
amplitude of approximately 30 ppbv and a nearly linear
increase between 1977 and 1983 of 18.0 ± 0.8 ppbv yr

1
,
slowing significantly after 1990.
I
NDEX
T
ERMS
:
0330
Atmospheric Composition and Structure: Geochemical cycles;
0394 Atmospheric Composition and Structure: Instruments and
techniques; 1610 Global Change: Atmosphere (0315, 0325);
0315 Atmospheric Composition and Structure: Biosphere/
atmosphere interactions; 0325 Atmospheric Composition and
Structure: Evolution of the atmosphere.
Citation:
Washenfelder,
R. A., P. O. Wennberg, and G. C. Toon, Tropospheric methane
retrieved from ground-based near-IR solar absorption spectra,
Geophys. Res. Lett.
,
30
(23), 2226, doi:10.1029/2003GL017969,
2003.
1. Introduction
[
2
] Methane (CH
4
) is the most abundant hydrocarbon in
the atmosphere and plays an important role in both radiative
and chemical processes. Between 1984 and 1996, the
globally averaged CH
4
mole fraction increased from

1625 to 1730 parts per billion by volume (ppbv)
[
Dlugokencky et al.
, 1998]. Although the rate of increase
is slowing, the cause of the variability has not been fully
explained. Additional constraints on CH
4
sources and sinks
are necessary to understand current behavior and to predict
future trends.
[
3
] Active in situ monitoring programs are in place,
including those undertaken by the National Oceanic and
Atmospheric Administration Climate Monitoring and
Diagnostics Laboratory (NOAA CMDL) [
Dlugokencky
et al.
, 1998] and the Global Atmospheric Gases Experi-
ment/Advanced Global Atmospheric Gases Experiment
(GAGE/AGAGE) [
Cunnold et al.
, 2002]. Although these
measurements are highly accurate, they have limited
spatial coverage.
[
4
] Space-based column measurements of CH
4
using
scattered sunlight in the near-infrared (near-IR) have been
proposed (e.g., SCIAMACHY and MOPITT) as a means
of providing better spatial coverage. The near-IR region is
a good candidate for space-based remote sensing because
(i) it is near the peak of the solar Planck function; (ii) the
column averaging kernels peak at the surface, facilitating
identification of CH
4
sources and sinks; (iii) thermal emis-
sion from the atmosphere and instrument are negligible
compared with reflected sunlight, simplifying calibration
and radiative transfer calculations. Unfortunately, CH
4
spectroscopy is poorly characterized in this region and
suffers from both missing weak lines and incomplete
quantum assignments [
Brown
,1992].Inthisletter,we
examine solar absorption spectra from the Kitt Peak
National Solar Observatory to determine the suitability of
the 2
n
3
band centered at 6001 cm

1
for remote sensing of
tropospheric CH
4
.
[
5
] This analysis provides a long-term CH
4
column time
series beginning in 1977. Sampling of CH
4
by the NOAA
CMDL network did not begin until 1983 and GAGE/
AGAGE network measurements were not initiated until
1985. Flask samples [e.g.,
Blake and Rowland
, 1986;
Khalil and Rasmussen
, 1983] extend the record back to
the mid-1970s. The scarcity of frequent, high precision
measurements between 1977 and 1983 makes the Kitt Peak
dataset especially valuable.
2. Determination of Tropospheric CH
4
[
6
] Column CH
4
exhibits variability driven by (i) changes
in surface pressure, (ii) changes in the tropospheric CH
4
volume mixing ratio (VMR), and (iii) changes in the amount
of stratospheric CH
4
due to changes in tropopause altitude.
The CH
4
mole fraction decreases significantly in the strato-
sphere due to oxidation by O(
1
D), OH, and Cl. A 30 ppbv
change in tropospheric CH
4
or a 30 hPa change in tropo-
pause altitude will each produce

1.5% variation in the sea
level CH
4
column. Thus, to accurately determine the tropo-
spheric CH
4
VMR, it is necessary to correct for variations in
both surface pressure and stratospheric contribution.
[
7
] Analysis of the pressure-broadened lineshape is com-
monly used to gain altitude information for gases retrieved
in the mid-IR. However, this method is not optimal for the
near-IR Kitt Peak spectra. For a typical CH
4
line at
GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 23, 2226, doi:10.1029/2003GL017969, 2003
C
o
p
y
r
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e
o
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y
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U
n
i
o
n
.
0
0
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0
1
7
9
6
9
ASC 13
-
-
1
6000 cm

1
, the Doppler width exceeds the air-broadened
width at

14 km, preventing retrieval of stratospheric
profile information. In addition, profile retrievals are limited
by the spectral resolution of the available Kitt Peak mea-
surements (typically
dn
= 0.02 cm

1
), knowledge of the
instrument lineshape, and lack of accurate air-broadening
parameters for the CH
4
2
n
3
band.
[
8
] In this analysis, we instead use simultaneous column
measurements of HF to more accurately quantify strato-
spheric CH
4
variations. Previous studies have demonstrated
that HF and CH
4
are inversely correlated in the stratosphere
[
Luo et al.
, 1995]. As stratospheric air ages and CH
4
is oxidized, the photolysis of chlorofluorocarbons (CFC)
initiates a chain of reactions, eventually yielding F atoms
which react with H
2
O and CH
4
to form HF in the stratosphere.
[
9
] Provided the relationship between the CH
4
and HF
VMRs is sufficiently linear, the CH
4
-HF slope in the
stratosphere can be applied directly to correct the CH
4
total
column for stratospheric variations. Mathematically, this
argument is shown by:
CH
4
VMR
¼
a
þ
b

HF
VMR
;
b
¼
@
CH
4
VMR
ðÞ
@
HF
VMR
ðÞ
:
ð
1
Þ
The integrated column is:
X
column
¼
Z
Ps
0
X
VMR
dp
ð
2
Þ
with pressure (p) in units of molecules cm

2
and P
s
equal to
surface pressure. Therefore, by substitution:
CH
4
column
¼
Z
Ps
0
a
þ
b

HF
VMR
ðÞ
dp
¼
a

P
s
þ
b

HF
column
:
ð
3
Þ
Since HF
VMR
= 0 in the troposphere, parameter
a
in (1) is
equal to the tropospheric CH
4
VMR, assuming that CH
4
is
well-mixed in the troposphere. Because the atmospheric O
2
VMR (0.2095) is highly constant, the relationship O
2 column
=
0.2095

P
s
can be used to eliminate P
s
from (3) yielding:
CH
4
trop VMR
¼
a
¼
0
:
2095

CH
4
column

b

HF
column
ðÞ
O
2
column
:
ð
4
Þ
Equation (4) includes both the surface pressure correction
using O
2column
and the stratospheric correction using
HF
column
. Dividing by O
2column
also removes possible
systematic errors in the spectra, temperature profile, or
calculated airmass that are common to CH
4
, HF, and O
2
.
This equation implicitly assumes that CH
4
and HF have
similar averaging kernels in the stratosphere (see S3)
1
.In
this letter, we determine tropospheric CH
4
VMRs by
simultaneously retrieving CH
4
, HF, and O
2
columns from
the Kitt Peak solar spectra and by employing two additional
datasets to determine the stratospheric CH
4
-HF relationship,
b, and its time dependence.
3. The Kitt Peak Spectra
[
10
] The spectra analyzed in this work have been described
previously [
Yang et al.
, 2002]. The dataset includes more than
400 high-resolution near-IR solar absorption spectra (
dn
=

0.02 cm

1
) obtained with the 1-m Fourier transform spec-
trometer (FTS) at the McMath telescope complex of the Kitt
Peak National Solar Observatory (31.9 N, 111.6 W, 2.09 km
above sea level) between 1977 and 1995. Each of these
spectra include the 4000–8000 cm

1
region necessary for
the simultaneous retrieval of CH
4
,O
2
, and HF. Many of these
observations were used by
Wallace and Livingston
[1990] to
determine the column-averaged dry air VMR of CH
4
and
CO
2
. Their work used equivalent widths to analyze 12
manifolds of the 2
n
3
CH
4
(
n
0
= 6001 cm

1
) band and 14
lines of the O
2
0-0
1

g

3

g

(
n
0
= 7882 cm

1
) band.
[
11
] Here, we reanalyze the Kitt Peak solar spectra using
an improved spectral retrieval algorithm with updated
spectroscopic linelists for CH
4
,O
2
,H
2
O, and solar absorp-
tion lines. We simultaneously fit the entire O
2
band
(containing more than 200 significant lines), the CH
4
2
n
3
P-branch (containing ten significant manifolds), and the
strong HF R(1) (1-0) line at 4038.96 cm

1
.
4. Spectral Analysis and Retrievals
[
12
] The line-by-line fitting algorithm used in this work
(GFIT) was developed at the Jet Propulsion Laboratory
(JPL) for the analysis of solar absorption spectra. The use of
the GFIT algorithm, temperature profiles for Kitt Peak, O
2
spectral parameters, and solar linelist have been described
previously [
Yang et al.
, 2002]. The atmospheric CH
4
and
HF a priori VMR profiles are based on JPL MkIV measure-
ments recorded during balloon flights from Ft. Sumner,
New Mexico, and Daggett, California (both at

34

N).
[
13
] Spectral parameters for the HF R(1) (1-0) line are
taken from the HITRAN database [
Rothman et al.
, 1998].
Line position, intensity, and ground-state energy parameters
for the CH
4
2
n
3
manifolds [
Margolis
,1988;
Margolis
,
1990] are taken from HITRAN, but different air-broadened
widths are employed. Since measurements of 2
n
3
linewidths
have never been reported, in HITRAN these lines are
assigned air-broadened widths based on measurements of
the
n
2
and
n
4
CH
4
bands [
Brown
, 1992]. When these
parameters were used to fit laboratory and atmospheric
spectra, however, the residuals showed that the assigned
widths are systematically large. Additionally, CH
4
columns
initially retrieved from the Kitt Peak spectra had unreason-
ably large, symmetric daily variations of about 6% that
peaked at noon. In our retrieval of CH
4
, we have substituted
broadening parameters from the
n
3
band as these are
expected to be more closely related to the 2
n
3
band than
the values assigned in HITRAN. (See S1 for a detailed
discussion and linelist
1
.) To further minimize the airmass
dependence of the analysis, we fit only the P-branch (5880–
5996 cm

1
). These manifolds are weaker than the R- and
Q-branch manifolds, making this region less susceptible to
errors in linewidth. The retrieved tropospheric CH
4
VMR is
observed to vary by about 1% between 1 and 10 airmasses.
1
Auxiliary material is available at ftp://ftp.agu.org/apend/gl/
2003GL017969.
ASC 13
-
2
WASHENFELDER ET AL.: CH
4
MEASUREMENTS
[
14
] Fits to the 2
n
3
P-branch include a full range of
ground-state energies, so retrieved column CH
4
is only
weakly dependent on the assumed temperature profile
(0.01% K

1
). Considering O
2
and HF temperature sensitiv-
ities as well (0.02% K

1
and 0.26% K

1
respectively), a
systematic error of 5 K at all levels within a temperature
profile would change the retrieved tropospheric CH
4
VMR
in (4) by

2 ppbv (

0.1%).
[
15
] A spectral fitting example for the CH
4
2
n
3
P-branch
is shown in Figure 1. A fit to the O
2
0–0
1

g

3

g

band
(
n
0
= 7882 cm

1
) has been shown previously in
Yang et al.
[2002] and a fit to the HF R(1) (1-0) line is included in S2
1
.
The residuals (model-observed) are also illustrated in
Figure 1. We have excluded from further analysis 32 obser-
vations (7.7%) that produce column errors greater than
3.0% for O
2
or CH
4
, as estimated from the spectral
residuals. An additional 94 observations (22.7%) that pro-
duce column errors greater than 9.0% for HF have also been
eliminated. The remaining 288 spectra typically have
column errors of 1.4–2.8% for O
2
, 1.7–2.7% for CH
4
,
and 3.5–6.7% for HF. The required HF precision is modest,
as it is used as a linear correction with relatively small
sensitivity. A 30 hPa change in tropopause pressure results
in a

1.5% change in the CH
4
column, while the HF
column changes by

15%.
[
16
] The retrieved slant column amounts were divided by
the calculated airmasses to determine vertical column
amounts. The airmass calculation includes the 226 m optical
path inside the telescope [
L. R. Brown
, private communica-
tion, 2002] and the effects of refraction. The column-aver-
aged CH
4
VMRs (0.2095

CH
4column
/O
2column
) without the
HF correction described in (4) are shown in Figure 2a.
[
17
] Monthly average CH
4
flask data from NOAA
CMDL’s Mauna Loa site are also shown in Figure 2a. The
dotted trend line through the May 1983 to 1995 data is the
twelve-month running average. The average seasonal cycle
is determined by subtracting the running average from the
data and binning the results by month. No evidence has
been found for changes in the amplitude of the CH
4
seasonal
cycle [
Dlugokencky et al.
, 1997], so this approximation is
reasonable. The trend prior to May 1983, for which no
Mauna Loa flask data are available, is a linear extrapolation
with a slope that is consistent with the reported global
tropospheric CH
4
increase of 18 ± 2 ppbv yr

1
during
1978 to 1983 [
Blake and Rowland
, 1986]. The average
Mauna Loa seasonal cycle was then applied to the linear
growth rate for 1977 to May 1983. The extrapolation of the
seasonal cycle is used here only as a visual guide.
[
18
] The retrieved Kitt Peak column-average CH
4
VMRs
are systematically lower (4%) than the Mauna Loa flask
samples. Some of this difference is expected as the CH
4
VMR is lower in the stratosphere, but geographical differ-
ences and uncertainty in the absolute CH
4
line strengths and
widths preclude meaningful comparison of the column-
average CH
4
VMRs.
5. Tropospheric CH
4
Volume Mixing Ratios
[
19
] The column-average CH
4
VMRs in Figure 2a
include significant variability driven by changes in tropo-
pause altitude. This is illustrated by the anti-correlation of
CH
4
in Figure 2a with retrieved HF columns in Figure 2b
(see S3). In many years the HF column varies by as much as
a factor of two between summer and winter. The seasonal
variation in HF is superimposed on its increasing burden due
to increasing CFC VMRs. Figure 2b shows the fluorine-
Figure 1.
Example of a spectral fit to the 5880–5996 cm

1
region, containing the CH
4
2
n
3
P-branch, of a Kitt Peak
spectrum measured at 70.94

SZA on 9 May 1981.
Diamonds are the measurements and black lines are the
fitted transmittance. Contributions from individual gases are
shown in color.
Figure 2.
(a) Time series of Kitt Peak column-averaged
CH
4
, determined from 0.2095

CH
4column
/O
2column
.
Mauna Loa flask samples (monthly average) are shown
with their twelve-month running average. No Mauna Loa
flask data exists prior to May 1983. A linear extrapolation
with slope 18.0 ppbv yr

1
is shown for this period, with the
average Mauna Loa seasonal cycle applied. (b) Time series
of Kitt Peak column HF. The fluorine-weighted CFC sum
(CFC-11 + 2

CFC-12 + 3

CFC-113 + 2

HCFC-22)
has been lagged by six years to account for atmospheric
transport into the stratosphere.
WASHENFELDER ET AL.: CH
4
MEASUREMENTS
ASC 13
-
3
weighted CFC trend (CFC-11 + 2

CFC-12 + 3

CFC-
113 + 2

HCFC-22) reconstructed from measurements by
the ALE/GAGE/AGAGE network at Cape Grim, Tasmania.
The fluorine-weighted CFC sum is lagged by six years to
account for atmospheric transport within the stratosphere.
Further details are given in S3
1
.
[
20
] To retrieve the tropospheric CH
4
VMR using (4), the
correlation of CH
4
with HF, b, is needed. This slope is
determined here from two datasets. The Halogen Occulta-
tion Experiment (HALOE) instrument on the Upper Atmo-
sphere Research Satellite (UARS) has been measuring CH
4
and HF simultaneously in solar occultations since 1991
using the gas filter correlation technique [
Russell et al.
,
1993]. The CH
4
-HF plots for that data are characterized by
tightly fitted curves for different latitude bands [
Luo et al.
,
1995]. Using sunrise and sunset data measured at tangent
latitudesof20–40

N, we have fitted a linear CH
4
-HF
relationship. The second CH
4
-HF dataset was recorded by
the JPL MkIV Interferometer during 8 balloon flights at
tangent latitudes between 32–38

N during 1990 to 1996.
The MkIV is an FTS that uses the solar occultation
technique to record mid-IR spectra [
Toon
, 1991].
[
21
] Figure 3 shows the slope of the CH
4
-HF correlation,
b, obtained from the HALOE and MkIV data for pressure
levels between 10 and 100 hPa. The value calculated from
the retrieved Kitt Peak CH
4
and HF columns is also shown.
The CH
4
-HF slope has increased significantly between
1977 and 1995, due to increasing HF VMRs. The slope
has been extrapolated back to 1977 using the time-lagged
VMR of fluorine-weighted CFCs, shown in Figure 2b.
Although this extrapolation is not ideal, it is necessary
due to the lack of simultaneous CH
4
and HF profile
measurements available from the 1970s and 1980s. Further
explanation of the HALOE, MkIV, and Kitt Peak b values,
as well as extrapolation of b using CFC data is given in S3
1
.
[
22
] Tropospheric CH
4
VMRs calculated from (4) are
shown in Figure 4a and provided in tabular form in S3.
The Mauna Loa data in Figure 4a are identical to those in
Figure 2a. The Kitt Peak tropospheric CH
4
VMRs are
scaled by 1.015 to bring their values into agreement with
the Mauna Loa data. This scaling was empirically deter-
mined to minimize bias between the Kitt Peak and Mauna
Loa data. The linear slope of the 1977 to 1983 Kitt Peak
tropospheric VMRs is 18.0 ± 0.8 ppbv yr

1
, consistent with
the value of 18 ± 2 reported by
Blake and Rowland
[1986]
for this period.
[
23
] The average seasonal cycle (and 2
s
variability) for
the 1983 to 1995 Mauna Loa data is shown in Figure 4b.
The Kitt Peak data were detrended by subtracting the
linear trend and twelve-month running average shown
in Figure 4a. Figure 4b shows that approximately ±2%
seasonal variation is evident in the Kitt Peak data, consistent
with the Mauna Loa data.
6. Conclusions
[
24
] Reanalysis of the high-resolution near-IR spectra
obtained at the Kitt Peak National Solar Observatory
demonstrates that tropospheric CH
4
VMRs can be retrieved
with 0.5% precision. However, our results are limited by
current CH
4
spectroscopy (linewidths, intensities, and miss-
ing weak lines) and by our ability to accurately separate
tropospheric and stratospheric variability using (4). The
largest errors in this analysis include noise in our HF
retrievals, the assumption of a linear CH
4
-HF relationship
in the stratosphere, and the difficulty of extrapolating this
relationship into the past. Despite these challenges, reanal-
ysis of the Kitt Peak spectra allows the tropospheric CH
4
record to be extended back to 1977. These results show that
high precision measurements of column CH
4
are possible
using ground-based FTS, and that this technique can be
used to validate future space-based observations. However,
to determine CH
4
sources and sinks from column measure-
Figure 3.
CH
4
-HF slope values, b, obtained from the
HALOE, MkIV, and Kitt Peak data. Slope values have been
extrapolated back to 1977 using the CFC sum shown in
Figure 2b.
Figure 4.
(a) Time series of Kitt Peak tropospheric CH
4
VMR, determined from (4). The data have been multiplied
by 1.015 (see text) to bring them into agreement with the
Mauna Loa data. (b) Detrended Kitt Peak tropospheric CH
4
VMR shown together with the average Mauna Loa seasonal
cycle (2
s
variability). Kitt Peak data prior to May 1983 are
represented by crosses and later data are represented by
triangles.
ASC 13
-
4
WASHENFELDER ET AL.: CH
4
MEASUREMENTS
ments, it will be necessary to separate the tropospheric and
stratospheric column contributions. This letter uses HF as a
stratospheric tracer to achieve this separation. In the future,
higher resolution spectra and precise laboratory measure-
ments of air-broadened widths may allow direct retrieval of
tropospheric CH
4
VMRs.
[
25
]
Acknowledgments.
We thank the Kitt Peak personnel who
acquired these spectra. We thank Linda Brown, Ming Luo, James Randerson,
and Zhonghua Yang for helpful discussions. We acknowledge the
NOAA CMDL and GAGE/AGAGE networks for the use of their data.
R.A.W. acknowledges support from NSF and the California Institute of
Technology. P.O.W. and G.C.T. acknowledge support from NASA.
Research at JPL, California Institute of Technology, is performed under
contract with NASA.
References
Blake, D. R., and F. S. Rowland, World-wide increase in tropospheric
methane, 1978–1983,
J. Atmos. Chem.
,
4
, 43–62, 1986.
Brown, L. R., Methane and its isotopes: Current status and prospects for
improvement,
J. Quant. Spectrosc. Ra.
,
48
, 617–628, 1992.
Cunnold, D. M., L. P. Steele, and P. J. Fraser, et al., In situ measurements of
atmospheric methane at GAGE/AGAGE sites during 1985–2000 and
resulting source inferences,
J. Geophys. Res.
,
107
(D14), doi:10.1029/
2001JD001226, 2002.
Dlugokencky, E. J., K. A. Masarie, P. M. Lang, and P. P. Tans, Continuing
decline in the growth rate of the atmospheric methane burden,
Nature
,
393
, 447–450, 1998.
Dlugokencky, E. J., K. A. Masarie, and P. P. Tans, et al., Is the amplitude of
the methane seasonal cycle changing?,
Atmos. Environ.
,
31
, 21–26, 1997.
Khalil, M. A. K., and R. A. Rasmussen, Sources, sinks, and seasonal cycles
of atmospheric methane,
J. Geophys. Res.
,
88
, 5131–5144, 1983.
Luo, M., R. J. Cicerone, and J. M. Russell III, Analysis of Halogen
Occultation Experiment HF versus CH
4
correlation plots: Chemistry and
transport implications,
J. Geophys. Res.
,
100
(D7), 13,927–13,937, 1995.
Margolis, J. S., Measured line positions and strengths of methane between
5500 and 6180 cm-1,
Appl. Opt.
,
27
, 4038–4051, 1988.
Margolis, J. S., Empirical values of the ground state energies for methane
transitions between 5500–6150 cm-1,
Appl. Opt.
,
29
, 2295–2302, 1990.
Rothman, L. S., et al., The HITRAN molecular spectroscopic database and
HAWKS (HITRAN atmospheric workstation): 1996 edition,
J. Quant.
Spectrosc. Ra.
,
60
, 665–710, 1998.
Russell, J. M., L. L. Gordley, and J. H. Park, et al., The Halogen Occulta-
tion Experiment,
J. Geophys. Res.
,
98
(D6), 10,777–10,797, 1993.
Toon, G. C., The Jet Propulsion Laboratory MkIV Interferometer,
Opt.
Photonics News
,
2
, 19–21, 1991.
Wallace, L., and W. Livingston, Spectroscopic observations of atmospheric
trace gases over Kitt Peak: 1. Carbon dioxide and methane from 1979 to
1985,
J. Geophys. Res.
,
95
(D7), 9823–9827, 1990.
Yang, Z., G. C. Toon, J. S. Margolis, and P. O. Wennberg, Atmospheric
CO
2
retrieved from ground-based near IR solar spectra,
Geophys. Res.
Lett.
,
29
(9), 1339, 10.1029/2001GL014537, 2002.























G. C. Toon, Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, CA, USA.
R. A. Washenfelder and P. O. Wennberg, Division of Engineering and
Applied Sciences, California Institute of Technology, Pasadena, CA 91125,
USA. (rebeccaw@its.caltech.edu)
WASHENFELDER ET AL.: CH
4
MEASUREMENTS
ASC 13
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5