Atmospheric Carbon Dioxide Dry Air Mole Fractions from 
the NOAA GML Carbon Cycle Cooperative Global Air Sampling 
Network, 1968-2022

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, CO  80305



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 Carbon Dioxide Dry 
   Air Mole Fractions from the NOAA GML Carbon Cycle Cooperative Global Air 
   Sampling Network, 1968-2022, Version: 2023-08-28, https://doi.org/10.15138/wkgj-f215


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. 2021-02-08 CO2 measurements from flask-air samples were recalculated onto the X2019 CO2 mole fraction scale by first reassigning all standards used on the CO2 analysis sytems to X2019. CO2 values on X2019 from air samples were caluclated by reprocessing the original raw files (i.e., files with raw analyzer output (typically voltages)) with the updated assignments for the standards. A detailed description of the scale revision from X2007 to X2019 is given in Hall et al. (2020), but the main reasons were to correct biases caused by CO2 absorption by O-rings in the manometric calibration system and to correct a virial coefficient used in the calculation. The magnitiude of the differences between X2019 and X2007 varies with CO2 abundance, and is typically between 0.1 and 0.2 ppm. Some differences from the late-2000s and early-2010s are larger (up to ~0.3 ppm) because of a mis-assigned standard on the system used to transfer the scale to working standards. Measurements prior to ~1980 from early analytical systems (instrument codes LR1 and LR2) could not be reprocessed onto X2019; they remain on a CO2 scale originally developed at Scripps Institution of Oceanography. 2020-07-15 Based on comparison with in situ measurements at SPO and SYO (from National Institute of Polar Research, Japan), a storage correction was applied to flask-air samples measured for CO2. The correction is ~0.2 ppm/yr at SPO and 0.08 ppm/yr at SYO. Uncertainty for the correction has not yet been determined and applied to the measurement uncertainty, but it is estimated to be ~0.05 ppm. 2018-07-30 1. Uncertainties (68% c.i.) were estimated for all CO2 measurements. Uncertainity entries of -999.99 occur when a reasonable uncertainty could not be calculated. 2. In 2017, began using N.. to flag samples not meeting pair agreement criterion. 2016-07-07 We discovered a small leak in the intake line of the portable sampler at Vestmannaeyjar, Iceland (ICE) that affected CO2. All samples from 1 July 2009 through 28 June 2016 were flagged N.. for CO2. 2012-08-03 Corrections for drifting standard gas cylinders have been applied to all samples measured on the following instruments through the specified date. Instrument ID Analysis Date L3 thru 2009-10 L8 thru 2010-06 L10 thru 2010-10 2011-10-13 Discrete (flask) CO2 mole fractions from 1980 to 2006 have all been recalculated to bring them onto the WMO X2007 Mole Fraction Scale (X2007). The data since 2007 were already on the X2007 scale. WMO X2007 is based on repeated manometric measurements of the NOAA primary standards (Zhao et al. 2006) and comparison of those results to similar measurements made over a period of more than 10 years at Scripps Institution of Oceanography. The difference between the X2007 and the previous scales propagated by SIO and NOAA (X83, X87, etc.) is ~0.2 ppm (X2007-previous) in the 1980s, decreasing to ~0.1 ppm in the late 1990s, and to 0.0 ppm by 2006. The recalculation of individual measurements was accomplished by first determining the X2007 values for the reference gases used to measure the air samples and then using those values with the raw data (NDIR voltages) to recompute mole fractions for each sample. 1999-01-01 Corrections have been applied to CO2 mole fractions measured on the flask analysis apparatus in use from July 1987 through April 1998. The corrections are +0.1 ppm to samples measured from July 1987 through 18 November 1993, and +0.24 ppm to samples measured from 19 November 1993 through 10 April 1998. Corrections for drifting reference gas cylinders have been applied to all samples measured during 1995 through April 2008 on system L3 and 1998 through May 2007 on systems S2 and L3. Files obtained previously from this site should be discarded and replaced with the revised files contained in this version. -------------------------------------------------------------------- 6. INTRODUCTION CO2 dry air mole fractions reported in these files were measured by a nondispersive infrared absorption analyzer or cavity ring-down spectrometer (since Aug., 2019) in air samples collected in glass flasks at NOAA GML Carbon Cycle Cooperative Global Air Sampling Network sites. Measurements are reported on the X2019 CO2 mole fraction scale. More information on this scale is available here: https://gml.noaa.gov/ccl/co2_scale.html Calibration procedures are given in Komhyr et al., 1983; Komhyr et al., 1985; Thoning et al., 1987, and Thoning et al., 1995; and Tans et al.,2017. Analysis and interpretation of the data have been reported by Komhyr et al., 1985; Conway et al., 1988; Tans et al., 1989a; Tans et al., 1990, and Conway et al., 1994. Carbon dioxide (CO2) in ambient and standard air samples was historically detected using a non-dispersive infrared (NDIR) analyzer. The measurement of CO2 in air was made relative to standards whose CO2 abundance was determined with high precision and accuracy. Because detector response is non-linear in the range of atmospheric levels, ambient samples are bracketed during analysis by a set of standards used to calibrate detector response. Measurements are reported in units of micromol/mol (10^-6 mol CO2 per mol of dry air or parts per million (ppm)). In August, 2019, we switched from a NDIR to a Cavity Ring-Down Spectrometer that analyzes CH4 and CO2. The analyzer response 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. ------------------------------------------------------------------- 7. DATA - GENERAL COMMENTS Uncertainties of the CO2 measurements are included. Key components of it are our ability to propagate the WMO XCO2 scale to working standards, the repeatability of the analyzers used for sample measurement, and the long-term reproducibility. Zhao and Tans (2006) determined that the internal consistency of working standards is +/- 0.02 ppm (68% confidence interval). The typical repeatability of the analyzers, based on repeated measurements of natural air from a high-pressure cylinder, is +/- 0.06 ppm for NDIR and +/- 0.02 ppm for CRDS. Long-term reproducibility is estimated to be +/- 0.03 ppm 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. 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 CO2 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 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/co2/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/co2/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 Ballantyne, A. P., C. B. Alden, J. B. Miller, P. Tans and J. W. C. White, (2012), Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years, Nature, 488, 7409, , 10.1038/nature11299 Conway, T.J., P.P. Tans, L.S. Waterman, K.W. Thoning, D.R. Kitzis, K.A. Masarie, and N. Zhang, 1994, Evidence for interannual variability of the carbon cycle from the NOAA/CMDL global air sampling network, J. Geophys. Res.,99, 22831-22855. Conway, T.J., P. Tans, L.S. Waterman, K.W. Thoning, K.A. Masarie, and R.H. Gammon, 1988, Atmospheric carbon dioxide measurements in the remote global troposphere, 1981-1984, Tellus, 40B, 81-115. Komhyr, W.D., L.S. Waterman, and W.R. Taylor, 1983, Semiautomatic nondispersive infrared analyzer apparatus for CO2 air sample analyses, J. Geophys. Res., 88, 1315-1322. Komhyr, W.D., R.H. Gammon, T.B. Harris, L.S. Waterman, T.J. Conway, W.R. Taylor, and K.W. Thoning, 1985, Global atmospheric CO2 distribution and variations from 1968-1982 NOAA/GMCC CO2 flask sample data, J. Geophys. Res., 90, 5567-5596. Tans, P.P., T.J. Conway, and T. Nakazawa, 1989a, Latitudinal distribution of the sources and sinks of atmospheric carbon dioxide from surface observations and an atmospheric transport model, J. Geophys. Res., 94, 5151-5172. Tans, P.P, K.W. Thoning, W.P. Elliott, and T.J. Conway, 1989b, Background atmospheric CO2 patterns from weekly flask samples at Barrow, Alaska: Optimal signal recovery and error esitmates, in NOAA Tech. Memo. (ERL ARL-173). Environmental Research Laboratories, Boulder, CO, 131 pp. Tans, P.P., I.Y. Fung, and T. Takahashi, 1990, Observational constraints on the global atmospheric CO2 budget, Science, 247, 1431-1438. Tans, Pieter P., Andrew M. Crotwell and Kirk W. Thoning, 2017, Abundances of isotopologues and calibration of CO2 greenhouse gas measurements, Atmospheric Measurement Techniques, 10, 7, 2669-2685, 10.5194/amt-10-2669-2017. Thoning, K.W., P. Tans, T.J. Conway, and L.S. Waterman, 1987, NOAA/GMCC calibrations of CO2-in-air reference gases: 1979-1985. NOAA Tech. Memo. (ERL ARL-150). Environmental Research Laboratories, Boulder, CO, 63 pp. 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. Thoning, K.W., T.J. Conway, N. Zhang, and D. Kitzis, 1995, Analysis system for measurement of CO2 mixing ratios in flask air samples, J. Atmos. and Oceanic Tech., 12, 1349-1356. Zhao, C., and P.P. Tans (2006), Estimating uncertainty of the WMO Mole Fraction Scale for carbon dioxide in air, J. Geophys. Res. 111, D08S09, doi: 10.1029/2005JD006003. -------------------------------------------------------------------