Atmospheric Methane Dry Air Mole Fractions from the
NOAA GML Carbon Cycle Cooperative Global Air
Sampling Network, 1983-2022

Version: 2023-08-28

1.       Data source and contacts
2.       Use of data
2.1      Citation
3.       License 
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
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. To ensure that GML
receives fair credit for their work please include relevant citation
text in publications. We encourage users to contact the data providers,
who can provide detailed information about the measurements and
scientific insight.  In cases where the data are central to a
publication, coauthorship for data providers may be appropriate.


Please reference these data as

   Lan, X., J.W. Mund, A.M. Crotwell, M.J. Crotwell, E. Moglia, 
   M. Madronich, D. Neff and K.W. Thoning (2023), Atmospheric Methane Dry Air Mole
   Fractions from the NOAA GML Carbon Cycle Cooperative Global Air Sampling Network, 
   1983-2022, Version: 2023-08-28, https://doi.org/10.15138/VNCZ-M766


These data were produced by NOAA and are not subject to copyright protection in the United States. NOAA waives any potential copyright and related rights in these data worldwide through the Creative Commons Zero 1.0 Universal Public Domain Dedication (CC0 1.0)

CC0 1.0 Universal -------------------------------------------------------------------- 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. -------------------------------------------------------------------- 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. +++++++++++++++++++++++++++++++ Parameter-specific notes: 2023-05-01 In Spring 2023, we moved to an internal quality control (QC) tagging system for the flask air samples. There are three categories of tags documenting issues associated with sample collection, measurement and representativity in the CCGG database. Tags are more specific than flags, which allows a more granular internal tracking and analysis of QC issues. Tags are converted to simplified 3 character flags in the data files for external data users. See section 7.5 for more details. 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. -------------------------------------------------------------------- 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). Measurements are reported in units of nanomol/mol (10^-9 mol CH4 per mol of dry air (nmol/mol) or parts per billion (ppb)) For measurements using gas chromatography measurements, aliquots of sample and standard gas are injected into a gas chromatograph (GC) alternatively 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. 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. The average repeatability has been approximately 3 ppb before 1991. 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.) Repeatability after 1991 has ranged from 1 to 2 ppb over the period of our GC 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 reference air in a high-pressure cylinder. All air samples are measured relative to 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). ------------------------------------------------------------------- 7. DATA - GENERAL COMMENTS Uncertainties of the CH4 measurements are included. Key components of it are our ability to propagate the WMO XCH4 scale to working standards, the repeatability of the analyzers used for sample measurement, and the long-term variability of the analyzer responses (reproducibility). The scale propagation uncertainty is estimated to be 0.2 ppb, based on the reproducibility determined for scale propagation in our calibration laboratory. Long-term reproducibility is estimated to be +/- 0.5 ppb based on repeated measurements of air from a high-pressure cylinder, or the long-term variability in the differences between measurements of test flasks on the flask-air analysis system and separate measurements of the test gas used to fill the test flasks. The three terms are added in quadrature (square root of the sum of the squares) to estimate the measurement uncertainty at 68% confidence interval. 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. ------------------------------------------------------------------- 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/. ------------------------------------------------------------------- 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 ------------------------------------------------------------------- 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. ------------------------------------------------------------------- 7.4 DATA - CONTENT See individual files for description of the provided variables and other dataset metadata. ------------------------------------------------------------------- 7.5 QC FLAGS Quality control 3-column flags indicate retained and rejected flask results as follows in datafiles. If the first character is not a period, the sample result should be rejected for scientific use due to sample collection and/or measurement issue. A second column character other than a period indicates a sample that is likely valid but does not meet selection for representativeness such as midday sampling or background air sampling. A third column flag other than a period indicates abnormal circumstances that are not thought to affect the data quality. Flag Description Retained ... good pair, no other issues Rejected M.. sample measurement issue C.. sample collection issue B.. both measurement and collection issues Selection .S. selection issue. High/low mole fraction thought to not represent background conditions for example. Informational ..M informational measurement tag or potential measurement issue ..C informational collection tag or potential collection issue The retained values comprise the data set that mostly represents CH4 distribution in the remote, well-mixed global surface atmosphere. Data with selection flag (with a 2nd column flag other than '.') are likely valid measurements, but represent poorly mixed air parcels influenced by local sources. Data selection is applied considering multiple measured gases from the same air sample , and sample conditions such as wind direction, wind speed, and back trajectory, in addition to identifying outliers using a curve fitting approach described in detail here: https://gml.noaa.gov/ccgg/mbl/crvfit/crvfit.html ------------------------------------------------------------------- 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 ------------------------------------------------------------------- 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 ------------------------------------------------------------------- 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. ------------------------------------------------------------------- 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. -------------------------------------------------------------------