These data were produced by NOAA and are not subject to copyright protection in the United States. NOAA waives any potential copyright and related rights in these data worldwide through the Creative Commons Zero 1.0 Universal Public Domain Dedication (CC0 1.0)
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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 or 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)
+++++++++++++++++++++++++++++++
Project-specific notes:
2022-07-21
Dataset is now provided in self describing ObsPack format with improved metadata.
Surface flask event data are available in NetCDF and ASCII text. Surface flask
monthly data are available in ASCII text. Shipboard data binned by 5 or 3 degrees
are now removed from surface flask event data, but still provided in monthly data.
This format change makes some previous notes irrelevant.
+++++++++++++++++++++++++++++++
Parameter-specific notes:
2023-05-01
In Spring 2023, we moved to an internal quality control
(QC) tagging system for the flask air samples.
There are three categories of tags documenting
issues associated with sample collection, measurement
and representativity in the CCGG database.
Tags are more specific than flags, which allows
a more granular internal tracking and analysis of QC issues.
Tags are converted to simplified 3 character flags in the data
files for external data users. See section 7.5 for more details.
2018-07-30
Repeatability of ECD for N2O degraded in October, 2017,
and is reflected in the uncertainties.
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6. INTRODUCTION
All samples were analyzed for N2O at NOAA GML in Boulder, Colorado
by gas chromatography with electron capture detection or Tunable
Infrared Laser Direct Absorption Spectroscopy (since Aug., 2019),
and each sample aliquot was referenced to the NOAA 2006A N2O
standard scale (see Hall et al., 2007;
http://www.esrl.noaa.gov/gmd/ccl/n2o_scale.html).
The average repeatability has ranged from 0.02 to 0.4 ppb.
Data files in the subdirectories "ccg/n2o/flask/event" and
"ccg/n2o/flask/month" list N2O dry air mole fractions in nmol/mol
based on measurements from the NOAA GML Carbon Cycle Cooperative Global
Air Sampling Network. The subdirectory "event" contains all measurements
from air samples collected at each site and the subdirectory "month"
contains monthly means determined from the data files in "event". Network
sampling sites are listed in section 7.1.
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7. DATA - GENERAL COMMENTS
For measurements using gas chromatography, aliquots of gas from ambient
air samples and a cylinder of reference gas are alternately injected
into a gas chromatograph (GC) with a sampling valve. Nitrous oxide (N2O)
is separated from other sample components using packed columns and
detected with an electron capture detector (ECD). The response of the ECD
to N2O is calibrated off-line about monthly relative to the reference with a
suite of gas standards. Each aliquot of sample is measured relative
to the same reference, and N2O values are calculated from the response
function. N2O measurements are reported in units of nanomol/mol
(10^-9 mol N2O per mol of dry air (nmol/mol) or parts per billion (ppb)).
In August, 2019, we replaced the GC/ECD method for N2O measurement with
a Tunable Infrared Laser Direct Absorption Spectroscopy (TILDAS) method.
The analyzer is calibrated off-line once per month With a suite of
standards 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, ranges from
~0.2 to 0.4 ppb for the GC to 0.01 to 0.02 ppb for the TILDAS analyzer
(1 sigma). The scale propagation uncertainty is estimated to be 0.12 ppb,
based on the reproducibility determined for scale propagation in our
calibration laboratory. Long-term reproducibility is estimated to be
+/- 0.1 ppb based on repeated measurements of air from a high-pressure
cylinder, or the long-term variability in the differences between measurements
of test flasks on the flask-air analysis system and separate
measurements of the test gas used to fill the test flasks.
These three components of uncertainty are added in quadrature
to calculate the measurement uncertainty.
The absolute accuracy of our standard scale is
estimated as +/-0.4 nmol/mol in the ambient atmospheric range of N2O
(95% confidence interval).
Pacific Ocean Cruise (POC, travelling between the US west coast
and New Zealand or Australia) flask-air samples were collected in
about 5 degree latitude intervals. For South China Sea (SCS), samples
were collected at about 3 degree latitude intervals.
Sampling intervals are approximately weekly for 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.
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://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 (optional); 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]_[optional 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 https://gml.noaa.gov/ccgg/obspack/labinfo.html).
5. [optional measurement group]
Identifies the group within the NOAA GML or the Institute of Arctic and Alpine
Research (INSTAAR) at the University of Colorado Boulder that made the
measurement.
It is possible to have multiple different 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)
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 (technically UTF-8 encoded) file is derived directly from the
netCDF file. The text file is also self-describing and can be viewed using
any ASCII or UTF-8 capable 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. Note that some non-ASCII characters (accents,
international character sets) may be present in various names and contact
information. These may require a UTF-8 capable text editor to view properly.
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7.4 DATA - CONTENT
See individual files for description of the provided variables and other
dataset metadata.
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7.5 QC FLAGS
Quality control 3-column flags indicate retained and rejected flask
results as follows in datafiles.
If the first character is not a period, the sample result should be
rejected for scientific use due to sample collection and/or measurement
issue. A second column character other than a period indicates a sample
that is likely valid but does not meet selection for representativeness
such as midday sampling or background air sampling. A third column flag
other than a period indicates abnormal circumstances that are not thought
to affect the data quality.
Flag Description
Retained ... good pair, no other issues
Rejected M.. sample measurement issue
C.. sample collection issue
B.. both measurement and collection issues
Selection .S. selection issue. High/low mole fraction
thought to not represent background
conditions for example.
Informational ..M informational measurement tag or
potential measurement issue
..C informational collection tag or
potential collection issue
The retained values comprise the data set that mostly represents
N2O distribution in the remote, well-mixed global surface atmosphere.
Data with selection flag (with a 2nd column flag other than '.') are
likely valid measurements, but represent poorly mixed air parcels
influenced by local sources. Data selection is applied using a curve
fitting approach described in detail here:
https://gml.noaa.gov/ccgg/mbl/crvfit/crvfit.html
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7.6 COLLECTION METHODS
A single-character code is used to identify 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
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7.7 DATA - MONTHLY AVERAGES
The monthly data files in https://gml.noaa.gov/aftp/data/trace_gases/n2o/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
All (ASCII text and netCDF) files are located in
"https://gml.noaa.gov/aftp/data/trace_gases/n2o/flask/surface/".
To transfer all files in a directory, it is more efficient to
download the tar or zipped files. Individual or zipped files can
be downloaded using your web browser by clicking the hyperlinked file
or right clicking hyperlink and using browser menu to 'save as' or similar.
Files can also be accessed by anonymous ftp at aftp.cmdl.noaa.gov.
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9. REFERENCES
Dlugokencky, E.J., L.P. Steele, P.M. Lang, and K.A. Masarie, 1994, The growth
rate and distribution of atmospheric methane, J. Geophys. Res., 99, 17,021-
17,043.
Thoning, K.W., P.P. Tans, and W.D. Komhyr, 1989, Atmospheric carbon dioxide
at Mauna Loa Observatory 2. Analysis of the NOAA GMCC Data, 1974-1985,
J. Geophys. Res., 94, 8549-8565.
Hall, B. D., G. S. Dutton and J. W. Elkins (2007), The NOAA nitrous oxide
standard scale for atmospheric observations, Journal of Geophysical
Research-Atmospheres, 112, D09305, doi:10.1029/2006JD007954.
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