A new instrument for studying seawater pCOz dynamics is described. The Submersible Autonomous Moored Instrument for CO, (SAMI-CO,) operates by equilibration of ambient seawater pCOz with a colorimetric pH indicator contained in a gas-permeable membrane. The indicator is periodically renewed to improve the stability and sensitivity typically reported for similar indicator-based pCOz sensors. The design combines off-the-shelf optical components, a miniature solenoid pump and valve, a low power data logger, and a fiber-optic flow cell to achieve low power consumption and easy assembly. SAMI-CO, is designed to operate down to 100 m and contains sufficient batteries and reagent for deployment up to 6 months while making 48 measurements per day. Extensive in situ field tests have been performed, including deployment in Woods Hole Harbor for ~30 d (May-June 1994). The field evaluation has confirmed that SAMI-CO, is capable of measuring seawater pC0, with exceptional long-term stability (no detectable drift in 1 month) and sensitivity comparable to ship-based equilibrator-infrared methods (f 1 patm). The time-series data obtained from this study show that pC0, can be highly variable in nearshore environments with up to lOOpatm changes detected over a 4-h period. The structurally rich data highlight the need for continuous mooringbased measurements of pC0, for understanding carbon cycling in natural waters.The oceans absorb a significant fraction of anthropogenitally produced CO2 (Sarmiento and Sundquist 1992), yet the processes that regulate CO, fluxes into the ocean are not well constrained (Garcon et al. 1992). The airsea exchange of CO, primarily depends upon the ApCO, (the difference in partial pressure of CO, between the air and sea) and the degree of agitation of the sea surface. Therefore, atmospheric pCO,, seawater pCO,, and winds or surface wave activity (Wallace and Wirick 1992) must be measured to estimate the gas flux.The primary variability in CO, fluxes arises from changes in winds and surface seawater pC02. Surface pC0, can have large spatial and temporal variability due to its dependence on many physical and biogeochemical processes (heat gain or loss, upwelling, gas-exchange, photosynthesis, respiration, etc.). One long-term goal of oceanographers is to establish robust relationships between surface seawater pC0, and other oceanographic properties (e.g. temperature, salinity, white-cap coverage, chlorophyll, oxygen) so that ocean models will be capable of predicting CO2 fluxes in different climate change scenarios. Ship-based studies of seawater pC0, provide only a brief snapshot of CO, dynamics and are biased toward relatively calm conditions. These limitations have made it difficult to obtain the long-term time-series measurements of seawater pC0, needed to establish statistically significant relationships with other oceanographic properties. AcknowledgmentsWe thank Hans J xnnasch for many helpful discussions during the development of SAMI-CO,. We also thank Gene Terray for assisting in the ...

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Calibrations are necessary for most chemical sensors because the response is not consistent between sensors nor stable over time. If chemical sensors could be designed to have identical behavior from sensor to sensor and no drift, the need for sensor calibrations would be reduced. In the present paper, the feasibility of calibration-free optical chemical sensors is explored. An indicator-based pCO 2 (partial pressure of CO 2 ) sensor is designed that has excellent sensor-to-sensor reproducibility and measurement stability. This superior level of performance is achieved by using the following strategy:(1) renewing the sensing solution, (2) allowing the sensing solution to reach equilibrium with the analyte, (3) calculating the response from a ratio of the indicator solution absorbances, and (4) through careful solution preparation, wavelength calibration, and stray light rejection. Three pCO 2 sensors are calibrated, and the response curves are essentially identical within the uncertainty of the calibration. Long-term laboratory and field studies are presented that show the response has no drift over extended periods (months). The theoretical response, determined from thermodynamic characterization of the indicator solution, also predicts the observed calibrationfree performance. Other absorbance-based sensors, such as optrodes, can be designed and operated in a similar fashion, making calibration-free optical chemical sensors available for a wide range of biomedical, industrial, and environmental applications.