The importance of crystalline phases in ice nucleation by volcanic ash
The importance of crystalline phases in ice nucleation by volcanic ash
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DOI：
10.5194/acp-19-5451-2019
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发表时间：
2019-04-25
影响因子：
6.3
通讯作者：
Murray, Benjamin J.
中科院分区：
文献类型：
--
作者：
Maters, Elena C.;Dingwell, Donald B.;Murray, Benjamin J.
众所周知，火山灰浸入过冷的水滴中时会形成冰核。一旦火山灰分散到大气中，这个过程可能会影响喷发羽流和云以及气象云的特性和动态。然而，尽管有人认为灰烬中的结晶成分可能发挥重要作用，但对灰烬冰核活性（INA）控制因素的了解仍然有限。在这里，我们采用了一种新颖的方法，使用九对火山灰及其重熔和淬火玻璃等效物来研究化学成分、结晶度和矿物学对浸泡模式下灰分 INA 的影响。在所有九对研究中，含晶体的火山灰在比相应的无晶体玻璃更高的温度下成核冰，这表明结晶相是灰分 INA 的关键。与干旱和半干旱地区沙漠尘埃的发现类似，长石矿物的存在是四种冰活性最强的火山灰样本的特征，尽管在含有斜长石和斜方辉石的样本中，即使不存在碱长石，也观察到高 INA。有证据表明，化学成分和灰分 INA 之间存在潜在的间接关系，长英质到中间成分的岩浆可能会产生含有冰活性长石或辉石矿物的灰分。化学成分、结晶度和矿物学之间的这种复杂的相互作用有助于解释文献中报道的火山灰 INA 的变异性。总体而言，通过证明结晶相在灰分 INA 中的重要性，我们的研究为更好地评估空气中的灰分在冰形成中的作用提供了重要的见解。其中的一个推论是，最大的火山喷发所释放的以玻璃为主的火山灰在影响冰核粒子群方面可能不如较小、更频繁的火山喷发所产生的结晶火山灰有效。
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 activity (INA) 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 INA in the immersion mode. For all nine pairs studied, the crystal-bearing tephra nucleated ice at warmer temperatures than the corresponding crystal-free glass, indicating that crystalline phases are key to ash INA. Similar to findings for desert dust from arid and semi-arid regions, the presence of feldspar minerals characterizes the four most ice-active tephra samples, although a high INA 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 INA, whereby a magma of felsic to intermediate composition may generate ash containing ice-active feldspar or pyroxene minerals. This complex interplay between chemical composition, crystallinity, and mineralogy could help to explain the variability in volcanic ash INA reported in the literature. Overall, by demonstrating the importance of crystalline phases in the INA of 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 volcanic eruptions might be less effective at impacting ice-nucleating particle populations than crystalline ash generated by smaller, more frequent eruptions.