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Atmospheric Carbon Dioxide Mixing Ratios from the
NOAA ESRL Carbon Cycle Cooperative Global Air
Sampling Network, 1968-2008

Version: 2009-07-15
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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 - Event
7.4      DATA - QC Flags
7.5      DATA - Monthly Averages
8.       Data retrieval
9.       References

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1. DATA SOURCE AND CONTACTS

National Oceanic and Atmospheric Administration (NOAA)
Earth System Research Laboratory (ESRL)
Global Monitoring Division (GMD)
Carbon Cycle Greenhouse Gases (CCGG)

Correspondence concerning these data should be directed to:

Pieter P. Tans and Thomas J. Conway
NOAA ESRL Global Monitoring Division
325 Broadway, GMD-1
Boulder, CO  80305
U.S.A.

email:    Pieter.Tans@noaa.gov
          Thomas.J.Conway@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
assure that ESRL receives fair credit for their work.  If the data 
are obtained for potential use in a publication or presentation, 
ESRL should be informed at the outset of the nature of this work.  
If the ESRL data are essential to the work, or if an important 
result or conclusion depends on the ESRL data, co-authorship
may be appropriate.  This should be discussed at an early stage in
the work.  Manuscripts using the ESRL data should be sent to ESRL
for review before they are submitted for publication so we can
insure that the quality and limitations of the data are accurately
represented.

2.1 CITATION

Please reference these data as 

   Conway, T.J., P.M. Lang, and K.A. Masarie (2009), Atmospheric
   Carbon Dioxide Dry Air Mole Fractions from the NOAA ESRL Carbon Cycle
   Cooperative Global Air Sampling Network, 1968-2008, Version: 2009-07-15,
   Path: ftp://ftp.cmdl.noaa.gov/ccg/co2/flask/event/. 

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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 ESRL 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

The 3-letter identification code for Lulin, Taiwan has been changed 
from LUL to LLN.

Corrections have been applied to CO2 mixing ratios 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 mixing ratios reported in these files were
measured by a nondispersive infrared absorption
technique in air samples collected in glass flasks at
NOAA ESRL Carbon Cycle Cooperative Global Air Sampling 
Network sites.  Descriptions of the sampling, measurement,
and calibration procedures are given in Komhyr et al.,
1983; Komhyr et al., 1985; Thoning et al., 1987, and 
Thoning et al., 1995.  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 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 reference
standards whose CO2 mixing ratio 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 reference
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)). Measurements are
directly traceable to the WMO CO2 mole fraction scale.
Measurement accuracy determined from repeated analyses of
CO2 in standard gas cylinders using an absolute manometric
technique is ~0.2 micromol/mol. Measurement precision
determined from repeated NDIR analysis of the same air is ~0.1
micromol/mol. Average agreement between pairs of flasks
sampled in series throughout the network is <0.2 micromol/mol. 

The Pacific Ocean Cruise (POC, travelling between the US west coast
and New Zealand or Australia) data have been merged and grouped into 
5 degree latitude bins.  For the South China Sea cruises (SCS) the 
data are grouped in 3 degree latitude bins.

Sampling frequencies are approximately weekly for the fixed sites
and average one sample every 3 weeks per latitude zone for POC and
about one sample every week per latitude for SCS.

The air samples are collected by two general methods:  flushing and
then pressurizing glass flasks with a pump, or opening a stopcock on
an evacuated glass flask.  During each sampling event, a pair of
flasks is filled.

7.1 DATA - SAMPLING LOCATIONS


Carbon Cycle Surface Flasks (ccg_surface).


The table includes the three letter code used to identify each 
site; the site name; latitude, longitude, and elevation (meters 
above sea level) of the sampling location; and country.

IMPORTANT NOTES: 
1.  Data for all species may not be available for all sites listed 
in the table.
2.  The exact location of a sampling site recorded in our database
may change or become better defined over time.  The latitude,
longitude, and altitude of a sample event is based on the best
information available at the time of sample collection.  Differences
in sample position associated with a particular site may be due
to the site moving or changes in technology that permit a more
accurate location determination.

To view near real-time data, manipulate and compare data, and create
custom graphs, please visit

http://www.esrl.noaa.gov/gmd/ccgg/iadv/.

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7.2 DATA - FILE NAME DESCRIPTION

Encoded into each file name are the sampling location, platform, and
strategy; measurement laboratory; file content; and gas identifier.
All file names use the following naming scheme:

 1     2               3     4         5          6           7
[site][data grouping]_[lab#][strategy][platform]_[qualifier].[gas]

1. [Sampling site]
   (ex) brw_, poc_, car_, amt_

2. [Grouping of data within the file]

   Data may be grouped by date, latitude, longitude, altitude, etc.
   
   If data are grouped by date then identifier has yyyy[mm][dd] format.
   (ex) brw2005_, amt200403_, lef20050315_

   If the sampling platform is an aircraft then the identifier is a
   3-character numeric field with units of 10^2 meters (hm) above sea level.
   (ex) car040_, haa005_

   If the sampling platform is a ship then the identifier is a 3-character 
   alphanumeric field with units of degrees (00-90).  Bins in the northern 
   and southern hemispheres are denoted as n## and s## respectively.  The 
   equatorial bin is denoted as 000.
   (ex) pocs25_, poc000_, scsn03_

3. [Measurement laboratory]

   A two character numeric field identifies the measurement laboratory (01-99).
   NOAA ESRL is lab number 01 (see https://om.cmdl.noaa.gov/globalview/labs/).

4. [Sampling strategy]

   A single alphanumeric character (0-9,a-z,A-Z) indicates the sampling strategy.

   _??C               Semi-continuous
   _??D               Discrete

5. [Sampling platform]

   A single alphanumeric character (0-9,a-z,A-Z) indicates the sampling platform.

   _???0              Land
   _???1              Ship
   _???2              Aircraft
   _???3              Tower

6. [Qualifier]

   An alphanumeric string describes the type of data included in the file.

   _????_event        Data from every collection event
   _????_mm           Computed monthly mean values
   _????_hr           Computed hourly averages (semi-continuous data only)
   _????_day          Computed daily averages (semi-continuous data only)
   _????_all          All Data

7. [Gas]

   Identifies the trace gas species.

   _????_???.co2      Carbon dioxide
   _????_???.ch4      Methane
   _????_???.co2c13   d13C (co2)

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7.3 DATA - EVENT

The data files "ccg/co2/flask/event/" use the following naming
scheme (see Section 7.2):

     xxx[data grouping]_01D[platform]_event.co2

(ex) pocn30_01D1_event.co2 contains all event data from 
     POC samples collected from a moving ship and grouped at 
     30N +/- 2.5 degrees.

(ex) brw_01D0_event.co2 contains all event data from
     BRW samples collected from a fixed position.

The data files contain multiple lines of header information
followed by one line for each atmospheric measurement.

Fields are defined as follows:

Field 1:    [SITE CODE] The three-character sampling location code (see above).

Field 2:    [YEAR] The sample collection date and time in UTC.
Field 3:    [MONTH]
Field 4:    [DAY]
Field 5:    [HOUR]
Field 6:    [MINUTE]

Field 7:    [FLASK ID] The sample container ID.

Field 8:    [METHOD] A single-character code that identifies 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 ESRL 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.

Field 9:    [TRACE GAS NAME] Gas identifier (e.g., co2, co2c13).

Field 10:   [MEASURED VALUE] Dry air mole fraction or isotopic composition.  
             Missing values are denoted by -999.99[9].

Field 11:   [QC FLAG] A three-character field indicating the results of our 
             data rejection and selection process, described in section 7.4.

Field 12:   [INSTRUMENT] A 2-character code that identifies the instrument 
             used for the measurement.

Field 13:   [YEAR] The measurement date and time in LT.
Field 14:   [MONTH]
Field 15:   [DAY]
Field 16:   [HOUR]
Field 17:   [MINUTE]

Field 18:   [LATITUDE] The latitude where the sample was collected, (negative (-)
             numbers indicate samples collected in the southern hemipshere).

Field 19:   [LONGITUDE] The longitude where the sample was collected, (negative (-)
             numbers indicate samples collected in the western hemisphere).

Field 20:   [ALTITUDE] The altitude where the sample was collected (masl).

Field 21:   [EVENT NUMBER] A long integer that uniquely identifies the sampling
             event.

Fields in each line are delimited by whitespace.

(ex)

   KEY 2004 12 17 15 00 2185-99 D co2 400.170 -.. L3 2005 01 03 13 52 25.6600 -80.2000 3.00 193357

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7.4 QC FLAGS

NOAA ESRL 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    COMMENT 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 determined.

If both the first and second column contain a period (e.g., "..." and "..I"),
the sample is RETAINED.

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 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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7.5 DATA - MONTHLY AVERAGES

The data files "ccg/co2/flask/month/" use the following naming
scheme (see Section 7.2):

     xxx[data grouping]_01D[platform]_mm.co2

(ex) pocn30_01D1_mm.co2 contains computed monthly mean values
     from POC samples collected from a moving ship and grouped at 
     30N +/- 2.5 degrees.

(ex) brw_01D0_mm.co2 contains computed monthly mean values 
     from BRW samples collected from a fixed position.

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

In these files a monthly mean value of -999.99 denotes months where there
are insufficient data to calculate a monthly mean.

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8. DATA RETRIEVAL

To transfer all files in a directory, it is more efficient to 
download the tar or zipped files.  

To transfer a tar file, use the following steps from the ftp prompt:

   1. ftp> binary                    ! set transfer mode to binary
   2. ftp> get filename.tar.gz       ! transfer the file
   3. ftp> bye                       ! leave ftp

   4. $ gunzip filename.tar.gz       ! unzip your local copy
   5. $ tar xvf filename.tar         ! unpack the file

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

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.

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.

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