A High Precision Manometric Analysis System to Define the WMO Mole Fraction Scale for CO2 in Dry Air
A better understanding of the global carbon cycle is central to resolving uncertainties surrounding the future rate of global change. A major tool in the study of the carbon cycle is provided by the detailed temporal and large-scale spatial patterns of the mixing ratio of CO2 in the atmosphere. A fundamental requirement is careful and continuing calibration for CO2. Without a solid and precise link to a common calibration scale, most of the global measurements would be useless in meeting the goals of the carbon budget study.
Most atmospheric measurements of CO2 are made using nondispersive infrared (NDIR) analyzers. This technique requires accurately calibrated reference gases. Since the mole fraction of CO2 in reference gas tanks may change with time, it is necessary that periodic calibrations of the gases in "primary" tanks be performed with a manometric calibration system in order to ensure preservation of the original "baseline" gas calibration levels. CMDL has been designated as the Central CO2 Laboratory (CCL) by the WMO to help determine the international scale against which CO2 measurements are reported. An independent manometric system has been developed to evaluate the primary standards over long periods of time to an uncertainty of 0.1 micromole/mole. (Figure 1).
Figure 1. CCL CO2 Manometric Calibration System.
A manometric apparatus that can be easily and repeatedly employed to perform calibrations has been developed by the CCGG group. This system is illustrated in Figure 2.
The apparatus is principally made of glass. It consists essentially of a 6 L Pyrex glass flask connected by means of a manifold to a 10-mL Pyrex glass volume. A pressure gauge of quartz spiral Bourdon tube type as a primary manometer is used to measure the CO2 and the carrier gas pressure precisely. Two additional intermediate size glass flasks are used for calibrations of the volume ratio of the 10 mL and the 6 L volumes. All of the components are enclosed in an oven, where the temperature is regulated to an accuray of 0.01° C. (Figure 3).
Two liquid nitrogen cold traps connected to the manifold are used for CO2 collection when the sample gas is extracted from the large volume. The quartz spiral pressure gauge is regularly calibrated with a dead-weight tester.
Principle of measurement method
To begin the manometric calibrating process, the glass manometric chamber including the 6 L large volume are evacuated to a residual pressure of less than 0.13 Pa (~1 mtorr). Sample air from a cylinder to be analyzed is passed through a 1 m long, 3 mm diameter copper coil immersed in an alcohol bath at -70° to remove water vapor. The gas then flows into the evacuated glass volume until the pressure in the volume reaches 80kPa. The gas is then allowed to come to equilibrium, and the temperature and pressure is measured. The sample air is then slowly pumped out through two liquid nitrogen traps, freezing out CO2, N2O and residual water vapor. The pressure of the gas entering the traps is maintained at 0.4 kPa (~ 3 torr). This takes about 40 minutes. The frozen CO2 and N2O in the traps is distilled several times to remove water vapor by surrounding the traps with a dry ice-alcohol mixture. The CO2 and N2O are then transferred to the 10 mL volume by placing liquid nitrogen around the small volume and removing the dry ice-alcohol mixture from the traps. The trapped CO2 and N2O are then thawed and the pressure and temperature are continuously measured while equilibrium is reached. This takes about one hour. The molar ratio of the CO2 in the original air sample can be calculated with the virial equation of state, using a predetermined ratio of the large to small volumes, and taking real gas compressibility into account.
Since the first manometric calibrations in 1996, when the reproducibility of the manometric analyses was 0.12 ppm, it has gradually improved to 0.03 ppm in 2006. Comparison of the manometric determinations with NDIR measurements using reference gases calibrated by the Scripps Institution of Oceanography relative to the WMO X93 scale showed differences of less than 0.1 micromole/mole in the ambient range (340-420 ppm).
The manometric calibration system is described in more detail in:
Zhao, C. L. and P. P. Tans (2006), Estimating uncertainty of the WMO mole fraction scale for carbon dioxide in air, Journal of Geophysical Research-Atmospheres, 111(D8), 10.1029/2005JD006003.
Zhao, C., P.P. Tans and K.W. Thoning, A high precision manometric system for absolute calibrations of CO2 in dry air. Journal of Geophysical Research102(D5):5885-5894 March 20, 1997