The importance of crystalline phases in ice nucleation by volcanic ash
Elena C. Maters, Donald B. Dingwell, Corrado Cimarelli, Dirk Müller, Thomas F. Whale & Benjamin J. Murray, 2019
Ice nucleation active site density (ns) as a function of temperature for 1 wt.% suspensions of tephra in water. The blue and red lines represent parameterisations for, respectively, alkali and plagioclase feldspar of non-pyroclastic origin reported in Harrison et al. (in prep.) from a compilation of literature data. The solid lines indicate mean values and the dashed lines indicate lower and upper limits corresponding to the standard deviation of the mean. Sample codes are 5 listed in Table 1.
Volcanic ash is known to nucleate ice when immersed in supercooled water droplets. This process may impact the properties and dynamics of the eruption plume and cloud, as well as those of meteorological clouds once the ash is dispersed in the atmosphere. However, knowledge of what controls the ice-nucleating effectiveness (INE) of ash remains limited, although it has been suggested that crystalline components in ash may play an important role. Here we adopted a novel approach using nine pairs of tephra and their remelted and quenched glass equivalents to investigate the influence of chemical composition, crystallinity and mineralogy on ash INE in the immersion mode. For all nine pairs studied, the crystal-bearing tephra nucleated ice at higher temperatures than the corresponding crystal-free glass, demonstrating that crystalline phases are key to ash INE. Similar to findings for desert dust from arid and semi-arid regions, the presence of feldspar minerals characterises the four most ice-active tephra samples, although a high INE is observed even in the absence of alkali feldspar in samples bearing plagioclase feldspar and orthopyroxene. There is evidence of a potential indirect relationship between chemical composition and ash INE, whereby a magma of felsic to intermediate composition may generate ash containing ice- active feldspar minerals. This complex interplay between chemical composition, crystallinity, and mineralogy could help partly to explain the variability in volcanic ash INE reported in the literature. Overall, by categorically demonstrating the importance of crystalline phases in the INE of volcanic ash, our study contributes insights essential for better appraising the role of airborne ash in ice formation. Among these is the inference that glass-dominated ash emitted by the largest explosive eruptions may be less effective at impacting ice-nucleating particle populations than crystalline ash generated by smaller, more frequent eruptions.
Droplet fraction frozen (fice) as a function of temperature for 1 wt.% suspensions of (a) tephra or (b) glass in water. The grey bands represent the spread of fice(T) measurements (mean values ± standard deviation) of the background water (i.e., containing no added sample). Ice nucleation active site density (ns) as a function of temperature for 1 wt.% suspensions of (c) tephra or (d) glass in water. For the sake of comparison, the grey curves represent theoretical upper limit 5 ns(T) values of the background water, calculated using the upper limit fice(T) measurements (mean values + standard deviation) of the background water, and assuming it contains particles with SSABET values equal to the lowest from the tephra and glass sets (1.4 and 0.9 m2 g-1, respectively). The tephra ns(T) values are well above this background but most of the glass ns(T) values should be regarded as upper limits. The uncertainty in ns(T) is shown as error bars for a subset of data points (of NUOteph and CIDglass) and omitted 10 from remaining data points for clarity, but is typical of all tephra and glass samples. Sample codes are listed in Table 1.
Volcanic ash produced by explosive eruptions can act as ice-nucleating particles (INPs), promoting heterogeneous freezing of supercooled water in the vertical eruption plume, the laterally dispersed eruption cloud, and the wider atmosphere (Isono et al., 1959a; 1959b; Hobbs et al., 1971; Rose et al., 2003). Ice formation in these contexts is poorly understood yet may exert a profound influence on eruption plume/cloud dynamics and electrification (e.g., Herzog et al., 1998; Cimarelli et al., 2016), sequestration of gaseous species (e.g., Textor et al., 2003; Guo et al., 2004a) and ash aggregation and sedimentation (e.g., Guo et al., 2004b; Van Eaton et al., 2015), as well as atmospheric cloud properties and lifetime (e.g., Komabayasi, 1957; Seifert et al., 2011) and thereby the hydrological cycle and climate (e.g., Isono and Komabayasi, 1954). Ongoing volcanic activity generates a recurrent flux of ash particles into the atmosphere (176-256 Tg a-1; Durant et al., 2010), whereas sporadic large eruptions can result in ash loadings greatly exceeding annual averages over very short (hour to day) time scales and transiently dominating INP populations (e.g., Isono et al., 1959a; 1959b; Hobbs et al., 1971).
By definition, volcanic ash consists of pyroclastic particles <2 mm in diameter usually dominated by aluminosilicate glass derived from the melt and/or aluminosilicate and Fe(-Ti) oxide minerals in the form of crystals suspended within the original melt (Heiken and Wohletz, 1992). Field and laboratory measurements present conflicting evidence as to the ice-nucleating effectiveness (INE) of ash, even for samples from the same volcano, and it is far from clear what drives this variation (Durant et al., 2008, and references therein; Mangan et al., 2017). Studies on desert dust from arid and semi-arid regions (1000-3000 Tg a-1 emitted; Penner et al., 2001) – considered one of the most important INP types globally (Hoose et al., 2010; Vergara- Temprado et al., 2017) – suggest that chemical composition, crystallinity and mineralogy can influence the abundance of ice- active surface sites on the solid particles (Murray et al., 2012, and references therein). Specifically, the presence of K-rich feldspar is thought to dominate the INE of dust (Atkinson et al., 2013; Yakobi-Hancock et al., 2013; Kaufmann et al., 2016).
Similar factors might influence ice nucleation by volcanic ash (Kulkarni et al., 2015; Schill et al., 2015; Genareau et al., 2018). To date however, the roles and potential interplay of differing physicochemical attributes in determining a solid particle’s INE remain poorly understood, having rarely been systematically investigated for any ice-nucleating material. Here we examine the influence of three properties dictated primarily by the state of the erupted source magma – chemical composition, crystallinity, and mineralogy – on the INE of volcanic ash in the immersion mode which is likely relevant to ash particles in the water-rich eruption plume/cloud and in mixed-phase atmospheric clouds (Textor et al., 2006; McNutt and Williams, 2010; Pruppacher and Klett, 2010; Murray et al., 2012). To assist in disentangling the individual effects of these properties on ice nucleation, we have adopted a novel approach of sample selection by using a wide range of natural tephra and their remelted and quenched glass equivalents. In this manner, we aim to contribute to improving understanding of the potential for airborne ash from different eruptions to impact ice formation above the volcanic vent and/or once dispersed in the ambient atmosphere.