Characterisation of atmospheric cloud particles using a light scattering probe

When investigating the composition and dynamics of the atmosphere's particulate matter, particle shape is an important parameter by which classification and possibly identification of particles may be achieved. Spherical droplets, cuboidal crystals typical of marine aerosols, and the wide variety of morphologies assumed by ice crystals, are examples where the determination of shape may be used in combination with size spectra measurements to provide experimental data upon which theoretical models of macroscopic and microscopic physical behavior of clouds and aerosols may be developed and tested. A specific example involves the study of ice microphysics and the behavior of droplets and ice crystals which can occur simultaneously within clouds. The radiative properties of these mixed-phase clouds can be radically dependent upon the relative proportions and size spectra of the two phases [Prabhakara et al., 1993]. There are several commercially available instruments designed to measure the size spectra of atmospheric particles, the FSSP - Forward Scatter Spectrometer Probe, (Particle Measurement Systems Inc. Boulder CO.), being perhaps the most widely used in alrbome platforms [Knollenberg, 1981; Dye and Baumgardner, 1984]. However, these instruments cannot provide information relating to particle shape. Other techniques, based on recording the silhouettes of particles traversing a light sheet [Knollenberg, 1970], or which record impact pits left by particles on deformable film [Amott et al., 1994], or based on holographic imaging of particles [Lawson and Cormack, 1995], or which allow visual examination of ice crystals impacting onto an optical window in front of a video camera microscope [Amott et al., 1995], are capable of discerning particle shapes. However these methods require particle-by-particle post-flight image examination to extract visual shape information, a process that can be laborious. Furthermore, in general they are unable to resolve the shapes of the smallest micrometre-sized particles which the FSSP often indicates are present in ice or mixed-phase clouds and which can have an important bearing on the cloud radiative properties.