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