An interesting piece of work from Þröstur Þorsteinsson at the Nordic Volcanological centre looks at the particle size distribution from the Eyjafjallajokull eruption.
Ash particles are normally in the 50-100 micron (0.05 to 0.1 millimetre) range. But at a site 50km east of the eruption, 24% of the ash falling to the ground was in the form of particles 10 microns or less in size. Studies of ash captured from the air show that for every one of the largest particles (about 300 microns) there are a million or more in the 2 micron range. So though the total volume of the eruption, put at about 0.14 cubic kilometres, is low, the amount of ash capable of travelling long distances is high.
While the measuring instruments only seem to go down to 2 microns or so, given the distribution profile it is a fair bet that Eyjafjallajokull is producing large amounts of nanoparticles. It would be good to see some more distribution data, perhaps using this type of instrument?