To quantify the size of tephra, two practical challenges must be addressed: the wide range of particle sizes (10⁻⁸ - 10¹ m) and the diversity of particle shape, density and optical properties. Here we use dynamic image analysis (DIA) to simultaneously characterise the size and shape of tephra samples from Mount Mazama, Krafla, Mount St. Helens and Campi Flegrei. The Camsizer X2 instrument used in this study, which has a measurement range of 0.8 μm – 8 mm, avoids the need to overlap different measurement methods and principles for fine (<125 μm) and coarse (>125 μm) particle sizes. Importantly, DIA does not require an assumption of particle properties. DIA also allows the measurement of grain size distributions (GSDs) using multiple size definitions. Quantification by particle long axis and the area equivalent sphere diameter, for example, make DIA GSDs compatible with the outputs of other methods such as laser diffraction and sieving. Parallel mass-based (sieving) and volume-based (DIA) GSDs highlight the effects of particle density variations on methods of size analysis; concentrations of dense crystals within a narrow size range, in particular, can affect mass-based GSDs and their interpretations. We also show that particle shape has an important effect on the apparent grain size of distal tephra. Extreme particle shapes, such as the platy glass shards typical of the distal Campanian Ignimbrite deposits, can appear coarser than other distal tephras if size is quantified according to the particle long axis. These results have important implications for ash dispersion models, where input GSDs assume that reported measurements are for volume-equivalent sphere diameters. We conclude that DIA methods are not only suitable for characterising, simultaneously, the size and shape of ash particles but also provide new insights into particle properties that are useful for both ash dispersion modelling and studies of explosive volcanism.

为了量化火山灰的大小，两个实际的挑战必须解决：在宽范围的粒度（10 ^-8 - 10 ¹m）以及颗粒形状，密度和光学性质的多样性。在这里，我们使用动态图像分析（DIA）来同时表征来自Mazama山，Krafla，St。Helens山和Campi Flegrei山的提菲拉样品的大小和形状。本研究中使用的Camsizer X2仪器的测量范围为0.8μm– 8 mm，避免了对细（<125μm）和粗（> 125μm）粒径的不同测量方法和原理进行重叠。重要的是，DIA不需要假设颗粒性质。DIA还允许使用多种尺寸定义来测量晶粒尺寸分布（GSD）。例如，通过颗粒长轴和面积等效球体直径进行定量，可使DIA GSD与其他方法（例如激光衍射和筛分）的输出兼容。平行的基于质量（筛分）和基于体积（DIA）的GSD强调了颗粒密度变化对尺寸分析方法的影响。在狭窄的尺寸范围内，致密晶体的浓度尤其会影响基于质量的GSD及其解释。我们还表明，颗粒形状对远侧提夫拉的表观晶粒尺寸有重要影响。如果根据颗粒长轴进行定量，则极端的颗粒形状（例如，Campanian Ignimbrite远端沉积物典型的板状玻璃碎片）看起来会比其他远侧特弗拉斯更粗糙。这些结果对灰分弥散模型具有重要意义，在灰分弥散模型中，输入的GSD假设报告的测量值是针对体积等效球体直径的。我们得出结论，DIA方法不仅适用于同时表征，