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Atmospheric Hydrogen Dry Air Mole Fractions from the NOAA GML Carbon
Cycle Cooperative Global Air Sampling Network, 2009-2021

Version: 2023-07-05
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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
7.6 DATA - Collection Methods
7.7 DATA - Monthly Averages
8. Data retrieval
9. References

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1. DATA SOURCE AND CONTACTS

National Oceanic and Atmospheric Administration (NOAA)
Global Monitoring Laboratory (GML)

Correspondence concerning these data should be directed to:

Gabrielle Petron
NOAA ESRL Global Monitoring Laboratory
325 Broadway, GML-1
Boulder, CO 80305
U.S.A.

Electronic Mail: gabrielle.petron@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

Petron, G., Crotwell, A., Crotwell, M., Kitzis, D., Madronich, M.,
Mefford, T., Moglia, E., Mund, J., Neff, D., Thoning, K., & Wolter, S.
(2023). Atmospheric Hydrogen Dry Air Mole Fractions from the NOAA GML Carbon
Cycle Cooperative Global Air Sampling Network, 2009-2021 [Data set].
NOAA GML CCGG Division. Version: 2023-07-05, https://doi.org/10.15138/WP0W-EZ08

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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, modeled mole fractions, 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 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.

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6. INTRODUCTION

Individual site data files provide H2 dry air mole fractions in
parts per billion (ppb) (ppb = 1 part in 10^-9 by mole
fraction = nmole/mole) based on measurements from the NOAA
GML Carbon Cycle Cooperative Global Air Sampling Network.

More information about the flask network can be found at:
https://gml.noaa.gov/ccgg/ggrn.php
A map and table at https://gml.noaa.gov/dv/site/?program=ccgg
list the flask network sampling locations, the 3 letter codes
used to identify them, and their latitudes, longitudes and
altitudes. H2 data from sites not provided in this directory
may be available from GML (contact Gabrielle Petron).

All air samples were analyzed for H2 at the NOAA GML laboratory
in Boulder using gas chromatography with a Helium Pulse Discharge
Detector (Novelli et al., 2009). All measurements are
referenced to the WMO/MPI X2009 calibration scale.

Between 2009 and July 2019, we used a single standard calibration
strategy as the response characteristics of the instrument,
Gas Chromatography with a Pulsed Discharged Helium Ionization Detector,
has been shown to be linear over a range of 0 to 2000 ppb H2
(Novelli et al., 2009).

Since August 2019, we use a multi-standard calibration strategy
(normalized to a reference air tank) for the flask analysis system
instrument, MAGICC-3. These calibration episodes were conducted biweekly early
on and are now performed every 4 to 5 weeks. Instrument drift
between calibration episodes is corrected by normalization to
a reference tank. The calibration response results for the H2 standards
normalized peak heights are valid until the next calibration episode.

One sigma total uncertainties are provided. They are calculated from two terms:
short term repeatability and calibration scale propagation uncertainty.
To get the total uncertainty variance, terms are added in quadrature.

Previous NOAA GML CCGG H2 data releases and publications included
flask air measurements using gas chromatography with a mercuric oxide
reduction detection and referenced to an internal H2 calibration scale
(Novelli et al., 1999). These results were not corrected for the transition
to the WMO H2 X2009 scale maintained by MPI-BGC. The NOAA H2 X1996 internal scale
is known to be biased relative to WMO H2 X2009 and unstable over time.
In addition, the GML older data is not corrected for the non-linear response
of the reduction gas analyzer (RGA) instruments, so may contain mole fraction
dependent offsets.

Petron et al. (in prep) will provide a detailed description
of the implementation of the WMO/MPI H2 X2009 scale at GML
and the reprocessing of the flask air GC-HePDD H2 measurements.

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7. DATA - GENERAL COMMENTS

Measurements are reported in units of 10^-9 mol H2 per mol
of dry air (nmol/mol) or parts per billion (ppb) relative
to the WMO/MPI H2 X2009 scale (Jordan and Steinberg, 2011).

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

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

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.

When samples are collected in pairs, the pair
difference is calculated, and samples with a
pair difference greater than 5 ppb are rejected.

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 has been
rejected. 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 significantly affect the data quality.

Flag Description

Retained ... good pair, no other issues
(D <= 5 ppb)

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 we feel best
represents the H2 distribution in the remote, well-mixed
global troposphere. It is possible, and even likely, that
some values flagged as non-background conditions
(with a 2nd column flag other than '.') are valid
measurements, but represent poorly mixed air parcels
influenced by local anthropogenic sources or strong local
biospheric sources or sinks. Users of these data should be
aware that data selection is a difficult but necessary aspect
of the analysis and interpretation of atmospheric trace gas
data sets, and the specific data selection scheme used may be
determined by the goals of a particular investigation.

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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/h2/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/h2/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

Jordan, A. and B. Steinberg, Calibration of atmospheric hydrogen
measurements, Atmos. Meas. Tech., 4, 509–521, 2011,
doi:10.5194/amt-4-509-2011

Novelli, P.C., A.M. Crotwell, and B.D. Hall, Application
of Gas Chromatography with a Pulsed Discharge Helium Ionization
Detector for Measurements of Molecular Hydrogen in the Atmosphere,
Environ. Sci. Technol. 2009, 43, 7, 2431–2436,
doi:10.1021/es803180g.

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