title: Measurement of light absorption by aquatic particles: improvement of the                 quantitative filter technique by use of an integrating sphere approach.
abstract: Determination of particulate absorption in natural waters is often made by measuring the transmittance of samples on glass-fiber filters with the so-called quantitative filter technique (QFT). The accuracy of this technique is limited due to variations in the optical properties of the sample/filter composite, and due to uncertainties in the path-length amplification induced by multiple scattering inside the filter. Some variations in the optical properties of the sample/filter composite can be compensated by additional measurements of the filter's reflectance (transmittance-reflectance method [T-R] [S. Tassan and G. M. Ferrari, Limnol. Oceanogr. 40, 1358 (1995)]). We propose a different, rarely used approach, namely to measure the filter's absorptance in the center of a large integrating sphere, to avoid problems with light losses due to scattering. A comparison with other QFTs includes a sensitivity study for different error sources and determination of path-length amplification factors for each measurement technique. Measurements with a point-source integrating-cavity absorption meter were therefore used to determine the true absorption. Filter to filter variability induced a much lower error in absorptance compared to a measured transmittance. This reduced error permits more accurate determination of the usually low absorption coefficient in the near IR spectral region. The error of the T-R method was lower than that of the transmittance measurement but slightly higher than that of an absorptance measurement. The mean path-length amplification was much higher for the absorptance measurement compared to the T-R method (4.50 versus 2.45) but was found to be largely independent of wavelength and optical density. With natural samples the path-length amplification was less variable for the absorptance measurement, reducing the overall error for absorption to less than ±14%, compared to ±25% for the T-R method.
authors.affiliation: Institute for Coastal Research, Helmholtz-Zentrum Geesthacht, Center for Materials and Coastal Research, Max-Planck Strasse 1, D-21502 Geesthacht, Germany. rroettgers@hzg.de
authors.forename: Rüdiger
authors.initials: R
authors.lastname: Röttgers
authors.forename: Steffen
authors.initials: S
authors.lastname: Gehnke
mesh_terms: D000042:Absorption; D000458:Cyanobacteria; D017377:Diatoms; D005374:Filtration / Q000379:methods*; D005618:Fresh Water / Q000032:analysis; D058087:Haptophyta; D008027:Light; D009391:Nephelometry and Turbidimetry / Q000379:methods*; D010316:Particle Size; D015203:Reproducibility of Results; D012542:Scattering, Radiation; D012623:Seawater / Q000032:analysis; D014873:Water Pollutants / Q000032:analysis*
chemical_list: D014873:Water Pollutants