Atmospheric carbon dioxide records from sites in the NOAA/CMDL air
sampling network
T.J. Conway, P.P. Tans, L.S. Waterman
National Oceanic and Atmospheric Administration,
Environmental Research Laboratories, Climate Monitoring and
Diagnostics Laboratory, 325 Broadway, Boulder, Colorado 80303-3328,
U.S.A.
Alert
Amsterdam Island
Ascension Island
Azores
Barbados
Barrow
Bermuda (East)
Bermuda (West)
Cape Grim
Cape Kumukahi
Cape Meares
Christmas Island
Cold Bay
Guam
Halley Bay
Izaña
Key Biscayne
Mace Head
Mauna Loa
Midway Islands
Mould Bay
Niwot Ridge
Pacific Ocean
Palmer Station
Qinghai Province
Samoa
Seychelles
Shemya Island
South Pole
Station "C"
Station "M"
Syowa Station
Tae-ahn Peninsula
Ulaan Uul
Virgin Islands
Period of Record
1968-92 (varies by site)
Methods
Starting in 1968, air samples were collected in cylindrical glass
flasks tapered at both ends to ground glass stopcocks lubricated
with hydrocarbon grease. At several sites from 1980 to 1985 samples
were also collected in spherical 5-L flasks equipped with a single
ground glass stopcock. These flasks were filled by the evacuation
method described below. In 1983, measurements of CH4 in the flask
samples were begun. Experiments at this time revealed that CO mixing ratios
increased with time in the greased flasks. In
1989, 0.5-L glass flasks equipped with glass piston Teflon O-ring
stopcocks were introduced into the network so CO could be measured
in addition to CO2 and CH4. In 1990, measurements of
13C/12C and 18 O/16O of CO2 in the
flask samples were begun. The precision of the isotopic measurements was
better with larger volume flasks, so in 1991 2.5-L glass flasks with two
Teflon O-ring stopcocks began to replace the 0.5-L flasks. In 1994, the
conversion of the network to 2.5-L flasks will be completed.
Flasks samples are always collected in pairs, once or twice per
week, on a schedule determined largely by the sample collector. The
sample collectors have been given guidelines concerning preferred
wind speeds, directions, and time of day for sample collection.
Whole air samples are collected with no attempt to remove water
vapor. Samples are dried during analysis using a cryogenic trap at
¬70°C.
From 1968 to 1980, collectors used a hand-held aspirator bulb to
pull air through the flasks. In 1980, a portable battery powered
pumping unit was introduced. This method allowed the sample
collector to move downwind while the flasks, connected in series,
were being flushed, enabled pressurization of the flasks, and
incorporated an intake line that could be extended to 2 m above the
ground. This device resulted in improved agreement between members
of flask pairs and decreased scatter in the measurements. To avoid
artifacts due to this inhomogeneity in the data quality, most CMDL
analyses of the flask data begin with the 1981 data.
The sampling method changed again in mid-1990 when an improved
portable sampler was introduced. While the sampling principles were
unchanged, the new sampler employed a single, larger battery; a
more rugged, higher capacity pump; a 5-m intake line; and a back
pressure regulator to control the pressure in the flasks.
The effect of the flask and sampler improvements has been an
increase in the percentage of sample pairs meeting a CO2 agreement
criterion of 0.5 ppm, from ~75% in the mid-1980s to
~90% in 1992. However, overlapped sampling was
conducted at several sites and no offsets due to the new flasks or
sampling equipment were observed.
At Barrow, Niwot Ridge, Mauna Loa, Cape Kumukahi, Christmas Island,
and Samoa, flask samples have also been collected in evacuated 3-L
flasks. This type of flask is also used on the containerships
making regular voyages in the Pacific Ocean between Los Angeles and
New Zealand. In this method two flasks are filled in rapid
succession by holding the flask into the wind, purging the dead
volume in the inlet to the flask, opening the stopcock, and
allowing the flask to fill with air to ambient atmospheric
pressure. In overlapped sampling at Mauna Loa and Niwot Ridge, no
significant difference was found between the 3-L flasks and the
pressurized flasks. At Barrow and Cape Kumukahi, there is an
indication of an offset of ~0.3 ppm, with the
evacuated flasks generally being higher.
From 1968 until 1980, flask samples were measured by transferring
the air from the flasks to a nondispersive infrared analyzer (NDIR)
manually by means of a mercury displacement pump
(Komhyr et al. 1983). In 1980, this apparatus was replaced by a
semiautomatic analysis apparatus described by Komhyr et al. (1983). In 1988, this
system was replaced by a more highly automated system that was
capable of higher precision CO2 measurements and able to handle up
to three times as many flasks per analysis day.
All the CMDL CO2 measurements are reported in the WMO X85 mole
fraction scale. The CMDL measurements are made using standard gases
traceable to the WMO Central CO2 Laboratory operated by
C.D. Keeling at Scripps Institution of Oceanography
(Thoning et al. 1987). From 1968 to 1980, the working standard
gases consisted of CO2 in N2. From 1980 to 1990, the secondary
standards were CO2 in natural air and the tertiary and working
gases were CO2 in synthetic air (N2, O2, and Ar)
(Komhyr et al. 1985a). Since 1990, only CO2 in natural air
standards have been used.
The data selection methods used to obtain the values from which
monthly and annual means are calculated have been discussed in
detail by Komhyr et al. (1985a), Conway et al. (1988), and
Conway et al. (1994), so only a brief description will be given
here. First, both members of sample pairs are flagged when the CO2 difference between them is greater than 0.5 ppm. Prior to 1989, one
value of a bad pair was sometimes retained, based on the results of
the curve-fitting procedure described below. Since 1989, both
members of bad pairs are automatically rejected. Samples that are
affected by improper sampling techniques, or analytical problems
are also flagged as rejected data. At this point a curve is fit to
the remaining data, and values lying more than
±3 residual standard deviations from the curve are
flagged as not representing well mixed, regionally representative
air masses. The curve-fitting procedure is repeated until no more
samples are flagged. The fitted curves are then used to calculate
monthly and annual means. Most analyses of the NOAA/CMDL flask CO2 data use only the retained data, but the samples flagged as not
representative of background conditions may still contain useful
information.
CITE AS: Conway, T.J., P.P. Tans, and L.S. Waterman.
1994. Atmospheric CO2 records from sites in the NOAA/CMDL air
sampling network. In
T.A. Boden, D.P. Kaiser, R.J. Sepanski, and F.W. Stoss (eds.),
Trends '93: A Compendium of Data on Global Change. ORNL/CDIAC-65.
Carbon Dioxide Information Analysis Center, Oak Ridge National
Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.
Carbon Dioxide Information Analysis Center (CDIAC)
NOTICE (March 2018): This website provides access to the CDIAC archive data temporarily.
It will be gradually transitioned into data packages in the new ESS-DIVE archive. This site will continue to operate in parallel during and after the transition, and will be retired at a future date. If you have any questions regarding the data or the transition, please contact ess-dive-support@lbl.gov.
The CDIAC data archive ceases operation at the end of September 2017. The data collected over 30 years of operation are transitioning to new archives. The U.S. Department of Energy’s (DOE) Environmental System Science Data Infrastructure for a Virtual Ecosystem (ESS-DIVE) archive is managing the transition. During the transition, this version of the CDIAC website provides access to the CDIAC data. ESS-DIVE is maintained by the Lawrence Berkeley National Laboratory and supported by the DOE’s Office of Science Biological and Environmental Research program (BER). If you have any questions about ESS-DIVE or the data transition, please contact ess-dive-support@lbl.gov or see http://ess-dive.lbl.gov.
ESS-DIVE is in the process of implementing the new archive and is expected to be fully functional and available in Spring 2018. ESS-DIVE is designed to provide long-term stewardship and use of data from observational, experimental, and modeling activities in the DOE in the Subsurface Biogeochemical Research (SBR) and Terrestrial Ecosystem Science (TES) Programs in the Environmental System Science (ESS) activity. ESS-DIVE data contributors will be able to archive and version their data and metadata, obtain data DOIs, search for and access ESS data and metadata via web and programmatic portals, and provide data and metadata in standardized forms.
If your data is generated by or used by a SBR or TES Program project and you would like to store your data in ESS-DIVE, please contact ess-dive-support@lbl.gov for assistance. Parts of the services including DOI assignment are already available.
Alert
Next Section
