Spectrally High resolution Infrared measurements for the characterisation of Volcanic Ash (SHIVA): a new way to study volcanic processes

火山爆发产生的火山灰掌握着火山导管内岩浆动力学的信息，这些导管位于发生碎裂并决定喷发方式的关键地带。虽然火山灰和矿渣的形态和岩石学特征提供了对爆炸性玄武岩喷发机制的见解，但不同机制之间转换的重要问题仍未得到解答。了解火山灰组成的三种方法是：羽流光谱法、羽流颗粒样品的化学分析和颗粒显微镜。第一种技术在提供洞察力的能力方面正在经历一个阶段性的变化，原因有两个。首先，通过NERC资助的项目，火山灰样本的高分辨率折射率数据正在变得可用。虽然这些数据对该项目并不重要，但它允许使用已知喷发（例如Grimsvötn）样品的折射率来生成光学特性，用于从卫星测量中检索火山灰云的特性。其次，高分辨率红外光谱仪现在以足够的频率观测地球，可以很好地观测到最近在偏远地区的火山喷发，例如2011年厄立特里亚的Nabro火山喷发。该项目将首先根据科学文献中的现有信息和新的灰烬测量数据建立一个灰烬光学特性（消光系数、单散射反照率和相位函数）数据库。这些性质将构成从高分辨率红外光谱中提取灰成分、光学厚度和有效半径的最佳估计类型的基础。本文将针对三种不同的观测类型开发检索算法：1)火山口附近火山灰的高分辨率红外透射光谱。这些光谱由我们的项目合作伙伴提供，他们将协助解释测量结果。2) MetOp卫星红外大气探测干涉仪（IASI）的高分辨率红外发射光谱。IASI是一种最低点观察傅立叶变换光谱仪，覆盖光谱范围645至2760 cm-1（3.62-15.5微米）。IASI的视场由四个直径为20公里的圆组成，在一个50 x 50公里的正方形内，名义上它可以在12小时内实现全球覆盖。3) ENVISAT卫星上被动大气探测迈克尔逊干涉仪（MIPAS）的高分辨率红外发射光谱。2002年至今，MIPAS测量了6-68公里切线高度范围内685-2410 cm-1（14.5 - 4.1微米）的大气边缘发射光谱。利用这些仪器，我们将观察几座不同火山在不同喷发阶段产生的火山灰。将通过将卫星对火山灰特性的估计与从火山灰样本中测得的火山灰特性以及与其他卫星结果进行比较，来验证卫星对火山灰特性的估计。我们有相关测量的喷发包括Eyjafjallajökull， Grimsvötn， Nabro和Puyehue。观察到的火山灰行为将根据产生火山灰的火山过程加以解释。

Ash from explosive volcanic eruptions holds information about magma dynamics within volcanic conduits, in the critical zone where fragmentation occurs and eruption style is decided. Although morphological and petrological features of ash and scoria have provided insights into the mechanisms of explosive basaltic eruptions, important questions on the transition between different mechanisms are still unanswered. Three approaches to understanding ash composition are: plume spectrometry, chemical analysis of bulk plume particle samples and particle microscopy. The first of these techniques is experiencing a step change in its ability to provide insight for two reasons. Firstly, high resolution refractive index data from volcanic ash samples is becoming available through a NERC funded project. While this data is not critical to the project, it allows the refractive indices of samples from a known eruption (e.g. Grimsvötn) to be used to generate the optical properties that are used in the retrieval of the properties of the volcanic ash cloud from satellite measurements. Secondly, high resolution infrared spectrometers now observe the Earth with sufficient frequency that recent eruption in remote locations have been well observed, e.g. the 2011 eruption of Nabro in Eritrea. The project will initially construct a database of ash optical properties (extinction coefficient, single scatter albedo and phase function) based on existing information in the scientific literature and new ash measurements. These properties will then form the basis of an optimal estimation type retrieval of ash composition, optical thickness and effective radius from high resolution infrared spectra. The retrieval algorithm will be developed for three different types of observation:1) High resolution infrared transmission spectra of ash near the volcanic vent. These spectra are provided by our project partners who will assist in the interpretation of the measurements.2) High resolution infrared emission spectra from the Infrared Atmospheric Sounding Interferometer (IASI) onboard MetOp satellite. IASI is a nadir viewing Fourier transform spectrometer that covers the spectral range 645 to 2760 cm-1 (3.62-15.5 microns). The IASI field of view consists of four circles of 20 km diameter inside a square of 50 x 50 km, and nominally it can achieve global coverage in 12 hours.3) High resolution infrared emission spectra from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onboard ENVISAT satellite. MIPAS measures atmospheric limb emission spectra from 685-2410 cm-1 (14.5 to 4.1 microns) over a tangent altitude range 6-68 km from 2002 to the present.Using these instruments we will observe volcanic ash generated by several different volcanoes and during different stages of eruptive behaviour. The satellite based estimates of ash properties will be validated by comparing them to ash properties measured from ash samples as well as by comparison with other satellite results. Eruptions for which we have correlative measurements include Eyjafjallajökull, Grimsvötn, Nabro and Puyehue. The observed ash behaviour will then be interpreted in terms of the ash-generating volcanic processes.