The spaceborne detection of volcanic ash clouds at infrared wavelengths helps to avoid regions with enhanced volcanic ash concentrations that pose a threat to aviation. Current volcanic ash data retrievals require detailed information on microphysical properties and the refractive index of volcanic ash, which are highly variable. Uncertainties in the latter currently limit the quality of volcanic ash nowcasts. Here, we introduce a novel method to calculate the complex refractive indices of volcanic ashes at wavelengths from 5 to 15 μm from measurements of their individual components based on generic petrological ash compositions. Thereby the refractive indices for volcanic glasses and bulk volcanic ashes of different chemical compositions are derived. The variability of the latter is mainly influenced by the silica content and the porosity and to a minor degree by the glass-to-crystals ratio. Calculating optical properties exhibits an equally large impact of bulk composition and grain size distribution, whereas particle shape is considered less important for particle sizes of the order 1 μm. Using these optical properties to determine brightness temperature differences between the 11 μm and 12 μm channels we show that the effect of ash composition is non-negligible for modern satellite instruments. Particularly, the dependence of the volcanic ash on the silica content (and to a much smaller extent on the glass-to-crystals ratio) is observable in its refractive index, its optical properties and the brightness temperature difference, indicating that composition might be retrievable to some degree by remote sensing methods.

太空探测火山灰云的红外波长有助于避免火山灰浓度升高的区域对航空构成威胁。当前的火山灰数据检索需要有关火山灰的微物理性质和折射率的详细信息，而火山灰的微物理性质和折射率是高度可变的。后者的不确定性目前限制了火山灰临近预报的质量。在这里，我们介绍了一种新颖的方法，该方法可根据普通岩石灰组成，通过测量其单个成分来计算5至15μm波长的火山灰的复折射率。由此得出不同化学成分的火山玻璃和火山灰的折射率。后者的可变性主要受二氧化硅含量和孔隙率的影响，而在较小程度上受玻璃晶体比的影响。计算光学性质对整体组成和粒度分布表现出同样大的影响，而对于1μm左右的粒度，认为颗粒形状不太重要。使用这些光学特性来确定11μm和12μm通道之间的亮度温差，我们表明灰分成分的影响对于现代卫星仪器而言是不可忽略的。尤其是，从火山灰的折射率，光学特性和亮度温度差中可以观察到火山灰对二氧化硅含量的依赖性（在较小程度上取决于玻璃与晶体的比率），