Reference Gas Preparation and Calibration
NOAA Technical Memorandum ERL-14
CMDL/CARBON CYCLE GREENHOUSE GASES GROUP STANDARDS
PREPARATION AND STABILITY
Duane Kitzis
Conglong Zhao
Cooperative Institute for Research in Environmental Sciences,
University of Colorado, Boulder, Colorado.
Climate Monitoring and Diagnostics Laboratory,
Boulder, Colorado.
September 1999 UNITED STATES NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION
DEPARTMENT OF COMMERCE
William M. Daley, D. James Baker Secretary,
Under Secretary for Oceans and Atmosphere/Administrator
NOTICE
Mention of a commercial company or product does not constitute an
endorsement by NOAA Environmental Research Laboratories. Use for publicity
or advertising purposes of information from this publication concerning
proprietary products or the tests of such products is not authorized.
______________________________________________________________
For sale by the National Technical Information Service, 5285 Port Royal Road
Springfield, VA 22061
CONTENTS
Abstract
1. Introduction
2. Site Description
3. Weather and Wind Trajectories
4. Cylinder and Pumping Facility Description
4.1. Calibrations
4.2. Cylinders
4.3. Pump
4.4. Drying System
4.5. Pressure Measurement and Compressor Control
4.6. Complete System
4.7. Air/Cylinder Measurement System
5. Air Standard Preparation
5.1. Conditioning
5.2. Filling
5.3. Targeting Above Ambient Levels
5.4. Targeting Below Ambient Levels
5.5. Adjusting Trace Gas Levels in a cylinder
5.6. Initial Calibration and Use of Trace Gases
6. References Duane Kitzis and Conglong Zhao Abstract
The NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) Carbon Cycle
Gases Group (CCGG) methods and materials for air standards preparation are presented
in detail. Atmospheric natural air standards are prepared in aluminum cylinders
as reference gases for trace gas measurement systems by CCGG and others. The
trace gas carbon dioxide (CO2) is primarily discussed, with some mention of
common processes for carbon monoxide (CO) and methane (CH4) where applicable.
Cylinders are conditioned with natural air and then pressurized to 135 atmospheres
using an oil-free compressor. The mixing ratios of CO2, CH4, CO, and the isotopic
ratios of CO2 can be altered by spiking the cylinder with low or high concentration
gas prior to filling with a balance of natural air. The air is dried to a dew
point of -80 C by passing the air through a very strong desiccant. The calculations
and spiking methods for CO2 are presented for targeting CO2 mixing ratios to
within 2 umol/mol. The stability of CO2 and CH4 in aluminum high-pressure cylinders
is better than the present analytical precision.
1. Introduction
High precision measurements of the trace gases CO2, CH4, and CO are being undertaken
by many laboratories of various nations in order to better understand the biogeochemical
cycles of these gasses. There can be no integration of all these various measurement
projects unless the data are all referenced to common well-defined calibration
scales. The consistency required for optimally usable atmospheric CO2 measurements
and intercomparison must be no worse than 0.1 umol/mol for the northern hemisphere
and 0.05 umol/mol in the southern hemisphere [World Meteorological Organization
(WMO), 1993]. Because of stringent quality controls necessary for the measurement
of long-lived trace gases, stable reference gas standards are an integral part
of any long-term measurement program. The Carbon Cycle Gases Group (CCGG) of
the Climate Monitoring and Diagnostics Laboratory (CMDL) has been involved with
making CO2 standards since the early 1970s [Miller, 1974] and over the years
has expanded to include methane, carbon monoxide, and the stable isotopes of
CO2. Currently the CCGG standards preparation facility at the Mountain Research
Station makes more than 200 standards per year and has enabled many laboratories
world wide to be on the same comparable CO2 and CO [Novelli et al., 1991, 1994]
scales as designated by the World Meteorological Organization [WMO, 1995]. Most
of the discussion in this paper is focused on CO2. The need for natural air
standards was recognized when standards were measured using different nondispersive
infrared (NDIR) analyzers. The comparative work done by Komhyr et al. [1985]
was prompted by calculations of CO2 pressure broadening in nitrogen and natural
air presented by others [e.g., Bischof, 1975; Griffith et al., 1982]. Because
of the difference in pressure broadening of the CO2 absorption lines by nitrogen
and by natural air, considerable corrections need to be applied to NDIR measurements
when using CO2-in-nitrogen reference standards to calibrate CO2-in-air measurements.
These corrections are strongly analyzer dependent and may not be constant in
time. From these comparisons it became apparent that standards of wholly natural
air needed to be created. Efforts started in 1979 [Thoning et al., 1987] and
have continued through the present. All standards are calibrated in the central
calibration laboratory at CCGG in Boulder, Colorado. The CMDL secondary standards,
to which all other NOAA standards are tied, were measured approximately every
3 years against the WMO designated primaries maintained by the Scripps Institution
of Oceanography (SIO). In 1995 the WMO designated the CCGG as the central calibration
laboratory. The world network is currently tied to the 15 CCGG primary standards
now designated as the new WMO primaries. The Boulder primary standards are calibrated
at regular intervals on our own manometric system [Zhao, 1997] and were intercompared
with the previous WMO calibration scale maintained at SIO. As of today, there
appears to be no significant offset between the CMDL scale and the SIO scale
in the range of ambient concentrations.
The air standard preparation facility is located in a biosphere
preserve at 3040 m altitude, 40 degree 02'N, 105 degree 32'W. This subalpine
forested area has very limited vehicle access and usually provides nonurban
continental air. A record of the CO2 mixing ratio and it's seasonal variations
at the site can be seen at the CCGG network site at Niwot Ridge (NWR).
The network site is located just to the west of the preparation facility
at 40 degree 03'N, 105 degree 35'W at 3475 m. An
aerial picture looking west over the area can be seen at the CCGG Web site
(www.cmdl.noaa.gov/cccg/index.html). The biosphere preserve is maintained by
the National Forest Service and the University of Colorado, Mountain Research
Station. The nearest paved road is approximately 10 km away at a much lower
elevation. Summer and winter transportation to the site over the last 3 km varies
due to snow coverage and conditions. The site is unreachable in winter except
via snow-tracked vehicles or skis. To the east of the site is the urban front
range of Colorado, 25 km away. The densely populated Denver metropolitan area
is 45 km to the southeast, and to the south is the city of Boulder watershed,
which is closed to all human traffic. To the north and west is the Arapahoe
National Forest and wilderness area. Basically south, west, and north is very
sparsely populated for 100 km or more.
3. Weather and Wind Trajectories
In general the site experiences clean well-mixed air from the western continent
of North America and beyond from the North Pacific Ocean. The daily averaged
wind speed and direction, from 1990 to 1997 Figure
2 shows that wind direction is predominantly from the west with a more variable
direction and weaker winds during the summer months. Some spring and summer
days have short periods of local upslope easterly sourced air with a component
of relatively polluted metropolitan air. This is very evident from high concentrations
of correlated CO and CO2. It is common for winter winds to average 5-10 m s-1.
4. Cylinder and Pumping Facility Description
4.1. Calibrations
The hierarchy of the CCGG CO2 standards is derived from the
CCGG central calibration laboratory for 15 CO2 primary standards. (Figure
4)
The original standards ranging from 264 to 520 umol/mol were measured
three times at SIO and on the CCGG manometric system [Zhao, 1997]. The calibration
results of these standards are currently used to provide the WMO (CO2 in air)
mole fraction scale. Currently, five new primary standards are being added at
CO2 mole fractions of approximately 700, 1000, 1500, 2000, 2500, and 3000 umol/mol.
The ten secondaries in the range of 290 to 420 umol/mol are calibrated using
the primaries. All internal and external standards are calibrated on a single
NDIR, which is referenced to these secondaries. For CO2 stability the calibration
histories of the various internal CMDL standards were checked for calibra-tion
precision and long-term drift. The mean difference of repeated measurements
for CO2 mole fractions between 325 and 425 umol/mol is 0.002 B1 0.014 (histogram
of distribution (Figure 5).
This represents
381 cases since 1988 where individual cylinders were recalibrated in less than
6 months. Thus we can say that the precision of our measurement is 0.014 umol/mol
(one standard deviation). Outside of this range the precision was found to be
0.1 for mole fractions from 200 to 325 umol/mol and 0.25 for mole fractions
from 425 to 500 umol/mol (histogram of distributions (Figure
6).
The loss in precision of relatively low and high CO2 mixing ratios is
because of the limitations of measuring toward the end of the range of the primary
standard scale and the slight degradation in measurement precision at high mole
fractions. This estimate of precision takes all cylinders into account and may
contain some cases of real drift and, therefore, is a worst-case scenario. The
absolute accuracy stated by the CO2 calibration laboratory is 0.1 umol/mol.
The precision and internal consistency of the standards in the range between
325 and 425 umol/mol is 0.04 umol/mo l over a 2-year period at the 2-sigma confidence
level. If a cylinder is recalibrated at the end of its useful volume, the precision
of a cylinder is limited by the reproducibility of our measurements (0.028 at
the two sigma level), and necessary assumptions about the history of the drift
between calibrations should include this uncertainty.
4.2. Cylinders
Brass, packless taper-threaded valves are used in almost all
of our cylinders with Teflon tape as the thread sealant. The Ceodeux valves
have a soft seat that does not become difficult to turn or seal over years of
use. Several types of cylinders and surface treatments were tested, and no evidence
was found of specific coatings to improve the CO, CO2, or CH4 stability over
bare, uncoated aluminum. Luxfer aluminum, alloy 6061, cylinders with their standard
cleaning preparation are the most used cylinder in CCGG. Currently Luxfer provides
a high heat treatment, proprietary acid wash (with no etching of the surface),
steam cleaning and forced-air drying. The steel cylinders used in the past exhibited
too much drift in CO2 mixing ratios, with half of these drifts on the order
of 0.05 umol/mol CO2 per year [Komhyr, 1985] could be at different rates or
of a different sign in each cylinder. Aluminum cylinders have much better stability
than steel for CO and CO2. Methane seems to be stable in any type of cylinder
we use, whether it is aluminum, surface treated, or steel as long as it has
been air dried to a very low dew point. The same cylinders are used for stable
CO2 isotopes and have exhibited no measurable drift [Trolier, 1996]. There have
been some cases of major drift (0.05 per mil 13C per week) of the CO2 stable
isotopes, attributed to the Teflon paste used as the cylinder valve sealant
and also to unknown regulator contamination. There may be some evidence for
long-term drift of CO in aluminum cylinders as exhibited in smaller high-pressure
cylinders [Paul Novelli, CMDL, personal communication, 1999]. This drift, however,
is on the order of the measurement precision and has not yet been well quantified.
A histogram of CO2 concentration drift (Figure
7) for 125 cylinders that have a calibration history greater than 2 years,
shows drift in both directions.
There are some cases (8%) with evidence of drift
greater than 0.045 umol/mol per year. For this reason, it is recommend that
standards be recalibrated during long use and at the end of their useful volume.
The drifts, as shown in the histogram, cannot be predicted. Drift in most of
our standards is often undetectable for histories of less than 2.5 years. In
some documented cases the drift was attributed to H2O mixing ratios greater
than 5 umoles mole-1, stainless steel valves, or valves with packing materials,
which preferentially absorb trace gases. There may be an increasing trend in
the CO2 with decreasing cylinder pressure below 20 atmospheres. This is different
in each cylinder or may not be present at all. All cylinders are hydrotested
after manufacturing and need to be retested periodically. All cylinders are
stamped with the month and year of the last hydrotest and, if out of date must
be retested before they can be shipped. Currently aluminum cylinders need hyrotesting
5 years after the stamped date. In the United States hydrotesting can only be
done by companies certified by the U. S. Department of Transportation. The process
consists of removing the valve and attaching the cylinder to a water/hydraulic
pump system that displaces all of the air in the cylinder. The cylinder is immersed
in a water bath of measured volume and the water in the cylinder is pressurized
to 1.67 times the working pressure of the cylinder. The elastic expansion is
measured via the outside water displacement, pressure is released in the cylinder,
and the permanent expansion is measured. The typical cylinder used (Size AL-150)
elastically expands by about 190 cc and is permanently expanded 0-2 cc after
the pressure is released. The cylinder is deemed unusable if it permanently
expands by more than 10% of the elastic expansion. We have never had a cylinder
fail the test. Hydrotesting is followed by a drying/passivation process from
Scott Marrin. After this process they are filled with 20 atmospheres of Ultrapure
air. Ultrapure air is whole air compressed at Scott Marrin and is highly filtered
with mole sieves to trap most trace gases and water. (Jack Marrin, Scott Marrin,
Inc., personal communication, 1999)
The compressor currently in service is a RIX SA6B. It is a
three-stage oilless, piston compressor commonly used for recharging scuba tanks
and is used as sold with no alterations. The three stages of compression are
to 6, 34, and 204 atmospheres respectively. The compression rings are made of
filled PTFE plastic and require no lubrication. There are moisture separators
between the second and third stage and at the output of the compressor. The
third stage is designed with a 102 atmosphere back pressure regulator to maintain
this minimum pressure against the piston. The flow at 3040 m altitude for this
compressor is 126 L min-1 at ambient pressure. The temperature of the compressed
air in the output-cooling coil can reach 121(C. These pumps are not airtight
and some alterations of trace gases may be observed. Others have made some modifications
to this pump to lower the air temperature from compression and improve its use
for ambient air sampling [Mak, 1994]. Some of the improvements may result in
some loss of pump performance however. Currently some methods to make the pump
run cooler, which do not affect the air stream, are being tried.
4.4. Drying System
The stability of many trace gases is sensitive to water vapor
and especially sensitive to liquid water. Therefore the drying step is important
in the preparation of any gas standards. The air exiting the compressor is passed
through a Balston, 1 micron coalescent filter with a manual drain. The air then
passes through two stainless steel tubes of 50 cm length in series. These have
a 2.5 cm cross section and are filled with Anhydrous Magnesium Perchlorate (Mg(ClO4)2
) (Anhydrone(r), J. T. Baker, Inc., a division of Mallinckrod & Baker, Inc.).
This system reduces the water vapor in the air stream to typically less than
1 umol/mol. After approximately 52,000 L of air have passed through the traps,
the upstream trap is replaced with the downstream trap, which is replaced with
a fresh one. The second moisture trap is for complete redundancy as one will
dry the air stream to less than 1 umol/mol. Water accumulated in the compressor
moisture separators and the coalescent filter is blown out before and after
each cylinder pressurization. Another possible desiccant is Aquasorb (Mallinckrodt).
Aquasorb is phosphorus pentoxide (P2O5). However, CO2 was found to increase
by 2-5 umol/mol with this new drying material. This problem and the safety issue
of cycling phosphoric acid in a high pressure system prompted the decision to
discontinue use.
4.5. Pressure Measurement and Compressor Control
The measurement, display, and control of the compressor is
done with an Omega series DP205-S controller with a PX302 pressure transducer.
The output solid-state relay, controlled by the alarm setting, activates a set
of Crydom DC control, solid-state opto-isolated relays. These relays control
the 220 V compressor power and are normally off unless turned on via the Omega
controller. The setting of the control is 135 atmospheres for a full cylinder.
4.6. Complete System
The air is drawn into the system
through a 10-m tall, 2.5-cm diameter stainless steel intake. All high
pressure flex hoses are stainless steel braided, PTFE lined with 1/4" VCR connections.
All connections are 1/4" NPT or 1/4" VCR with silver coated nickel gaskets.
A 0.5-m length between the compressor and the Balston filter allows for some
extra cooling of the air and, therefore, better condensation before going through
a coalescent filter. This was found to improve the performance of the filter
significantly. All parts are secured to a rack for safety. The popoff valves
and the check valve are for safety. Popoff valves are set to 170 atmospheres
on either end in case of a trap clogging, and the check valve will prevent the
cylinder from dumping all the air volume in the case of a line break. Seven
micron filters are in line at the end to prevent dirt from getting to the safety
valve or the pressure transducer.
4.7. Air/Cylinder Measurement System
Carbon dioxide is measured on site with a Licor NDIR, and
CO is measured with a TECO NDIR. The system requires a set of six standards
ranging from 300 to 450 umol/mol CO2 and one 200 nmol/mol CO span gas. These
standards can be used to bracket ambient air measurements or test prepared standards.
New standards are also tested for H2O content with a MEECO Aquamatic that uses
a P2O5 conductivity method and is set up to measure from 1 to 200 umol/mol H2O.
The maximum allowable H2O mixing ratio in a filled cylinder is 5 umol/mol. Most
cylinders test between 0.5 and 1.0 umol/mol. This corresponds to dew points
less than -75 degree C and usually less than -80 degree C when expanded, and
less than -30 degree C under full pressure.
5. Air Standard Preparation
5.1. Conditioning
When a cylinder is to be refilled and contains air with the
trace gases near ambient, conditioning is not usually necessary. New cylinders
have Ultrapure air and cylinders with undesirable concentrations left in them
must be purged of the old air to have the walls conditioned to ambient air.
Drifts have persisted for months due too evacuating the cylinder down to vacuum
before filling. Drifts due to spiking, discussed below, have been eliminated
by filling the cylinder immediately after spiking and filling the cylinder laid
down. There is no evidence to show drifting due to the filling process described
below. As a result of these experiences and to remain conservative about long-term
unknown effects, the following steps have been adopted by CCGG as procedure:
(1.) Blow off the residual air down to ambient pressure. This must be slow enough
to avoid water condensation at the valve. (2.) Fill to 20 atmospheres and blow
down to ambient pressure two times to purge old air. This uses mass flow to
flush out the original air. (3.) Fill to 33 atmospheres and let sit for at least
a week for wall equilibration. From this flushing procedure the original air
left in the cylinder is less than 0.01% of the volume at the time of spiking
and less than 5E-5% after the final filling. Drifts in the stable isotopes of
CO2 have been observed, even after this procedure, and have been corrected via
commercial rolling of the cylinder.
5.2. Filling
The 33 atmospheres of air is blown down to ambient pressure.
The standard can now be filled with ambient air or the trace gas can be adjusted
via one of the next two steps. The cylinders are rated to 153 atmospheres and
are filled to 149 atmospheres in about 35 to 45 minutes. When the air cools
the pressure in the tank decreases to 136 atmospheres.
5.3. Targeting Above Ambient Levels
For increased standards above ambient we use one of several
cylinders with very high concentrations of the trace gas of interest. A small
amount of the spiking gas is put in a 1 liter transfer volume at higher than
ambient pressure. This over pressure is allowed to flow into the cylinder until
pressures are equilibrated. The formula for increased trace gas values is as
follows: Trace gas increase K * (Target Concentration - Ambient Concentration)
where K is a constant for trace gas increase/pressure of the
spiking gas in the transfer volume. The over pressures used in the transfer
volume are on the order of 0.1 atm of over pressure per increased umol/mol CO2
in a standard 29.5-L cylinder. Because the composition of the spiking gas varies,
this value varies and must be found empirically with each cylinder of spike
gas. Once a few cylinders are spiked and calibrated, the constant K is the same
over a wide range of increased trace gases. Examples of spike gas concentrations
are 10% CO2 in air, 1.5% CO in air, and 2.5% CH4 in air.
5.4. Targeting Below Ambient Levels
Lowering a trace gas below ambient levels requires that some
volume of air with none of the trace gas, be put in the cylinder.
Pf * Vf = Pz * Vz + Pa * Va
where
Pf is the final filling pressure of the standard at room temperature.
Vf is the final concentration of the trace gas. Pz is the pressure of the zero
gas put in the standard first to lower the concentration below ambient. Vz
is the concentration of the zero gas. Pa Pf - Pz is the balance pressure
of ambient air necessary to fill the standard to 149 atmospheres. Va is the
ambient concentration of the trace gas. The zero air is created with specific
chemical traps for each trace gas. Carbon dioxide can be removed with sodium
hydroxide, Ascarite (Thomas). Carbon monoxide is removed with Schutze reagent.
The zero air from both trapping agents was tested and found to be less than
5 umol/mol for CO2 and immeasurable trace amounts for CO. The isotopes of CO2
were manipulated with gases of light or heavy isotopes. The same type of trap
holder is used as for the desiccant traps. Specific trace gas traps can be installed
between the Mg(ClO4)2 filled tubes. The second trap will catch any residual
moisture released by the trace gas traps. The zero air is put directly into
the cylinder if very low final concentrations are targeted. The more precise
method requires the filling of an intermediate ballast with the zero air and
then transferring the partial pressure necessary to the standard cylinder. The
necessary partial pressure of the zero gas is calculated from the above formula,
rewritten as: Pf * Vf = [Pf - Pz] * Va
Pf * [Va - Vf] / Va = Pz
The standard cylinder is filled to the pressure P(z) from
a ballast of whole air (with the specific trace gas removed) and then filled
to the final pressure with dry natural air of ambient concentrations.
5.5. Adjusting Trace Gas Levels in a Cylinder
Occasionally the targeted concentration is missed by an amount
that requires adjustment. If the concentration of the standard is closer to
ambient than the target concentration, it is easiest to blow this off and start
over. Experimenting with introducing small volumes of high pressure (high or
low concentrations) into the air stream have proven very time consuming due
to the time necessary for the cylinder to become evenly mixed before obtaining
a usable measurement from the standard. If, however the final concentration
turns out to be farther away from ambient than the desired targeted trace gas
concentration, it is possible to adjust the standard toward the desired concentration.
The same partial pressure formula stated previously can be used adding a term
for the gas already in the cylinder and blowing the cylinder down to the necessary
partial pressure. When the cylinder is filled to the final pressure again with
ambient levels of the trace gas, the concentration will be adjusted toward the
targeted value fairly precisely. In the case where the concentration in the
cylinder is too low and must be increased Pf * [Va - Vf]/[Va - Vm] = Pm
where Vm is the concentration of the gas in the cylinder,
and Pm is the pressure to lower the standard to, before refilling with ambient
air. In the case where the concentration in the cylinder is too high and can
be lowered Pf * [Vf - Va]/[Vm - Va] = Pm
5.6. Initial Calibration and Use of Trace Gases
After the standard has been filled, checked for low water
content, and for targeting of the trace gas it is transported to the central
calibration laboratory in Boulder. The standard is then calibrated three times
with approximately 1-week intervals between each calibration. For CO2 these
measurements are done with a NDIR analyzer, comparing them to secondary CMDL
standards that are closely tied to the primary WMO standards. The trace gas
mixing ratio reported includes measurement repeatability as well as a check
on cylinder stability. These standards are used at our base line observatories,
international collaborative projects, and many national laboratories as tertiary
standards. There is evidence that as the volume of pressurized air is used up,
the trace gas can come off the walls in disproportionate amounts. This has sometimes
been seen in cylinders with CO2 when a standard is used at pressures below 20
atm. Thus CO2 mole fraction may increase with pressure loss. This effect is
variable and may not be measurable in all tanks. For a representative final
calibration, use is discontinued when the cylinder pressure becomes lower than
20 atm.
Bischof, W., The influence of the carrier gas on the infrared
gas analysis of atmospheric CO2, Tellus, 27, 59-61, 1975.
Griffith, D.W.T., Calculations of carrier gas effects in nondispersive
infrared analyzers, I. Theory, Tellus, 34, 376-384, 1982.
Komhyr, W.D., T.B. Harris, and L.S. Waterman, Calibration
of Non-dispersive Infrared CO2 Analyzers with CO2-in-Air Reference Gases, J.
Atmos. Ocean Technol., 2(1), 1985.
Mak, J., and C.M. Brenninkmeijer, Compressed Air Technology
for Isotopic Analysis of Atmospheric Carbon Monoxide, J. Atmos. Ocean. Tech.,
11(2), 425-431, 1994.
Miller, J.M. (Ed.), Geophysical Monitoring for Climatic Change,
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1974.
Novelli, P.C., L.P. Steel, and J.W. Elkins, The development
and evaluation of a gravimetric reference scale for measurements of atmospheric
carbon monoxide, J. Geophys. Res., 96(D7), 22,461-22,476, 1991.
Novelli, P.C., J.E. Collins, Jr., R.C. Myers, G.W. Sachse,
and H.E. Scheel, Reevaluation of the NOAA/CMDL carbon monoxide reference scale
and comparisons with CO reference gasses at NASA-Langley and the Fraunhofer
Institut, J. Geophys. Res., 99(D6), 12,833-12,839, 1994.
Thoning, K.W., P. Tans, T.J. Conway, and L.S. Waterman, NOAA/GMCC
calibrations of CO2-in-air reference gases: 1979-1985, NOAA Tech. Memo. ERL
ARL-150, 63 pp., NOAA Environ. Res. Labs., Boulder, Colorado, 1987.
Trolier, M., J.W. White, P.P. Tans, K.A. Masarie, and P.A.
Gemery, Monitoring the isotopic composition of atmospheric CO2: Measurements
from the NOAA Global Air Sampling Network, J. Geophys. Res., 101(D20), 25,897-25,916,
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World Meteorological Organization Global Atmosphere Watch,
Report of the Seventh WMO Meeting of Experts on Carbon Dioxide Concentration
and Isotopic Measurement Techniques, WMO TD-No. 669, No. 88 Rome, Italy, 1993.
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Activity Centers (QA/SACs) of the Global Atmosphere Watch, WMO/TD No. 689, No.
104, Garmisch-Partenkirchen, Germany, 1995.
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precision manometric system for absolute calibrations of CO2
in dry air, J. Geophys. Res., 102(D5), 5885-5894, 1997. 14
CMDL/CARBON CYCLE GASES GROUP STANDARDS
PREPARATION AND STABILITY
Figure 3 shows back trajectories at the filling
site. These trajectories are averages from 1993 to 1997 and are divided into
6 of the most common patterns. All all are predominantly from the west with
spring and summer
showing more continental air as compared with the longer winter
trajectories. Currently trajectories can
be observed on the CMDL web site.(www.cmdl.noaa.gov)
4.3. Pump
Return to CCGG Reference Gas page.
