Spectral absorption properties of mineral particles in western Lake Erie: Insights from individual particle analysis

Abstract

The light‐absorbing attributes of minerogenic particle populations were determined for clay‐mineral‐enriched conditions at 14 sites in the western basin of Lake Erie following a wind‐driven re‐suspension event. Estimates of spectral absorption properties of the minerogenic particle assemblages, namely the imaginary part of the refractive index (n′), the mean absorption efficiency factor (<Qa,m>), the mean package effect index (<>), and the mass‐specific absorption coefficient (), are presented. These were based on laboratory measurements of non‐algal particulate absorption (aNAP), minerogenic particle size distributions (PSD) with an individual particle analysis technique, and iterative Mie theory calculations to retrieve n′(λ). There were wide spatial differences in magnitudes of these estimates. The n′(λ) values are generally consistent with those obtained with a different methodology. Absorption by minerogenic particles was dominated by clay mineral particles in the 2–20 µm size range. The effects of n′ and PSD variations on both <Qa,m> and <> were well‐parameterized in the multiplicative format of optical thickness, and were qualitatively consistent with the literature and Mie theoretical predictions for monodispersed spheres. Spatial differences in n′ and PSD were both important drivers of the variations in <Qa,m> and <>. Mie theory calculations indicate that absorption by these minerogenic particles causes substantial decreases in their spectral scattering, backscattering, and backscattering ratio, from the red to the blue spectral regions, relative to the case of non‐absorbing.