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CARBON DIOXIDE SENSOR

An autonomous CO2 sensor was built around the Licor 820 IR instrument, a single path, dual wavelength, non-dispersive infrared gas analyzer that allows measurement of absolute concentrations of CO2 in air. This instrument has been adapted at MBARI for buoy deployment as part of the TAO/TOGA buoy array in the equatorial Pacific and for numerous coastal buoys and drifters where the primary focus is the measurement of sea surface pCO2. Measurement precision and accuracy was improved by almost an order of magnitude for the O-buoy deployment where the measurement of atmospheric pCO2 is the main goal.  A 3-point field calibration is done via a soda lime chamber to generate a zero standard and two small high pressure CO2 gas standards provided by NOAA’s Earth Systems Research Laboratory (ESRL). The two gas standards span the annual range of pCO2 that has been observed at the NOAA Barrow Observatory in recent years. During the calibration and air sampling modes, the measurement cell is flushed for 1 min. and then the gas flow is stopped, measurements of temperature, pressure and humidity accompany each CO2 measurement under flow and no-flow conditions. The air gas stream is dried and filtered before entering the system.  The precision of the system that is currently deployed is about 0.1 ppm and the accuracy depends on the stability of calibration gases. Calibration gases in large cylinders have a stability of about 0.02 ppm/yr. The CO2 system is controlled by a low power CPU that schedules the pumps and valves, collects and formats the data and also stores all the information in flash memory before passing it on to the supervisory computer for transmission. We have chosen a sampling frequency of 8 measurements per day; this frequency will allow us to resolve significant events while conserving power. Standard gas calibrations are performed 4 times per day. Prior to deployment this system underwent extensive testing at temperatures as low as -30oC even thought the actual temperature experienced during a deployment should be near the freezing point of seawater.