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.
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).
Measurements are reported in units of nanomol/mol (10^-9 mol CH4 per mol
of dry air (nmol/mol) or parts per billion (ppb))
For measurements using gas chromatography measurements, aliquots of sample
and standard gas are injected into a gas chromatograph (GC) alternatively
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.
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. The average repeatability has been approximately 3 ppb before 1991.
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.)
Repeatability after 1991 has ranged from 1 to 2 ppb over the period of our GC
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 reference air in a high-pressure cylinder. All air
samples are measured relative to 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).
-------------------------------------------------------------------
7. DATA - GENERAL COMMENTS
Uncertainties of the CH4 measurements are included.
Key components of it are our ability to propagate the WMO XCH4
scale to working standards, the repeatability of the analyzers
used for sample measurement, and the long-term variability of
the analyzer responses (reproducibility). The scale propagation
uncertainty is estimated to be 0.2 ppb, based on the reproducibility
determined for scale propagation in our calibration laboratory.
Long-term reproducibility is estimated to be +/- 0.5 ppb based on
repeated measurements 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. The
three terms are added in quadrature (square root of the sum of the
squares) to estimate the measurement uncertainty at 68% confidence
interval.
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
CH4 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 considering
multiple measured gases from the same air sample , and sample
conditions such as wind direction, wind speed, and back trajectory,
in addition to identifying outliers 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/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.
-------------------------------------------------------------------