Pore structure 3-D imaging by synchrotron micro-tomography of graupel grains

Abstract

Three dimensional air bubble structure including size distribution, concentration and spatial distribution are important clues in identifying the growth regime of graupel and hailstone. For imaging of the bubble structure, a cryo-stage was developed to adapt to the standard setup of the SLS X04SA tomography beamline (actually replaced by the TOMCAT beamline) at the Swiss Light Source synchrotron facility to the requirements of ice particle micro-tomography. The cryo-stage setup provides for the first time 3-D-data on the individual inner pore shape delineation down to μm spatial (voxel) resolution of sub-mm small naturally as well as wind tunnel rimed graupel particles. Special care must be taken for maintaining a cooling chain between sampling and measurement. It must be kept at liquid nitrogen temperature (77 K) until measurement of the original structure at the μm spatial scale. However, even at that temperature there is no chance to preserve any ice bubble structure at sub-μm spatial resolution due to the Kelvin effect. In natural graupel grains, Y-shaped morphology of air-filled pores was found. This morphology transformed into smaller and rounded voids well-known from literature when the ice particle was annealed for as short as half an hour at 265 K and must, therefore, be regarded as artificial rather than representing the in situ pore structure. With the new synchrotron tomography approach, quantitative information on the in situ pore structure statistics within individual samples representative for a known or, thus, deduced growth mode or history can be derived, in particular if combined with airplane sampling in the troposphere at in situ growth conditions