Inversion of Light Scattering Measurements to Obtain Biogeochemical Parameters

Ocean Optics XVI, Santa Fe, New Mexico, 18–22 Nov., 2002J. Ronald V. Zaneveld1 (Presenter), Michael S. Twardowski2, Kusiel S. Shifrin1, W. Scott Pegau1, Emmanuel Boss3, and Ilia Zolotov1
1Oregon State University, Ocean. Admin. Bldg 104, Corvallis, OR 97331
2WET Labs, Inc., Dept. of Research, 165 Dean Knauss Drive, Narragansett, RI 02882
3University of Maine, School of Marine Sciences, 5741 Libby Hall, Orono, ME 04469

MS Word version (11 pp)

INTRODUCTION

Renewed emphasis has been placed recently on the measurement of the volume scattering function, its moments, and its spectrum in natural waters. It is thus useful to review the methods of inversion of this data to obtain information regarding the nature of the particles. There is a very large literature on scattering inversions, so that this extended abstract cannot be complete. We have chosen to highlight a number of issues that could be addressed in the next few years using newly available instrumentation.

The interaction of an electromagnetic wave and a particle depends on the electromagnetic properties of the particle. For simple particles such as homogeneous spheres, these properties can be summarized by the size and complex index of refraction (m = n – in'). For more irregular particles, such as phytoplankton, in principle a complete description of the structure of the particle is necessary, that is internal complex index of refraction distribution and shape. Bulk measurements such as those obtained with light scattering, attenuation or absorption instrumentation linearly sum the properties of the individual particles if their concentration is not too high. It is obvious that inversion can at best reproduce the parameters that went into the original measurement. Therefore inversion of bulk inherent optical properties (Mobley, 1994 for a review) cannot produce more than the parameters mentioned above. As will be further discussed below, researchers nonetheless have presented inversions to obtain Particulate Organic Carbon (POC), Total Suspended Mass (TSM), etc. The derivation of these parameters therefore depend on secondary (often empirical) relationships with the particulate size, shape, and index of refraction characteristics.

Inversion of light scattering by a collection of natural particles to obtain the details of all the particles is clearly not possible as the number of particulate parameters will always exceed the number of measured scattering parameters. Such an exact inversion is possible however, if the number of parameters is small. Polystyrene beads can be obtained commercially that have a very narrow, known size distribution and a known index of refraction. For a collection of this kind of particle nearly exact inversions are possible. The fact that that is possible is no guarantee that the same inversion method will be correct for mixtures of far more complex natural particles.