Characterisation of a point-source integrating cavity absorption meter for applications in optical oceanography

• Main Author: Lefering, Katharina
• Published: University of Strathclyde 2016
• Subjects: 551.46
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.698556

Many biogeochemical and physical processes in the aquatic environment are driven by the spectral light absorption properties of the water body and the constituents dissolved and suspended within. Improving our knowledge on absorption processes in marine waters is of great interest to optical oceanographers as absorption influences the structure of underwater light fields. The determination of high quality absorption data are important for accurate modelling of underwater radiative transfer processes and the interpretation and derivation of ocean colour remote sensing products. Accurate measurements of spectral absorption coefficients, however, are challenging because instruments and methods are affected by scattering by marine particles and can suffer from significant systematic errors. Röttgers and co-workers (2005) introduced a point-source integrating cavity absorption meter (PSICAM) in which sample absorption is measured inside an integrating sphere using a totally diffuse light field. This set-up has been shown to be insensitive to scattering errors and therefore ideally suited for absorption determinations of marine waters. Initial characterisation and a sensitivity analysis confirmed the superior performance of the PSICAM compared to other spectrophotometric techniques but also highlighted remaining limitations in accuracy at UV/blue wavelengths. PSICAM data were subsequently used to develop and validate corrections for established absorption measurements, in particular the determination of particulate absorption coefficients with the filter pad technique and the determination of in situ absorption measurements with submersible AC-9 instruments. The latter can be used to populate radiative transfer models and simulate underwater light fields. An optical closure study demonstrated consistency between in situ measurements of radiometry and inherent optical properties coupled into radiative transfer model outputs, confirming high accuracy of input absorption data and output model parameters. The ability to model underwater and water-leaving light fields correctly is important for ecosystem modelling application and the validation of satellite remote sensing data. A preliminary analysis of the potential to simultaneously measure spectral fluorescence and absorption was carried out. This demonstrated both the magnitude of inelastic scattering effects on current PSICAM performance and potential towards further development of the system that could benefit primary production studies.