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Atmospheric Methane from quasi-continuous measurements at 
Barrow, Alaska and Mauna Loa, Hawaii, 1986-present.
 
National Oceanic and Atmospheric Administration (NOAA) 
Global Monitoring Laboratry (GML) 
Carbon Cycle Greenhouse Gases (CCGG) Group 

Version: 2024-07-19
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CONTENTS

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

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1. DATA SOURCE AND CONTACTS

These directories contain atmospheric CH4 in units of dry-air mole 
fraction from 2 NOAA GML observatories: Barrow, Alaska (BRW), 
and Mauna Loa, Hawaii (MLO). 

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

Dr. Xin Lan
NOAA Global Monitoring Laboratory
325 Broadway, R/GML-1
Boulder, Colorado, 80305 USA
Electronic Mail: xin.lan@noaa.gov


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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. 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.



2.1 CITATION

Please reference these data as

   K.W. Thoning, X. Lan, A.M. Crotwell, and J.W. Mund (2024), 
   Atmospheric methane from quasi-continuous measurements at 
   Barrow, Alaska and Mauna Loa, Hawaii, 1986-present. Version: 2024-07-19,  
   National Oceanic and Atmospheric Administration (NOAA),
   Global Monitoring Laboratory (GML), Boulder, Colorado, USA
   https://doi.org/10.15138/ve0c-be70

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3. LICENSE

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 CH4 measurements resumed on July 5, 2023. 2023-07-27 Barrow: Small revisions to the data from 2013 to 2022 were made based on improved assignments to the standard gases and improved determination of the analyzer response. These revisions were less than 0.5 ppb. Mauna Loa: Small revisions to the data from April 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 0.5 ppb. 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 CH4 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. 2021-08-31 Barrow: Transitioned to new intake lines on a new tower. Intake height changed from 16.46 m to 35.05 m. 2020-08-19 For measurements made with laser-based spectrometers (instrument = LGR* and PC*), value_std_dev can now be defined when nvalue (number of measurements) = 1. See Introduction for details. 2020-03-16 Measurements of CH4 at Mauna Loa, Hawaii were switched from a GC to a CRDS in April, 2019. Calibration strategy and measurement frequency changed. 2018-03-16 Measurement uncertainty added to all CH4 hourly averages. The column/variable previously labeled value_unc is now labeled value_std_dev and contains the hourly average standard deviation. See Section 6 (Introduction) for measurement uncertainty calculation. The daily and monthly average standard deviations (formerly value_unc) are now labled value_std_dev. 2016-01-20 All CH4 measurements are reported on the NOAA X2004A scale. https://gml.noaa.gov/ccl/ch4_scale.html 2014-08-12 Barrow in situ measurements through 2013 are now available. 2014-07-29 Barrow in situ measurements were restarted in April 2013. The 2013 data are not included in this update due to new instrumentation. 2013 data will be available sometime in the next few months. 2013-03-29 The CH4 in situ measurements at Barrow were suspended in June 2012. They will resume in 2013. -------------------------------------------------------------------- 6. INTRODUCTION NOAA GML began quasi-continuous measurements of atmospheric CH4 at Barrow, Alaska (BRW) in January, 1986 and at Mauna Loa, Hawaii (MLO) in April, 1987. Since the start of these measurements, our methods have evolved as described below. Analytical: Our first CH4 measurement systems at BRW and MLO were based on gas chromatography (GC) with flame ionization detection (FID) (for details, see Dlugokencky et al., 1995). At Barrow, from January 1986 through April 1996, 2 to 3 individual measurements were made each hour. From May 1996 through May 2012, 4 measurements were made each hour. At Mauna Loa, from April 1987 through November 1995, 2 to 3 individual measurements were made each hour, and from December 1995 to April 2019, 4 atmospheric measurements were made each hour. Our next-generation analytical systems use laser-based spectrometers to measure CH4. We began using a new system (off-axis, integrated cavity output spectroscopy, ICOS) at BRW in April 2013. There is a gap in the CH4 measurements at BRW from June, 2012 until April, 2013. 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. At MLO, we switched to a laser- based, cavity ringdown spectrometer (CRDS) in April 2019, and now use a similar data processing scheme to BRW. Calibration: GC/FID systems were calibrated with a single-point calibration; each aliquot of ambient air was bracketed by aliquots of standard gas, and CH4 in each air sample was calculated based on the ratio of the sample chromatographic peak response to the average response of the bracketing standards, and the assigned value for the standard cylinder. For laser-based spectrometers, the analyzer response is calibrated with a suite of standards every two weeks relative to a reference cylinder, and the reference cylinder is measured hourly to track and correct short term analyzer drift. As a quality assurance step, a well-calibrated "target" cylinder is measured up to twice per day. NOAA methane measurements are reported on the gravimetrically-prepared NOAA X2004A CH4 standard scale (see Dlugokencky et al., 2005; https://gml.noaa.gov/ccl/ch4_scale.html). Quality control: All measurements by GC were first edited using a rule-based editing algorithm (Masarie et al., 1991) to exclude measurements obtained when the analytical instrument was not working optimally. New algorithms were developed to edit data from the off-axis ICOS instrument at BRW since April, 2013 and the CRDS at MLO since April, 2019. Hourly averages: For GC/FID systems, hourly averages were calculated from all valid measurements within an hour and reported in our data files with the number of measurements (nvalue) and standard deviation (value_std_dev). For n=1, value_std_dev cannot be calculated. For spectrometer systems, 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 with a "..." selection flag. A ".C." flag is assigned to data identified as non-background. The criteria for determining background conditions are site specific. The background criteria for Barrow are when the wind is from the clean air sector (020-110 degrees) and wind speed is greater than 1 m/s for at least one hour prior to inclusion. The background criteria for Mauna Loa are during predominatly "downslope" meteorological conditions as indicated by local time of day, 0000-0659. See Dlugokencky et al. [1995] for details. Uncertainties: Measurement uncertainties (value_unc) are calculated for each measurement (individual aliquot on GC and 5-minute average on optical spectrometer) based on analytical repeatability and reproducibility, and our ability to propagate the WMO CH4 mole fraction standard scale. For GCs, analytical repeatability is based on the stability of standard or reference aliquots averaged over the period a particular instrument was used. It varies with analytical instrument from 5.5 to 0.9 ppb. For laser-based spectrometers, it is based on the standard deviations of the bracketing measurements of reference gas. Reproducibility is based on a comparison of near- simultaneous flask-air samples measured independently in Boulder and quasi-continuous hourly averaged measurements at MLO and BRW observatories. It is assessed from the median difference, and it ranges from 0.1 to 0.9 ppb. The scale propagation term is based on the uncertainty we assign to standards, 0.5 ppb. 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. For the hourly average uncertainty, repeatability is divided by sqrt n, where "n" is the number of observations used in the calculation. Reproducibility and scale propagation terms are applied once. Total uncertainty is calculated by adding the individual terms in quadrature (square root of the sum of the squares). Methane hourly average values outside the range of standards are assigned a default uncertainity (-999.99). ------------------------------------------------------------------- 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 we provide hourly, daily, and monthly averaged files. Daily averages are derived directly from the hourly data. Monthly averages are calculated from values extracted at one-day intervals from a smooth curve (Thoning et al., 1989) fitted to 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/ch4/in-situ/surface/". Files are named as follows (see Section 7.2 for details): ch4_[site]_surface-insitu_1_ccgg_HourlyData.[file type] ch4_[site]_surface-insitu_1_ccgg_DailyData.[file type] ch4_[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 ... - no flag applied *.. - Unable to compute a mole fraction or average .C. - data are non-background ------------------------------------------------------------------- 8. DATA RETRIEVAL Users may transfer individual files or a single zipped file: https://gml.noaa.gov/aftp/data/trace_gases/ch4/in-situ/surface/ Zipped files contain the README file and either netCDF files or ASCII text files depending on the zipped file name. (ex) ch4_mlo_surface-insitu_1_ASCIItext.zip (ex) ch4_mlo_surface-insitu_1_netCDF.zip Zipped files can be expanded using standard operating system utilities by double clicking downloaded file. ------------------------------------------------------------------- 9. REFERENCES 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 CMDL atmospheric dry air methane mole fractions to a gravimetrically-prepared standard scale, J. Geophys. Res., 110, D18306, doi : 10.1029/2005JD006035. Dlugokencky, E.J., L.P. Steele, P.M. Lang, and K.A. Masarie, Atmospheric methane at Mauna Loa and Barrow observatories: presentation and analysis of in situ measurements, J. Geophys. Res., 100, 23,103-23,113, 1995. Masarie, K.A., L.P. Steele, and P.M. Lang, A rule-based expert system for evaluating the quality of long-term, in situ, gas chromatographic measurements of atmospheric methane, NOAA Tech. Memo. ERL CMDL-3, NOAA Environ. Res. Lab., Boulder, Colorado, 1991. 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. -------------------------------------------------------------------