Near-source observations of aerosol size distributions in the eruptive plumes from Eyjafjallajökull volcano, March-April 2010

Near-source observations of aerosol size distributions (<15 km from the vent) were made during three eruptive phases of Eyjafjallajökull in 2010: Phase I - basaltic lava fountaining; Phase II - andesitic phreatomagmatic explosions (mass discharge rate of the order of 10⁵ kg s^-1); Phase III - andesitic magmatic explosions (mass discharge rate of the order of 10³ kg s^-1). Two methods were used, photometric measurements of suspended aerosol mass (with size distributions retrieved by inverse modelling); and sampling of ash fallout during Phases II and III (with size distributions measured by laser diffraction). The suspended aerosol in Phase I plume was dominated by particles sized <0.4 μm in diameter. During Phase II, the suspended aerosol mass contained a high number of fine (<1 μm) particles (an internal mixture of soluble aerosol and very fine ash) and coarse (>1 μm) ash particles. The number ratio of fine and coarse particles fluctuated strongly within short intervals of time, indicative of changes in the fragmentation energy. The near-source ash fallout was poorly sorted (interquartile range 16-80 μm); ash grains sized <1 μm contributed up to 7% of total volume. The high water content and electrical charge in the plume are believed to have enhanced the deposition through particle aggregation. In Phase III the eruption became drier and less explosive. The suspended aerosol mass was strongly bimodal with high particle abundances at ∼0.2 and 1-4 μm. The ash fallout was better sorted (interquartile range 60-120 μm; no grains <1 μm), attributed to limited particle aggregation.