Light-absorbing properties of mineral particles in the Great Lakes

Abstract

Abstract The light-absorbing attributes of minerogenic particle populations are presented for 32 sites from five areas of the Laurentian Great Lakes based on laboratory measurements of non-algal particulate absorption along with a particle size distribution (PSD) metric and total projected area of minerogenic particles per unit volume of water (PAV m ) determined with an individual particle analysis (IPA) technique. IPA established clay mineral and calcite dominance as the primary and secondary composition cases, respectively, for the sampled populations. Estimates of spectral absorption properties of these populations are presented, including the mean absorption efficiency ( Q a,m >), the imaginary part of the refractive index ( n ′), and the mean package effect index ( Q a ⁎ ,m >). Substantial differences in these attributes among the samples are documented; those for Q a,m > and Q a ⁎ ,m > were explained well by the effects of optical thickness, which represent the multiplicative effects of n ′ and a PSD metric. Greater package effect is attributed to increases in n ′. Mie theory calculations, based on the n ′ estimates and the IPA characterizations, depict substantial decreases in the spectral backscattering coefficient ( b b,m ) for the minerogenic particle populations from the red to blue spectral regions, relative to the case of non-absorption. Related recommendations for mechanistic remote sensing initiatives to represent the effects of minerogenic particles are presented, including (1) use of PAV m as the optically active constituent metric, (2) use of Q a,m > as the corresponding cross-section (interim value of 0.33 or 0.39), and (3) representation of the spectral character for b b,m reported here.