Volcanic ash ice-nucleating activity can be enhanced or depressed by ash-gas interaction in the eruption plume
Elena C.Maters, Corrado Cimarelli, Ana S.Casas, Donald B.Dingwell & Benjamin J. Murray, 2020
Schematic of the eruption plume and cloud, meteorological clouds, and altitude-temperature profile of the international standard atmosphere (ISA). Note that the adsorption and condensation zones in the eruption plume (Óskarsson, 1980) are not shown to scale with respect to the ISA profile; the altitudes at which they occur depend on factors such as the volcano summit elevation, eruption magnitude, and air entrainment rate. The hot core of the plume, where ash-gas interaction takes place, is the context of ‘eruption plume processing’ in this study.
Volcanic ash can trigger ice nucleation when immersed in supercooled water. This will impact several processes (e.g., electrification, aggregation, precipitation) in the eruption plume and cloud and in the wider atmosphere upon ash dispersal. Previous studies show that ash bulk properties, reflecting the chemistry and phase state of the source magma, likely contribute to the ice-nucleating activity (INA) of ash. However, it remains unexplored how interaction with magmatic gases in the hot eruption plume, which inevitably leads to altered ash surface properties, affects the ash INA. Here we demonstrate that the INA of tephra is raised by exposure to H2O(g) mixed with SO2(g) at both 800 and 400 ◦C, but is substantially reduced by exposure to H2O(g) alone or mixed with HCl(g) at the same temperatures. In contrast, the INA of K-feldspar and quartz is reduced by all three eruption plume processing treatments. The decrease in INA of all silicates after heating with H2O(g) might relate to a loss of ice-active sites by surface dehydroxylation and/or oxidation. In the presence of HCl(g) or SO2(g), respectively, metal chloride or sulphate salts form on the tephra surfaces only. While NaCl and CaCl2 seem to have no effect on the tephra INA, CaSO4 is inferred to create ice-active sites, potentially through a particular combination of surface chemistry and topography. Overall, our findings suggest a complex interplay of bulk mineralogy and surface alteration in influencing ice nucleation by volcanic ash, and highlight the general sensitivity (enhancement or depression) of ash INA to interaction with magmatic gases in the eruption plume.
Dissolved Na, K, Ca, Mg, Fe, SO2−4, and Cl−concentrations per unit solid surface area in water leachates of the a) non-treated, b) H2O-treated, c) H2O −SO2-treated, and d) H2O −HCl-treated silicate samples. The samples were leached in water at 1 wt.% and under constant gentle rotation for 30 min, prior to filtration and analyses of the leachate. Sample codes are listed in Table1.
Explosive volcanic eruptions generate mixed glassy and crystalline silicate ash that can serve as ice-nucleating particles (INPs). In the vertical eruption plume and the laterally dispersed eruption cloud, the freezing of supercooled water may impact a range of processes including plume/cloud electrification, gas scavenging, and ash aggregation (Van Eaton et al., 2015; Prata et al., 2020). In the wider atmosphere, ice formation on airborne ash may modify the properties and lifetime of clouds, thereby affecting precipita- tion and the Earth’s radiation balance (Seifert et al., 2011). Volcanic activity worldwide produces a recurrent flux of ash to the atmosphere (176-256 Tg a−1 ) (Durant et al., 2010), while sporadic large eruptions release substantial quantities of ash at one time (e.g., ∼500 Tg on 18 May 1980 from Mt. St. Helens volcano, USA) (Nathenson, 2017), potentially boosting local to regional INP populations (Seifert et al., 2011; Hobbs et al., 1971).
The ability of ash to nucleate ice (its ice-nucleating activity; INA) when immersed in supercooled water (i.e. in the immersion mode) is influenced by various bulk properties. For example, broad correlations have been observed between the INA and the chemical composition of a range of milled ash samples; with INA increasing with K2O content and decreasing with MnO, TiO2, FeO/Fe2O3, MgO, and/or CaO contents (Genareau et al., 2018; Maters et al., 2019). These trends are thought to reflect an indirect influence on INA of the source magma composition, which dictates the types of crystalline phases likely to end up in the erupted ash and contribute to INA (Maters et al., 2019). Indeed, several studies highlight the role of mineralogy in explaining variation in INA by ash samples from different volcanoes and/or eruptions, with alkali (here ‘K-’, indicating K-rich) feldspars, plagioclase (here ‘Na/Ca-’) feldspars, pyroxenes, and potentially quartz inferred to impart a high INA to ash (Maters et al., 2019; Schill et al., 2015; Jahn et al., 2019). However, ice nucleation is an interfacial process and the surface properties of ash differ from the bulk properties, due in part to interactions with magmatic gases including H2O(g), SO2(g), HCl(g) and HF(g) at high temperatures (∼200-800 ◦C) in the eruption plume (Óskarsson, 1980; Delmelle et al., 2007). This ‘eruption plume processing’ emplaces surficial metal sulphate and halide salts and modifies the surface chemical reactivity of ash (Óskars- son, 1980; Delmelle et al., 2007; Maters et al., 2016), but its influence on the INA of ash in particular is not known (Fig. 1).
Based on field measurements showing no increase in back- ground INP concentrations following several volcanic eruptions, it has been suggested that exposure to magmatic SO2(g) deactivates the INA of ash particles (Langer et al., 1974; Schnell et al., 1982). Numerous experimental studies on mineral dusts have investigated the effects of gaseous and aqueous acids on INA. Treatment of montmorillonite with SO2(g) had no impact on its ability to nucleate ice from water in the vapour phase (i.e. in the deposition mode) (Salam et al., 2008), while exposure of Arizona test dust (ATD) to HNO3 vapour reduced, promoted, or had no effect on INA below, near, or above water saturation, respectively (Sullivan et al.,2010a). Treatment of kaolinite, illite, K-feldspar and ATD with H2SO4 vapour and heat (70-250 ◦C) reduced deposition and/or immersion mode INA (Sullivan et al., 2010b; Wex et al., 2014; Augustin-Bauditz et al., 2014). Additionally, contact with dilute al- kali salt solutions (K2SO4, Na2SO4, KCl, NaCl; 10−4 to 1 M) has been shown to decrease the INA of some silicates including K- and Na/Ca-feldspars compared to when they are immersed in pure wa- ter (Whale et al., 2018; Kumar et al., 2018, 2019a). This has been proposed to reflect a suppressed exchange, in the presence of dis- solved K+ and Na+, of alkali cations from the silicate surfaces with H+/H3O+ from solution, a process that may be important in ice nucleation by these materials (Kumar et al., 2018, 2019a). Further, dissolved SO2− may form complexes with Al and thereby promote Al dissolution and -OH group removal from the silicates, potentially blocking or destroying surface sites at which ice nucleation occurs (ice-active sites) (Kumar et al., 2018, 2019a). Collectively, these findings suggest that in-plume exposure to acidic gases and emplacement of soluble metal sulphate and halide salts might de- press the INA of volcanic ash. However, the conditions simulated in various ‘atmospheric processing’ studies on dust differ substantially from the extreme thermal and chemical conditions to which ash is exposed in an eruption plume, calling for dedicated investigation of the effects of high temperature ash-gas interaction on the INA of ash.
Heating of silicates (e.g., silica and soda-lime-silica glass, Na/Ca-feldspar, quartz, kaolinite) >400 ◦C can result in condensation of adjacent Si-OH groups to form Si-O-Si bridges, producing a more hydrophobic and compact silica-like surface (Schaeffer et al., 1986; Temuujin et al., 1999; D’Souza and Pantano, 2002). This removal of surface -OH groups (also known as dehydroxylation) can be enhanced by the presence of H2O vapour (Temuujin et al., 1999) and can become irreversible as the heating temperature is raised >400 ◦C (D’Souza and Pantano, 2002). Powdered K-feldspar can also be converted to an amorphous state when ‘highly heated,’ although no temperature is stated in the patent describing this effect (Swayze, 1907). Additionally, Fe-containing silicates undergo oxidation of Fe2+ to Fe3+ at high temperatures (550-1100 ◦C), accompanied by an outward diffusion of divalent cations (e.g., Mg2+, Fe2+, Ca2+) (Wu et al., 1988; Cooper et al., 1996). Oxidation of Fe2+ likely reduces the mobility of alkali cations such as Na+ and K+, as these cations may become fixed in a charge-compensating role to accommodate newly-formed Fe3+ in the silicate network (Cooper et al., 1996; Pelte et al., 2000).
Exposure of silicates (e.g., multi-oxide silicate glasses, Na/Ca- feldspar, pyroxene, Kilauea basalt, Mono Craters obsidian) to SO2(g) and HCl(g) at high temperatures can lead to the formation of metal sulphate and chloride salts as reaction products (Delmelle et al., 2018; King et al., 2018; Renggli et al., 2019). For example, SO2(g) interacts with soda-lime-silica glass and volcanic glasses at 400 to 800 ◦C to produce primarily Na2SO4 and CaSO4, respectively (Douglas and Isard, 1949; Ayris et al., 2013; Casas et al., 2019). These sulphate compounds are argued to reflect the reaction of adsorbed SO2 with alkali or alkaline earth cations (e.g., Na+, K+, Ca2+, Mg2+) at the silicate surface. Further, HCl(g) interacts with soda-lime-silica, mixed-alkali borosilicate, and volcanic tephrite and phonolite glasses at 300 to 700 ◦C to produce primarily NaCl (Schaeffer et al., 1986; Sung et al., 2009; Ayris et al., 2014). This is proposed to involve the reaction of Cl− from adsorbed HCl with Na+ at the glass surface. Differences in cation diffusivities, which vary as a complex function of factors such as silicate composition and heating temperature, are thought to dictate which particular salts form Renggli et al. (2019). Cation extraction by reaction with SO2(g) or HCl(g) at high temperatures generates a compacted, cation-depleted and silica-enriched surface beneath the salts (Renggli et al., 2019; Ayris et al., 2014; Li et al., 2010). This process of ‘dealkalisation’ is often exploited in the glass industry to enhance surface resistance to chemical weathering, for instance by blocking further diffusive exchange of cations (Schaeffer et al., 1986).
While these studies shed light on how various high temperature gas-solid interactions might alter the surface properties of volcanic ash, the influence of such interactions on the ability of ash to nucleate ice has yet to be explored. Here we expose three compositionally distinct tephra samples, a K-feldspar sample, and a quartz sample to gas mixtures comprising H2O(g), SO2(g), and/or HCl(g) under a heating sequence of 800 and then 400 ◦C, and assess the INA of the non-treated and treated samples in the immersion mode us- ing cold stage droplet assays. By uniquely combining in-plume gas- solid interaction simulations with ice nucleation measurements of various silicate materials, this experimental study contributes new insights on the potential impacts of eruption plume processing on the INA of volcanic ash before it disperses in the eruption cloud and wider atmosphere.