Atmospheric Carbon Monoxide Dry Air Mole Fractions from 
the NOAA GML Carbon Cycle Cooperative Global Air Sampling 
Network, 1988-2021

Reference scale:  WMO CO_X2014A

Version: 2022-07-28

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
7.7      DATA - Monthly Averages
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:

Gabrielle Petron
NOAA Global Monitoring Laboratory
325 Broadway, GML-1
Boulder, Colorado, 80305 USA

Electronic Mail: gabrielle.petron@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

   G. Petron, A.M. Crotwell, M.J. Crotwell, E. Dlugokencky, 
   M. Madronich, E. Moglia, D. Neff, K. Thoning, S. Wolter, J.W. Mund (2022), 
   Atmospheric Carbon Monoxide Dry Air Mole Fractions 
   from the NOAA GML Carbon Cycle Cooperative Global Air Sampling Network, 
   1988-2021, Version: 2022-07-28, https://doi.org/10.15138/33bv-s284


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 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 


Project-specific notes:


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.


In 2021, impacts of the COVID-19 pandemic on flask shipping and sampling continues
at a few sites. Network sampling coverage remained reasonably good throughout 2021.


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 2020.


Sample lat/lon were revised for PSA.  All prior entries
were set to the correct lat/lon.


Sample elevation was revised for ALT.  All prior entries
were set to the correct elevation.


Sample elevation was corrected for AMY.  All prior entries
were set to the correct elevation.


Method codes and sample locations were edited for accuracy.
Edited sites are: ZEP, BKT, OXK, CGO, RPB, ASC, CHR, ICE,


Latitude and longitude were adjusted for 3 sites:
Old: 82.4508  -62.5056   205.00   200.00
New: 82.4508  -62.5072   195.00   190.00

Old: 52.5178    1.1389   236.00    56.00
New: 52.5177    1.1386   236.00    56.00

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


Incorrect sample dates from Ulaan Uul, Mogolia (UUM) from
20 Aug. 2013 through 30 Sept. 2015 were corrected on
13 May 2016.


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.


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.


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).


The 3-letter site identification code for Ushuaia, Argentina (TDF) was
changed to USH to be consistent with the WMO GAWSIS.


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.


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.


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:

The carbon monoxide data contained in these files supercede 
all previous measurement results. Data are referenced to the 
most recent WMO scale - WMO CO_X2014A.

Released in December 2015, the new scale was developed using the
calibration results of secondary standards against six sets 
of primary standards. Three of these were not previously 
available. The additional primary standards allow better 
estimation of drift in the secondary standards.  

The conversion of previous results to the new scale used the
archived instrument signals from the sample and standard
measurements. The original calibration curves used for the GC-HgO 
instruments were reconstructed using the raw instrument signals 
of the standards with their new CO assignements on the X2014A 
scale. The sample mole fractions were recalculated against the 
new calibration curves. 

A description of CO_X2014A and the revision of previous measurements
are available at: http://www.esrl.noaa.gov/gmd/ccl/co_scale.html

Until the 2017 data release, if a flask pair differed 
by less than 3 ppb the samples were accepted, otherwise they were 
flagged and not used in further analysis (Novelli et al., 1998). 
Given the VUV measurement noise in recent years and the variability 
in atmospheric CO, we use a CO pair agreement 
criteria of 4 ppb for some pairs analyzed between 2016 and 2019. 
This allowed us to retain more sample pairs while still requiring 
a reasonable agreement between paired flasks. Data from the new 
TILDAS instrument use a strict 3 ppb pair agreement criteria.

At a few sites with in-situ measurement systems, a flask pair is collected 
using the in-situ sampling line ("through analyzer" or method "S" flasks) 
in addition to the flask pair collected with the portable sampling unit (PSU).
By default, the S flasks have been flagged "S.." for CO results. SMO S flask 
CO data are now available from 2009-06-05 onwards.

One member of the flask pair from the Cape Grim Observatory (CGO) is first 
analyzed by CSIRO (the comparison flask). For several years, we often noticed 
large pair differences for CO, with the comparison flask being higher 
than its mate. This suggested a potential contamination 
in the comparison flask for CO. 
A significant number of CGO CO flask pairs have been flagged prior to mid 2019 
due to large pair differences. To retain valid samples and measurements, 
only the CGO comparison flasks with suspected contamination were flagged 
"N.." (or "N.I") for CO. The CGO CO results for flasks analyzed solely at NOAA 
have been retained (first column "." flag) if all other data quality criteria 
(besides flask pair agreement) were met. The CGO comparison flask issue 
was resolved by CSIRO in early 2019.

The flask data will be reprocessed and uncertainty estimates
will be available with the next CO calibration scale revision, 
which we hope to complete soon.


For the past three decades CO has been measured in discrete samples
of air collected as part of the NOAA Collaborative Global Air
Sampling Network. Three analytical methods were used.  From 1988 to 
2008 instruments based on gas chromatography and HgO reduction detection
were used (GC-HgO, instruments from Trace Analytical Inc.)  
The instruments had a nonlinear response over the range 
of the remote troposphere (Novelli et al., 1991).  Response curves 
composed of 4-8 standards defined instrument response (Novelli et al., 
1994, 1998).

In 2008 an analyzer based on CO fluorescence in the vacumm ultra violet
(VUV, Gerbig et al.,1996) replaced one of the two GCs on the measurement 
system. The remaining GC and the VUV instrument were calibrated using a 
common set of reference gases, ranging from 50 to 350 ppb. In 2010 the 
last GC was replaced by a VUV instrument.  

Since August 2019, all samples are analyzed for CO by Tunable 
Infrared Laser Direct Absorption Spectroscopy. The TILDAS instrument 
is calibrated regularly using 11 standards, ranging from 23 to 486 ppb.
In this range, the instrument response is linear.

The air samples were collected on site in evacuated glass flasks 
or using a portable air sampling pump package. Two air samples were 
collected in series nearly simultaneously, constituting a pair (Lang 
et al., 1989 a,b). Quality of the air samples is evaluated by the 
difference between the two flasks. For most pairs, if they differed 
by < three ppb the samples were accepted, otherwise they are flagged 
and not used in further analysis (Novelli et al., 1998). If one or both 
flasks have a sampling or analysis issue, the corresponding data 
is flagged with N or A in the flag's first column and it should not 
be used. Data flags are described in details in Section 7.4.

A review of the four sets of primary standards prepared between 
1989-2000 suggested our working standards were drifting upward at 
rates of ~0.5-1 ppb year. A time dependant correction was applied to 
all air samples measured through 12/2000 (Novelli et al., 2003). It 
is now believed primary standards prepared in 1999/2000 were biased.  
They were assigned new mole fractions based on their calibrations
versus primary standards prepared in 2006, 2011 and 2015 and the 
flask air measured against these standards were re-calculated. 

Instruments used in the analysis are identified in the data string
by their ID.  The GC-HgO instruments were R2,CS, R5 and R6.  
              The VUV instruments are V2, V3 and V4.
              The TILDAS instrument is AR2.

The flask data will be reprocessed when the CO calibration scale is updated 
and uncertainty estimates will be provided.


Carbon monoxide mixing ratios in these files are reported 
in units of nmol/mol (10^-9 mole CO per mole of dry air 
or as part per billion by mole fraction (ppb)) relative
to the NOAA/WMO CO scale (Novelli et al., 1991, Novelli 
et al., 2003).  The reproducibility of the measurements, 
estimated from repeated analysis of air contained in a 
high-pressure cylinder, is ~1-2 ppb using GC-HgO, 0.5 
ppb or better for the VUV instruments, and 0.1 ppb
for the TILDAS.

Two air samples are collected on site nearly simultaneously 
in glass flasks and constitute a pair. Samples are typically
collected using a semi-automated unit which incorporates a
powerful battery, a high capacity pump, ~5 m intake line,
and a back pressure regulator to control the pressure of 
air in the flasks.  Two air samples are collected in series         
and pressured to ~1.5 atm.  At a few sites air is sampled 
in evacuated glass flasks. Over time several versions of glass 
flasks and automatic samplers have been used; they are 
indentified in the data string.  

Sampling frequencies are approximately weekly for land sites.
For close to three decades ships transecting the Pacific Ocean 
collected one sample pair every three weeks per five degree 
latitude zone. Cruise tracks for the Pacific sampling program 
ran between the US west coast and New Zealand or Australia. 
In the South China Sea one bin sample pair was collected 
per week per 3 degree latitude. 

Details of the Cooperative Air Sampling Program can be found 
at https://gml.noaa.gov/ccgg/flask.html                                             

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.


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 NOAA GML or INSTAAR that makes the actual measurement.
   See Section 5 (UPDATE NOTES) for details.


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.


All (ASCII text and netCDF) files are located in 

Files are named as follows (see Section 7.2 for details):

     co_[site]_surface-flask_1_[group]_[selection].[file type]


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:

         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       

         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


The monthly data files in https://gml.noaa.gov/aftp/data/trace_gases/co/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


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. 


Lang, P.M., L.P. Steele, R.C. Martin, and K.A. Masarie,
  Atmospheric methane data for the period 1983-1985 from
  the NOAA/GMCC global cooperative flask sampling network,
  NOAA Technical Memorandum ERL CMDL-1, 1990a.

Lang, P.M., L.P. Steele, and R.C. Martin, Atmospheric
  methane data for the period 1986-1988 from the NOAA/CMDL
  global cooperative flask sampling network, NOAA Technical
  Memorandum ERL CMDL-2, 1990b.

Gerbig, C., S. Schmitgen et al., An improved fast-response
  vacuum-UV resonance fluorescence CO instrument, J. Geophys.
  Res., 104, 1699-1704, 1999.

Novelli, P.C., J.E. Elkins, and L.P. Steele, The development
  and evaluation of a gravimetric reference scale for
  measurements of atmospheric carbon monoxide, J. Geophys.
  Res., 96, 13,109-13,121, 1991.

Novelli, P.C., L.P. Steele, and P.P. Tans, Mixing ratios of
  carbon monoxide in the troposphere, J. Geophys. Res., 97,
  20,731-20,750, 1992.

Novelli, P.C., J.E. Collins, Jr, R.C. Myers, G.W. Sachse,
  and H.E. Scheel, Reevaluation of the NOAA/CMDL carbon
  monoxide reference scale and comparisons to CO reference
  gases at NASA-Langley and the Fraunhofer Institute, 99,
  12,833- 12,839, 1994.

Novelli, P.C., K.A. Masarie, and P.M. Lang, Distributions
  and recent changes in carbon monoxide in the lower
  troposphere, J. Geophys. Res., 103, 19,1015- 19,033, 1998.

Novelli, P.C., K.A. Masarie, P.M. Lang, B.D. Hall, R.C. Myers,
  and J.W. Elkins, Re-analysis of tropospheric CO trends:
  Effects of the 1997-1998 wild fires, J. Geophys. Res., 108,
  D15 : 4464, doi:10.1029/2002JD003031, 2003.