Theoretical and experimental approaches to improve the accuracy of particulate absorption coefficients derived from the quantitative filter technique

The quantitative filter technique (QFT), whereby particles are concentrated onto glass-fiber filters and analyzed in a spectrophotometer, is used extensively to estimate the absorption coefficients of aquatic particles. A number of empirically derived correction factors (beta) have been developed to account for the amplified optical path length associated with the highly scattering glass-fiber filters. Published results are inconsistent, and beta remains the largest source of uncertainty in estimated absorption coefficients. In this study, path-length amplification was estimated from the average cosine of diffusely traveling photons in the filter pad using a theoretical approach. This amplification factor, combined with variability in blank filter pad optical density, explains many of the confounding observations in the literature. Absorption coefficients for phytoplankton cultures and held samples were estimated from a modified QFT using the new model for path-length amplification and tested against absorption coefficients measured with a nine-wavelength absorption and attenuation meter (ac9, WETLabs). A linear regression between the modeled and measured particulate absorption coefficients was highly significant (r(2) > 0.99, n = 99), with estimated slope and intercept not significantly different from 1 and 0, respectively (P < 0.001). The model outperforms published, empirically derived correction factors over a broad range of absorption coefficients and particulate compositions. Results indicate that the modified QFT combined with the new model for path-length amplification yields accurate estimates of spectral particulate absorption coefficients regardless of the concentration or composition of the particulate material.