|Converting volume concentration (VC) to mass concentration, total suspended solids or suspended sediment concentration requires some assumptions. First of all, the reason why the LISST does not output mass right away is that this is not what is being measured using laser diffraction. When the laser beam encounters a particle it scatters in a manner that is proportional to the particle cross-sectional area. Essentially, the laser beam encounters the cross-sectional area of the particle. Now, the ratio of particle surface area to particle volume decreases with increasing particle size (i.e. with increasing particle volume). Therefore, on a per unit volume concentration basis, the scattering is inversely proportional to the size of the particles. This is true for all laser particle sizers, not just Sequoia’s LISST instruments. The only way to get mass concentration directly is by actually weighing the sediment in a sample – typically this means filtering it.
In principle it is very straightforward to convert volume to mass, by multiplying the volume of the particles with the density of the particles. For a suspension of fully dispersed (unflocculated) mineral grains the density of all particles will be 2.65 g/cm3 or very close to this value. In this case the VC (with units of µl/l) could simply be multiplied with 2.65 in order to yield mass concentration in units of mg/l.
Major problems arise when the density of the particles in suspension are not all the same. There are several reasons why this might be:
- Flocculation. If particles flocculate their density decreases. The more they flocculate, the smaller the density. A floc with a diameter of a few hundred micro meters can easily have a density very close to that of water (1 g/cm3), because most of its volume *is* water.
- Mixture of organic / inorganic particles. If the suspended particles are made up of a mixture of organic (e.g diatoms) and inorganic particles, then the densities will differ, as the organic particles tend to have (much) smaller densities than the mineral grains.
- Different minerals. If the suspended particles in your sample are made of of minerals with varying densities, the same thing will happen.
It is possible to – occasionally – take a water sample, filter it, get the mass concentration and then compare that to the VC from the same sample, thereby getting an average density for the particles for that sample. However, if just some of the particles are flocculated, then using an average density to convert volume to mass will invariably cause an overestimation of the mass in the larger size classes, and a general overestimation of the mass concentration. (Of course, for the *exact* sample that is being used to obtain the relationship everything will be OK!).
The best way to convert VC to mass concentration is to take a water sample for mass concentration now and then, and divide the mass concentration with the volume concentration from the LISST in order to get the average density of the particles in that paricular sample. Do this over time and you will get an idea of how much the density of the particles generally varies. If the variability is small, then do an average of all the densities and use this value to for future runs. One may find a seasonal variability of the densities, with lower densities in the summer and higher in the winter (due to more organic particles in the summer), in which case it would be necessary to use a seasonally varying density to convert LISST volume to mass.
The following articles may be of interest if you are interested in learning more about this topic:
Fennessy et al (1994): INSSEV: An instrument to measure the size and settling velocity of flocs in situ. Marine Geology 117: 107-117.
Fennessy et al. (1997): Estimation of settling flux spectra in estuaries using INSSEV. In ‘Cohesive Sediments’ (Eds: Burt, Parker, Watts), John Wiley & Sons, pp. 87-104.
Fugate D, and Chant B (2006): Aggregate settling velocity of combined sewage overflow. Marine Pollution Bulletin 52: 427-432.
Khelifa A, and Hill PS (2006): Models for effective density and settling velocity of flocs. Journal of Hydraulic Research 44(3): 390-401.
Mikkelsen OA, and Pejrup M (2001): The use of a LISST-100 laser particle sizer for in-situ estimates of floc size, density and settling velocity. Geo-Marine Letters 20: 187-195. doi:10.1007/s003670100064
Finally, you may be interested in this article here on our website: ‘Comparing volume concentration and mass concentration.’