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Atmospheric Methane Dry Air Mole Fractions from the
NOAA GML Carbon Cycle Cooperative Global Air
Sampling Network, 1983-2019

Version: 2020-07-24
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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)
Earth System Research Laboratories (ESRL)
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 ESRL Global Monitoring Laboratory
325 Broadway, R/GML-1
Boulder, Colorado, 80305 USA

E-Mail: xin.lan@noaa.gov
        ed.dlugokencky@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.,  A.M. Crotwell, J.W. Mund, M.J. Crotwell, and
   K.W. Thoning (2020), Atmospheric Methane Dry Air Mole Fractions from the
   NOAA GML Carbon Cycle Cooperative Global Air Sampling Network, 1983-2019,
   Version: 2020-07, https://doi.org/10.15138/VNCZ-M766

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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:

2020-07-16

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:

2018-07-30

Uncertainties recalculated after reassessment of
uncertainty terms. Uncertainties on early data are
now larger than before.

2015-07-07

We updated and extended the WMO CH4 X2004 scale to X2004A.
Details are described on a dedicated web page: 
http://www.esrl.noaa.gov/gmd/ccl/ch4_scale.html. 

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6. INTRODUCTION

All samples were analyzed for methane at NOAA ESRL in Boulder, Colorado
by gas chromatography with flame ionization detection or by cavity ring-
down spectroscopy (since August, 2019), and each sample aliquot was 
referenced to the NOAA 2004A methane standard scale
(see www.esrl.noaa.gov/gmd/ccl/ch4_scale.html; Dlugokencky et al., 2005).
Through most of the period 1983-1991, one flask of a sample pair was analyzed
for methane, and, whenever the overpressure was sufficient, at least two 
aliquots were analyzed.  We have used the difference in methane mole fraction 
between the first and second aliquot to establish the repeatability of the 
measurement.  Over the full period of the record described here, the average 
repeatability has been approximately 2 ppb.  In October 1991, our analysis 
procedure was altered; we began analyzing a single aliquot from both members 
of the flask pair.  The principle reasons for the change were to simplify 
flask handling procedures (the carbon dioxide and carbon monoxide projects 
also measure both flasks of the sample pair) and to have flask pair agreement 
(the difference in methane mole fraction between the two flasks collected 
simultaneously) as an additional diagnostic to use in evaluating the quality 
of the data.  The repeatability of the analytical instrument is now assessed 
by two approaches: approximately monthly measurements of target tanks, and 
assessing the relative stability of the standard or reference gas aliquots 
during each day of flask measurements.  (See Steele et al., 1987, Lang et 
al., 1990a,b, Dlugokencky et al., 1994b, and Dlugokencky et al., 2005 for 
details of the sampling network, equipment, standards, and procedures.)

Uncertainties are reported for each measurement based on analytical
repeatability, reproducibility, and our ability to propagate the WMO CH4 mole
fraction standard scale. Analytical repeatability is based on one of the
methods above or on the average absolute value of pair agreement between
pairs collected nearly simultaneously. It varies with analytical instrument
from 0.2 to 2.3 ppb. Propagation of the scale is based on the reproducibility 
determined for scale propagation in our calibration laboratory. It has a
fixed value of 0.5 ppb based on subsequent calibrations of the same cylinder
at least one year after the first. Reproducibility is based on long-term
variations in measurements of target cylinders, typically ~0.3 ppb. The
three terms are added in quadrature (square root of the sum of the squares)
to estimate the measurement uncertainty at 68% confidence interval.

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7. DATA - GENERAL COMMENTS

Aliquots of sample and standard gas are injected into a gas
chromatograph (GC) with a sampling valve.  Methane (CH4) is 
separated from other sample constituents using packed columns 
and detected using flame ionization (FID). This process is 
highly automated for field and laboratory operations. Instrument 
response of the sample must be compared to a standard of known 
CH4 content. Measurements are reported in units of nanomol/mol 
(10^-9 mol CH4 per mol of dry air (nmol/mol) or parts per billion 
(ppb)) relative to the NOAA 2004A CH4 standard scale.  Repeatability
of our measurements, based on repeated analysis of air from a 
high-pressure cylinder, has ranged from 1 to 3 nmol/mol
over the period of our measurements. 

In August, 2019, we switched from GC/FID 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 ~0.2 ppb (1 sigma). 

The uncertainity of our CH4 standard scale (NOAA 2004A) near 1800 ppb 
is estimated at +/- 0.2%, or about 3 nmol/mol.

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 ESRL is lab number 1 (see http://www.esrl.noaa.gov/gmd/ccgg/obspack/labinfo.html).

5. [measurement group]

   Identifies the group within NOAA 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/ch4/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 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    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.


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7.6 DATA - MONTHLY AVERAGES

The monthly data files in ftp://aftp.cmdl.noaa.gov/data/trace_gases/ch4/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

Dlugokencky, E.J., et al. (2009), Observational constraints on 
  recent increases in the atmospheric CH4 burden, Geophys. Res. Lett.,
  36, L18803, doi:10.1029/2009GL039780.

Dlugokencky, E.J., R.C. Myers, P.M. Lang, K.A. Masarie, A.M. Crotwell,
  K.W. Thoning, B.D. Hall, J.W. Elkins, and L.P. Steele, 2005, Conversion
  of NOAA atmospheric dry air methane mole fractions to a 
  gravimetrically-prepared standard scale, J. Geophys. Res., 110, D18306,
  doi:10.1029/2005JD006035.

Dlugokencky, E.J., B.P. Walter, K.A. Masarie, P.M. Lang, and E.S. Kasischke,
  2001, Measurements of an anomalous global methane increase during 1998,
  Geophys. Res. Lett., 28, 499-502.                                           

Dlugokencky, E.J., K.A. Masarie, P.M. Lang, and P.P. Tans, 1998, 
  Continuing decline in the growth rate of atmospheric methane,
  Nature, 393, 447-450. 

Dlugokencky, E.J., K.A. Masarie, P.M. Lang, P.P. Tans, L.P. Steele, and 
  E.G. Nisbet, 1994a, A dramatic decrease in the growth rate of atmospheric 
  methane in the northern hemisphere during 1992, Geophys. Res. Lett., 21, 
  45-48.

Dlugokencky, E.J., L.P. Steele, P.M. Lang, and K.A. Masarie, 1994b, The growth 
  rate and distribution of atmospheric methane, J. Geophys. Res., 99, 17,021-
  17,043.

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

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

Lang, P.M., L.P. Steele, L.S. Waterman, R.C. Martin, K.A. Masarie, and 
  E.J. Dlugokencky, 1992, NOAA/CMDL Atmospheric methane data for the period
  1983-1990 from shipboard flask sampling, NOAA Technical Memorandum ERL CMDL-4.

Steele, L.P., P.J. Fraser, R.A. Rasmussen, M.A.K. Khalil, T.J. Conway, A.J. 
  Crawford, R.H. Gammon, K.A. Masarie, and K.W. Thoning, 1987, The global 
  distribution of methane in the troposphere, J. Atmos. Chem, 5, 125-171.

Steele, L.P. and P.M. Lang, 1991, Atmospheric methane concentrations-the 
  NOAA/CMDL global cooperative flask sampling network, 1983-1988, 
  ORNL/CDIAC-42, NDP-038.

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