Atmospheric SF6 Dry Air Mole Fractions from the
NOAA GML Carbon Cycle Cooperative Global Air
Sampling Network, 1997-2022

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) Group

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 Sulfur Hexafluoride 
   Dry Air Mole Fractions from the NOAA GML Carbon Cycle Cooperative Global Air 
   Sampling Network, 1997-2022, Version: 2023-08-28, https://doi.org/10.15138/p646-pa37


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. -------------------------------------------------------------------- 6. INTRODUCTION All samples were analyzed for SF6 at NOAA GML in Boulder, Colorado by gas chromatography with electron capture detection, and each sample aliquot was referenced to the NOAA X2014 SF6 standard scale The average repeatability of the analytical system is approximately 0.04 ppt. Data files in the subdirectories "ccg/sf6/flask/event" and "ccg/sf6/flask/month" list SF6 dry air mole fractions in pmol/mol based on measurements from the NOAA GML Carbon Cycle Cooperative Global Air Sampling Network. The subdirectory "event" contains all measurements from air samples collected at each site and the subdirectory "month" contains monthly means determined from the data files in "event". Network sampling sites are listed in section 7.1. ------------------------------------------------------------------- 7. DATA - GENERAL COMMENTS For measurements using gas chromatography, aliquots of gas from ambient air samples and a cylinder of reference gas are alternately injected into a gas chromatograph (GC) with a sampling valve. Sulfur hexafluoride (SF6) is separated from other sample components using packed columns and detected with an electron capture detector (ECD). The response of the ECD to SF6 is calibrated off-line about monthly relative to the reference with a suite of standards. Each aliquot of sample is measured relative to the same reference, and SF6 values are calculated from the response function. SF6 measurements are reported in units of picomol/mol (10^-12 mol SF6 per mol of dry air (pmol/mol) or parts per trillion (ppt)) relative to the NOAA 2014 SF6 standard scale. Reproducibility of our measurements (1 sigma) is approximately 0.01 nmol/mol, based on repeated analysis 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 SF6 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/sf6/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/sf6/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 Dlugokencky, E.J., L.P. Steele, P.M. Lang, and K.A. Masarie, 1994, The growth rate and distribution of atmospheric methane, J. Geophys. Res., 99, 17,021- 17,043. 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. Hall, B. D., G. S. Dutton, D. J. Mondeel, J. D. Nance, M. Rigby, J. H. Butler, F. L. Moore, D. F. Hurst and J. W. Elkins, (2011), Improving measurements of SF6 for the study of atmospheric transport and emissions, Atmospheric Measurement Techniques, 4, 11, , 10.5194/amt-4-2441-2011. -------------------------------------------------------------------