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Stable Isotopic Composition of Atmospheric Methane (13C) from the NOAA
GML Carbon Cycle Cooperative Global Air Sampling Network, 1998-2020.



Version: 2021-10-19
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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

University of Colorado
Institute of Arctic and Alpine Research (INSTAAR)

Correspondence concerning these data should be directed to:

Sylvia Englund Michel
Institute of Arctic and Alpine Research
Campus Box 450
University of Colorado, Boulder, CO 80309-0450 USA

telephone: (303) 735-5850
facsimile: (303) 492-6388
email: sylvia.michel@colorado.edu

or

Reid Clark	
Institute of Arctic and Alpine Research
Campus Box 450
University of Colorado, Boulder, CO 80309-0450 USA

telephone: (303)492-5495
facsimile: (303) 492-6388
email: Justin.Reid.Clark@Colorado.edu

or

Bruce Vaughn
Institute of Arctic and Alpine Research
Campus Box 450
University of Colorado, Boulder, CO 80309-0450 USA

telephone: (303) 492-7985
facsimile: (303) 492-6388
email: Bruce.Vaughn@colorado.edu



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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 

   Michel, S.E., Clark, J.R., and Vaughn, B.H. (2021), University of Colorado, 
   Institute of Arctic and Alpine Research (INSTAAR). Stable Isotopic Composition 
   of Atmospheric Methane (13C) from the NOAA GML Carbon Cycle Cooperative Global 
   Air Sampling Network, 1998-2020. Version: 2021-10-19 
   https://doi.org/10.15138/9p89-1x02
   

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3. RECIPROCITY

Use of these data implies an agreement to reciprocate.
Laboratories making similar measurements agree to make their
own data available to the general public and to the scientific
community in an equally complete and easily accessible form.
Modelers are encouraged to make available to the community,
upon request, their own tools used in the interpretation
of the GML data, namely well documented model code, transport
fields, and additional information necessary for other
scientists to repeat the work and to run modified versions.
Model availability includes collaborative support for new
users of the models.

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4. WARNINGS

Every effort is made to produce the most accurate and precise
measurements possible.  However, we reserve the right to make
corrections to the data based on recalibration of standard gases
or for other reasons deemed scientifically justified.

We are not responsible for results and conclusions based on use
of these data without regard to this warning.


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5. UPDATE NOTES

+++++++++++++++++++++++++++++++
Lab-wide notes:

2011-10-07

We introduced the term "measurement group", which identifies
the group within NOAA and Institute of Arctic and Alpine Research (INSTAAR)
University of Colorado Boulder that made the measurement.  We can 
now have multiple groups measuring some of the same trace gas species 
in our discrete samples.  

Measurement groups within NOAA and INSTAAR are 

  ccgg:  NOAA Carbon Cycle Greenhouse Gases group (CCGG)
  hats:  NOAA Halocarbons and other Atmospheric Trace Species group (HATS)
  arl:   INSTAAR Atmospheric Research Laboratory (ARL)
  sil:   INSTAAR Stable Isotope Laboratory (SIL)
  curl:  INSTAAR Laboratory for Radiocarbon Preparation and Research (CURL)

We also changed the file naming convention (see section "DATA - FILE 
NAME DESCRIPTION").

+++++++++++++++++++++++++++++++
Project-specific notes:

2021-07-29

In 2020, the COVID-19 pandemic impacted flask shipping and sampling 
frequency at many sites causing isolated gaps and some delayed 
processing into the 2nd half of 2021.  In spite of this, network 
sampling coverage remained reasonably good throughout the year.  

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:

*************************************
January 2013 - Second position flags moved to first and third positions:

Due to confusion resulting from our second position flags, they were moved to either first or third position. 
All flags are applied automatically by code except for '!' hand flags. Please see flagging section for more information.


May 2014 - Additional first position flags added

To increase the quality of the data, we decided to flag data for which there was a small sample peak (less than 3 nA) 
with a first position 'P' flag. We also decided to flag data for which there was significant (greater than 0.25 permil) 
drift in the reference gas with a 'D' flag. 

October 2021 - Amount effect correction added for data from 2018-2020. 

Third-position 'r' and 'R' flags were introduced to account an amount effect, where isotope delta values were influenced by differences in 
peak size between the sample and standard peak. Third-position 'r' flags were assigned to all data for which a correction was applied - 
this includes all data measured 2018-2020. Third-position 'R' flags were assigned to samples with a pronounced amount effect, where the 
difference between the sample and standard peak size exceeded 2.5 nA. Both 'r' and 'R' flagged data have expanded uncertainty due to the 
uncertainty of the data correction calculation, in addition to the standard analytical uncertainty.

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6. INTRODUCTION

The data files in this archive list measurements of the stable isotopic
composition (13C) of atmospheric methane.  The isotopic analysis was
performed at the Stable Isotope Laboratory, CU-INSTAAR, using samples
of air provided by the NOAA Global Monitoring Laboratory Global Greenhouse Gas Reference Network.

This release contains data for flask samples beginning in 1998 and extending through the end of 2020.  
Data are given for all sites where at least 18 months of data are available through the end of 2020.

The subdirectory 'event' contains all measurements from flask samples
collected at each site.  Monthly values are given only in those months 
where there are data, and no interpolation is made for missing months.   
A listing of the sampling sites in the network is given in the table 
below; all NOAA sites for which CH4 isotope data has been collected 
up to 2020 are included.  Data from recently initiated sites will be 
included in future releases.  See Trolier et al., 1996 or Conway et al., 
1994 for details of the sampling network, equipment and procedures.


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7. DATA - GENERAL COMMENTS

Isotope data are reported as isotope delta values: the ratio of minor to major isotopes relative to a standard. The 'delta' notation is

d13C(CH4) = [ (13C/12C)sam / (13C/12C)std - 1 ] 

and is expressed in units of 'permil'. The standard for carbon isotopes is VPDB.


All of the samples have been analyzed at the Stable Isotope Laboratory at 
CU-INSTAAR in Boulder, Colorado, using either a Micromass Optima or Elementar Isoprime isotope-ratio mass spectrometer coupled to a methane 
a custom-built trapping system, a gas chromatograph, and a combustion furnace.  Measurement precision is approximately 0.06 permil for 13C 
(where 'precision' is taken as the long term reproducibility of a surveillance cylinder of ambient air. Our isotopic scale for d13C of CH4 
is tied to Dr. Stan Tyler's lab at the University of California Irvine. Current efforts to validate this scale and compare with other labs 
in the isotopic community are ongoing. For more information regarding calibrations of methane-in-air isotopes, see the recent review by 
Umezawa et al., 2017.

For more information on our methodology, please refer to the following publications:

Vaughn, B.H., J. Miller, D. F. Ferretti, J. W. C. White 2004: Stable isotope measurements of atmospheric CO2 and CH4. Handbook of Stable Isotope 
Analytical Techniques, vol 1, ch.14, Elsiever, 1248 p. 

Miller, J. B., K. A. Mack, R. Dissly, J. W. C. White, E. J. Dlugokencky, 
and P. P. Tans 2002: Development of analytical methods and measurements of 13C/12C in atmospheric CH4 from the NOAA Climate Monitoring and 
Diagnostics Laboratory Global Air Sampling Network. J Geophys. Res., v107, NO. D13.

In 2013 we added a metric of uncertainty for isotopes of methane. This is not a "true" calculation of uncertainty in a metrological sense - 
instead it is a metric of short- to medium-term reproducibility of a surveillance cylinder. This air cylinder is well characterized by long-term 
measurement, and in addition to tracking our ties to VPDB over the long term, it alerts us to any problems with the mass spectrometer or extraction 
system. We use the standard deviation of ten runs' worth of surveillance cylinder measurements as the uncertainty metric. Usually four samples are run 
a day, of which all but the first are used (due to known irregularities caused by the tank regulator). For each run, the uncertainty is calculated from 
data from that run, regardless of its flagging, and the previous nine runs of unflagged data. (For example, if the current run is flagged, that surveillance
cylinder data will be used in the uncertainty calculation; however, the next day's run will not use the flagged data in its uncertainty calculation.) 
As of Dec 2013, uncertainty is calculated with each run, and has been back-calculated to 2006. 

In May 2017, a post combustion column was added to remove doubly-charged Kr ions which interferes with measurement of the different masses of CH4-
derived CO2. Extensive tests of cylinders with different CH4/Kr ratios before and after implementation of the PC column show that there is very 
little effect of Krypton on samples with methane mole fraction over 1400 ppb.  

A data correction has been applied for all 2018-2020 data to reduce a positive bias introduced by our sample preparation system. During this time 
there was a clog in a valve that caused reduced flow through our methane trapping system, causing smaller sample peaks in our analyzer, especially 
in low-pressure flasks. The difference in peak heights between samples and standards exacerbated an "amount effect" problem, where the peak height 
influenced the measured isotope delta value. With ongoing extensive testing, that began in 2019, we discovered that our amount effect was variable, 
with an average of -0.019 permil/nA and a standard deviation of 0.047 permil/nA. To correct for it, we applied a Monte Carlo approach: we applied 
100 randomly-chosen slopes from from the list of measured corrections and calculated the mean of the corrected values. The standard deviation 
of the 100 iterations was added in quadrature to the analytical uncertainty to account for the error in the correction.  Data for which the amount 
effect correction has been applied has a third position 'r'. A third position 'R' flag was added where the sample peak height differs from the 
standard peak height by more than 2.5 nA, where the correction was significant. This correction is imperfect, but does reduce a positive bias in 
the data suggested by intercomparisons with other reserach groups. If further evidence suggests a better way to handle this effect, we may revisit 
the correction for this time period. Evidence suggests that the amount effect comes from the combustion process, though investigations continue. 
In late-2020, the obstruction in the system was resolved and we now reliably control for peak size across sample types; therefore the amount effect 
is now within measurement noise.


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

https://gml.noaa.gov/dv/site/?program=ccgg.

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

https://gml.noaa.gov/dv/iadv/.

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7.2 DATA - FILE NAME DESCRIPTION

Encoded into each file name are the parameter (trace gas identifier); sampling 
site; sampling project; laboratory ID number; measurement group; and optional 
qualifiers that further define the file contents.

All file names use the following naming scheme:

         1      2         3               4                   5            
[parameter]_[site]_[project]_[lab ID number]_[measurement group]_[optional 

         6           7
qualifiers].[file type]


1. [parameter]

   Identifies the measured parameter or trace gas species.

   (ex)
   co2      Carbon dioxide
   ch4      Methane
   co2c13   d13C (co2)
   merge    more than one parameter

2. [site]

   Identifies the sampling site code.

   (ex)
   brw
   pocn30
   car
   amt

3. [project]
   
   Identifies sampling platform and strategy.

   (ex)
   surface-flask
   surface-pfp
   surface-insitu
   aircraft-pfp
   aircraft-insitu
   tower-insitu

4. [lab ID number]

   A numeric field that identifies the sampling laboratory (1,2,3, ...).
   NOAA GML is lab number 1 (see http://www.esrl.noaa.gov/gmd/ccgg/obspack/labinfo.html).

5. [measurement group]

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

   (ex)
   ccgg
   hats
   arl
   sil

6. [optional qualifiers]

   Optional qualifier(s) may indicate data subsetting or averaging.
   Multiple qualifiers are delimited by an underscore (_).  A more detailed
   description of the file contents is included within each data file.

   (ex)
   event         All measurement results for all collected samples (discrete (flask) data only).
   month         Computed monthly averages all collected samples (discrete (flask) data only).
   hour_####     Computed hourly averages for the specified 4-digit year (quasi-continuous data only)
   HourlyData    Computed hourly averages for entire record (quasi-continuous data only)
   DailyData     Computed daily averages for entire record (quasi-continuous data only)
   MonthlyData   Computed monthly averages for entire record (quasi-continuous data only)

7. [file type]
   
   File format (netCDF, ASCII text). 

   (ex) 

   txt           ASCII text file
   nc            netCDF4 file

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7.3 DATA - EVENT WITH SINGLE PARAMETER

The event data files in ftp://aftp.cmdl.noaa.gov/data/trace_gases/ch4c13/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.

Reject flags (1st flag character)

A   known problem in analysis (poor precision of measured standard air)
a   analysis problem (specifics unknown)
!   Hand-flagged by INSTAAR researcher: analytical problem.
E   specific analysis problem due to extraction manifold problem
B   specific analysis problem due to transfer pump

n   known sampling problem --   taken from NOAA CH4 data base
+   bad pair, high  member
-   bad pair, low member
.   good flask

Data measured after 2006 may have these flags:
H   mean of trap (cylinder of known isotopic value run as sample) in same 
    run as sample was more than 0.3 permil above the long-term
    mean 
L   mean trap value in same run as sample was more than 0.3 permil
    below the long term mean 
P   small peak (less than 3 nA) - too low for accurate analysis
Y   peak too large (greater than 18 nA) - swamps collectors
D   too much drift; more than 0.25 permil drift in the reference gas 
   

Data measured before 2006 may have these flags:
D   a second aliquot taken more than 20 days after the first
2   a 'D' flag applied over another hard flag (e.g., D+n=2 or D+A=2)
M   a third or greater aliquot taken more than 20 days after the first
3   a 'M' flag applied over another hard flag (e.g., T+n=2 or T+A=2)

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Non-background flags  (2nd flag character)

X   outlier by more than 3-sigma from a smooth curve fit to CH4C13 data
x   outlier flag taken from CH4 data (ie, outlier in CH4 concentration)
Z   hand-flagged by NOAA (not INSTAAR) researchers
.   good flask
-----------------------------------------------------------------------------
Retain flags (3rd flag character)

S   flask analyzed singly, without its mate
.   good flask
I   intercomparison sample
i   intercomparison sample flag overwriting previous (now lost) flag
o   No surveillance cylinder data associated with this sample
r   Correction for “amount effect” applied to these data
R   Correction for “amount effect” is significant, because peak 
    height of sample differs from standard by more than 2.5 nA
T   trap samples had unusually high variance (0.2 permil)


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7.6 DATA - MONTHLY AVERAGES

The monthly data files in ftp://aftp.cmdl.noaa.gov/data/trace_gases/ch4c13/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

Conway, T.J., Tans, P.P., Waterman, L.S., Thoning, K.W., Kitzis, D.R.,
Masarie, K.A., and Zhang, N. 1994. Evidence for interannual variability 
of the carbon cycle from the NOAA/CMDL global air sampling network, JGR 99, 22831-22855.

Miller, J. B., Mack, K. A., Dissly, R., White, J. W. C., Dlugokencky, E.
J. and Tans, P. P. 2002. Development of analytical methods and
measurements of 13C/12C in atmospheric CH4 from the NOAA/CMDL global air
sampling network. J. Geophys. Res. 107, doi: 10.1029/2001JD000630.

Thoning, K.W., Tans,  P.P., and Komhyr, W.D. 1989,  Atmospheric carbon 
dioxide at Mauna Loa Observatory 2.  Analysis of the NOAA GMCC Data, 
1974-1985, J. Geophys. Res., 94, 8549-8565.

Umezawa, T., Brenninkmeijer, C., Rockmann, T., van der Veen, C., Tyler, 
S. C., Fujita, R., Morimoto, S., Aoki, S., Sowers, T., Schmitt, J., and 
Bock, M. (2018). Interlaboratory comparison of d13C and dD measurements 
of atmospheric CH4 for combined use of data sets from different laboratories. 
Atmospheric Measurement Techniques.

Vaughn, B., Ferretti, D., Miller, J. and White, J. 2004. Stable isotope
measurements of atmospheric CO2 and CH4. Handbook of Stable Isotope Analytical 
Techniques, vol 1, ch. 14, Elsevier, 1248 p.


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