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Atmospheric Carbon Dioxide Dry Air Mole Fractions from 
the NOAA GML Carbon Cycle Cooperative Global Air Sampling 
Network, 1968-2019

Version: 2021-03-05
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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 with single parameter
7.4      DATA - Event with multiple parameters
7.5      DATA - QC Flags
7.6      DATA - Monthly Averages
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, 1968-2019, Version: 2021-02,
   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:

2020-10-22

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

2020-07-16

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

2019-09-09

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

2016-07-07

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).

2013-08-27

Coordinates of some of the sample locations were revised 
to reflect improved information. Changes are minor. 

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 seconds (e.g., 2011 03 15 23 06 12).  See section 7.3 
for details.

2010-10-01

The format of the NOAA GML 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.

Pacific Ocean Cruise (POC, travelling between the US west coast
and New Zealand or Australia) data were 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 intervals 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.

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.

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7.1 DATA - SAMPLING LOCATIONS

For a summary of sampling locations, please visit

http://www.esrl.noaa.gov/gmd/dv/site/site_table.html.

IMPORTANT NOTES: 
1.  Data may not be available for all species at all sites listed 
in the table.
2.  The exact locations of a sampling sites 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/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 - EVENT WITH SINGLE PARAMETER

The event data files in ftp://aftp.cmdl.noaa.gov/data/trace_gases/co2/flask/surface/ 
use the following naming scheme (see Section 7.2):

     [parameter]_[site]_[project]_[lab ID number]_[measurement group]_[optional qualifiers].txt

(ex) CH4_pocn30_surface-flask_1_ccgg.txt contains CH4 ccgg measurement
     results 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.txt contains CO2 ccgg measurement 
     results for all surface flask samples collected at Barrow, Alaska.

The data files contain multiple lines of header information followed by one 
record for each atmospheric measurement of a single parameter or trace gas species.

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:    [SECOND]

Field 8:    [FLASK ID] The sample container ID.

Field 9:    [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 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.

Field 10:   [TRACE GAS NAME] Gas identifier (e.g., co2, co2c13).

Field 11:   [MEASUREMENT GROUP] Identifies the group within NOAA and INSTAAR 
	     making the actual measurement (e.g., ccgg, hats, arl).  

Field 12:   [MEASURED VALUE] Dry air mole fraction or isotopic composition.  
             Missing values are denoted by -999.99[9].

Field 13:   [ESTIMATED UNCERTAINTY] Estimated uncertainty of the reported
             measurement value.  Missing values are denoted by -999.99[9].

Field 14:   [QC FLAG] A three-character field indicating the results of our 
             data rejection and selection process, described in section 7.5.

Field 15:   [INSTRUMENT] A 2-character code that identifies the instrument 
             used for the measurement.

Field 16:   [YEAR] The measurement date and time in LT.
Field 17:   [MONTH]
Field 18:   [DAY]
Field 19:   [HOUR]
Field 20:   [MINUTE]
Field 21:   [SECOND]

Field 22:   [LATITUDE] The latitude where the sample was collected, (negative (-)
             numbers indicate samples collected in the Southern Hemipshere).

Field 23:   [LONGITUDE] The longitude where the sample was collected, (negative (-)
             numbers indicate samples collected in the Western Hemisphere).

Field 24:   [ALTITUDE] The altitude of the sample inlet (masl). The reported altitude
            is the surface elevation plus sample intake height.

Field 25:   [ELEVATION] Surface elevation (masl).

Field 26:   [INTAKE HEIGHT] Air sample collection height above ground level (magl).

Field 27:   [EVENT NUMBER] A long integer that uniquely identifies the sampling
             event.

Fields in each line are delimited by whitespace.

(ex)
   BRW 1971 10 19 01 20 00 48-71 N co2 CCGG 319.520 -999.990 N.. L1 1971 12 15 08 46 
   00 71.3200 -156.6000 13.00 11.00 2.00 26142

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7.4 DATA - EVENT WITH MULTIPLE PARAMETERS

On special request we can distribute a "merged" file, which
includes for each sampling event, measurement results for muliple 
parameters or trace gas species.  A merged file does not include all 
information found in a single parameter data file.  For example,
merged files exclude measurement uncertainty, analysis instrument 
ID and date and time for each parameter.  Thus, the single parameter
data file is our most comprehensive data archive. 

The format of a merged file is slightly different from a single parameter event file.
A "merged" file will have the word "merge" in the parameter field of the file name.  
The file name does not inform on the number of parameters included in the file.

Merged data files use the following naming scheme (see Section 7.2):

     merge_[site]_[project]_[lab ID number]_[measurement group]_[optional qualifiers].txt

(ex) merge_pocn30_surface-flask_1_ccgg.txt contains ccgg measurement results for two or
     more parameters for all surface flask samples collected on the Pacific Ocean Cruise 
     sampling platform and grouped at 30N +/- 2.5 degrees.

(ex) merge_brw_surface-flask_1_ccgg.txt contains ccgg measurement results for two or more
     parameters for all surface flask samples collected at Barrow, Alaska.

The data files contain multiple lines of header information followed by one 
record for each atmospheric measurement of a single parameter or trace gas species.

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:    [SECOND]

Field 8:    [FLASK ID] The sample container ID.

Field 9:    [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 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.

Field 10:   [LATITUDE] The latitude where the sample was collected, (negative (-)
             numbers indicate samples collected in the Southern Hemipshere).

Field 11:   [LONGITUDE] The longitude where the sample was collected, (negative (-)
             numbers indicate samples collected in the Western Hemisphere).

Field 12:   [ALTITUDE] The altitude of the sample inlet (masl). The reported altitude
            is the surface elevation plus sample intake height.

Field 13:   [ELEVATION] Surface elevation (masl).

Field 14:   [INTAKE HEIGHT] Air sample collection height above ground level (magl).

Field 15:   [EVENT NUMBER] A long integer that uniquely identifies the sampling
             event.


There is a group of 4 fields for each parameter and measurement group included in the 
merge file.

Field ##+1: [TRACE GAS NAME] Gas identifier (e.g., co2, co2c13).

Field ##+2: [MEASUREMENT GROUP] Identifies the group within NOAA and INSTAAR 
	     making the actual measurement (e.g., ccgg, hats, arl).

Field ##+3: [MEASURED VALUE] Dry air mole fraction or isotopic composition.  
             Missing values are denoted by -999.99[9].

Field ##+4: [QC FLAG] A three-character field indicating the results of our 
             data rejection and selection process, described in section 7.5.

Fields in each line are delimited by whitespace.

(ex)

   BRW 2015 11 02 20 05 00 4827-99 P 71.3230 -156.6114 16.00 11.00 5.00 397570 
   co2 CCGG 400.480 ... co CCGG 105.340 ...

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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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7.6 DATA - MONTHLY AVERAGES

The monthly data files in ftp://aftp.cmdl.noaa.gov/data/trace_gases/co2/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

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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

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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