Atmospheric Carbon Dioxide Record from In Situ Measurements at Baring Head

Graphics    Data

Investigators

M.R. Manning, A.J. Gomez, K.P. Pohl
National Institute of Water and Atmospheric Research, Ltd.,
Climate Division,
Gracefield Road, Gracefield,
P.O. Box 31-311,
Lower Hutt, New Zealand

Period of Record

1970-93

Methods

Determinations of atmospheric CO₂ mixing ratios are made using a Siemens Ultramat-3 nondispersive infrared (NDIR) gas analyzer. The NDIR CO₂ analyzer is connected via a gas manifold consisting of stainless steel tubing and computer-controlled solenoid switches to 12 gas cylinders and 2 sample air lines. The NDIR analyzer compares ambient air CO₂ mixing ratios relative to known CO₂ mixing ratios in tanks of compressed reference gases. The analyzer operates in a differential mode, with a "zero" reference gas of CO₂ mixing ratio 20 to 30 parts per million (ppm) below ambient air CO₂ levels flowing continuously through one cell of the analyzer at ~10 mL/min. When atmospheric CO₂ is measured, a diaphragm pump pulls air through a sampling line at ~5 L/min. A small fraction of this (180 mL/min) is dried cryogenically to a temperature of approximately ¬70° Celsius and passed through the sample cell of the CO₂ analyzer. Both the "zero" and sample gas are exhausted into the observatory building.

Operation of the analyzer system is controlled by a computer that switches gas from cylinders or sample air lines into the analyzer sample gas cell. For each calibration gas measurement, the computer monitors the analyzer voltage every second until the standard deviation of 60 consecutive values is below 0.02 ppmv. In some cases, stability is not reached within 5 min, so the computer repeats the previous gas followed by the gas that did not stabilize. All data for the final 60 s of each calibration gas are recorded in a computer file, whether or not stability was achieved. In the early years of measurement at Baring Head, CO₂-in-nitrogen calibration gases, prepared by the Scripps Institution of Oceanography (SIO) were used. Since 1985 both CO₂-in-natural-air mixtures prepared at SIO and CO₂-in-synthetic-air mixtures prepared commercially have been used for calibration. In all cases concentrations were assigned by SIO. No significant difference has been observed between these two types of calibration gas, but since 1992 only natural-air mixtures have been used. Further details of sampling methods and calibration gases are available as unpublished reports from the authors.

A small amount of data are removed from the record where there is some evidence of malfunction. This is assisted by using at least one of the calibration gases as a "surveillance" gas and noting any periods where its apparent concentration differs from that assigned by SIO. Data management procedures allow the choice of "surveillance" and "calibration" gases to be made retrospectively. Baseline data are selected from the remaining data based on steadiness of the CO₂ concentration and on wind direction. At Baring Head maritime well mixed air masses come from the Southerly direction, and a baseline event is normally defined as one in which the local wind direction is from the South and the standard deviation of minute-by-minute CO₂ concentrations is <<0.1 ppmv for 6 or more hours.

The CO₂ concentrations shown here are given in a preliminary version of the X93 SIO 1993 mole fraction scale. This scale is identical with the SIO X85 scale for all measurements before 1986.

Baring Head, New Zealand
Seashore coastal cliff
41°24' S, 174°54' E
85 m above MSL

Trends

The annual atmospheric CO₂ concentration at Baring Head, New Zealand (including data from Makara for the period November 1970 to March 1972) rose from 325.06 parts per million by volume (ppmv) in 1971 to 354.90 ppmv in 1993. The monthly CO₂ record from Baring Head is typical of Southern Hemisphere sites: it shows small seasonal amplitude, maxima occur from August to October, and minima occur from February to April.

References

• Lowe, D.C., C.D. Keeling, and P.R. Guenther. 1979. The concentration of atmospheric carbon dioxide at Baring Head, New Zealand. Tellus 31:58-67.
• Manning, M.R., and K.P. Pohl. 1986. Atmospheric CO₂ monitoring in New Zealand 1971-1985. Report No. INS-R-350, DSIR Institute of Nuclear Sciences.
• Manning, M.R., and K.P. Pohl. 1987. A review of analyzer calibration procedures and calibration gas mixtures used in the New Zealand CO₂ monitoring programme. Report No. INS-R-363, DSIR Institute of Nuclear Sciences.
• Manning, M.R., K.P. Pohl, and A.J. Gomez. 1994. Status of New Zealand CO₂ measurement programme. In G.I. Pearman and J.T. Peterson (eds.), Report of the seventh WMO meeting of experts on carbon dioxide concentration and isotopic measurement techniques, (Rome, 7-10 September 1993) World Meteorological Organization, Geneva.