MEASUREMENT OF FUGACITY OF CO2 IN SURFACE-WATER USING CONTINUOUS AND DISCRETE SAMPLING METHODS

Instrumentation and methodology is described which is used for measure ment of the fugacity (or partial pressure) of carbon dioxide (fCO(2) o r pCO(2)) in surface seawater. Two separate instruments were developed for the measurements. One is an underway system which measures the mi xing ratio of CO2, XCO(2), in a headspace in equilibrium with surface seawater continuously pumped into a 24 1 equilibration chamber. The ot her is a discrete system in which 460 mi aliquots of water are equilib rated with a 120 mi headspace. Both systems use a non-dispersive infra red analyzer as detector. In the underway instrument the average XCO(2 ) in the headspace of an equilibration chamber is measured at near in- situ temperature over 20 min each hour. At a cruising speed of 13 knot s this translates into a space averaged fCO(2) value over 8 km. The un derway system is ideally suited for mapping of the surface water fugac ity over large geographic regions. Samples from the discrete instrumen t are analyzed at 20 degrees C. The primary function of the system is for measurement of subsurface fCO(2) values. The discrete system is al so well suited for determining the relationship between the fugacity o f CO2 and other (carbon) parameters sub-sampled from the same aliquot. To calculate the fCO(2) in water for in-situ conditions from the mixi ng ratio in the headspace of the flask of the discrete system, small c arbon mass balance and, sometimes significant, temperature corrections have to be applied. Comparison of 100 surface values obtained in the South Atlantic using the underway and discrete systems shows that the average difference of pCO(2) values for the two systems ranges from -4 .3 mu atm to -8.6 mu atm, depending on the temperature correction, wit h a standard deviation of 4 mu atm. The differences show scatter of up the 15 mu atm which we attribute to a mismatch between the point samp les for the discrete system and the integrated samples for the underwa y system.