Atmospheric Methane Dry Air Mole Fractions from
NOAA GML Tall Tower and other Continental Sites 

Version: 2022-11-01

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
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:

Xin Lan
NOAA Global Monitoring Laboratory
325 Broadway, GML-1
Boulder, CO  80305

email: Xin.Lan@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.
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


Please reference these data as

   Andrews, A., Crotwell, A., Crotwell, M., Handley, P., Higgs, J., Kofler, J.,
   Lan, X., Legard, T., Madronich, M., McKain, K., Miller, J., Moglia, E., Mund,
   J., Neff, D., Newberger, T., Petron, G., Turnbull, J., Vimont, I., Wolter, S., &
   NOAA Global Monitoring Laboratory. (2022). NOAA Global Greenhouse Gas Reference
   Network Flask-Air PFP Sample Measurements of CH4 at Tall Tower and other
   Continental Sites, 2005-Present [Data set]. NOAA GML.
   https://doi.org/10.15138/35JE-6D55 Version: 2022-11-01.


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.


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.

Lab-wide notes:


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:


Datasets are provided in the self describing ObsPack format.
See https://gml.noaa.gov/ccgg/obspack/ for details.
Surface pfp event data are available in NetCDF and ASCII text.

Parameter-specific notes:


We updated and extended the WMO CH4 X2004 scale to X2004A.
Details are described on a dedicated web page: 


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)


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.

Air samples are collected in programmable flask packages (PFP) using
programmable compressor packages (PCP). The PCP contains battery-powered pumps
for flushing and pressuring the flasks. The PFP contains twelve 0.7L cylindrical
borosilicate glass flasks with glass valves on each end sealed with Teflon
O-rings, a stainless-steel manifold on each side of the flasks, and a data
logging and control system. 


For a summary of sampling locations, please visit


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



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.

   co2      Carbon dioxide
   ch4      Methane
   co2c13   d13C (co2)
   merge    more than one parameter

2. [site]

   Identifies the sampling site code.


3. [project]
   Identifies sampling platform and strategy.


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
  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.

   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). 


   txt           ASCII text file
   nc            netCDF4 file


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.


See individual files for description of the provided variables and other 
dataset metadata.


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

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.


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 PFP and PCP
 air sampling devices. The air sample is contained in a 0.7 liter borosilicate
 glass container (flask) fitted with 2 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 2
 diaphragm type pumps connected in series and controlled with a microprocessor.
 The flow rate is approximately 15 L/min at sea level. (Karion, A., Sweeney, C.,
 Wolter, S., Newberger, T., Chen, H., Andrews, A.,Kofler, J., Neff, D. and Tans,
 P., 2013. Long-term greenhouse gas measurements from aircraft. Atmos. Meas.
 Tech. 6, 511-526.)

    B - The manual filling of a whole air sample similar to method R but the air is
dried using one of several techniques. In one method, e.g. site=ACT, the air was
dried with a two stage chiller cooled to 5 degrees C. 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. At CRV tower, a two-stage chiller set to -26
degrees C is used. Baier et al. (2019): "Multi-species assessment of factors
influencing regional carbon enhancements during the wintertime ACT-America
campaign", https://doi.org/10.1029/2019JD031339.
    C - Air samples are filled using the PFP/PCP air collection system similar to
method A but the samples are dried before sample collection. These samples are
not pre-filled with ambient air before flushing and pressurizing the sample
    I - Air is sampled from a 15L spherical stainless steel mixing volume after
being filled with ambient air that was pulled through an M&C Techgroup Model ECP
20-2 gas chiller using an integrating compressor and mass flow controller.  The
two stainless steel dryer traps are plumbed in series, held at approximately 5
degrees C and the condensed water is removed with a peristaltic pump.  The
integration starts with a high flow rate of 3.8 SLM and decreases over time to
0.29 SLM over 1 hour.  The resulting one hour time averaged ambient air sample
is compressed into a 0.75 liter borosilicate glass container (flask) fitted with
2 Glass Expansion high vacuum glass valves and PTFE O-rings. The flask is
typically filled to 40 psia. (See Turnbull, J.C., Guenther, D., Karion, A.,
Sweeney, C., Anderson, E., Andrews, A.E., Kofler, J., Miles, N.L., Newberger,
T., Richardson, S.J., Tans, P.P., 2012. An integrated flask sample collection
system for greenhouse gas measurements. Atmos. Meas. Tech. 5, 2321-2327.)
    R - Air is collected with the PFP/PCP sampling apparatus, similar to method 'A',
but the glass containers are flushed and filled with ambient air and then vented
and flushed just before the actual air sample collection. This is a glass
conditioning step (known as prefilling) that addresses glass wall effects of the
gasses of interest.


All (ASCII text and netCDF) files are located in 

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. 


Tans, P.P. et al., Carbon Cycle (Group Report), Climate Monitoring
  and Diagnostics Laboratory, No. 23, Summary Report 1994-1995,
  Hoffman, D.J., J.T. Peterson and R.M. Rosson, eds, US Department
  of Commerce, Boulder, Colorado, 1996.