Atmospheric Methane Dry Air Mole Fractions from the
NOAA GML Carbon Cycle Aircraft Vertical Profile Network

Version: 2023-08-11

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
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:

Kathryn McKain
NOAA Global Monitoring Laboratory
325 Broadway, GML-1
Boulder, CO  80305

email:    kathryn.mckain@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

   McKain, K., Sweeney, C., Baier, B., Crotwell, A., Crotwell, M., Handley, P.,
   Higgs, J., Legard, T., Madronich, M., Miller, J. B., Moglia, E., Mund, J., Neff,
   D., Newberger, T., Wolter, S., & NOAA Global Monitoring Laboratory. (2023). NOAA
   Global Greenhouse Gas Reference Network Flask-Air PFP Sample Measurements of
   CO2, CH4, CO, N2O, H2, SF6 and isotopic ratios collected from aircraft vertical
   profiles [Data set]. Version: 2023-08-11. https://doi.org/10.15138/39HR-9N34


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: 2022-10-27 Datasets are provided in the self describing ObsPack format. See https://gml.noaa.gov/ccgg/obspack/ for details. Aircraft pfp event data are available in NetCDF and ASCII text. -------------------------------------------------------------------- 6. INTRODUCTION Methane dry air mole fractions reported in these files were measured in air samples collected at NOAA GML Carbon Cycle Aircraft Vertical Profile Network sites and during aircraft campaigns. Since its inception in 1992, the Global Greenhouse Gas Reference Network’s aircraft program has been dedicated to collecting air samples in vertical profiles at sites mostly over North America. The program's mission is to capture seasonal and inter-annual changes in trace gas mixing ratios throughout the boundary layer and free troposphere (up to 8000m/26,000 ft). Most Aircraft Network flights collect 12 flask samples at different fixed altitude levels using an automated Programmable Flask Package (PFP). These samples are stored in glass flasks for later analysis of carbon dioxide (CO2), carbon monoxide (CO), nitrous xxide (N2O), methane (CH4), molecular hydrogen (H2), and sulfur hexafluoride (SF6), as well as isotopes of CO2 and CH4, and multiple halo- and hydrocarbons. Data collected by the aircraft program provide a view of how the large-scale horizontal and vertical distribution of the measured trace gases change throughout a given year over the continent (Sweeney et al. 2015). The large-scale, three-dimensional picture of how trace mixing ratios change throughout the year has provided a means to estimate the contribution of the North America continent to the atmospheric concentration of long-lived tracers like carbonyl sulfide and CO2 (e.g. Montzka et al. 2007, Crevoisier et al. 2010) and serves as an essential benchmark for forward and inverse modeling (i.e. Stephens et al. 2007, Yang et al. 2007, Peters et al. 2007) and to calibrate and evaluate remote sensing retrievals (e.g. Wunch et al. 2010, Emmons et al. 2004). See https://gml.noaa.gov/ccgg/aircraft/ and Sweeney et al. 2015 for more details. ------------------------------------------------------------------- 7. DATA – MEASUREMENT METHODS All samples were analyzed for CH4 at NOAA GML in Boulder, Colorado by gas chromatography with flame ionization detection (prior to August 2019) or by cavity ring-down spectroscopy (since August, 2019). Each sample aliquot was referenced to the NOAA 2004A methane standard scale (see Dlugokencky et al. 2005, www.esrl.noaa.gov/gmd/ccl/ch4_scale.html). Measurements are reported in units of nanomol/mol (10^-9 mol CH4 per mol of dry air (nmol/mol) or parts per billion (ppb)). Prior to August 2019, aliquots of sample and standard gas were injected into a gas chromatograph (GC) with a sampling valve. Methane was separated from other sample constituents using packed columns and detected using flame ionization (FID). This process was highly automated for field and laboratory operations. Instrument response of the sample was compared to a standard of known CH4 content. Repeatability of our measurements, based on repeated analysis of air from a high-pressure cylinder, 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. ------------------------------------------------------------------- 7.1 DATA - SAMPLING LOCATIONS For a summary of sampling locations, please visit: https://gml.noaa.gov/dv/site/?program=ccgg or https://gml.noaa.gov/ccgg/aircraft/. 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 DATA - 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 DATA - COLLECTION METHODS A single-character code is used to identify the sample collection method. The codes are: A - The automated or manual filling of a whole air sample using the Programmable Flask Package (PFP) and Programmable Compressor Package (PCP)air sampling devices. The air sample is contained in a 0.7 liter borosilicate glass container (flask) fitted with two Glass Expansion high vacuum glass piston valves sealed with PTFE O-rings. The flasks are typically flushed with 10 liters (aircraft samples) or 75 liters (tower samples) of ambient air and then pressurized to 40 psia. Air is delivered through an intake tube using two diaphragm pumps connected in series and controlled with a microprocessor. The flow rate is approximately 15 L/min at sea level. See Sweeney et al. 2015 and Andrews et al. 2014 for more details. C - Air samples are collected as with method ‘A’, but the sample air is dried before sample collection. Flasks are not pre-filled with ambient air before flushing and collecting the air sample. R - Air samples are collected as with method 'A', but with an additional glass conditioning step known as “prefilling”. During prefilling, the glass containers are flushed and filled with ambient air and then vented and flushed immediately before air sample collection. This is a glass conditioning step that addresses glass wall effects of the gasses of interest (Andrews et al. 2014). B - Air samples are collected as with method ‘R’, but with an additional drying step using one of several techniques. In one method, e.g. site=ACT, air samples were dried with a two-stage chiller cooled to 5 degrees C; those taken below 3000 m MSL were also pressurized to 40 psia to further facilitate air sample drying (Baier et al. 2019). At tower sampling sites, M&C chillers with temperatures set to approximately 3 degrees C and sub-ambient pressure due to high flow are used (Andrew et al. 2014). At CRV tower, a two-stage chiller set to -26 degrees C is used. ------------------------------------------------------------------- 8. DATA RETRIEVAL All (ASCII text and netCDF) files are located in "https://gml.noaa.gov/aftp/data/trace_gases/ch4/pfp/aircraft/". 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 Andrews A. E., J. D. Kofler, M. E. Trudeau, J. C. Williams, D. H. Neff, K. A. Masarie, D. Y. Chao, D. R. Kitzis, P. C. Novelli, C. L. Zhao, E. J. Dlugokencky, P. M. Lang, M. J. Crotwell, M. L. Fischer, M. J. Parker, J. T. Lee, D. D. Baumann, A. R. Desai, C. O. Stanier, S. F. J. De Wekker, D. E. Wolfe, J. W. Munger, P. P. Tanks (2014) CO2, CO, and CH4 measurements from tall towers in the NOAA Earth System Research Laboratory’s Global Greenhouse Gas Reference Network: instrumentation, uncertainty analysis, and recommendations for future high- accuracy greenhouse gas monitoring efforts, Atmos. Meas. Tech., 7, 647–687, DOI: 10.5194/amt-7-647-2014. Baier B. C., C. Sweeney, Y. Choi, K. J. Davis, J. P. DiGangi, S. Feng, A. Fried, H. Halliday, J. Higgs, T. Lauvaux, B. R. Miller, S. A. Montzka, T. Newberger, J. B. Nowak, P. Patra, D. Richter, J. Walega, P. Weibring (2019) Multi-species assessment of factors influencing regional carbon enhancements during the wintertime ACT-America campaign", DOI: 10.1029/2019JD031339. Crevoisier C., C. Sweeney, M. Gloor, J. L. Sarmiento, and P. P. Tans (2010) Estimate of regional US carbon sinks from three-dimensional atmospheric CO2 sampling Proceedings of the National Academy of Sciences of the United States of America 107(43): 18348-18353 DOI: 10.1073/pnas.0900063107. 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. Emmons, L. K., M. N. Deeter, J. C. 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C. Krol and P. P. Tans (2007) An atmospheric perspective on North American carbon dioxide exchange: CarbonTracker, Proceedings of the National Academy of Sciences of the United States of America 104(48): 18925-18930, DOI: 10.1072/pnas.07089861074. Stephens, B. B., K. R. Gurney, P. P. Tans, C. Sweeney, W. Peters, L. M. P. Bruhwiler, P. Ciais, M. Ramonet, P. Bousquet, T. Nakazawa, S. Aoki, T. Machida, G. Inoue, N. Vinnichenko, J. Lloyd, A. Jordan, M. Heimann, O. Shibistova, R. Langenfelds, L. P. Steele, R. J. Francey and A. S. Denning (2007) Weak northern and strong tropical land carbon uptake from vertical profiles of atmospheric CO2, Science, 316(5832), 1732-1735, DOI: 10.1126/science.1137004. Sweeney, C., A. Karion, S. Wolter, T. Newberger, D. Guenther, J. A. Higgs, A. E. Andrews, P. M. Lang, D. Neff, E. Dlugokencky, J. B. Miller, S. A. Montzka, B. R. Miller, K. A. Masarie, S. C. Biraud, P. C. Novelli, M. Crotwell, A. M. Crotwell, K. Thoning, and P. P. Tans (2015) Seasonal climatology of CO2 across North America from aircraft measurements in the NOAA/ESRL Global Greenhouse Gas Reference Network, J. Geophys. Res. Atmos., 120, DOI: 10.1002/2014JD022591. Wunch, D., G. C. Toon, P. O. Wennberg, S. C. Wofsy, B. B. Stephens, M. L. Fischer, O. Uchino, J. B. Abshire, P. Bernath, S. C. Biraurd, J. F. L. Blavier, C. Boone, K. P. Bowman, E.V . Browell, T. Campos, B. J. Connor, B. C. Daube, N. M. Deutscher, M. Diao, J. W. Elkins, C. Gerbing, E. Gottlieb, D. W. T. Griffith, D. F. Hurst, R. Jimenez, G. Keppel-Aleks, E. A. Kort, R. Macatangay, T. Machida, H. Matsueda, F. Moore, I. Morino, S. Park, J. Robinson, C. M. Roehl, Y. Sawa, V. Sherlock, C. Sweeney, T. Tanaka, and M. A. Zondlo (2010). Calibration of the Total Carbon Column Observing Network using aircraft profile data, Atmospheric Measurement Techniques 3(5): 1351-1362, DOI: 10.5194/amt-3-1351-2010. Yang, Z., R. A. Washenfelder, G. Keppel-Aleks, P. O. Wennberg, N. Y. Krakauer, J. T. Randerson, P. P. Tans, and C. Sweeney (2007) New constraints on Northern Hemisphere growing season net flux. Geophysical Research Letters, 34 (12), DOI: 10.1029/2007GL029742. -------------------------------------------------------------------