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**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
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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  CMDL/CARBON CYCLE GASES GROUP STANDARDS   PREPARATION AND STABILITY 
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.

2. Site Description 
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. 

  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. 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)
  4.3. Pump
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.

6. References 
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,   Summary Report 1972, No. 1, pp. 22-23, NOAA, Environ. Res. Labs., Boulder Colorado,   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,   1996.
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.
World Meteorological Organization Global Atmosphere Watch,   Report of the Fourth WMO Meeting of Experts on the Quality Assurance/Science   Activity Centers (QA/SACs) of the Global Atmosphere Watch, WMO/TD No. 689, No.   104, Garmisch-Partenkirchen, Germany, 1995.
Zhao, C.L., P. Tans, and K.W. Thoning, A high   precision manometric systemfor absolute calibrations   of CO2 in dry air, J. Geophys. Res., 102(D5), 5885-5894, 1997. 
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