Articles | Volume 15, issue 13
Atmos. Chem. Phys., 15, 7523–7536, 2015
Atmos. Chem. Phys., 15, 7523–7536, 2015

Research article 10 Jul 2015

Research article | 10 Jul 2015

Deposition and immersion-mode nucleation of ice by three distinct samples of volcanic ash

G. P. Schill1,a, K. Genareau2, and M. A. Tolbert1 G. P. Schill et al.
  • 1Department of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO, USA
  • 2Department of Geological Sciences, University of Alabama, Tuscaloosa, AL, USA
  • anow at: Department of Atmospheric Sciences, Colorado State University, Fort Collins, CO, USA

Abstract. Ice nucleation of volcanic ash controls both ash aggregation and cloud glaciation, which affect atmospheric transport and global climate. Previously, it has been suggested that there is one characteristic ice nucleation efficiency for all volcanic ash, regardless of its composition, when accounting for surface area; however, this claim is derived from data from only two volcanic eruptions. In this work, we have studied the depositional and immersion freezing efficiency of three distinct samples of volcanic ash using Raman microscopy coupled to an environmental cell. Ash from the Fuego (basaltic ash, Guatemala), Soufrière Hills (andesitic ash, Montserrat), and Taupo (Oruanui eruption, rhyolitic ash, New Zealand) volcanoes were chosen to represent different geographical locations and silica content. All ash samples were quantitatively analyzed for both percent crystallinity and mineralogy using X-ray diffraction. In the present study, we find that all three samples of volcanic ash are excellent depositional ice nuclei, nucleating ice from 225 to 235 K at ice saturation ratios of 1.05 ± 0.01, comparable to the mineral dust proxy kaolinite. Since depositional ice nucleation will be more important at colder temperatures, fine volcanic ash may represent a global source of cold-cloud ice nuclei. For immersion freezing relevant to mixed-phase clouds, however, only the Oruanui ash exhibited appreciable heterogeneous ice nucleation activity. Similar to recent studies on mineral dust, we suggest that the mineralogy of volcanic ash may dictate its ice nucleation activity in the immersion mode.

Short summary
Fine volcanic ash can influence cloud glaciation and, therefore, global climate. In this work we examined the heterogeneous ice nucleation properties of three distinct types of volcanic ash. We find that, in contrast to previous studies, these volcanic ash samples have different ice nucleation properties in the immersion mode. In the deposition mode, however, they nucleate ice with similar efficiency. We show that this behavior may be due to their mineralogy.
Final-revised paper