Atmospheric Carbon Monoxide Dry Air Mole Fractions from 
the NOAA GML Carbon Cycle Cooperative Global Air Sampling 
Network, 1988-2022

Reference scale:  WMO CO_X2014A

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:

Gabrielle Petron
NOAA Global Monitoring Laboratory
325 Broadway, GML-1
Boulder, Colorado, 80305 USA

Electronic Mail: gabrielle.petron@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

   G. Petron, A.M. Crotwell, M.J. Crotwell, E. Dlugokencky, 
   M. Madronich, E. Moglia, D. Neff, K. Thoning, S. Wolter, J.W. Mund (2023), 
   Atmospheric Carbon Monoxide Dry Air Mole Fractions 
   from the NOAA GML Carbon Cycle Cooperative Global Air Sampling Network, 
   1988-2022, Version: 2023-08-28, https://doi.org/10.15138/33bv-s284


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: The carbon monoxide data contained in these files supercede all previous measurement results. Data are referenced to the most recent WMO scale - WMO CO_X2014A. Released in December 2015, the new scale was developed using the calibration results of secondary standards against six sets of primary standards. Three of these were not previously available. The additional primary standards allow better estimation of drift in the secondary standards. The conversion of previous results to the new scale used the archived instrument signals from the sample and standard measurements. The original calibration curves used for the GC-HgO instruments were reconstructed using the raw instrument signals of the standards with their new CO assignements on the X2014A scale. The sample mole fractions were recalculated against the new calibration curves. A description of CO_X2014A and the revision of previous measurements are available at: http://www.esrl.noaa.gov/gmd/ccl/co_scale.html Until the 2017 data release, if a flask pair differed by less than 3 ppb the samples were accepted, otherwise they were flagged and not used in further analysis (Novelli et al., 1998). Given the VUV measurement noise in recent years and the variability in atmospheric CO, we use a CO pair agreement criteria of 4 ppb for some pairs analyzed between 2016 and 2019. This allowed us to retain more sample pairs while still requiring a reasonable agreement between paired flasks. Data from the new TILDAS instrument use a strict 3 ppb pair agreement criteria. At a few sites with in-situ measurement systems, a flask pair is collected using the in-situ sampling line ("through analyzer" or method "S" flasks) in addition to the flask pair collected with the portable sampling unit (PSU). By default, the S flasks have been rejected for CO results. SMO S flask CO data are now available from 2009-06-05 onwards. SPO S flask CO data are available from 1993-04-19 onwards. One member of the flask pair from the Cape Grim Observatory (CGO) is first analyzed by CSIRO (the comparison flask). For several years, we often noticed large pair differences for CO, with the comparison flask being higher than its mate. This suggested a potential contamination in the comparison flask for CO. A significant number of CGO CO flask pairs have been flagged prior to mid 2019 due to large pair differences. To retain valid samples and measurements, only the CGO comparison flasks with suspected contamination were flagged "N.." (or "N.I") for CO. The CGO CO results for flasks analyzed solely at NOAA have been retained (first column "." flag) if all other data quality criteria (besides flask pair agreement) were met. The CGO comparison flask issue was resolved by CSIRO in early 2019. The flask data will be reprocessed and uncertainty estimates will be available with the next CO calibration scale revision, which we hope to complete soon. 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. -------------------------------------------------------------------- 6. INTRODUCTION For the past three decades CO has been measured in discrete samples of air collected as part of the NOAA Collaborative Global Air Sampling Network. Three analytical methods were used. From 1988 to 2008 instruments based on gas chromatography and HgO reduction detection were used (GC-HgO, instruments from Trace Analytical Inc.) The instruments had a nonlinear response over the range of the remote troposphere (Novelli et al., 1991). Response curves composed of 4-8 standards defined instrument response (Novelli et al., 1994, 1998). In 2008 an analyzer based on CO fluorescence in the vacumm ultra violet (VUV, Gerbig et al.,1996) replaced one of the two GCs on the measurement system. The remaining GC and the VUV instrument were calibrated using a common set of reference gases, ranging from 50 to 350 ppb. In 2010 the last GC was replaced by a VUV instrument. Since August 2019, all samples are analyzed for CO by Tunable Infrared Laser Direct Absorption Spectroscopy. The TILDAS instrument is calibrated regularly using 11 standards, ranging from 23 to 486 ppb. In this range, the instrument response is linear. The air samples were collected on site in evacuated glass flasks or using a portable air sampling pump package. Two air samples were collected in series nearly simultaneously, constituting a pair (Lang et al., 1989 a,b). Quality of the air samples is evaluated by the difference between the two flasks. For most pairs, if they differed by < three ppb the samples were accepted, otherwise they are flagged and not used in further analysis (Novelli et al., 1998). If one or both flasks have a sampling or analysis issue, the corresponding data is flagged with N or A in the flag's first column and it should not be used. Data flags are described in details in Section 7.4. A review of the four sets of primary standards prepared between 1989-2000 suggested our working standards were drifting upward at rates of ~0.5-1 ppb year. A time dependant correction was applied to all air samples measured through 12/2000 (Novelli et al., 2003). It is now believed primary standards prepared in 1999/2000 were biased. They were assigned new mole fractions based on their calibrations versus primary standards prepared in 2006, 2011 and 2015 and the flask air measured against these standards were re-calculated. Instruments used in the analysis are identified in the data string by their ID. The GC-HgO instruments were R2,CS, R5 and R6. The VUV instruments are V2, V3 and V4. The TILDAS instrument is AR2. The flask data will be reprocessed when the CO calibration scale is updated and uncertainty estimates will be provided. ------------------------------------------------------------------- 7. DATA - GENERAL COMMENTS Carbon monoxide mixing ratios in these files are reported in units of nmol/mol (10^-9 mole CO per mole of dry air or as part per billion by mole fraction (ppb)) relative to the NOAA/WMO CO scale (Novelli et al., 1991, Novelli et al., 2003). The reproducibility of the measurements, estimated from repeated analysis of air contained in a high-pressure cylinder, is ~1-2 ppb using GC-HgO, 0.5 ppb or better for the VUV instruments, and 0.1 ppb for the TILDAS. Two air samples are collected on site nearly simultaneously in glass flasks and constitute a pair. Samples are typically collected using a semi-automated unit which incorporates a powerful battery, a high capacity pump, ~5 m intake line, and a back pressure regulator to control the pressure of air in the flasks. Two air samples are collected in series and pressured to ~1.5 atm. At a few sites air is sampled in evacuated glass flasks. Over time several versions of glass flasks and automatic samplers have been used; they are indentified in the data string. Sampling frequencies are approximately weekly for land sites. For close to three decades ships transecting the Pacific Ocean collected one sample pair every three weeks per five degree latitude zone. Cruise tracks for the Pacific sampling program ran between the US west coast and New Zealand or Australia. In the South China Sea one bin sample pair was collected per week per 3 degree latitude. Details of the Cooperative Air Sampling Program can be found at https://gml.noaa.gov/ccgg/flask.html 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 CO 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/co/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/co/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 Lang, P.M., L.P. Steele, R.C. Martin, and K.A. Masarie, Atmospheric methane data for the period 1983-1985 from the NOAA/GMCC global cooperative flask sampling network, NOAA Technical Memorandum ERL CMDL-1, 1990a. Lang, P.M., L.P. Steele, and R.C. Martin, Atmospheric methane data for the period 1986-1988 from the NOAA/CMDL global cooperative flask sampling network, NOAA Technical Memorandum ERL CMDL-2, 1990b. Gerbig, C., S. Schmitgen et al., An improved fast-response vacuum-UV resonance fluorescence CO instrument, J. Geophys. Res., 104, 1699-1704, 1999. Novelli, P.C., J.E. Elkins, and L.P. Steele, The development and evaluation of a gravimetric reference scale for measurements of atmospheric carbon monoxide, J. Geophys. Res., 96, 13,109-13,121, 1991. Novelli, P.C., L.P. Steele, and P.P. Tans, Mixing ratios of carbon monoxide in the troposphere, J. Geophys. Res., 97, 20,731-20,750, 1992. Novelli, P.C., J.E. Collins, Jr, R.C. Myers, G.W. Sachse, and H.E. Scheel, Reevaluation of the NOAA/CMDL carbon monoxide reference scale and comparisons to CO reference gases at NASA-Langley and the Fraunhofer Institute, 99, 12,833- 12,839, 1994. Novelli, P.C., K.A. Masarie, and P.M. Lang, Distributions and recent changes in carbon monoxide in the lower troposphere, J. Geophys. Res., 103, 19,1015- 19,033, 1998. Novelli, P.C., K.A. Masarie, P.M. Lang, B.D. Hall, R.C. Myers, and J.W. Elkins, Re-analysis of tropospheric CO trends: Effects of the 1997-1998 wild fires, J. Geophys. Res., 108, D15 : 4464, doi:10.1029/2002JD003031, 2003. -------------------------------------------------------------------