The influence of schlieren on in situ optical measurements used for particle characterization

In pycnoclines, the density differences can cause light scattering - schlieren - even though only few particulate scatterers may be present. This may pose problems for the interpretation of results obtained with instruments relying on light scattering and transmission, for example the LISST (Laser In Situ Scattering and Transmissometry) particle sizer, and various cameras. Here, the influence of schlieren on in situ forward light scattering, beam attenuation and image analysis is evaluated using a LISST-100 and a digital floc camera. Automated image analysis routines detect schlieren as particles, causing an apparent increase in particle size and volume. Re-analysis omitting schlieren-affected parts of the images reveals no increase. LISST beam attenuation and Volume Scattering Function (VSF) measurements indicate that schlieren can cause increases in beam attenuation due to a marked increase in the VSF at angles smaller than similar to 1.5 degrees-2 degrees, and falsely indicate accumulation of suspended particles in the pycnocline. Light scattering caused by density differences can also cause multiple scattering, which produces an apparent decrease in particle size derived from the LISST. Schlieren is visible in images when the buoyancy frequency exceeds similar to 0.12 s(-1). Buoyancy frequencies above 0.025 s(-1) may cause increases in beam attenuation due to scattering from the density gradients and complete extinction of beam transmission can occur at buoyancy frequencies above similar to 0.20 s(-1). All instruments measuring light scattering can potentially be affected by density differences, and results obtained in waters where buoyancy frequencies exceeds 0.025 s(-1) should be interpreted carefully.