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Atmospheric Methane from quasi-continuous measurements at 
Barrow, Alaska and Mauna Loa, Hawaii, 1986-2020

National Oceanic and Atmospheric Administration (NOAA)
Global Monitoring Laboratry (GML)
Carbon Cycle Greenhouse Gases (CCGG) Group

Version: 2021-03-31
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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 - File Types
7.4      DATA - Content
7.5      DATA - QC Flags
8.       Data retrieval
9.       References

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1. DATA SOURCE AND CONTACTS

These directories contain atmospheric CH4 in units of dry-air mole 
fraction from 2 NOAA GML observatories: Barrow, Alaska (BRW), 
and Mauna Loa, Hawaii (MLO). 

Correspondence concerning these data should be directed to:

Dr. Edward J. Dlugokencky
NOAA Global Monitoring Laboratory
325 Broadway, R/GML-1
Boulder, Colorado, 80305 USA
Telephone: 303 497-6228
Electronic Mail: ed.dlugokencky@noaa.gov

John W. Mund
NOAA ESRL GML

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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, K.W. Thoning, and J.W. Mund (2021),
   Atmospheric methane from quasi-continuous measurements at Barrow, 
   Alaska and Mauna Loa, Hawaii, 1986-2020, Version: 2021-03,  
   https://doi.org/10.15138/ve0c-be70
   FTP path:
   ftp://aftp.cmdl.noaa.gov/data/greenhouse_gases/ch4/in-situ/surface/

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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

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

2020-08-19

For measurements made with laser-based spectrometers (instrument = 
LGR* and PC*), value_std_dev can now be defined when nvalue (number 
of measurements) = 1. See Introduction for details.

2020-03-16

Measurements of CH4 at Mauna Loa, Hawaii were switched from 
a GC to a CRDS in April, 2019. Calibration strategy and 
measurement frequency changed.

2018-03-16

Measurement uncertainty added to all CH4 hourly averages.
The column/variable previously labeled value_unc is now
labeled value_std_dev and contains the hourly average
standard deviation.  See Section 6 (Introduction) for
measurement uncertainty calculation.

The daily and monthly average standard deviations
(formerly value_unc) are now labled value_std_dev. 

2016-01-20

All CH4 measurements are reported on the NOAA X2004A scale.
www.esrl.noaa.gov/gmd/ccl/ch4_scale.html

2014-08-12

Barrow in situ measurements through 2013 are now available.

2014-07-29

Barrow in situ measurements were restarted in April 2013.  The 2013 data 
are not included in this update due to new instrumentation.  2013 data 
will be available sometime in the next few months.

2013-03-29

The CH4 in situ measurements at Barrow were suspended in June 2012.
They will resume in 2013.

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6. INTRODUCTION

NOAA GML began quasi-continuous measurements of atmospheric CH4 
at Barrow, Alaska (BRW) in January, 1986 and at Mauna Loa, Hawaii
(MLO) in April, 1987. Since the start of these measurements, 
our methods have evolved as described below.

Analytical:
Our first CH4 measurement systems at BRW and MLO were based on gas 
chromatography (GC) with flame ionization detection (FID) (for 
details, see Dlugokencky et al., 1995). At Barrow, from January 
1986 through April 1996, 2 to 3 individual measurements were made 
each hour. From May 1996 through May 2012, 4 measurements were made 
each hour. At Mauna Loa, from April 1987 through November 1995, 2 
to 3 individual measurements were made each hour, and from December 
1995 to April 2019, 4 atmospheric measurements were made each hour. 

Our next-generation analytical systems use laser-based spectrometers 
to measure CH4. We began using a new system (off-axis, integrated 
cavity output spectroscopy, ICOS) at BRW in April 2013. There is a gap
in the CH4 measurements at BRW from June, 2012 until April, 2013. Data 
are saved as 10-second averages, which are averaged into "5-minute 
averages". Our measurement sequence starts with 210 seconds of 
flushing, so only the last 90 seconds (9 10-second averages) are used 
to calculate each 5-minute average.  At MLO, we switched to a laser-
based, cavity ringdown spectrometer (CRDS) in April 2019, and now use 
a similar data processing scheme to BRW. 

Cwalibration:
GC/FID systems were calibrated with a single-point calibration; each 
aliquot of ambient air was bracketed by aliquots of standard gas, and 
CH4 in each air sample was calculated based on the ratio of the sample 
chromatographic peak response to the average response of the 
bracketing standards, and the assigned value for the standard 
cylinder.
For laser-based spectrometers, the analyzer response is calibrated 
with a suite of standards every two weeks relative to a reference 
cylinder, and the reference cylinder is measured hourly to track and 
correct short term analyzer drift. As a quality assurance step, a 
well-calibrated "target" cylinder is measured up to twice per day.

NOAA methane measurements are reported on the gravimetrically-prepared 
NOAA X2004A CH4 standard scale (see Dlugokencky et al., 2005; 
https://www.esrl.noaa.gov/gmd/ccl/ch4_scale.html).

Quality control:
All measurements by GC were first edited using a rule-based 
editing algorithm (Masarie et al., 1991) to exclude measurements 
obtained when the analytical instrument was not working optimally.
New algorithms were developed to edit data from the off-axis ICOS 
instrument at BRW since April, 2013 and the CRDS at MLO since 
April, 2019.

Hourly averages:
For GC/FID systems, hourly averages were calculated from all valid 
measurements within an hour and reported in our data files with the 
number of measurements (nvalue) and standard deviation (value_std_dev). 
For n=1, value_std_dev cannot be calculated.
For spectrometer systems, 5-minute averages are used to calculate 
hourly averages, but value_std_dev includes variability in the 5-min 
averages, so it is greater than the normal sample SD of the 5-min 
averages that go into the hourly mean, and when nvalue=1 (i.e., one 
5-min average in the hourly mean), value_std_dev is still calculated 
and reported. 

Selection for background:
Valid hourly averaged data are selected to distinguish samples of 
regionally representative air (background) from samples influenced 
by local sources and sinks (non-background). Background hourly 
data are identified with a "..." selection flag. A ".C." flag is 
assigned to data identified as non-background. The criteria for 
determining background conditions are site specific. The background 
criteria for Barrow are when the wind is from the clean air sector 
(020-110 degrees) and wind speed is greater than 1 m/s for at 
least one hour prior to inclusion. The background criteria 
for Mauna Loa are during predominatly "downslope" meteorological 
conditions as indicated by local time of day, 0000-0659. 
See Dlugokencky et al. [1995] for details.

Uncertainties:
Measurement uncertainties (value_unc) are calculated for each measurement 
(individual aliquot on GC and 5-minute average on optical spectrometer) 
based on analytical repeatability and reproducibility, and our ability to 
propagate the WMO CH4 mole fraction standard scale. For GCs, analytical 
repeatability is based on the stability of standard or reference aliquots 
averaged over the period a particular instrument was used. It varies with 
analytical instrument from 5.5 to 0.9 ppb. For laser-based spectrometers, 
it is based on the standard deviations of the bracketing measurements of 
reference gas.  Reproducibility is based on a comparison of near-
simultaneous flask-air samples measured independently in Boulder and 
quasi-continuous hourly averaged measurements at MLO and BRW 
observatories. It is assessed from the median difference, and it ranges 
from 0.1 to 0.9 ppb. The scale propagation term is based on the 
uncertainty we assign to standards, 0.5 ppb. All terms are given as 68% 
confidence intervals. Although we still list "value_std_dev" of hourly 
averages (standard deviation of values that go into an hourly average as 
described above, which includes natural variability and measurement 
uncertainty), we now list measurement uncertainty as a separate term. 
For the hourly average uncertainty, repeatability is divided by sqrt n, 
where "n" is the number of observations used in the calculation. 
Reproducibility and scale propagation terms are applied once. Total 
uncertainty is calculated by adding the individual terms in quadrature 
(square root of the sum of the squares). Methane hourly average values 
outside the range of standards are assigned a default uncertainity 
(-999.99). 


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7. DATA - GENERAL COMMENTS


7.1 DATA - SAMPLING LOCATIONS

For a summary of sampling locations, please visit

https://www.esrl.noaa.gov/gmd/dv/site/index.php.

Note: Data for all species may not be available for all sites listed 
in the table.

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

https://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 - FILE TYPE

We now provide some NOAA Global Monitoring Laboratory measurements
in two unique file formats (netCDF and ASCII text). The Network
Common Data Form (NetCDF) is a self-describing, machine-independent
data format that supports creation, access, and sharing of array-oriented
scientific data.  To learn more about netCDF and how to read netCDF
files, please visit http://www.unidata.ucar.edu. 

The ASCII text file is derived directly from the netCDF file.  The
text file is also self-describing and can be viewed using any text
editor.  "Self-describing" means the file includes enough information
about the included data (called metadata) that no additional file is 
required to understand the structure of the data and how to read and 
use the data.

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7.4 DATA - CONTENT

For each observatory we provide hourly, daily, and monthly
averaged files. Daily averages are derived directly from the hourly
data. Monthly averages are calculated from values extracted at one-day 
intervals from a smooth curve (Thoning et al., 1989) fitted to the 
daily averages. Higher resolution data (sub-hourly) are available 
upon request.

All (ASCII text and netCDF) files are located in 
"ftp://aftp.cmdl.noaa.gov/data/trace_gases/ch4/in-situ/surface/[site]/".

Files are named as follows (see Section 7.2 for details):

     ch4_[site]_surface-insitu_1_ccgg_HourlyData.[file type]
     ch4_[site]_surface-insitu_1_ccgg_DailyData.[file type]
     ch4_[site]_surface-insitu_1_ccgg_MonthlyData.[filetype]

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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.

SUMMARY OF SELECTION FLAGS

  ... - no flag applied
  *.. - Unable to compute a mole fraction or average
  .C. - data are non-background

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8. DATA RETRIEVAL

Users may transfer individual files from a directory or may choose to
transfer a single zipped file.  Zipped files contain the README file 
and either netCDF files or ASCII text files depending on the zipped file
name.

(ex) ch4_mlo_surface-insitu_1_ASCIItext.zip
(ex) ch4_mlo_surface-insitu_1_netCDF.zip

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., 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 CMDL atmospheric dry air methane mole fractions to a
  gravimetrically-prepared standard scale, J. Geophys. Res., 110, D18306,
  doi : 10.1029/2005JD006035.

Dlugokencky, E.J., L.P. Steele, P.M. Lang, and K.A. Masarie,
  Atmospheric methane at Mauna Loa and Barrow observatories: 
  presentation and analysis of in situ measurements, J. Geophys. Res., 
  100, 23,103-23,113, 1995. 
 
Masarie, K.A., L.P. Steele, and P.M. Lang, A rule-based expert system
  for evaluating the quality of long-term, in situ, gas chromatographic
  measurements of atmospheric methane, NOAA Tech. Memo. ERL CMDL-3,
  NOAA Environ. Res. Lab., Boulder, Colorado, 1991.

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

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