Ash mists and brown snow: Remobilization of volcanic ash from recent Icelandic eruptions

Recent eruptions in Iceland and Chile have demonstrated that volcanic ash problems persist long after an eruption. For this reason, ash dispersion models are being extended to include ash remobilization. Critical to these models is knowledge of the ash source and the particle sizes that can be mobilized under different wind and moisture conditions. Here we characterize the physical and chemical characteristics of ash deposited on new snow in Reykjavik, Iceland, following a blizzard on 6 March 2013. Morphological, textural, and compositional analyses indicate resuspension from multiple eruptive deposits, including both Grimsvotn (2011) and Eyjafjallajokull (2010) eruptions. Grain size measurements show a mode of 32–63 µm, with particles as large as 177 µm; there is little mass in the very fine fraction, ≤10 µm (PM10). We compare our observations to predictions using the Lagrangian particle dispersion model, NAME (UK Met Office). The model output is consistent with observations in that it forecasts resuspension from both Eyjafjallajokull and Grimsvotn source regions, and shows ash deposition coincident with the timing of observed deposition in Reykjavik. The modeled deposit in Reykjavik predicts, however, a substantially lower proportion of Grimsvotn ash than observed. This discrepancy has highlighted the need to reassess the assumptions used in the simulations, particularly regarding the source area and precipitation thresholds. Furthermore, we suggest that modification of ash deposits in the form of erosion, redeposition, compaction, or cementation may influence the dynamics of resuspension over time, thus influencing the ability of model simulations to accurately forecast remobilization events.