Particle characterisation

Satellite image of sea ice coloured by albedo

Suspended in the waters of rivers, estuaries, coastal seas and the open oceans are tiny particles of sediment (mud, silt and sands), small animals (zooplankton) and tiny plants (phytoplankton) as well as a range of dead and decaying detritus. Characterizing what these particles are and quantifying variations in abundance, size, shape and composition aids understanding of such processes as aggregation/dis-aggregation, particle transport and sedimentation, pollutant and carbon sequestration, opto-acoustic scattering and absorption, primary production and zooplankton feeding and reproduction.

Suspended particles range in length from a few microns to many millimeters. Electrical molecular forces and sticky biological glues bind small particles together into bigger, loosely bound aggregates which are relatively fragile and can be broken apart by strong turbulence or invasive sampling. Simple measurements such as size, shape, composition and settling speed are not easily obtained from suspended particles since these characteristics are constantly changing as particles aggregate and dis-aggregate.

Plymouth University has a strong history of instrument development for particle imaging and research work centered around particle characterization across a range of environments.

Projects

Particles from space (2007 - 2010)

To model the behaviour and impact of suspended particles, which affect the scattering and absorption of light underwater, we need to know more about their size and the way this varies geographically and with time. The aim of this project was to develop a new way of measuring the mean size of marine particles by using satellite data. The advantage of such remote sensing is that particle size can be mapped virtually instantaneously over large areas - the whole of the Irish Sea, for example - and changes with time can be observed. There are limitations: only the size of particles near the sea surface can be determined, and only under cloud-free skies; even so this is a substantial step forward in observational capability. The principle of the technique is that the scattering of light by a given concentration of particles increases as the particles become smaller, we have observed that particle composition - especially excess density - also plays an important role. Simultaneous measurements of concentration and scattering can therefore be used to derive an estimate of mean particle size.

Submicron Particles (2010-2013)

An important question, to which the answer is not at all clear at present, is exactly how large the particles are that are mainly responsible for scattering the light that is 'seen' by ocean remote sensing satellites. It is often assumed that the size distribution of particles in the ocean follows a 'Junge' distribution, in which the number of particles increases rapidly as the size of the particles decrease. With this assumption, and using an optical theory which assumes spherical particles, it has been shown that most of the light scattering is performed by particles smaller than 1 micron in diameter. This means that the particles seen in satellite imagery are mostly very small with slow settling speeds and long residence times in the surface of the ocean and has important implications for those who interpret these images and who use them to verify numerical models of particles in the ocean. There have never been any direct, in-situ observations of the numbers of such small particles predicted by the Junge distribution. Underwater photographs of undisturbed samples of seawater show that particles tend to gather together in 'flocs' and the measurements of particle size distribution which support the Junge distribution use a disruptive technique which potentially breaks up flocs and hence possibly over-estimates the number of small particles. Using holographic cameras designed and developed at Plymouth University (Figure 1 & Figure 2), focused images of small particles suspended in water have been made at a range of locations in the Irish Sea and UK west coast continental shelf as well as at inland freshwater locations. This technique images particles down to about 0.7 microns in size. We also make improved measurements of absorption and scattering by particles using optical back-scattering instrumentation with our collaborators at the University of Strathclyde.

Figure 1: Commercial holocam (the LISST-HOLO, Sequoia Scientific Inc) being deployed alongside ADV and LISST-100X on a benthic frame in the Menai Strait, Wales, UK.

Figure 2:The sea surface in visible light images (such as this SeaWIFS image, courtesy of NEODAAS) is often bright because of light scattering by particles suspended near the surface.