April 11, 2020
Author: Emmanuel Boss
Complex Particles (Aggregates)
Aggregates are delicate aquatic particles comprised of primary particles held together by polymers. A large fraction of an aggregate (90% by volume) is often water and hence its physical (e.g. settling speed) and optical properties could be signiﬁcantly diﬀerent from the constituent particle comprising it. When mechanically disturbed (e.g. due to pumping through an instrument, sampling through the spigot of a Niskin bottle, or in the wake of a rosette) fragile aggregates may break.
It is expected that aggregates diﬀer in optical properties from the particles that comprise them and from a solid particle of the same size for two primary reasons: First, the packing of particles within aggregates is often dense enough that coherent interactions between scattered waves emanating from individual particles within the aggregate will cause a diﬀerent scattering pattern than the simple superposition of scattering by the individual particles in suspension. Second, aggregate porosity is observed to grow with increasing aggregate size, so the cross-sectional areas of aggregates can be signiﬁcantly larger than that derived by assuming that the solid mass is packed into a sphere of the same density as the component particles.
Indeed aggregates have been found to contribute signiﬁcantly to backscattering with little change in backscattering/mass as they formed in lab or ﬁeld (Hou et al. 1997, Hatcher et al. 2001, Folly et al. 2004). In a recent study (Boss et al. 2009c) we ﬁnd aggregation to help explain the relative constancy of scattering/mass or attenuation/mass found in coastal areas. These observation are inconsistent with traditional modeling of marine particles (e.g. using Mie theory), which assumes particles are solid, that predict a decrease in scattering eﬃciency per mass with size.
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