USING A POINT-SOURCE INTEGRATING-CAVITY ABSORPTION METER TO MEASURE COLOURED DISSOLVED ORGANIC MATTER ABSORPTION OF DUTCH NATURAL WATERS

摘要

A Point-Source Integrating-Cavity Absorption Meter (PSICAM) is an integrating cavity in which light is emitted by a central point-source, reflected by a Lambertian cavity wall. The light flux at the cavity wall is measured using a spectrometer. The light field within the cavity is isotropic and theoretically there is no loss of light due to scattering by particles in the medium. In a standard cuvette light scattered by particles out of the collimated beam is lost to the measuring device. Furthermore the average path length of photons in the cavity is longer than using a cuvette, as they can collide and be reflected by the cavity wall a number of times before being absorbed. Thus a PSICAM is capable of accurately detecting lower absorptions than a normal spectrometer system. In this research an experimental PSICAM was calibrated using the reflective properties of the materials used and its operational range was determined by numerical modelling. Coloured Dissolved Organic Matter (CDOM) absorption, from 20 samples collected in the Dutch Vecht area wetlands covering a broad range of aquatic environments, was measured using both PSICAM and a standard spectrophotometer cuvette. The exponentially fitted g440 absorptions measured in the PSICAM were on average 50% lower than the cuvette absorptions. The difference between cuvette and PSICAM was found to be systematic and nearly spectrally neutral. This result is likely due to the absence of scattering in the PSICAM cavity. The overall accuracy of CDOM absorption should be investigated further by optical modelling.