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Stable Isotopic Composition of Atmospheric Methane (13C) from the NOAA
GML Carbon Cycle Cooperative Global Air Sampling Network, 1998-2021.

Version: 2022-12-15
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CONTENTS

1. Data source and contacts
2. Use of data
2.1 Citation
3. Reciprocity
4. Warnings
5. Update notes
6. Introduction
7. DATA - General Comments
7.1 DATA - Sampling Locations
7.2 DATA - File Name Description
7.3 DATA - File Types
7.4 DATA - Content
7.5 DATA - QC Flags
7.6 DATA - Collection Methods
7.7 DATA - Monthly Averages
8. Data retrieval
9. References

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1. DATA SOURCE AND CONTACTS

University of Colorado
Institute of Arctic and Alpine Research (INSTAAR)

Correspondence concerning these data should be directed to:

Sylvia Englund Michel
Institute of Arctic and Alpine Research
Campus Box 450
University of Colorado, Boulder, CO 80309-0450 USA

telephone: (303) 735-5850
facsimile: (303) 492-6388
email: sylvia.michel@colorado.edu

Reid Clark
Institute of Arctic and Alpine Research
Campus Box 450
University of Colorado, Boulder, CO 80309-0450 USA

telephone: (303)492-5495
facsimile: (303) 492-6388
email: Reid.Clark@Colorado.edu

Bruce Vaughn
Institute of Arctic and Alpine Research
Campus Box 450
University of Colorado, Boulder, CO 80309-0450 USA

telephone: (303) 492-7985
facsimile: (303) 492-6388
email: Bruce.Vaughn@colorado.edu

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2. USE OF DATA

These data are made freely available to the public and the
scientific community in the belief that their wide dissemination
will lead to greater understanding and new scientific insights.
The availability of these data does not constitute publication
of the data. NOAA relies on the ethics and integrity of the user to
ensure that GML receives fair credit for their work. If the data
are obtained for potential use in a publication or presentation,
GML should be informed at the outset of the nature of this work.
If the GML data are essential to the work, or if an important
result or conclusion depends on the GML data, co-authorship
may be appropriate. This should be discussed at an early stage in
the work. Manuscripts using the GML data should be sent to GML
for review before they are submitted for publication so we can
ensure that the quality and limitations of the data are accurately
represented.

2.1 CITATION

Please reference these data as

Michel, S.E., Clark, J.R., Vaughn, B.H., Crotwell, M., Madronich, M.,
Moglia, E., Neff, D., Mund, J. (2022), University of Colorado, Institute of
Arctic and Alpine Research (INSTAAR). Stable Isotopic Composition of
Atmospheric Methane (13C) from the NOAA GML Carbon Cycle Cooperative Global
Air Sampling Network, 1998-2021. Version: 2022-12-15
https://doi.org/10.15138/9p89-1x02

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3. RECIPROCITY

Use of these data implies an agreement to reciprocate.
Laboratories making similar measurements agree to make their
own data available to the general public and to the scientific
community in an equally complete and easily accessible form.
Modelers are encouraged to make available to the community,
upon request, their own tools used in the interpretation
of the GML data, namely well documented model code, transport
fields, modeled mole fractions, and additional information
necessary for other scientists to repeat the work and to run
modified versions. Model availability includes collaborative
support for new users of the models.

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4. WARNINGS

Every effort is made to produce the most accurate and precise
measurements possible. However, we reserve the right to make
corrections to the data based on recalibration of standard gases
or for other reasons deemed scientifically justified.

We are not responsible for results and conclusions based on use
of these data without regard to this warning.

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5. UPDATE NOTES
+++++++++++++++++++++++++++++++
Lab-wide notes:

2011-10-07

We introduced the term "measurement group", which identifies
the group within NOAA or Institute of Arctic and Alpine Research (INSTAAR)
University of Colorado Boulder that made the measurement. We can
now have multiple groups measuring some of the same trace gas species
in our discrete samples.

Measurement groups within NOAA and INSTAAR are

ccgg: NOAA Carbon Cycle Greenhouse Gases group (CCGG)
hats: NOAA Halocarbons and other Atmospheric Trace Species group (HATS)
arl: INSTAAR Atmospheric Research Laboratory (ARL)
sil: INSTAAR Stable Isotope Laboratory (SIL)
curl: INSTAAR Laboratory for Radiocarbon Preparation and Research (CURL)

+++++++++++++++++++++++++++++++

Project-specific notes:

2022-07-21

Dataset is now provided in self describing ObsPack format with improved metadata.
Surface flask event data are available in NetCDF and ASCII text. Surface flask
monthly data are available in ASCII text. Shipboard data binned by 5 or 3 degrees
are now removed from surface flask event data, but still provided in monthly data.
This format change makes some previous notes irrelevant.

2022-07-20

In 2021, impacts of the COVID-19 pandemic on flask shipping and sampling continues
at a few sites. Network sampling coverage remained reasonably good throughout 2021.

2021-07-29

In 2020, the COVID-19 pandemic impacted flask shipping and sampling
frequency at many sites causing isolated gaps and some delayed
processing into the 2nd half of 2021. In spite of this, network
sampling coverage remained reasonably good throughout 2020.

2020-10-22

Sample lat/lon were revised for PSA. All prior entries
were set to the correct lat/lon.

2020-07-16

Sample elevation was revised for ALT. All prior entries
were set to the correct elevation.

2019-09-09

Sample elevation was corrected for AMY. All prior entries
were set to the correct elevation.

2017-07-27

Method codes and sample locations were edited for accuracy.
Edited sites are: ZEP, BKT, OXK, CGO, RPB, ASC, CHR, ICE,
KEY, KUM, and TAP

2016-08-26

Latitude and longitude were adjusted for 3 sites:
ALT
Old: 82.4508 -62.5056 205.00 200.00
New: 82.4508 -62.5072 195.00 190.00

TAC
Old: 52.5178 1.1389 236.00 56.00
New: 52.5177 1.1386 236.00 56.00

ZEP
Old: 78.9067 11.8889 479.00 474.00
New: 78.9067 11.8883 479.00 474.00

Collaborator name corrected for UTA:
Old: U.S. National Weather Service [NWS]
New: Beth Anderson/NWS Cooperative Observer

2016-07-07

Incorrect sample dates from Ulaan Uul, Mongolia (UUM) from
20 Aug. 2013 through 30 Sept. 2015 were corrected on
13 May 2016.

2016-07-07

Since 24 Jan 2015, air samples from Negev Desert, Israel (WIS)
are collected at 29.9731N, 35.0567E, 156 masl; the old location
was 30.8595N, 34.7809E, 482 masl.

2016-07-07

Since 03 Dec 2015, sampling in Natal, Brazil (NAT) was moved
from 5.5147S, 35.2603W, 20 masl to 5.7952S, 35.1853W, 87 masl.

2015-11-20

Updated the content and format of event files to include elevation in
meters above sea level (masl) and sample collection intake height in
meters above ground level (magl). Elevation plus collection intake
height equals altitude, which has always been included in the NOAA
distribution. In adding these 2 fields, the event number column
has moved. The new format is described in Sections 7.3 and 7.4.

Users may find minor changes (from earlier distributions) to reported
monthly mean values for the beginning months of data records. These
changes are due to minor corrections to our curve fitting methods as
first described by Thoning et al. (JGR,1989).

2015-08-03

The 3-letter site identification code for Ushuaia, Argentina (TDF) was
changed to USH to be consistent with the WMO GAWSIS.

2013-08-27

Coordinates of some of the sample locations have changed.
These changes improve the specified location based on new
information. Changes tend to be minor and do not necessarily
reflect a change in the actual sampling location.

2011-10-01

The data file format has been modified to include the measurement group
and, additionally, the sample collection and analysis times
now include second information (e.g., 2011 03 15 23 06 12). See
section 7.3 for details.

2010-10-01

The format of the NOAA ESRL data records has been changed to include
an estimate of the uncertainty associated with each measurement. The
determination of the estimate is trace gas specific and described in
section 6 (INTRODUCTION).

+++++++++++++++++++++++++++++++
Parameter-specific notes:

*************************************
January 2013 - Second position flags moved to first and third positions:

Due to confusion resulting from our second position flags, they were
moved to either first or third position. All flags are applied automatically
by code except for '!' hand flags. Please see flagging section for more
information.

May 2014 - Additional first position flags added

To increase the quality of the data, we decided to flag data for which
there was a small sample peak (less than 3 nA) with a first position 'P'
flag. We also decided to flag data for which there was significant (greater
than 0.25 permil) drift in the reference gas with a 'D' flag.

October 2021 - Amount effect correction added for data from 2018-2020.

Third-position 'r' and 'R' flags were introduced to account an amount
effect, where isotope delta values were influenced by differences in peak
size between the sample and standard peak. Third-position 'r' flags were
assigned to all data for which a correction was applied - this includes all
data measured 2018-2020. Third-position 'R' flags were assigned to samples
with a pronounced amount effect, where the difference between the sample and
standard peak size exceeded 2.5 nA. Both 'r' and 'R' flagged data have
expanded uncertainty due to the uncertainty of the data correction
calculation, in addition to the standard analytical uncertainty.

Nov 2022 - Adjustments to flagging

We reduced the peak size that is flagged from 3 nA to 2 nA. However, we
added a flag starting in 2021 when the peak size is more than 1 nA different
than the standard peak size. This addresses the 'amount effect' that was
problematic previously. We also increased the allowable drift from 0.25
permil to 0.3 permil between standards.

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6. INTRODUCTION

The data files in this archive list measurements of the stable isotopic
composition (13C) of atmospheric methane. The isotopic analysis was
performed at the Stable Isotope Laboratory, CU-INSTAAR, using samples of air
provided by the NOAA Global Monitoring Laboratory Global Greenhouse Gas
Reference Network.

This release contains data for flask samples beginning in 1998 and
extending through the end of 2021. Data are given for all sites where at
least 18 months of data are available.

The subdirectories txt and nc contain all measurements from flask samples
collected at each site. The txt directory also contains monthly values given
only in those months where there are data, and no interpolation is made for
missing months. All NOAA sites for which CH4 isotope data has been collected
up to 2021 are included. Data from recently initiated sites will be included
in future releases. See Trolier et al., 1996 or Conway et al., 1994 for
details of the sampling network, equipment and procedures.

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7. DATA - GENERAL COMMENTS

Isotope data are reported as isotope delta values: the ratio of minor to
major isotopes relative to a standard. The 'delta' notation is

d13C(CH4) = [ (13C/12C)sam / (13C/12C)std - 1 ]

and is customarily expressed as a multiple of 1000 in units of 'permil'.
The scale for carbon isotopes is the VPDB scale and is realized through
calcite reference materials. Our local realization of the VPDB scale is
through to Dr. Stan Tyler's lab at the University of California Irvine. He
measured d13C-CH4 of methane in air relative to CO2 produced by reaction of
NBS-19.

All of the samples have been analyzed at the Stable Isotope Laboratory at
CU-INSTAAR in Boulder, Colorado, using either a Micromass Optima or Elementar
Isoprime isotope-ratio mass spectrometer coupled to a methane a custom-built
trapping system, a gas chromatograph, and a combustion furnace. Measurement
precision is approximately 0.06 permil for 13C (where 'precision' is taken as
the long term reproducibility of a surveillance cylinder of ambient air.
Current efforts to validate this scale and compare with other labs in the
isotopic community are ongoing. For more information regarding calibrations
of methane-in-air isotopes, see the recent review by Umezawa et al., 2017.

For more information on our methodology, please refer to the following
publications:

Vaughn, B.H., J. Miller, D. F. Ferretti, J. W. C. White 2004: Stable
isotope measurements of atmospheric CO2 and CH4. Handbook of Stable Isotope
Analytical Techniques, vol 1, ch.14, Elsiever, 1248 p.

Miller, J. B., K. A. Mack, R. Dissly, J. W. C. White, E. J. Dlugokencky,
and P. P. Tans 2002: Development of analytical methods and measurements of
13C/12C in atmospheric CH4 from the NOAA Climate Monitoring and Diagnostics
Laboratory Global Air Sampling Network. J Geophys. Res., v107, NO. D13.

In 2013 we added a metric of uncertainty for isotopes of methane. This is
not a "true" calculation of uncertainty in a metrological sense - instead it
is a metric of short- to medium-term reproducibility of a surveillance
cylinder. This air cylinder is well characterized by long-term measurement,
and in addition to tracking our ties to VPDB over the long term, it alerts us
to any problems with the mass spectrometer or extraction system. We use the
standard deviation of ten runs' worth of surveillance cylinder measurements
as the uncertainty metric. Usually four samples are run a day, of which all
but the first are used (due to known irregularities caused by the tank
regulator). For each run, the uncertainty is calculated from data from that
run, regardless of its flagging, and the previous nine runs of unflagged
data. (For example, if the current run is flagged, that surveillance cylinder
data will be used in the uncertainty calculation; however, the next day's run
will not use the flagged data in its uncertainty calculation.) As of Dec
2013, uncertainty is calculated with each run, and has been back-calculated
to 2006.

In May 2017, a post combustion column was added to remove doubly-charged
Kr ions which interferes with measurement of the different masses of CH4-
derived CO2. Extensive tests of cylinders with different CH4/Kr ratios before
and after implementation of the PC column show that there is very little
effect of Krypton on samples with methane mole fraction over 1400 ppb.

A data correction has been applied for all 2018-2020 data to reduce a
positive bias introduced by our sample preparation system. During this time
there was a clog in a valve that caused reduced flow through our methane
trapping system, causing smaller sample peaks in our analyzer, especially in
low-pressure flasks. The difference in peak heights between samples and
standards exacerbated an "amount effect" problem, where the peak height
influenced the measured isotope delta value. With ongoing extensive testing
that began in 2019, we discovered that our amount effect was variable, with
an average of -0.019 permil/nA and a standard deviation of 0.047 permil/nA.
To correct for it, we applied a Monte Carlo approach: we applied 100
randomly-chosen slopes from the list of measured corrections and
calculated the mean of the corrected values. The standard deviation of the
100 iterations were added in quadrature to the analytical uncertainty to
account for the error in the correction. Data for which the amount effect
correction has been applied has a third position 'r'. A third position 'R'
flag was added where the sample peak height differs from the standard peak
height by more than 2.5 nA, where the correction was significant. This
correction is imperfect, but does reduce a positive bias in the data
suggested by intercomparisons with other research groups. If further evidence
suggests a better way to handle this effect, we may revisit the correction
for this time period. Evidence suggests that the amount effect comes from the
combustion process, though investigations continue. In late-2020, the
obstruction in the system was resolved and we now reliably control for peak
size across sample types; therefore the amount effect is now within
measurement noise.

Pacific Ocean Cruise (POC, travelling between the US west coast
and New Zealand or Australia) flask-air samples were collected in
about 5 degree latitude intervals. For South China Sea (SCS), samples
were collected at about 3 degree latitude intervals.

Sampling intervals are approximately weekly for fixed sites
and average one sample every 3 weeks per latitude zone for POC and
about one sample every week per latitude for SCS.

Historically, samples have been collected using two general methods:
flushing and then pressurizing glass flasks with a pump, or opening a
stopcock on an evacuated glass flask; since 28 April 2003, only the
former method is used. During each sampling event, a pair of flasks
is filled.

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7.1 DATA - SAMPLING LOCATIONS

For a summary of sampling locations, please visit

https://gml.noaa.gov/dv/site/?program=ccgg.

Note: Data for all species may not be available for all sites listed
in the table.

To view near real-time data, manipulate and compare data, and create
custom graphs, please visit

https://gml.noaa.gov/dv/iadv/.

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7.2 DATA - FILE NAME DESCRIPTION

Encoded into each file name are the parameter (trace gas identifier); sampling
site; sampling project; laboratory ID number; measurement group (optional); and optional
qualifiers that further define the file contents.

All file names use the following naming scheme:

1 2 3 4 5
[parameter]_[site]_[project]_[lab ID number]_[optional measurement group]_[optional

6 7
qualifiers].[file type]

1. [parameter]

Identifies the measured parameter or trace gas species.

(ex)
co2 Carbon dioxide
ch4 Methane
co2c13 d13C (co2)
merge more than one parameter

2. [site]

Identifies the sampling site code.

(ex)
brw
pocn30
car
amt

3. [project]

Identifies sampling platform and strategy.

(ex)
surface-flask
surface-pfp
surface-insitu
aircraft-pfp
aircraft-insitu
tower-insitu

4. [lab ID number]

A numeric field that identifies the sampling laboratory (1,2,3, ...).
NOAA GML is lab number 1 (see https://gml.noaa.gov/ccgg/obspack/labinfo.html).

5. [optional measurement group]

Identifies the group within the NOAA GML or the Institute of Arctic and Alpine
Research (INSTAAR) at the University of Colorado Boulder that made the
measurement.
It is possible to have multiple different groups measuring some of the same
trace gas species in our discrete samples.

Measurement groups within NOAA and INSTAAR are

6. [optional qualifiers]

Optional qualifier(s) may indicate data subsetting or averaging.
Multiple qualifiers are delimited by an underscore (_). A more detailed
description of the file contents is included within each data file.

(ex)
event All measurement results for all collected samples (discrete (flask) data only).
month Computed monthly averages all collected samples (discrete (flask) data only).
hour_#### Computed hourly averages for the specified 4-digit year (quasi-continuous data only)
HourlyData Computed hourly averages for entire record (quasi-continuous data only)
DailyData Computed daily averages for entire record (quasi-continuous data only)
MonthlyData Computed monthly averages for entire record (quasi-continuous data only)

7. [file type]

File format (netCDF, ASCII text).

(ex)

txt ASCII text file
nc netCDF4 file

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7.3 DATA - FILE TYPE

We now provide some NOAA Global Monitoring Laboratory measurements
in two unique file formats; netCDF and ASCII text.

The Network Common Data Form (NetCDF) is a self-describing, machine-independent
data format that supports creation, access, and sharing of array-oriented
scientific data. To learn more about netCDF and how to read netCDF
files, please visit http://www.unidata.ucar.edu.

The ASCII text (technically UTF-8 encoded) file is derived directly from the
netCDF file. The text file is also self-describing and can be viewed using
any ASCII or UTF-8 capable text editor. "Self-describing" means the file
includes enough information about the included data (called metadata)
that no additional file is required to understand the structure of the data
and how to read and use the data. Note that some non-ASCII characters (accents,
international character sets) may be present in various names and contact
information. These may require a UTF-8 capable text editor to view properly.

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7.4 DATA - CONTENT

See individual files for description of the provided variables and other
dataset metadata.

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7.5 QC FLAGS

NOAA GML uses a 3-column quality control flag where each column
is defined as follows:

column 1 REJECTION flag. An alphanumeric other
than a period (.) in the FIRST column indicates
a sample with obvious problems during collection
or analysis. This measurement should not be interpreted.

column 2 SELECTION flag. An alphanumeric other than a
period (.) in the SECOND column indicates a sample
that is likely valid but does not meet selection
criteria determined by the goals of a particular
investigation.

column 3 INFORMATION flag. An alphanumeric other than a period (.)
in the THIRD column provides additional information
about the collection or analysis of the sample.

WARNING: A "P" in the 3rd column of the QC flag indicates
the measurement result is preliminary and has not yet been
carefully examined by the PI. The "P" flag is removed once
the quality of the measurement has been assessed.

Reject flags (1st flag character)

A known problem in analysis (poor precision of measured standard air)
a analysis problem (specifics unknown)
! Hand-flagged by INSTAAR researcher: analytical problem.
E specific analysis problem due to extraction manifold problem
B specific analysis problem due to transfer pump

n known sampling problem -- taken from NOAA CH4 data base
+ bad pair, high member
- bad pair, low member
. good flask

Data measured after 2006 may have these flags:
H mean of trap (cylinder of known isotopic value run as sample) in same
run as sample was more than 0.3 permil above the long-term
mean
L mean trap value in same run as sample was more than 0.3 permil
below the long term mean
P small peak (less than 2 nA) - too low for accurate analysis
Y peak too large (greater than 18 nA) - swamps collectors
D too much drift; more than 0.3 permil drift in the reference gas
(this limit was previously set at 0.25 permil)

Data measured before 2006 may have these flags:
D a second aliquot taken more than 20 days after the first
2 a 'D' flag applied over another hard flag (e.g., D+n=2 or D+A=2)
M a third or greater aliquot taken more than 20 days after the first
3 a 'M' flag applied over another hard flag (e.g., T+n=2 or T+A=2)

Data measured 2020 and after may have this flag
Z standard and sample peaks are more than 1 nA different

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Non-background flags (2nd flag character)

X outlier by more than 3-sigma from a smooth curve fit to CH4C13 data
x outlier flag taken from CH4 data (ie, outlier in CH4 concentration)
Z hand-flagged by NOAA (not INSTAAR) researchers
. good flask
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Retain flags (3rd flag character)

S flask analyzed singly, without its mate
. good flask
I intercomparison sample
i intercomparison sample flag overwriting previous (now lost) flag
o No surveillance cylinder data associated with this sample
T trap samples had unusually high variance (0.2 permil)

Data measured 2018-2020 may have these third position flags
r Correction for 'amount effect' applied to these data
R Correction for 'amount effect' is significant, because peak
height of sample differs from standard by more than 2.5 nA

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7.6 COLLECTION METHODS

A single-character code is used to identify the sample collection method.
The codes are:

P - Sample collected using a portable, battery
powered pumping unit. Two flasks are
connected in series, flushed with air, and then
pressurized to 1.2 - 1.5 times ambient pressure.
D - Similar to P but the air passes through a
condenser cooled to about 5 deg C to partially
dry the sample.
G - Similar to D but with a gold-plated condenser.
T - Evacuated flask filled by opening an O-ring sealed
stopcock.
S - Flasks filled at NOAA GML observatories by sampling
air from the in situ CO2 measurement air intake system.
N - Before 1981, flasks filled using a hand-held
aspirator bulb. After 1981, flasks filled using a
pump different from those used in method P, D, or G.
F - Five liter evacuated flasks filled by opening a
ground glass, greased stopcock

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7.7 DATA - MONTHLY AVERAGES

The monthly data files in https://gml.noaa.gov/aftp/data/trace_gases/ch4c13/flask/surface/
use the following naming scheme (see Section 7.2):

[parameter]_[site]_[project]_[lab ID number]_[measurement group]_month.txt

(ex) CH4_pocn30_surface-flask_1_ccgg_month.txt contains CH4 ccgg monthly
mean values for all surface flask samples collected on the Pacific
Ocean Cruise sampling platform and grouped at 30N +/- 2.5 degrees.

(ex) CO2_brw_surface-flask_1_ccgg_month.txt contains CO2 ccgg monthly
mean values for all surface flask samples collected at Barrow, Alaska.

Monthly means are produced for each site by first averaging all
valid measurement results in the event file with a unique sample
date and time. Values are then extracted at weekly intervals from
a smooth curve (Thoning et al., 1989) fitted to the averaged data
and these weekly values are averaged for each month to give the
monthly means recorded in the files. Flagged data are excluded from the
curve fitting process. Some sites are excluded from the monthly
mean directory because sparse data or a short record does not allow a
reasonable curve fit. Also, if there are 3 or more consecutive months
without data, monthly means are not calculated for these months.

The data files contain multiple lines of header information
followed by one line for each available month.

Fields are defined as follows:

Field 1: [SITE CODE] The three-character sampling location code (see above).

Field 2: [YEAR] The sample collection year and month.
Field 3: [MONTH]

Field 4: [MEAN VALUE] Computed monthly mean value

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8. DATA RETRIEVAL

All (ASCII text and netCDF) files are located in
"https://gml.noaa.gov/aftp/data/trace_gases/ch4c13/flask/surface/".

To transfer all files in a directory, it is more efficient to
download the tar or zipped files. Individual or zipped files can
be downloaded using your web browser by clicking the hyperlinked file
or right clicking hyperlink and using browser menu to 'save as' or similar.

Files can also be accessed by anonymous ftp at aftp.cmdl.noaa.gov.

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9. REFERENCES

Conway, T.J., Tans, P.P., Waterman, L.S., Thoning, K.W., Kitzis, D.R.,
Masarie, K.A., and Zhang, N. 1994. Evidence for interannual variability
of the carbon cycle from the NOAA/CMDL global air sampling network, JGR 99, 22831-22855.

Miller, J. B., Mack, K. A., Dissly, R., White, J. W. C., Dlugokencky, E.
J. and Tans, P. P. 2002. Development of analytical methods and
measurements of 13C/12C in atmospheric CH4 from the NOAA/CMDL global air
sampling network. J. Geophys. Res. 107, doi: 10.1029/2001JD000630.

Thoning, K.W., Tans, P.P., and Komhyr, W.D. 1989, Atmospheric carbon
dioxide at Mauna Loa Observatory 2. Analysis of the NOAA GMCC Data,
1974-1985, J. Geophys. Res., 94, 8549-8565.

Umezawa, T., Brenninkmeijer, C., Rockmann, T., van der Veen, C., Tyler,
S. C., Fujita, R., Morimoto, S., Aoki, S., Sowers, T., Schmitt, J., and
Bock, M. (2018). Interlaboratory comparison of d13C and dD measurements
of atmospheric CH4 for combined use of data sets from different laboratories.
Atmospheric Measurement Techniques.

Vaughn, B., Ferretti, D., Miller, J. and White, J. 2004. Stable isotope
measurements of atmospheric CO2 and CH4. Handbook of Stable Isotope Analytical
Techniques, vol 1, ch. 14, Elsevier, 1248 p.

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