MEASUREMENT OF DISCRETE PCO
Hua Chen (CIMAS)
Measurement of the partial pressure of CO2 in subsurface samples
(discrete pCO2) in order to determine the anthropogenic
CO2 input and to characterize water masses.
In order to fully understand the oceanic inorganic carbon
cycle, and in specific to
separate the anthropogenic CO2 invasion from the
biogeochemical cycling of carbon, at least two of
the four carbon system parameters (total dissolved inorganic carbon (DIC),
total Alkalinity (TAlk), pCO2, and pH) must be
measured. pCO2 has the largest dynamic range of the four
parameters and is an excellent
parameter to use in calculations.
An aliquot of water is equilibrated with a headspace of known CO2
concentration. After equilibration the headspace gas is
analyzed using a standardized infra-red analyzer or gas chromatograph.
(Wanninkhof and Thoning, 1993)
Oceanic CO2 increases caused by anthropogenic invasion will
manifest itself by increases in pCO2 and DIC while TAlk remains
constant. To the contrary, increases caused by respiration will cause DIC,
pCO2 and TAlk to increase. The large dynamic signal of discrete
pCO2 from 200 microatmosphere at the surface to 2000
microatmosphere in thermocline waters (Figure 1) and good precision (<0.5 %
of full signal) of the measurement make it an ideal parameter to measure.
As part of the OACES
base line long lines effort the measurements are performed in each ocean
basin. Since pCO2 will increase ten times faster than DIC due
to anthropogenic invasion we expect a large change in this
property when repeat occupations are performed five to ten years after the
Chen, H., R. Wanninkhof, R.A. Feely, and D. Greeley,
Measurement of fugacity of carbon dioxide in
sub-surface water: an evaluation of a method based on infrared analysis,
NOAA Technical Memorandum ERL AOML-85, 1995.
Wanninkhof, R., and K. Thoning, Measurement of fugacity of
CO2 in surface water using continuous
and discrete sampling methods, Mar. Chem., 44 (2-4), pp. 189-205, 1993.
Wanninkhof R. and Feely R.A. (1998) fCO2 dynamics in the Atlantic,
Pacific, and South Indian Ocean. Marine Chemistry accepted "CO2
in the Oceans, special issue".
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