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Atmospheric Methane Dry Air Mole Fractions from quasi-continuous
measurements at Barrow, Alaska and Mauna Loa, Hawaii, 1986-2017
National Oceanic and Atmospheric Administration (NOAA)
Earth System Research Laboratory (ESRL)
Global Monitoring Division (GMD)
Carbon Cycle Greenhouse Gases (CCGG)
Version: 2018-05-04
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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 dry-air mole
fractions from 2 NOAA ESRL observatories: Barrow, Alaska
(BRW), and Mauna Loa, Hawaii (MLO).
Correspondence concerning these data should be directed to:
Dr. Edward J. Dlugokencky
NOAA ESRL Global Monitoring Division
325 Broadway, GMD-1
Boulder, Colorado, 80305 USA
Telephone: 303 497-6228
Electronic Mail: ed.dlugokencky@noaa.gov
Patricia M. Lang
NOAA ESRL GMD
John W. Mund
NOAA ESRL GMD
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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
insure that ESRL receives fair credit for their work. If the data
are obtained for potential use in a publication or presentation,
ESRL should be informed at the outset of the nature of this work.
If the ESRL data are essential to the work, or if an important
result or conclusion depends on the ESRL data, co-authorship
may be appropriate. This should be discussed at an early stage in
the work. Manuscripts using the ESRL data should be sent to ESRL
for review before they are submitted for publication so we can
insure 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, P.M. Lang, J.W. Mund and, M.E.
Rhodes (2018), Atmospheric Methane Dry Air Mole Fractions from
quasi-continuous measurements at Barrow, Alaska and Mauna Loa,
Hawaii, 1986-2017, Version: 2018-05-04, Path:
ftp://aftp.cmdl.noaa.gov/data/trace_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 ESRL 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:
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
Data are first edited using a rule-based editing algorithm
(Masarie et al., 1991) to exclude measurements obtained when the
analytical instrument was not working optimally.
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. In April 2013, we re-started
CH4 measurements at Barrow with a laser-based optical analyzer. Data
are saved as 10-second averages, which are averaged into 5-minute
averages. The 5-minute averages are used to calculate hourly averages,
and these are compared with wind speed and direction to constrain Barrow
data for background sampling conditions. The CH4 analyzer response is
calibrated every two weeks relative to a reference cylinder, and the
reference cylinder is measured hourly to track short term analyzer
drift. A well-calibrated "target" cylinder is measured about daily.
At Mauna Loa, from April 1987 through November 1995, 2 to 3 individual
measurements were made each hour. From December 1995 to present, 4
measurements are made each hour.
See Dlugokencky et al. [1995] for the experimental details.
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 speeds are 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.
NOAA methane measurements are reported on the gravimetrically-prepared
NOAA X2004A CH4 standard scale (see Dlugokencky et al., 2005).
Measurement uncertainties 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. 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 0.2 ppb. 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 was 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
"SD" of hourly averages (standard deviation of values that go into an hourly
average, 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 1500-2600 ppb 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
http://www.esrl.noaa.gov/gmd/dv/site/site_table.html.
Note: Data for all species may not be available for all sites listed
in the table.
To view near real-time data, manipulate and compare data, and create
custom graphs, please visit
http://www.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]
We provide some NOAA Global Monitoring Division measurements
in two unique file formats (netCDF and 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 Division 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 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.
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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