Development and modelling of a point source integrating cavity absorption meter (PSICAM)
The absorption coefficient is a fundamental parameter in understanding the underwater light field, for solving the Radiative Tranfer Equation and understanding/interpreting remotely sensed data from the ocean. Measuring the absorption coefficient is particularly complicated in coastal areas where the optical properties of the water body are the result of a complex mixture of dissolved and particulate components, but mainly because of the interfering effect that scattering has upon the measurements. A great variety of in situ instruments and laboratory techniques have been developed to measure total absorption or the absorption by the various fractions that constitute the total absorption. They are, however, all affected by scattering and empirical corrections need to be applied. Among the instruments to measure absorption, a promising one appeared to be one based on an integrating cavity. Kirk (1995, 1997) outlined the principle and theory of an absorption meter based on an integrating sphere: a Point Source Integrating Cavity Absorption Meter (PSICAM). He argued that owing to its design, a PSICAM would be insensitive to scattering. A novel Monte Carlo code was written to simulate the behaviour of a PSICAM of various cavity radiuses. The results of the simulations carried out with this code showed that such an absorption meter should indeed be unaffected by scattering even with high levels of scatterers. One important disadvantage deduced from numerical modelling for a PSICAM is its sensitivity to the reflectivity of the integrating cavity. Several prototype PSICAMs of increasing quality were built and tested with scattering-free standard solutions. A major difficulty in the development of the prototype was found to be the calibration of the integrating sphere reflectivity. A final laboratory instrument made of a Spectralon sphere was built and tested with artificial and natural water samples containing different levels of scattering particles and compared with existing in situ and laboratory techniques: the ac-9 transmissometer and the filter paper technique for particulate absorption as well as measurement of Coloured Dissolved Organic Matter. Compared with the ac-9 transmissometer, the PSICAM showed remarkable agreement even for water with very high content of Suspended Particulate Matter. Very good correlations were obtained when compared with traditional CDOM measurement. In some cases, significant discrepancies occurred with filter paper measurements of particulate absorption. From laboratory to in situ experiments the PSICAM proved to be a reliable instrument assuming that the instrument was regularly and carefully calibrated. Finally, the PSICAM was deployed during a cruise around the Antarctic Peninsula where total and dissolved absorption measurements were carried out together with chlorophyll absorption measurements after extraction in acetone.