--------------------------------------------------------------------- Atmospheric Methane Dry Air Mole Fractions from the NOAA GML Carbon Cycle Cooperative Global Air Sampling Network, 1983-2021 Version: 2022-11-21 -------------------------------------------------------------------- CONTENTS 1. Data source and contacts 2. Use of data 2.1 Citation 3. Reciprocity 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 -------------------------------------------------------------------- 1. DATA SOURCE AND CONTACTS 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 Dr. Edward J. Dlugokencky NOAA Global Monitoring Laboratory 325 Broadway, R/GML-1 Boulder, Colorado, 80305 USA Email: xin.lan@noaa.gov ed.dlugokencky@noaa.gov -------------------------------------------------------------------- 2. USE OF DATA 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. The availability of these data does not constitute publication of the data. NOAA relies on the ethics and integrity of the user to ensure that GML receives fair credit for their work. If the data are obtained for potential use in a publication or presentation, GML should be informed at the outset of the nature of this work. If the GML data are essential to the work, or if an important result or conclusion depends on the GML data, co-authorship may be appropriate. This should be discussed at an early stage in the work. Manuscripts using the GML data should be sent to GML for review before they are submitted for publication so we can ensure that the quality and limitations of the data are accurately represented. 2.1 CITATION Please reference these data as Lan, X., E.J. Dlugokencky, J.W. Mund, A.M. Crotwell, M.J. Crotwell, E. Moglia, M. Madronich, D. Neff and K.W. Thoning (2022), Atmospheric Methane Dry Air Mole Fractions from the NOAA GML Carbon Cycle Cooperative Global Air Sampling Network, 1983-2021, Version: 2022-11-21, https://doi.org/10.15138/VNCZ-M766 -------------------------------------------------------------------- 3. RECIPROCITY Use of these data implies an agreement to reciprocate. Laboratories making similar measurements agree to make their own data available to the general public and to the scientific community in an equally complete and easily accessible form. Modelers are encouraged to make available to the community, upon request, their own tools used in the interpretation of the GML data, namely well documented model code, transport fields, modeled mole fractions, and additional information necessary for other scientists to repeat the work and to run modified versions. Model availability includes collaborative support for new users of the models. -------------------------------------------------------------------- 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. 2022-07-20 In 2021, impacts of the COVID-19 pandemic on flask shipping and sampling continues at a few sites. Network sampling coverage remained reasonably good throughout 2021. 2021-07-29 In 2020, the COVID-19 pandemic impacted flask shipping and sampling frequency at many sites causing isolated gaps and some delayed processing into the 2nd half of 2021. In spite of this, network sampling coverage remained reasonably good throughout 2020. 2020-10-22 Sample lat/lon were revised for PSA. All prior entries were set to the correct lat/lon. 2020-07-16 Sample elevation was revised for ALT. All prior entries were set to the correct elevation. 2019-09-09 Sample elevation was corrected for AMY. All prior entries were set to the correct elevation. 2017-07-27 Method codes and sample locations were edited for accuracy. Edited sites are: ZEP, BKT, OXK, CGO, RPB, ASC, CHR, ICE, KEY, KUM, and TAP 2016-08-26 Latitude and longitude were adjusted for 3 sites: ALT Old: 82.4508 -62.5056 205.00 200.00 New: 82.4508 -62.5072 195.00 190.00 TAC Old: 52.5178 1.1389 236.00 56.00 New: 52.5177 1.1386 236.00 56.00 ZEP Old: 78.9067 11.8889 479.00 474.00 New: 78.9067 11.8883 479.00 474.00 Collaborator name corrected for UTA: Old: U.S. National Weather Service [NWS] New: Beth Anderson/NWS Cooperative Observer 2016-07-07 Incorrect sample dates from Ulaan Uul, Mogolia (UUM) from 20 Aug. 2013 through 30 Sept. 2015 were corrected on 13 May 2016. 2016-07-07 Since 24 Jan 2015, air samples from Negev Desert, Isreal (WIS) are collected at 29.9731N, 35.0567E, 156 masl; the old location was 30.8595N, 34.7809E, 482 masl. 2016-07-07 Since 03 Dec 2015, sampling in Natal, Brazil (NAT) was moved from 5.5147S, 35.2603W, 20 masl to 5.7952S, 35.1853W, 87 masl. 2015-11-20 Updated the content and format of event files to include elevation in meters above sea level (masl) and sample collection intake height in meters above ground level (magl). Elevation plus collection intake height equals altitude, which has always been included in the NOAA distribution. In adding these 2 fields, the event number column has moved. The new format is described in Sections 7.3 and 7.4. Users may find minor changes (from earlier distributions) to reported monthly mean values for the beginning months of data records. These changes are due to minor corrections to our curve fitting methods as first described by Thoning et al. (JGR,1989). 2015-08-03 The 3-letter site identification code for Ushuaia, Argentina (TDF) was changed to USH to be consistent with the WMO GAWSIS. 2013-08-27 Coordinates of some of the sample locations have changed. These changes improve the specified location based on new information. Changes tend to be minor and do not necessarily reflect a change in the actual sampling location. 2011-10-01 The data file format has been modified to include the measurement group and, additionally, the sample collection and analysis times now include second information (e.g., 2011 03 15 23 06 12). See section 7.3 for details. 2010-10-01 The format of the NOAA ESRL data records has been changed to include an estimate of the uncertainty associated with each measurement. The determination of the estimate is trace gas specific and described in section 6 (INTRODUCTION). +++++++++++++++++++++++++++++++ Parameter-specific notes: 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). 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. Over the full period of the record described here, the average repeatability has been approximately 2 ppb. 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.) Uncertainties are reported for each measurement based on analytical repeatability, reproducibility, and our ability to propagate the WMO CH4 mole fraction standard scale. Analytical repeatability is based on one of the methods above or on the average absolute value of pair agreement between pairs collected nearly simultaneously. It varies with analytical instrument from 0.2 to 2.3 ppb. Propagation of the scale is based on the reproducibility determined for scale propagation in our calibration laboratory. It has a fixed value of 0.5 ppb based on subsequent calibrations of the same cylinder at least one year after the first. Reproducibility is based on long-term variations in measurements of target cylinders, typically ~0.3 ppb. The three terms are added in quadrature (square root of the sum of the squares) to estimate the measurement uncertainty at 68% confidence interval. ------------------------------------------------------------------- 7. DATA - GENERAL COMMENTS Aliquots of sample and standard gas are injected into a gas chromatograph (GC) 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. Instrument response of the sample must be compared to a standard of known CH4 content. Measurements are reported in units of nanomol/mol (10^-9 mol CH4 per mol of dry air (nmol/mol) or parts per billion (ppb)) relative to the NOAA 2004A CH4 standard scale. Repeatability of our measurements, based on repeated analysis of air from a high-pressure cylinder, has ranged from 1 to 3 nmol/mol over the period of our 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 air, reference in a high-pressure cylinder. All air samples are measured relative 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). The uncertainty of our CH4 standard scale (NOAA 2004A) near 1800 ppb is estimated at +/- 0.2%, or about 3 nmol/mol. 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 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. ------------------------------------------------------------------- 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. -------------------------------------------------------------------