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Atmospheric Carbon Dioxide Mixing Ratios from the
NOAA CMDL Carbon Cycle Cooperative Global Air
Sampling Network, 1968-2004

Version: 2005-09-16.1139
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CONTENTS

1.       Data source and contacts
2.       Use of data
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 - Monthly
8.       Data retrieval
9.       References

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1. DATA SOURCE AND CONTACTS

National Oceanic and Atmospheric Administration (NOAA)
Climate Monitoring and Diagnostics Laboratory (CMDL)
Carbon Cycle Greenhouse Gases (CCGG)

Correspondence concerning these data should be directed to:

Pieter P. Tans and Thomas J. Conway
NOAA Climate Monitoring and Diagnostics Laboratory
325 Broadway, R/CMDL1
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.  CMDL relies on the ethics and integrity of the user to
assure that CMDL receives fair credit for their work.  If the data
are obtained for potential use in a publication or presentation,
CMDL should be informed at the outset of the nature of this work.
If the CMDL data are essential to the work, or if an important
result or conclusion depends on the CMDL data, co-authorship may
be appropriate.  This should be discussed at an early stage in
the work.  Manuscripts using the CMDL data should be sent to CMDL
for review before they are submitted for publication so we can
insure that the quality and limitations of the data are accurately
represented.

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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 CMDL 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

Please note that all event files now include the wind speed and wind
direction as recorded by the field operator during sampling collection.

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 2004.

Data selection (flagging) for 2003 and 2004 is not yet final.  
Only minor changes are expected.  These files will be updated
when final data selection is complete.

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 CMDL 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 NDIR analyses of
CO2 in standard gas cylinders using an absolute manometric
technique is ~0.2 micromol/mol. Measurement precision
determined from repeated 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


Table of NOAA CMDL cooperative air sampling sites.

The table includes the three letter code used to identify each 
site; the site name; latitude, longitude, and altitude (meters 
above sea level) of the sampling location; the start and end 
years for the co2 measurements; and sampling status.

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

http://www.cmdl.noaa.gov/ccgg/iadv/.

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7.2 DATA - FILE NAME DESCRIPTION

Encoded into each file name is 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_, scs_

2. [Grouping of data within the file]

   If not specified then the sampling site is at a single fixed position.
   (ex) brw_, asc_

   If the sampling platform is a ship and binned by latitude 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 CMDL is lab number 01.

4. [Sampling strategy]

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

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

5. [Sampling platform]

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

   _???0              Single Fixed Position
   _???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)

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
     from BRW samples collected from a fixed position.

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

Each line includes the

  SITE CODE, YEAR, MONTH, DAY, HOUR, MINUTE,
  FLASK ID, METHOD, GAS NAME, MEASURED VALUE, 
  QC FLAG, INSTRUMENT, YEAR, MONTH, DAY, HOUR, 
  MINUTE, LATITUDE, LONGITUDE, ALTITUDE, WIND 
  DIRECTION, WIND SPEED, and EVENT NUMBER.

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 25.66 -80.20 3.00 4.6 90 193357

Fields are defined as follows ...

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

  [YEAR, MONTH, DAY, HOUR, MINUTE] The sample collection date and time in UTC.

  [FLASK ID] The flask ID number.  This field may contain letters 
   as well as numbers.

  [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 CMDL 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.

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

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

  [QC FLAG] A three-character field indicating the results of our 
   data editing and selection process, described in section 7.3.1.

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

  [YEAR, MONTH, DAY, HOUR, MINUTE] The measurement date and time in LT.

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

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

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

  [WIND SPEED] Wind Speed (m/s) as reported by the field operator
   during sample collection.  Please note: CCCG has no means by which
   to assess the reliability of these recorded observations.

  [WIND DIRECTION] Wind Direction (degrees) as reported by the field 
   operator during sample collection.  Please note: CCGG has no means
   by which to assess the reliability of these recorded observations.

  [EVENT NUMBER] A long integer that uniquely identifies each CCGG
   flask sampling event.

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7.3.1 QC FLAGS

*************************************************************
WARNING: A QC flag assignment of "PRE" indicates the measurement
result is preliminary and has not yet been carefully examined by
the PI.  The "PRE" flag is removed once the quality of the
measurement has been determined.
*************************************************************

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.  (NOTE: if either the
first or second of these three characters is
not a period, the sample has been rejected.):

            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


Rejected    .X.         rejected as not
                        representative of 
                        background 
                        conditions (>3
                        sigma from a fitted 
                        curve)

            .Z.         rejected manually as
                        not representative of 
                        background conditions
	                (this is necessary to
                        prevent distortion of
                        the curve used for 
                        automated data selection)

            +..         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

            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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7.4 DATA - MONTHLY MEANS

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 do 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.

Each line includes the

  SITE CODE, YEAR, MONTH, and MEAN VALUE.

Fields in each line are delimited by whitespace.

(ex)

   KEY 2004 12   401.17

Fields are defined as follows ...

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

  [YEAR, MONTH] Year and month.

  [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

In addition to the text files found in each directory, there are
2 compressed tar files and 1 zipped file that contain all files
in the corresponding directory, e.g. event.tar.Z and event.zip 
contain all the files in the directory 'event'.  To transfer all
files in a directory, it is more efficient to download the tar
or zipped files.  To transfer the tar file, use the following 
steps from the ftp prompt:

   1. ftp> binary                 ! set transfer mode to binary
   2. ftp> get event.tar.Z        ! transfer the file
   3. ftp> bye                    ! leave ftp
   4. $ uncompress event.tar.Z    ! uncompress your local copy
   5. $ tar xvf event.tar         ! unpack the file

This will create a directory called 'event' and place each of
the files in this directory.  Repeat the steps for each tar file.

To transfer the zipped files to your local system follow steps
1-3 (above) and the "unzip".

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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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