Abstract
Fine particles of ash emitted during volcanic eruptions may sporadically influence cloud properties on a regional or global scale as well as influencing the dynamics of volcanic clouds and the subsequent dispersion of volcanic aerosol and gases. It has been shown that volcanic ash can trigger ice nucleation, but ash from relatively few volcanoes has been studied for its ice nucleating ability. In this study we quantify the efficiency with which ash from the Soufriere Hills volcano on Montserrat nucleates ice when immersed in supercooled water droplets. Using an ash sample from the 11th February 2010 eruption, we report ice nucleating efficiencies from 246 to 265 K. This wide range of temperatures was achieved using two separate droplet freezing instruments, one employing nanolitre droplets, the other using microlitre droplets. Soufriere Hills volcanic ash was significantly more efficient than all other ash samples that have been previously examined. At present the reasons for these differences are not understood, but may be related to mineralogy, amorphous content and surface chemistry.
Highlights
Water droplets in tropospheric clouds can supercool to below 236 K before freezing homogeneously, but suitable aerosol particles can catalyse ice formation at much higher temperatures
We take the pragmatic view that when an ice nucleating particles (INPs) is immersed inside a droplet, it is freezing in the immersion mode and it is in this mode that we study the ice nucleating ability of volcanic ash
In this study we ground the ash sample to break up the largest of the particles. This may have exposed fresh highly active feldspar surfaces, the results presented here may represent an upper limit to the ice nucleating ability of Soufriere Hills ash, a similar grinding process was used by Schill et al [23] who reported very low activity for two out of three of their volcanic ash samples
Summary
Water droplets in tropospheric clouds can supercool to below 236 K before freezing homogeneously, but suitable aerosol particles can catalyse ice formation at much higher temperatures. We take the pragmatic view that when an INP is immersed inside a droplet, it is freezing in the immersion mode and it is in this mode that we study the ice nucleating ability of volcanic ash.
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