1NILU – Norwegian Institute for Air Research, P.O. Box 100, 2027 Kjeller, Norway
2Research Department, Swedish Meteorological and Hydrological Institute, Folkborgsvägen 17, 601 76 Norrköping, Sweden
3Department of Earth and Space Science, Chalmers University of Technology, 412 96 Gothenburg, Sweden
4Department of Physics, P.O. Box 48, 00014 University of Helsinki, Helsinki, Finland
5Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
Received: 19 Aug 2013 – Published in Atmos. Meas. Tech. Discuss.: 16 Oct 2013
Abstract. The reverse absorption technique is often used to detect volcanic ash clouds from thermal infrared satellite measurements. From these measurements effective particle radius and mass loading may be estimated using radiative transfer modelling. The radiative transfer modelling usually assumes that the ash particles are spherical. We calculated thermal infrared optical properties of highly irregular and porous ash particles and compared these with mass- and volume-equivalent spherical models. Furthermore, brightness temperatures pertinent to satellite observing geometry were calculated for the different ash particle shapes. Non-spherical shapes and volume-equivalent spheres were found to produce a detectable ash signal for larger particle sizes than mass-equivalent spheres. The assumption of mass-equivalent spheres for ash mass loading estimates was found to underestimate mass loading compared to morphologically complex inhomogeneous ash particles. The underestimate increases with the mass loading. For an ash cloud recorded during the Eyjafjallajökull 2010 eruption, the mass-equivalent spheres underestimate the total mass of the ash cloud by approximately 30% compared to the morphologically complex inhomogeneous particles.
Revised: 17 Feb 2014 – Accepted: 21 Feb 2014 – Published: 04 Apr 2014
Kylling, A., Kahnert, M., Lindqvist, H., and Nousiainen, T.: Volcanic ash infrared signature: porous non-spherical ash particle shapes compared to homogeneous spherical ash particles, Atmos. Meas. Tech., 7, 919-929, doi:10.5194/amt-7-919-2014, 2014.