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Atmospheric Carbon Dioxide Dry Air Mole Fractions from
the NOAA GML Carbon Cycle Cooperative Global Air Sampling
Network, 1968-2020
Version: 2021-07-30
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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
8. Data retrieval
9. References
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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:
Edward J. Dlugokencky or
Xin (Lindsay) Lan
NOAA Global Monitoring Laboratory
325 Broadway, R/GML-1
Boulder, CO 80305
U.S.A.
email: Xin.Lan@noaa.gov
Ed.Dlugokencky@noaa.gov
Pieter.Tans@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.
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
Dlugokencky, E.J., J.W. Mund, A.M. Crotwell, M.J. Crotwell, and
K.W. Thoning (2021), Atmospheric Carbon Dioxide Dry Air Mole
Fractions from the NOAA GML Carbon Cycle Cooperative Global Air
Sampling Network, 1968-2020, Version: 2021-07-30,
https://doi.org/10.15138/wkgj-f215
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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, 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.
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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.
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5. UPDATE NOTES
+++++++++++++++++++++++++++++++
Lab-wide notes:
2011-10-07
We introduced the term "measurement group", which identifies
the group within NOAA and 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)
We also changed the file naming convention (see section "DATA - FILE
NAME DESCRIPTION").
+++++++++++++++++++++++++++++++
Project-specific notes:
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 the year.
2021-07-28
In 2020, the covid-19 pandemic impacted flask shipping and sampling
frequency differently for different sites. For the most part
and thanks to everyone's efforts, the network sampling coverage
remained quite good in 2020.
Several sites still have samples from 2020 on site and they will be
analyzed once received.
2019-07-15
Surface flask event data is now provided in self describing NetCDF and ASCII text
formats with improved metadata. See section 7.3 for details.
We will provide a zip download of legacy text formatted files for a transitional period.
Monthly means data continues to be provided in text format.
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:
2021-02-08
CO2 measurements from flask-air samples were recalculated onto the
X2019 CO2 mole fraction scale by first reassigning all standards used
on the CO2 analysis sytems to X2019. CO2 values on X2019 from air
samples were caluclated by reprocessing the original raw files (i.e.,
files with raw analyzer output (typically voltages)) with the updated
assignments for the standards. A detailed description of the scale
revision from X2007 to X2019 is given in Hall et al. (2020), but the
main reasons were to correct biases caused by CO2 absorption by
O-rings in the manometric calibration system and to correct a virial
coefficient used in the calculation. The magnitiude of the differences
between X2019 and X2007 varies with CO2 abundance, and is typically
between 0.1 and 0.2 ppm. Some differences from the late-2000s and
early-2010s are larger (up to ~0.3 ppm) because of a mis-assigned
standard on the system used to transfer the scale to working standards.
Measurements prior to ~1980 from early analytical systems (instrument codes
LR1 and LR2) could not be reprocessed onto X2019; they remain on a CO2
scale originally developed at Scripps Institution of Oceanography.
2020-07-15
Based on comparison with in situ measurements at SPO and SYO
(from National Institute of Polar Research, Japan), a storage
correction was applied to flask-air samples measured for CO2.
The correction is ~0.2 ppm/yr at SPO and 0.08 ppm/yr at SYO.
Uncertainty for the correction has not yet been determined and
applied to the measurement uncertainty, but it is estimated to
be ~0.05 ppm.
2018-07-30
1. Uncertainties (68% c.i.) were estimated for all CO2
measurements. Uncertainity entries of -999.99 occur
when a reasonable uncertainty could not be calculated.
2. In 2017, began using N.. to flag samples not meeting
pair agreement criterion.
2016-07-07
We discovered a small leak in the intake line of the portable
sampler at Vestmannaeyjar, Iceland (ICE) that affected CO2.
All samples from 1 July 2009 through 28 June 2016 were
flagged N.. for CO2.
2012-08-03
Corrections for drifting reference gas cylinders have been
applied to all samples measured on the following instruments
through the specified date.
Instrument ID Analysis Date
L3 thru 2009-10
L8 thru 2010-06
L10 thru 2010-10
2011-10-13
Discrete (flask) CO2 mole fractions from 1980 to 2006 have all
been recalculated to bring them onto the WMO X2007 Mole Fraction
Scale (X2007). The data since 2007 were already on the X2007 scale.
WMO X2007 is based on repeated manometric measurements of the NOAA
primary standards (Zhao et al. 2006) and comparison of those results
to similar measurements made over a period of more than 10 years at
Scripps Institution of Oceanography. The difference between the X2007
and the previous scales propagated by SIO and NOAA (X83, X87, etc.)
is ~0.2 ppm (X2007-previous) in the 1980s, decreasing to ~0.1 ppm in
the late 1990s, and to 0.0 ppm by 2006. The recalculation of individual
measurements was accomplished by first determining the X2007 values for
the reference gases used to measure the air samples and then using those
values with the raw data (NDIR voltages) to recompute mole fractions
for each sample.
1999-01-01
Corrections have been applied to CO2 mole fractions measured
on the flask analysis apparatus in use from July 1987 through
April 1998. The corrections are +0.1 ppm to samples measured
from July 1987 through 18 November 1993, and +0.24 ppm to samples
measured from 19 November 1993 through 10 April 1998.
Corrections for drifting reference gas cylinders have been
applied to all samples measured during 1995 through April 2008
on system L3 and 1998 through May 2007 on systems S2 and L3.
Files obtained previously from this site should be discarded and
replaced with the revised files contained in this version.
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6. INTRODUCTION
CO2 dry air mole fractions reported in these files were
measured by a nondispersive infrared absorption
analyzer or cavity ring-down spectrometer (since Aug., 2019)
in air samples collected in glass flasks at
NOAA GML Carbon Cycle Cooperative Global Air Sampling
Network sites. Measurements are reported on the X2019 CO2
mole fraction scale. Calibration procedures are given in
Komhyr et al., 1983; Komhyr et al., 1985; Thoning et al.,
1987, and Thoning et al., 1995; and Tans et al.,2017.
Analysis and interpretation of the data have been reported
by Komhyr et al., 1985; Conway et al., 1988; Tans et al.,
1989a; Tans et al., 1990, and Conway et al., 1994.
The file co2_flask_system.ps is a postscript file
explaining the operation of the flask-air analysis system used
from July 1987 through April 1998.
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7. DATA - GENERAL COMMENTS
Carbon dioxide (CO2) in ambient and standard air samples is
detected using a non-dispersive infrared (NDIR) analyzer.
The measurement of CO2 in air is made relative to standards
whose CO2 mole fraction is determined with high precision and
accuracy. Because detector response is non-linear in the range
of atmospheric levels, ambient samples are bracketed during
analysis by a set of standards used to calibrate
detector response. Measurements are reported in units of
micromol/mol (10^-6 mol CO2 per mol of dry air or parts per
million (ppm)).
In August, 2019, we switched from a NDIR 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
~0.02 ppm (1 sigma).
Measurements are directly traceable to the WMO X2019 CO2 in air
mole fraction scale.
Uncertainties of the CO2 measurements are now included.
Key components of it are our ability to propagate the WMO XCO2
scale to working standards, the repeatability of the analyzers
used for sample measurement, and agreement between pairs of
samples collected simultaneously. Zhao and Tans (2006)
determined that the internal consistency of working standards
is +/- 0.02 ppm (68% confidence interval). The typical
repeatability of the analyzers, based on repeated
measurements of natural air from a cylinder, is +/- 0.03 ppm.
Average agreement between pairs of samples flushed in series
across the entire sampling network is +/- 0.07 ppm.
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/.
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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; 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]_[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 http://www.esrl.noaa.gov/gmd/ccgg/obspack/labinfo.html).
5. [measurement group]
Identifies the group within NOAA GML or INSTAAR that makes the actual measurement.
See Section 5 (UPDATE NOTES) for details.
(ex)
ccgg
hats
arl
sil
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
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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 file is derived directly from the netCDF file. The
text file is also self-describing and can be viewed using any 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.
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7.4 DATA - CONTENT
All (ASCII text and netCDF) files are located in
"ftp://aftp.cmdl.noaa.gov/data/trace_gases/co2/flask/surface/".
Files are named as follows (see Section 7.2 for details):
co2_[site]_surface-flask_1_ccgg_event.[file type]
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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.
Samples are collected in pairs, the pair
difference is calculated, and samples with
a pair difference greater than 0.5 ppm ("bad
pairs") are flagged. Through 1988 one or both
members of a bad pair were sometimes retained if
they fell within +/- 3 residual standard deviations
from a fitted curve. From 1989 to the present, both
members of bad pairs have been automatically
rejected. Retained and rejected flasks are
flagged as follows:
Flag Description
RETAINED ... (3 periods) good pair
(D <= 0.5 ppm)
..H high member of bad
pair; retained
..L low member of bad
pair; retained
..I sample has also been measured
by another lab as part of
an intercomparison experiment
SELECTED .X. flagged automatically as an outlier,
greater than 3 sigma from a fitted curve
.Z. flagged manually as an outlier
(this is necessary to prevent
distortion of the curve used for
automated data selection)
REJECTED +.. high member of bad
pair; rejected
-.. low member of bad
pair; rejected
*.. off scale or broken
flask; rejected
N.. rejected due to
error in sampling
or analysis
A.. rejected due to
error in analysis
T.. sample collected as
part of a methods
test; not used in
data analysis
The retained values comprise the data set that we feel
best represents the CO2 distribution in the remote,
well-mixed global or regional troposphere. These
are the values we use to calculate long-term trends
and interannual and seasonal variations in our studies
of the global carbon cycle. It is possible, and even
likely, that some of the values flagged as not
representative of background conditions are valid
measurements, but represent poorly mixed air masses
influenced by local or regional anthropogenic sources
or strong local biospheric sources or sinks. Users of
these data should be aware that data selection is a
difficult but necessary aspect of the analysis and
interpretation of atmospheric trace gas data sets,
and the specific data selection scheme used may be
determined by the goals of a particular investigation.
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8. DATA RETRIEVAL
Users may transfer individual files from a directory or may choose to
transfer a single zipped file. Zipped files contain the README file
and either netCDF files or ASCII text files depending on the zipped file
name.
(ex) co2_mlo_surface-flask_1_ASCIItext.zip
To transfer a zipped file, use the following steps from the ftp prompt:
1. ftp> binary ! set transfer mode to binary
2. ftp> get filename.zip ! transfer the file
3. ftp> bye ! leave ftp
4. $ unzip filename.zip ! uncompress your local copy
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9. REFERENCES
Ballantyne, A. P., C. B. Alden, J. B. Miller,
P. Tans and J. W. C. White, (2012), Increase
in observed net carbon dioxide uptake by land and
oceans during the past 50 years, Nature, 488,
7409, , 10.1038/nature11299
Conway, T.J., P.P. Tans, L.S. Waterman, K.W. Thoning,
D.R. Kitzis, K.A. Masarie, and N. Zhang, 1994, Evidence
for interannual variability of the carbon cycle from the
NOAA/CMDL global air sampling network, J. Geophys. Res.,99,
22831-22855.
Conway, T.J., P. Tans, L.S. Waterman, K.W. Thoning,
K.A. Masarie, and R.H. Gammon, 1988, Atmospheric
carbon dioxide measurements in the remote global
troposphere, 1981-1984, Tellus, 40B, 81-115.
Komhyr, W.D., L.S. Waterman, and W.R. Taylor, 1983,
Semiautomatic nondispersive infrared analyzer
apparatus for CO2 air sample analyses, J. Geophys.
Res., 88, 1315-1322.
Komhyr, W.D., R.H. Gammon, T.B. Harris, L.S. Waterman,
T.J. Conway, W.R. Taylor, and K.W. Thoning, 1985,
Global atmospheric CO2 distribution and variations
from 1968-1982 NOAA/GMCC CO2 flask sample data, J.
Geophys. Res., 90, 5567-5596.
Tans, P.P., T.J. Conway, and T. Nakazawa, 1989a,
Latitudinal distribution of the sources and sinks of
atmospheric carbon dioxide from surface observations
and an atmospheric transport model, J. Geophys. Res.,
94, 5151-5172.
Tans, P.P, K.W. Thoning, W.P. Elliott, and T.J. Conway,
1989b, Background atmospheric CO2 patterns from weekly
flask samples at Barrow, Alaska: Optimal signal recovery
and error esitmates, in NOAA Tech. Memo. (ERL ARL-173).
Environmental Research Laboratories, Boulder, CO, 131 pp.
Tans, P.P., I.Y. Fung, and T. Takahashi, 1990,
Observational constraints on the global atmospheric
CO2 budget, Science, 247, 1431-1438.
Tans, Pieter P., Andrew M. Crotwell and Kirk W. Thoning,
2017, Abundances of isotopologues and calibration of CO2
greenhouse gas measurements, Atmospheric Measurement
Techniques, 10, 7, 2669-2685, 10.5194/amt-10-2669-2017.
Thoning, K.W., P. Tans, T.J. Conway, and L.S.
Waterman, 1987, NOAA/GMCC calibrations of CO2-in-air
reference gases: 1979-1985. NOAA Tech. Memo. (ERL
ARL-150). Environmental Research Laboratories,
Boulder, CO, 63 pp.
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.
Thoning, K.W., T.J. Conway, N. Zhang, and D. Kitzis, 1995,
Analysis system for measurement of CO2 mixing ratios in
flask air samples, J. Atmos. and Oceanic Tech., 12, 1349-1356.
Zhao, C., and P.P. Tans (2006), Estimating uncertainty of the
WMO Mole Fraction Scale for carbon dioxide in air, J. Geophys.
Res. 111, D08S09, doi: 10.1029/2005JD006003.
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