The reverse absorption technique is often used to detect volcanic ash cloudsfrom thermal infrared satellite measurements. From these measurementseffective particle radius and mass loading may be estimated using radiativetransfer modelling. The radiative transfer modelling usually assumes that theash particles are spherical. We calculated thermal infrared opticalproperties of highly irregular and porous ash particles and compared thesewith mass- and volume-equivalent spherical models. Furthermore, brightnesstemperatures pertinent to satellite observing geometry were calculated forthe different ash particle shapes. Non-spherical shapes and volume-equivalentspheres were found to produce a detectable ash signal for larger particlesizes than mass-equivalent spheres. The assumption of mass-equivalent spheresfor ash mass loading estimates was found to underestimate mass loadingcompared to morphologically complex inhomogeneous ash particles. Theunderestimate increases with the mass loading. For an ash cloud recordedduring the Eyjafjallajökull 2010 eruption, the mass-equivalent spheresunderestimate the total mass of the ash cloud by approximately 30%compared to the morphologically complex inhomogeneous particles.
展开▼
