Atmospheric Carbon Dioxide Dry Air Mole Fractions from 
quasi-continuous measurements at Barrow, Alaska; Mauna 
Loa, Hawaii; American Samoa; and South Pole, 1973-2023.
National Oceanic and Atmospheric Administration (NOAA) 
Global Monitoring Laboratory (GML) 
Carbon Cycle Greenhouse Gases (CCGG)

Version: 2024-02-12

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
8.       Data retrieval
9.       References


These directories contain atmospheric Carbon Dioxide (CO2)
Dry Air Mole Fractions from quasi-continuous measurements 
at Barrow, Alaska (BRW); Mauna Loa, Hawaii (MLO); American 
Samoa (SMO); and South Pole (SPO), 1973-2023.

Correspondence concerning these data should be directed to:

Kirk W. Thoning
NOAA Global Monitoring Laboratory
325 Broadway, GML-1
Boulder, Colorado, 80305 USA
Telephone: 720 263-1317
Electronic Mail: kirk.w.thoning@noaa.gov


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

   K.W. Thoning, A.M. Crotwell, and J.W. Mund (2024),
   Atmospheric Carbon Dioxide Dry Air Mole Fractions from 
   continuous measurements at Mauna Loa, Hawaii, Barrow, Alaska,
   American Samoa and South Pole. 1973-2023 Version 2024-02-12
   National Oceanic and Atmospheric Administration (NOAA),
   Global Monitoring Laboratory (GML), Boulder, Colorado, USA


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 +++++++++++++++++++++++++++++++ Project-specific notes: 2024-02-08 =================================================================== Mauna Loa: Because of power loss to the observatory due to the eruption of the Mauna Loa volcano in Novermber 2022, no data are available for January to June 2023. Partial power was restored using solar power and backup batteries in July 2023, and CO2 measurements resumed on July 5, 2023. 2023-07-27 =================================================================== Barrow: Small revisions to the data from 2017 to 2022 were made based on improved assignments to the standard gases and improved determination of the analyzer response. These revisions were less than 0.2 ppm. Mauna Loa: Small revisions to the data from June 2019 to 2022 were made based on improved assignments to the standard gases and improved determination of the analyzer response. These revisions were less than 0.2 ppm. Due to the eruption of the Mauna Loa Volcano, measurements from Mauna Loa Observatory were suspended as of Nov. 29, so there is no data for the end of November 2022 and all of December 2022. Mauna Kea: Measurements of CO2 began at Mauna Kea, Hawaii, approximately 21 miles north of Mauna loa, in December 2022, to replace the measurements that were suspended at Mauna Loa. 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. 2020-08-24 =================================================================== The README files were updated to include an 'Introduction' section with brief explanations of measurement methods and calculations. 2020-06-30 =================================================================== A mistake was discovered in the assigned value of a working tank for BRW in 2004. The value was corrected and the data recalculated. This affected data only from June 3, 2004 to January 12, 2005. 2019-04-16 =================================================================== Measurements of CO2 at Mauna Loa, Hawaii were switched from a NDIR to a CRDS in June, 2019. Calibration strategy and measurement frequency changed. Measurement uncertainty was added to CO2 hourly averages for all sites for 2019, for BRW and SPO for 2018, and for BRW for 2017. 2017-01-24 =================================================================== The method used to apply flags to the hourly average data was changed for the years 1973-2014, to agree with the method used for the 2015 and 2016 data. This means that daily and monthly averages for those earlier years may have changed by small amounts for data packages from previous years. Hourly values were not affected by this change, only the flags. See https://gml.noaa.gov/ccgg/about/co2_measurements.html for more details. =================================================================== 2011-10-13 Quasi-continuous CO2 dry air mole fractions from 1980 to 2006 have all been recalculated to bring them into the WMO X2007 Mole Fraction Scale (X07). The data since 2007 were already on the X07 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 X07 and the previous scales propagated by SIO and NOAA (X83, X87, etc.) is ~0.2 ppm (X07-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 X07 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. =================================================================== 2003-11-28 Added year 2002 data for all sites. Updated tar.gz files to include new data. =================================================================== 2003-03-13 The hourly average files now have a new format, which includes the standard deviation of the hourly average. See the README for details. Individual text files also now have a .co2 extension on the file name. =================================================================== 2002-11-27 Revised Maunal Loa 2001 data. There was a mistake in the November 2001 data values, resulting in them being ~0.5 ppm to low. =================================================================== 2002-10-01 Added year 2000 data for Barrow, Samoa and South Pole. Added year 2001 data for all sites. =================================================================== 2001-10-10 Added year 2000 data for Mauna Loa. Updated mlo.in-situ.tar.Z files to include new data. =================================================================== 2000-10-05 Added data through 1999 for all sites. Updated tar.Z files to include new data. Updated tar.Z files to include new data. =================================================================== 1999-11-29 Added data through 1998 for all sites. Updated tar.Z files to include new data. =================================================================== 1996-10-04 Changed the archive for the Mauna Loa data. The formats of the Mauna Loa files are different from the other sites. See the README for details. Added Mauna Loa data for 1993, 1994, and 1995. Updated tar.Z files to include new data. =================================================================== 1996-03-04 Changed the archive for all sites. The formats of the files are different from before. See the README for details. Added data for 1993, 1994, and 1995. Updated tar.Z files to include new data. =================================================================== 1994-05-18 Added data for Barrow, 1992. Added data for Mauna Loa, 1992. Added data for Samoa, 1992. Added data for South Pole, 1991. Added data for South Pole, 1992. Changes to this data based on recalibration of reference gases will occur in the future. Updated tar.Z files to include new data. =================================================================== 1992-08-12 Minor corrections to Samoa and Barrow data were made. Samoa: 77-04-09 hour 00 CO2 changed from 300.45 to 999.99 77-04-09 hour 00 Flag changed from SI to I 81-11-20 hour 00 CO2 changed from 861.97 to 999.99 81-11-20 hour 01 CO2 changed from 861.97 to 999.99 82-01-07 hour 19 CO2 changed from 902.43 to 999.99 82-01-07 hour 20 CO2 changed from 902.43 to 999.99 82-01-07 hour 21 CO2 changed from 902.43 to 999.99 82-01-07 hour 22 CO2 changed from 902.43 to 999.99 82-01-07 hour 23 CO2 changed from 902.43 to 999.99 82-01-08 hour 00 CO2 changed from 340.46 to 999.99 82-01-08 hour 01 CO2 changed from 340.11 to 999.99 Barrow: 82-06-20 hour 09 CO2 changed from 99.99 to 999.99 =================================================================== 1992-07-28 Compressed tar files are now available for transfer. There are four files, one for each observatory. Each tar file contains all the files in the directory for that station, and is named sta.in-situ.tar.Z where sta is the three letter station code, 'brw', 'mlo', 'smo', or 'spo'. =================================================================== 1992-07-27 Preliminary data from Mauna Loa for 1991 is now available. Changes to this data based on recalibration of reference gases will occur in the future. =================================================================== 1992-07-27 Corrections to the Samoa data from January 1, 1988 to August 31, 1991 were made due to better determinations of the reference gases used during this period. Files affected were smo88, smo89, smo90, smoday88, smoday89, smoday90, smomm. The data for 1991 were added to the archive. The new files are smo91 and smoday91. Data starting on September 1, 1991 are still subject to revision. =================================================================== 1992-07-22 Preliminary data from Barrow for 1991 is now available. Changes to this data based on recalibration of reference gases will occur in the future. =================================================================== 1992-07-22 The types of flags used are currently under review. Many flags that were seldom used or were redundant in thier meaning will be replaced with more common flags. The goal is to eventually have only 8 base flags total; ' ', C, I, V, D, A, DN, and NC. Any additional flags based on specific selection schemes for each site will be noted in this file. =================================================================== 1992-07-22 The data selection for Barrow is not consistent for the data set. Data starting in 1981 has been given a more rigourous selection than prior to this date, i.e., there are no D or A selection flags before 1981. The data for Samoa from 1976 through 1987 have had an additional selection based on wind direction and wind speed. (See Waterman et al., 1989). A 'WD' code stands for data that has been flagged because the wind direction was not from the clean air sector. A 'WS' speed is for data where the wind speed is below a minimum value. There are no 'A' flags in the data for this time period. -------------------------------------------------------------------- 6. INTRODUCTION NOAA GML began continuous measurements of atmospheric CO2 at Barrow, Alaska (BRW) in July, 1973 and at Mauna Loa, Hawaii (MLO) in April, 1974, at the South Pole in November, 1975 and at American Samoa in January 1976. Since the start of these measurements, our methods have evolved as described below. Analytical: The first CO2 measurements at all sites were based on Non-dispersive Infrared (NDIR) CO2 analyzers (see e.g. Komhyr et al, 1989). Initially the output from the analyzer was averaged into a single value per hour, then as two ~20 minute averages per hour in 2014. Our next-generation analytical systems use laser-based spectrometers to measure CO2. We began using a new system (off-axis, integrated cavity output spectroscopy, ICOS) at BRW in April 2013. This system was operated in parallel with the NDIR system so that comparison measurements could be made. In January 2017, the NIDR system was stopped, and the ICOS system continued as the sole CO2 measurement system. Data are saved as 10-second averages, which are averaged into "5-minute averages". Our measurement sequence starts with 210 seconds of flushing, so only the last 90 seconds (9 10-second averages) are used to calculate each 5-minute average. In January of 2018 the data processing of the South Pole NDIR data was switched to a scheme similar to BRW. At MLO, we switched to a laser-based, cavity ringdown spectrometer (CRDS) in June 2019, and now use a similar data processing scheme to BRW. Calibration: NDIR systems were first calibrated with two reference gases each hour, then with three reference gases each hour starting in 1998. A linear or quadratic response of the analyzer was determined from these reference gases, and the mole fraction of the ambient air sample was determined from this response. For laser-based spectrometers, the analyzer response is calibrated with a suite of standards every two weeks relative to a reference gas, and the reference gas is measured hourly to track and correct short term analyzer drift. As a quality assurance step, a well-calibrated "target" gas is measured up to twice per day. For the SPO NDIR starting in January 2018, the analyzer response is calibrated with a suite of 4 standards twice a day relative to a reference gas. The reference gas is measured twice an hour to track and correct short term analyzer drift. NOAA carbon dioxide measurements are reported on the WMO X2019 CO2 standard scale. (https://gml.noaa.gov/ccl/co2_scale.html). Quality control: All measurements are inspected manually to determine periods when the analytical instrument was not working optimally. Automated algorithms have been recently introduced to assist in the determination of these periods. Hourly averages: Starting in about 1986 (depending on site), minute average or 10 second average values of the analyzer output are recorded, and an hourly average and standard deviation of the ambient mole fraction CO2 is calculated (prior to about 1986, the standard deviation was not available). The value_std_dev takes into account the analyzer standard deviation, so normally it is not 0 even if nvalue=1. A value_std_dev value of 0.0 or -99.99 would indicate times when the analyzer output standard deviation for the hour is not available. For spectrometer systems and for the SPO NDIR system starting in January 2018, 5-minute averages are used to calculate hourly averages, but value_std_dev includes variability in the 5-min averages, so it is greater than the normal sample SD of the 5-min averages that go into the hourly mean, and when nvalue=1 (i.e., one 5-min average in the hourly mean), value_std_dev is still calculated and reported. Selection for background: Valid hourly averaged data are selected to distinguish samples of regionally representative air (background) from samples influenced by local sources and sinks (non-background). Background hourly data are identified where the first two characters are ".." for the selection flag. A couple selection steps are conducted, involving withing hour variability and hour to hour changes. For SMO and SPO, wind direction is also included as a selection criteria. For MLO, data during prevalent upslope hours of the day are flagged with a '.U.' flag, due to depeletion of CO2 by local vegetation nearby. See also section 7.5 Uncertainties: Measurement uncertainty (value_unc) was added to CO2 hourly averages for all sites for 2019, for BRW and SPO for 2018, and for BRW for 2017. The uncertainty is based on analytical repeatability and reproducibility, and our ability to propagate the WMO CO2 mole fraction standard scale. Repeatability is based on the stability of reference gas measurements, and reproducibility is based on long term measurements of a known 'target' gas. The scale propagation term is based on the uncertainty we assign to standards, 0.03 ppm. All terms are given as 68% confidence intervals. Although we still list "value_std_dev" of hourly averages (standard deviation of values that go into an hourly average as described above, which includes natural variability and measurement uncertainty), we now list measurement uncertainty as a separate term. Total uncertainty is calculated by adding the individual terms in quadrature (square root of the sum of the squares). ------------------------------------------------------------------- 7. DATA - GENERAL COMMENTS 7.1 DATA - SAMPLING LOCATIONS For a summary of sampling locations, please visit https://gml.noaa.gov/dv/site/index.php?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 For each observatory record we provide an hourly, daily, and monthly average file. Daily averages are derived directly from the hourly data. Monthly averages are derived from the daily averages. Higher resolution data (sub-hourly) are available upon request. All (ASCII text and netCDF) files are located in "https://gml.noaa.gov/aftp/data/trace_gases/co2/in-situ/surface/". Files are named as follows (see Section 7.2 for details): co2_[site]_surface-insitu_1_ccgg_HourlyData.[file type] co2_[site]_surface-insitu_1_ccgg_DailyData.[file type] co2_[site]_surface-insitu_1_ccgg_MonthlyData.[filetype] ------------------------------------------------------------------- 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. SUMMARY OF SELECTION FLAGS Missing data will have a value of -999.99 for the mole fraction. Times are specified in Coordinated Universal Time (UTC). Hours are specified as the beginning of the hour, for example, hour 5 corresponds to 5 AM to 6 AM UTC. The selection process is done to distinguish "background" mole fractions, that is, the values that we believe are not contaminated by local sources or sinks of CO2. The selection process depends on the station (see references). The selection code is a three character code, where the first character indicates missing data or instrumental problems, the second character indicates selection due to non-background criteria. The third character is not used for data selection. The available codes are: ... - No code applied. Data are considered 'background' *.. - Unable to compute a mole fraction or average I.. - No data available due to instrument calibration or malfunction. .V. - Large variability of CO2 mole fraction within one hour .D. - Hour-to-hour difference in mole fraction > 0.25 ppm .U. - Rejected, diurnal variation (upslope) in CO2 (Mauna Loa only) .S. - Single hour bracketed by flagged data. .N. - No unflagged data within +/- 8 hours. Assume non-background. .W. - Wind direction out of clean air sector. Assume non-background. Missing data will have a value of -999.99 for the hour, and "background" values will be designated by a '.' character for the first two characters. See also the 'Update_notes' file for information regarding other codes. ------------------------------------------------------------------- 8. DATA RETRIEVAL Users may transfer individual files or a single zipped file: https://gml.noaa.gov/aftp/data/trace_gases/co2/in-situ/surface/ Zipped files contain the README file and either netCDF files or ASCII text files depending on the zipped file name. (ex) co2_mlo_surface-insitu_1_ASCIItext.zip (ex) co2_mlo_surface-insitu_1_netCDF.zip Zipped files can be expanded using standard operating system utilities by double clicking downloaded file. ------------------------------------------------------------------- 9. REFERENCES Peterson, J.T., W.D. Komhyr, L.S. Waterman, R.H. Gammon, K.W. Thoning, and T.J. Conway, Atmospheric CO2 variations at Barrow, Alaska, 1973-1982, J. Atmos. Chem., 4, 491-510, 1986. Herbert, G.A., E.R. Green, J.M. Harris, G.L. Koenig, S.J. Roughton, and K.W. Thaut, Control and monitoring instrumentation for the continuous measurement of atmospheric CO2 and meteorological variables, J. Atmos. Oceanic Technol., 3, 414-421, 1986. Gillette, D.A., W.D. Komhyr, L.S. Waterman, L.P. Steele, and R.H. Gammon, The NOAA/GMCC continuous CO2 record at the South Pole, 1975-1982, J. Geophys. Res., 92, 4231-4240, 1987. Halter, B.C., Harris, J.M., and Conway, T.J., Component signals in the record of atmospheric carbon dioxide concentation at American Samoa, J. Geophys. Res., 93, 15914-15918, 1988. Komhyr, W.D., T.B. Harris, L.S. Waterman, J.F.S. Chin, and K.W. Thoning, Atmospheric carbon dioxide at Mauna Loa Observatory: 1. NOAA Global Monitoring for Climatic Change measurements with a nondispersive infrared analyzer, 1974-1985, J. Geophys. Res., 94, 8533-8547, 1989. Thoning, K.W., P.P. Tans, and W.D. Komhyr, Atmospheric carbon dioxide at Mauna Loa Observatory, 2. Analysis of the NOAA/GMCC data, 1974-1985., J. Geophys. Res. ,94, 8549-8565, 1989. Thoning, K.W. Selection of NOAA/GMCC CO2 data from Mauna Loa Observatory, In The Statistical Treatment of CO2 Data Records, NOAA Tech. Mem. (ERL-ARL-173), Environ. Res. Lab., 131 pp., 1989 Waterman, L.S., D. W. Nelson, W.D. Komhyr, T.B. Harris, and K.W. Thoning, Atmospheric carbon dioxide measurements at Cape Matatula, American Samoa, 1976-1984., J. Geophys. Res. , 94, 14817-14829, 1989. 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. -------------------------------------------------------------------