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Atmospheric Methane Dry Air Mole Fractions from the
NOAA GML Carbon Cycle Cooperative Global Air
Sampling Network, 1983-2021
Version: 2022-11-21
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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
National Oceanic and Atmospheric Administration (NOAA)
Global Monitoring Laboratory (GML)
Carbon Cycle Greenhouse Gases (CCGG)
Correspondence concerning these data should be directed to:
Dr. Xin Lan
Dr. Edward J. Dlugokencky
NOAA Global Monitoring Laboratory
325 Broadway, R/GML-1
Boulder, Colorado, 80305 USA
Email:
xin.lan@noaa.gov
ed.dlugokencky@noaa.gov
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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
Lan, X., E.J. Dlugokencky, J.W. Mund, A.M. Crotwell, M.J. Crotwell, E. Moglia,
M. Madronich, D. Neff and K.W. Thoning (2022), Atmospheric Methane Dry Air Mole
Fractions from the NOAA GML Carbon Cycle Cooperative Global Air Sampling Network,
1983-2021, Version: 2022-11-21, https://doi.org/10.15138/VNCZ-M766
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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, Mogolia (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, Isreal (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:
2018-07-30
Uncertainties recalculated after reassessment of
uncertainty terms. Uncertainties on early data are
now larger than before.
2015-07-07
We updated and extended the WMO CH4 X2004 scale to X2004A.
Details are described on a dedicated web page:
http://www.esrl.noaa.gov/gmd/ccl/ch4_scale.html.
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6. INTRODUCTION
All samples were analyzed for methane at NOAA GML in Boulder, Colorado
by gas chromatography with flame ionization detection or by cavity ring-
down spectroscopy (since August, 2019), and each sample aliquot was
referenced to the NOAA 2004A methane standard scale
(see www.esrl.noaa.gov/gmd/ccl/ch4_scale.html; Dlugokencky et al., 2005).
Through most of the period 1983-1991, one flask of a sample pair was analyzed
for methane, and, whenever the overpressure was sufficient, at least two
aliquots were analyzed. We have used the difference in methane mole fraction
between the first and second aliquot to establish the repeatability of the
measurement. Over the full period of the record described here, the average
repeatability has been approximately 2 ppb. In October 1991, our analysis
procedure was altered; we began analyzing a single aliquot from both members
of the flask pair. The principle reasons for the change were to simplify
flask handling procedures (the carbon dioxide and carbon monoxide projects
also measure both flasks of the sample pair) and to have flask pair agreement
(the difference in methane mole fraction between the two flasks collected
simultaneously) as an additional diagnostic to use in evaluating the quality
of the data. The repeatability of the analytical instrument is now assessed
by two approaches: approximately monthly measurements of target tanks, and
assessing the relative stability of the standard or reference gas aliquots
during each day of flask measurements. (See Steele et al., 1987, Lang et
al., 1990a,b, Dlugokencky et al., 1994b, and Dlugokencky et al., 2005 for
details of the sampling network, equipment, standards, and procedures.)
Uncertainties are reported for each measurement based on analytical
repeatability, reproducibility, and our ability to propagate the WMO CH4 mole
fraction standard scale. Analytical repeatability is based on one of the
methods above or on the average absolute value of pair agreement between
pairs collected nearly simultaneously. It varies with analytical instrument
from 0.2 to 2.3 ppb. Propagation of the scale is based on the reproducibility
determined for scale propagation in our calibration laboratory. It has a
fixed value of 0.5 ppb based on subsequent calibrations of the same cylinder
at least one year after the first. Reproducibility is based on long-term
variations in measurements of target cylinders, typically ~0.3 ppb. The
three terms are added in quadrature (square root of the sum of the squares)
to estimate the measurement uncertainty at 68% confidence interval.
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7. DATA - GENERAL COMMENTS
Aliquots of sample and standard gas are injected into a gas
chromatograph (GC) with a sampling valve. Methane (CH4) is
separated from other sample constituents using packed columns
and detected using flame ionization (FID). This process is
highly automated for field and laboratory operations. Instrument
response of the sample must be compared to a standard of known
CH4 content. Measurements are reported in units of nanomol/mol
(10^-9 mol CH4 per mol of dry air (nmol/mol) or parts per billion
(ppb)) relative to the NOAA 2004A CH4 standard scale. Repeatability
of our measurements, based on repeated analysis of air from a
high-pressure cylinder, has ranged from 1 to 3 nmol/mol
over the period of our measurements.
In August, 2019, we switched from GC/FID to a Cavity Ring-Down
Spectrometer that analyzes CH4 and CO2. The analyzer is calibrated
off-line with a suite of standards once per month relative to a
dry, natural air, reference in a high-pressure cylinder. All air
samples are measured relative the same reference, and CH4 and CO2
values in measured samples are calculated based on their ratio to
the reference. Repeatability of the analyzer, based on repeated
analysis of air from a high-pressure cylinder, is ~0.2 ppb (1 sigma).
The uncertainty of our CH4 standard scale (NOAA 2004A) near 1800 ppb
is estimated at +/- 0.2%, or about 3 nmol/mol.
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
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)
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.
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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/ch4/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/ch4/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
Lan X, Nisbet EG, Dlugokencky EJ, Michel SE. 2021, What do we know about
the global methane budget? Results from four decades of atmospheric
CH4 observations and the way forward. Phil. Trans. R. Soc. A 379:20200440.
https://doi.org/10.1098/rsta.2020.0440
Lan, X., Basu, S., Schwietzke, S., Bruhwiler, L. M. P., Dlugokencky,
E. J., Michel, S. E., et al. 2021, Improved constraints on global
methane emissions and sinks using δ13C-CH4. Global Biogeochemical
Cycles, 35, e2021GB007000.
https://doi.org/10.1029/2021GB007000
Dlugokencky, E.J., et al. 2009, Observational constraints on
recent increases in the atmospheric CH4 burden, Geophys. Res. Lett.,
36, L18803, doi:10.1029/2009GL039780.
Dlugokencky, E.J., R.C. Myers, P.M. Lang, K.A. Masarie, A.M. Crotwell,
K.W. Thoning, B.D. Hall, J.W. Elkins, and L.P. Steele, 2005, Conversion
of NOAA atmospheric dry air methane mole fractions to a
gravimetrically-prepared standard scale, J. Geophys. Res., 110, D18306,
doi:10.1029/2005JD006035.
Dlugokencky, E.J., B.P. Walter, K.A. Masarie, P.M. Lang, and E.S. Kasischke,
2001, Measurements of an anomalous global methane increase during 1998,
Geophys. Res. Lett., 28, 499-502.
Dlugokencky, E.J., K.A. Masarie, P.M. Lang, and P.P. Tans, 1998,
Continuing decline in the growth rate of atmospheric methane,
Nature, 393, 447-450.
Dlugokencky, E.J., K.A. Masarie, P.M. Lang, P.P. Tans, L.P. Steele, and
E.G. Nisbet, 1994a, A dramatic decrease in the growth rate of atmospheric
methane in the northern hemisphere during 1992, Geophys. Res. Lett., 21,
45-48.
Dlugokencky, E.J., L.P. Steele, P.M. Lang, and K.A. Masarie, 1994b, The growth
rate and distribution of atmospheric methane, J. Geophys. Res., 99, 17,021-
17,043.
Lang, P.M., L.P. Steele, R.C. Martin, and K.A. Masarie, 1990a, Atmospheric
methane data for the period 1983-1985 from the NOAA/GMCC global cooperative
flask sampling network, NOAA Technical Memorandum ERL CMDL-1.
Lang, P.M., L.P. Steele, and R.C. Martin, 1990b, Atmospheric methane data for
the period 1986-1988 from the NOAA/CMDL global cooperative flask sampling
network, NOAA Technical Memorandum ERL CMDL-2.
Lang, P.M., L.P. Steele, L.S. Waterman, R.C. Martin, K.A. Masarie, and
E.J. Dlugokencky, 1992, NOAA/CMDL Atmospheric methane data for the period
1983-1990 from shipboard flask sampling, NOAA Technical Memorandum ERL CMDL-4.
Steele, L.P., P.J. Fraser, R.A. Rasmussen, M.A.K. Khalil, T.J. Conway, A.J.
Crawford, R.H. Gammon, K.A. Masarie, and K.W. Thoning, 1987, The global
distribution of methane in the troposphere, J. Atmos. Chem, 5, 125-171.
Steele, L.P. and P.M. Lang, 1991, Atmospheric methane concentrations-the
NOAA/CMDL global cooperative flask sampling network, 1983-1988,
ORNL/CDIAC-42, NDP-038.
Steele, L.P., E.J. Dlugokencky, P.M. Lang, P.P. Tans, R.C. Martin, and
K.A. Masarie, 1992, Slowing down of the global accumulation of atmospheric
methane during the 1980's, Nature, 358, 313.
Thoning, K.W., P.P. Tans, and W.D. Komhyr, 1989, Atmospheric carbon dioxide
at Mauna Loa Observatory 2. Analysis of the NOAA GMCC Data, 1974-1985,
J. Geophys. Res., 94, 8549-8565.
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