Algorithms for determining the absorption coefficient for aquatic particulates using the quantitative filter technique

Particulates in aquatic environments dominate variability in inherent and apparent optical properties. Dilute concentrations have made the quantitative determination of particulate absorption a relatively difficult problem. A quantitative filter technique (QFT) has been described previously1 ,2 which allows determination of the particulate absorption coefficient (a(A)) for samples concentrated on filters by correcting for the pathlength amplification effect (,8). DetaiI of the generality, accuracy and precision of the procedure have not been reported previously. A spectrophotometer equipped with an integrating sphere accessory was used to study the optical density of phytoplankton suspensions and colored polystyrene beads, OD(A), and the optical density of the same suspensions on a variety of common filter types, OD(A). The ratio of OD(A)/OD5(A) confirms that multiple scattering in filters leads to variable 6, a non-linear function of OD.(A). Studies of OD(A) when the filters are mounted in different positions in a single spectrophotometer, and for standard and integrating sphere spectophotometers indicate that the procedures presented are of general applicability. With proper care in baseline correction and sample preparation, spectra of a(A) can be calculated from measurements of OD\) for aquatic particles of diverse size and refractive index with an accuracy of better than i A comparison of filter types commonly used for aquatic research indicates that different algorithms are required for estimation of a(A) from OD(A) measurements using different filter types.