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The Effects of Physical Parameters on the Absorption Coefficient of Natural Waters

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The Effects of Physical Parameters on the Absorption Coefficient of Natural Waters AN ABSTRACT OF THE THESIS OF W. Scott Pegau for the degree of Doctor of Philosophy in Oceanography presented on February 23, 1996. Title: The Effects of Physical Parameters on the Absorption Coefficient of Natural Waters. Abstract approved: Redacted for privacy J. Ronald V. Zaneveld The absorption coefficient is one of the fundamental parameters used for studies in hydrologic optics. The significance of this parameter makes it important to understand how to properly measure it and how the absorption of components, such as water, depend on physical parameters such as temperature and salinity. A discussion is provided of a majority of the techniques available for measuring the absorption coefficient. Results from a comparison of the techniques indicates that natural variability in the optical properties is an important factor to consider when comparing or reporting measured values of the absorption coefficient. The agreement between methods was found to be best at longer wavelengths where the absorption by water is a significant portion of the total absorption coefficient. In order to test the accuracy of measurements of the absorption coefficient a practical TOP closure equation is developed. Application of the equation to field measurements is limited by our ability to measure the volume scattering function. The dependence of the absorption coefficient of water on temperature and salinity is investigated. It was found that the absorption coefficient in the near infrared portion of the spectrum is strongly dependent on temperature and salinity. In the visible region the only portion with a temperature dependence > 0.00 1 m1/°C is in the neighborhood of 610 nm. A model of the absorption coefficient of water is used to develop a spectrally continuous temperature dependence curve. The only measured visible wavelength with a significant dependence was at 412 nm. It was found that in some spectral regions the temperature and salinity had opposite effects on the absorption coefficient. The temperature dependence of the absorption coefficient in the near infrared region creates a difference in the absorption coefficients of water and ice. Two optical techniques are developed and tested to measure frazil concentrations. One of the methods uses the difference in absorption properties of water and ice to measure frazil concentrations. This technique should allow frazil measurements to be made even when other types of particles are present in the water column. The Effects of Physical Parameters on the Absorption Coefficient of Natural Waters by W. Scott Pegau A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Completed February 23, 1996 Commencement June 1996 Doctor of Philosophy thesis of W. Scott Pegau presented on February 23, 1996 APPROVED: Redacted for privacy ijor Professor, representing Oceanography Redacted for privacy bean of College of Oceanic and At'mospheric Sciences Redacted for privacy Dean of Gradu1e School I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. Redacted for privacy W. Scott Pegau, Author ACKNOWLEDGMENTS I thank my major advisor, Dr. J. Ronald V. Zaneveld, for his guidance in research, teaching, and other aspects of life as a researcher. Because of the well rounded education and unselfish approach to research he has provided, I feel that the transition from being a student to being on my own will go as smoothly as possible. I also value the kindness he and his family have shown to my wife and me. In all aspects it has been a rewarding experience working with him. I am grateful to Dr. Clayton Paulson for including me in his Arctic research and his work in developing the frazil measurement techniques. He has provided invaluable help in expanding my research directions as well as providing insights to classroom material when I had difficulties. I thank the other members of my committee, Dr. Timothy Cowles, Dr. Barry Sherr, and Dr. Robert Olson, their time and effort as committee members is greatly appreciated. I look forward to interacting with my committee members in my future research endeavors. I thank all of the co-authors on the various chapters of this thesis for sharing their data with me and their helpful comments on the text. The efforts of Dr. James Mueller are especially appreciated. His many comments on content and style greatly improved the comparison of absorption measurement paper. I thank Steve Daly and Jim Lever for their cooperation in providing use of the facilities of the Cold Regions Research and Engineering Laboratory to test the optical frazil measurement techniques. I thank the Office of Naval Research Environmental Optics program managers, Dr. Curt Mobley, Dr. Gary Gilbert, and Dr. Steven Ackleson, for providing funding for a majority of this research. Parts of the research have also been funded by the High Latitude section of ONR, NASA, and NSF. Most importantly I must thank my wife, Cathy Pegau, for being so understanding when I had to work late on a project or for a class. I know of no other person who would be as understanding when I had long stretches of time traveling to meetings or cruises. She was even willing to drive me to and from the airports in Portland and Eugene at ungodly hours with a minimum of fuss. To her I give my deepest love. CONTRIBUTION OF AUTHORS Dr. Ronald Zaneveld was involved in the analysis and writing of all of the enclosed chapters. The data, method description, and error analysis of the spectrophotometer data used in Chapter two was provided by Dr. Joan Cleveland. Dr. Willard Wells, William Doss, and Robert Stone provided similar analysis of the Compound Radiometer. Dr. James Mueller provided the sections concerning the Tethered Optical Profiler and Gershun's equation. Dr. Alan Weidemann and Dan Kennedy were responsible for the sections involving the Integrating Cavity Absorption Meter. Robert Maffione provided the sections involving the Isotropic Point Source. Dr. Charles Trees provided information on pigment composition during the experiment. All of the above co-authors helped in developing the analysis and discussion of the collected data. Dr. Mueller provided much assistance in developing the final version of this manuscript. Dr. Kenneth Voss provided the General Angle Scattering Meter data used in Chapter 3 along with comments on the analysis provided in the manuscript. Deric Gray was responsible for making the laboratory measurements used in Chapter 5. Dr. Clayton Paulson provided the field data used in Chapter 6. He also helped in collection and analysis of the laboratory data. He has provided strong guidance in the writing of the manuscript. TABLE OF CONTENTS Page 1. GENERAL INTRODUCTION............................................................................... 1 2. A COMPARISON OF METHODS FOR THE MEASUREMENT OF THE ABSORPTION COEFFICIENT IN NARURAL WATERS......................... 8 2.1 ABSTRACT............................................................................................. 9 2.2 INTRODUCTION ................................................................................... 9 2.3 METHODS..............................................................................................12 2.3.1 Sampling Site............................................................................14 2.3.2 Ancillary Measurements...........................................................17 2.3.2.1 Beam attenuation at 660 nm and Chlorophyll-a Fluorescence..............................................................17 2.3.2.2 Chlorphyll a Concentrations......................................17 2.3.3 In Situ Absorption Measurements.............................................18 2.3.3.1 Reflecting Tube Absorption Meter (RTAM).............18 2.3.3.2 Gershun's Equation...................................................19 2.3.3.3 Tethered Optical Profiling System (TOPS)...............21 2.3.3.4 Isotropic Point Source (IPS)......................................25 2.3.3.5 Compound Radiometer (CR).....................................28 2.3.4 Laboratory Methods..................................................................29 2.3.4.1 Integrating Cavity Absorption Meter (ICAM) ...........29 2.3.4.2 Spectrophotometer.....................................................31 2.3.5 Instrumental Errors...................................................................33 2.3.5.1 RTAM Error Analysis ...............................................33 2.3.5.2 TOPS Error Analysis.................................................35 2.3.5.3 IPS Error Analysis.....................................................36 TABLE OF CONTENTS (Continued) Page 2.3.5.4 CR Error Analysis.....................................................38 2.3.5.5 ICAM Error Analysis................................................39 2.3.5.6 Spectrophotometer Error Analysis............................40 2.4 RESULTS ..................................................................................................42 2.4.1 Meteorological and Limnological Setting.................................42 2.4.2 Vertical Profiles of Absorption.................................................47 2.5 DISCUSSION...........................................................................................57 2.5.1 Temporal Variability of Bio-Optical Properties.......................57 2.5.2 Vertical Profiles........................................................................61
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