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

Instrumentation and methodology is described which is used for measurement of the fugacity (or partial pressure) of carbon dioxide (fCO(2) or pCO(2)) in surface seawater. Two separate instruments were developed for the measurements. One is an underway system which measures the mixing ratio of CO2, XCO(2), in a headspace in equilibrium with surface seawater continuously pumped into a 24 1 equilibration chamber. The other is a discrete system in which 460 mi aliquots of water are equilibrated with a 120 mi headspace. Both systems use a non-dispersive infrared 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 knots this translates into a space averaged fCO(2) value over 8 km. The underway system is ideally suited for mapping of the surface water fugacity over large geographic regions. Samples from the discrete instrument are analyzed at 20 degrees C. The primary function of the system is for measurement of subsurface fCO(2) values. The discrete system is also well suited for determining the relationship between the fugacity of CO2 and other (carbon) parameters sub-sampled from the same aliquot. To calculate the fCO(2) in water for in-situ conditions from the mixing ratio in the headspace of the flask of the discrete system, small carbon 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, with 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 samples for the discrete system and the integrated samples for the underway system.