Quantitative shape measurements of distal volcanic ash
Quantitative shape measurements of distal volcanic ash
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远端火山灰形状的定量测量
DOI：
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发表时间：
2003
期刊：
影响因子：
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通讯作者：
G. Bluth
中科院分区：
文献类型：
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作者：
C. Riley;W. Rose;G. Bluth
[1]大规模火山喷发产生的细火山灰（<200微米）在大气中停留时间较长（1小时或以上），并且可以从火山源头输送到很远的地方，从而对飞机和公众健康构成危害。灰烬颗粒具有不规则形状，因此需要有关颗粒形状、尺寸和终端速度的数据来了解不规则形状颗粒如何影响传输过程和辐射传输测量。在这项研究中，开发了一种方法来表征三种不同成分的灰分样品的颗粒形状、尺寸和终端速度。使用图像分析技术测量了 2500 个颗粒的形状和大小，这些颗粒来自（1）1974 年 10 月 14 日 Fuego 喷发（玄武岩）的远端沉降物（~100 km），（2）1992 年 8 月 18 日 Spurr 喷发（安山岩）的次级极大值（~250 km），以及（3）内布拉斯加州中新世 Ash Hollow 部分（流纹岩）。使用空气淘析装置将样品分为 10 至 19 个终端速度组 (0.6-59.0 cm/s)。使用激光衍射测量样品的粒度分布。长径比、费雷特直径和周长测量被发现是颗粒形状如何影响终端速度的最有用的描述符。这些测量值显示颗粒形状与球体（模型和算法中常用）有很大不同。在相同终端速度下，灰颗粒的直径比理想球体大 10-120%，这表明不规则的颗粒形状大大增加了阻力。气体吸附得出的表面积比基于测量尺寸和简单几何形状计算的表面积高 1 至 2 个数量级，表明颗粒形状高度不规则。表面积的校正因子是从灰分样品测量中得出的，因此可以校正通过假设球形颗粒形状计算出的表面积，以反映更实际的值。
[1] Large-scale volcanic eruptions produce fine ash (<200 μm) which has a long atmospheric residence time (1 hour or more) and can be transported great distances from the volcanic source, thus, becoming a hazard to aircraft and public health. Ash particles have irregular shapes, so data on particle shape, size, and terminal velocities are needed to understand how the irregular-shaped particles affect transport processes and radiative transfer measurements. In this study, a methodology was developed to characterize particle shapes, sizes, and terminal velocities for three ash samples of different compositions. The shape and size of 2500 particles from (1) distal fallout (∼100 km) of the 14 October 1974 Fuego eruption (basaltic), (2) the secondary maxima (∼250 km) of the 18 August 1992 Spurr eruption (andesitic), and (3) the Miocene Ash Hollow member, Nebraska (rhyolitic) were measured using image analysis techniques. Samples were sorted into 10 to 19 terminal velocity groups (0.6–59.0 cm/s) using an air elutriation device. Grain-size distributions for the samples were measured using laser diffraction. Aspect ratio, feret diameter, and perimeter measurements were found to be the most useful descriptors of how particle shape affects terminal velocity. These measurement values show particle shape differs greatly from a sphere (commonly used in models and algorithms). The diameters of ash particles were 10–120% larger than ideal spheres at the same terminal velocity, indicating that irregular particle shape greatly increases drag. Gas-adsorption derived surface areas are 1 to 2 orders of magnitude higher than calculated surface areas based on measured dimensions and simple geometry, indicating that particle shapes are highly irregular. Correction factors for surface area were derived from the ash sample measurements so that surface areas calculated by assuming spherical particle shapes can be corrected to reflect more realistic values.