24N98 Leg 2 Readme File Ship: NOAA research vessel Ronald H. Brown Cruise Start: Las Palmas, Canary Islands, January 23 1998 Cruise End: Miami, FL, February 24, 1998 Chief Scientist: Kitack Lee System Operator: Dana Greeley Expocode: 33RO19980123 Method: Infrared absorption of dried gas. For details of the system see: Measurement of fugacity of Carbon Dioxide in surface water and air using continuous sampling methods. Wanninkhof and Thoning, 1993 in Marine Chemistry 44, 189-205, And: Feely, R.A., R. Wanninkhof, H.B. Milburn, C.E. Cosca, M. Stapp, and P.P. Murphy, A new automated underway system for making high precision pCO2 measurements onboard research ships, Analytica Chim. Acta, 377, 185-191, 1998. The three standard gases come from CMDL in Boulder and are directly traceable to the WMO scale. Sampling Cycle: The system runs on an hourly cycle during which 3 standard gases, 3 air samples from the bow tower and 8 surface water samples (from the equilibrator head space) are analyzed on the following schedule: Mins. after hour Sample 4 Low Standard 8 Mid Standard 12 High Standard 16.5 Water 21 Water 25.5 Water 30 Water 34 Air 38 Air 42 Air 46.5 Water 51 Water 55.5 Water 60 Water Units: All xCO2 values are reported in parts per million (ppm) and fCO2 values are reported in microatmospheres (uatm) assuming 100 % humidity at the equilibrator temperature. Notes for Leg 2: 1. Prior to this cruise, the operation of the Valco multi-port valve was changed in an attempt to reduce the number of mistrips that had been occurring about 1.5% of the time. Previously, the Valco reset to the home position after each measurement and then stepped to the correct position. For this cruise, the Valco was set to the following configuration: Position 1 Water Position 2 Air Position 3 Water Position 4 Std 1 (low) Position 5 Std 2 (mid) Position 6 Std 3 (high) The valve was then stepped only at the end of each phase, or 6 times per hour. While this reduced the number of mistrips, the result when the valve failed to trip was that the valve would be in the wrong position for every succeeding phase until it was reset after the Std 3 phase. In all, there were 34 mistrips in about 787 hours of operation. Because of the configuration, when the valve failed to step std 1 gas would flow during the std 2 phase and std 2 gas would flow during the std 3 phase. To correct these, I pasted the voltage values into the appropriate phase for std 1 & 2. For std 3, I used 3 different methods depending on the trend of the voltage response in std 1 and std 2 phases from the previous hour's reading to the next hour's reading. In most cases, I averaged the change in voltage from the previous std 1 and std 2 readings and added the result to the previous std 3 voltage. In cases where the voltage change in std 1 and std 2 was of opposite sign, I used the same method but extrapolated back from the next hour's std readings. In 2 cases I simply averaged the last and next hour's std 3 voltages. 2. Data from the ship's computer system (SST, salinity, wind speed, direction, and fluorometer) were merged into the system's data file with an approximate 3 minute delay factor. This is to account for the time it takes for seawater to travel from the bow intake to the equilibrator in the Hydro Lab. 3. Fluorometer data was added to the feed from the ship's computer system before the start of this cruise. 4. A separate power supply was added to the system to drive the equilibrator thermistor. The thermistor was recalibrated and a new equation for converting voltage to temperature was added to the program. Mercury thermometer measurements taken during leg 1 verify that the new equation is accurate. For questions or comments contact: Bob Castle 4301 Rickenbacker Causeway Miami, FL 33149 305-361-4418 castle@aoml.noaa.gov