Atmospheric Methane Dry Air Mole Fractions from the
NOAA GML Carbon Cycle Cooperative Global Air
Sampling Network, 1983-2021

Version: 2022-11-21

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


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



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


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


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.


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.

Lab-wide notes:


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:


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.


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.


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.


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


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


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


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


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

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

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


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


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.


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.


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).


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


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.


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.


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:


Uncertainties recalculated after reassessment of
uncertainty terms. Uncertainties on early data are
now larger than before.


We updated and extended the WMO CH4 X2004 scale to X2004A.
Details are described on a dedicated web page: 


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.


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.


For a summary of sampling locations, please visit


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



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.

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

2. [site]

   Identifies the sampling site code.


3. [project]
   Identifies sampling platform and strategy.


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
  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.

   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). 


   txt           ASCII text file
   nc            netCDF4 file


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.


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


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

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.


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       
    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


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


All (ASCII text and netCDF) files are located in 

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. 


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. 

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. 

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,

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, 

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-

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.