, B . ; Folch , A . ; Hensch , M . ; Matthias , V . : Volcanic ash over Europe during the eruption of Eyjafjallajökull on

15 During the eruption of Eyjafjallajökull on Iceland in April/May 2010, air traffic over Europe 16 was repeatedly interrupted because of volcanic ash in the atmosphere. This completely 17 unusual situation in Europe leads to the demand of improved crisis management, e.g. 18 European wide regulations of volcanic ash thresholds and improved forecasts of theses 19 thresholds. However, the quality of the forecast of fine volcanic ash concentrations in the 20 atmosphere depends to a great extent on a realistic description of the erupted mass flux of fine 21 ash particles, which is rather uncertain. Numerous aerosol measurements (ground based and 22 satellite remote sensing, and in situ measurements) all over Europe have tracked the volcanic 23 ash clouds during the eruption of Eyjafjallajökull offering the possibility for an 24 interdisciplinary effort between volcanologists and aerosol researchers to analyse the release 25 and dispersion of fine volcanic ash in order to better understand the needs for realistic 26 volcanic ash forecasts. In this introductory paper, we provide a general introduction into 27 magma fragmentation processes during explosive volcanic eruptions, describe the evolution 28 of the eruption of Eyjafjallajökull, present the possibilities of ground based in-situ and remote 29 measurements and numerical model studies of volcanic ash and summarise open questions 30 and future directions. 31 32