Light absorption measurements of aquatic particles: status and prospects

Optical models in support of environmental or biogeochemistry studies using remote sensors require precise description of the individual contributions of the various substances contained in the upper water layers. This implies a demand for accurate measurements of optical properties, such as the light absorption coefficient of particulate matter suspended in aquatic environments, which includes phytoplankton, organic and inorganic detritus. Oceanographers use two basic approaches to measure absorption by marine particles. First, in situ instruments such as the ac-9 from WetLabs, Inc., if deployed with and without a particle filter, provide estimates of particulate absorption at several discrete wavelengths as a difference between the measured total absorption and the measured absorption by soluble materials. This approach became available only a few years ago. The second approach involves the concentration of particles on filters with subsequent absorption measurements in the laboratory using standard dual-beam laboratory spectrophotometers. The authors' attention is focused solely on the latter approach. Although in recent years considerable progress has been achieved, a general consensus on the most effective methodology for measuring particulate absorption has not been reached. In view of the importance of the topic, the NASA SeaWiFS Project Office sponsored a workshop, held at the Scripps Institution of Oceanography in December 1996, to address methodological issues. The goal of the workshop was to compare measurement protocols adopted by various laboratories, to assess and analyze different methodologies from the point of view of accuracy, ease of use, and practicality, and to provide recommendations for improving the measurements. The present paper briefly reviews some of the methodological issues for measuring light absorption by aquatic particles.