A NEW OPTICAL SENSOR FOR P(CO2) MEASUREMENTS IN SEAWATER

A knowledge of the distribution of the partial pressure of CO2 at the surface of the ocean is needed to quantify the absorption of atmospheric CO2 by gas exchange. Owing to the large space and time variability of this quantity at the surface of the ocean, an experimental approach aiming to make time-series measurements from unattended platforms must be envisaged. In this context we have developed a P(CO2) sensor which meets the requirements for being installed on a buoy for approximately 1 year. The principle of the sensor is based on a colorimetric method. The selected dye is thymol blue. Its spectra and variation of its pK vs. temperature have been determined. A dedicated optical spectrophotometer has been built making absorbance measurements at three wavelengths. The sensor includes a sensitive volume enclosed with two silicone membranes and filled with thymol blue in an ionic medium. It is related to the spectrophotometer with silica fiber optics. Two calibration experiments were made over a fugacity of CO2 (f(CO2)) range extending from 340 muatm to 590 muatm. They lasted 3 and 4 days, respectively and showed that in the range 300-600 muatm a variation of + 1 muatm is seen by the sensor and that the standard error of the fit of the two calibration curves is, respectively, +/- 3 muatm and +/- 5 muatm.