2.2. Carbon Dioxide

2.2.1. In Situ Carbon Dioxide Measurements

The mole fraction of atmospheric CO₂ was measured with continuously operating non-dispersive infrared analyzers (NDIR) at the four CMDL observatories during 1996 and 1997 as in previous years.  Monthly and annual mean CO₂ concentrations (expressed in the WMO mole fraction scale) are given in Table 2.2.  These values are provisional, pending final calibrations of station standards.  Preliminary selected monthly average CO₂ mole fractions for the entire record through 1997 are plotted versus time for the four observatories in Figure 2.1.

TABLE 2.2.   Provisional 1996 and 1997 Monthly Mean CO₂ Mole fractions From Continuous Analyzer Data (mmol mol^-1, Relative to Dry Air WMO Mole Fraction Scale)

Fig. 2.1.   Preliminary selected monthly mean CO₂ mole fractions in dry air expressed in mmol mol^-1 at the four CMDL observatories.

A new data acquisition system was installed at the Barrow Observatory, Barrow, Alaska (BRW) in May 1996.  This system uses a Hewlett-Packard Unix workstation to control not only the CO₂ NDIR measurements but the CH₄ and CO in situ gas chromatograph systems as well.  Data are downloaded daily from BRW to Boulder over the Internet, as well as recorded on optical disks at BRW for backup.  A new data acquisition system was installed at the South Pole Observatory, Antarctica (SPO) in January 1997.  This installation was done at the same time that the CO₂ in situ measurement system was moved into the new SPO Clean Air Facility.  There are no CH₄ or CO gas chromatographs at SPO; only the CO₂ system is controlled by the new data acquisition system.

At both BRW and SPO modifications were made to the CO₂ system to reduce the amount of reference gas usage.  The glass H₂O cryotrap was relocated between the inlet air pumps and the gas manifold with a smaller auxiliary cryotrap added in between the gas manifold and the NDIR analyzer.  With this setup the first cryotrap dries only the ambient air samples, and the second cryotrap dries the reference gases and the dried ambient air.  The volume of gas that the reference gases need to flush away after each gas change was greatly reduced making lower flow rates possible.  The flow rate was reduced to ~150 cc min^-1 from 300 cc min^-1.  This change increased the life expectancy of the working reference gas tanks to about 6 months. 

The SPO CO₂ NDIR analyzer experienced intermittent noise and stability problems throughout 1996 and 1997.  Without success, a number of components were tested or exchanged in an attempt to pinpoint the cause of the problems.  This noise problem continues to occur randomly, can last from a few minutes to many hours, and resulted in a loss of about 16% of the data for 1996 and 9% of the data for 1997.  A replacement analyzer is planned for installation at SPO during austral summer 1998. 

At SMO the old data acquisition unit had several hardware problems during the second half of 1997.  It failed completely at the end of September and was repaired and restarted in November.  

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