Measurement of light absorption by chromophoric dissolved organic matter (CDOM) using a type-II liquid capillary waveguide: assessment of an achievable accuracy.

• Rüdiger Röttgers, Michael Novak, and Mathias Belz
• received 12/21/2023; accepted 04/12/2024; posted 04/12/2024; Doc. ID 516580
• Abstract: Light absorption by chromophoric dissolved organic matter (CDOM) in the ocean isoften measured using liquid waveguide capillary cells coupled to spectral array detectors. Thistype of optical setup is affected by several sources of uncertainties related to the waveguide andthe detector. Uncertainties from the waveguide arise from error in the effective path length andthe effects of water salinity, while errors related to the detector are due to non-linearity in theresponse, internal stray light, and wavelength accuracy. Here, uncertainties in themeasurements of the spectral absorption coefficient of CDOM due to the optical setup itselfwere investigated in detail. The related systematic errors were very often significant (2 - 15%)and larger than expected from simple measurement uncertainty (1%). However, they can becorrected by characterizing the detector's response for non-linearity and stray light, regularlyperforming calibrations for the detector's wavelength response, and routinely measuring thewaveguide's effective path length. Including such corrections and timely calibrations reducesthe uncertainties related to the spectrophotometric measurements to about 2%. Uncertaintiesrelated to the necessary handling of samples are not included here.