The HYSPLIT distribution models will often show significant areal coverage. Is there a threshold particle density that defines the plume boundaries and if so, is it one that corresponds to a known aircraft danger/non-danger threshold ?
There is no known aircraft danger/non-danger threshold. From what we know, pilots simply want to avoid the ash plume. HYSPLIT does not calculate actual ash concentrations because when an eruption occurs, the quantity of ash in the eruption is unknown. HYSPLIT assumes an initial quantity of one unit, and outputs concentration with respect to that. A threshold ash concentration is chosen to define the visual ash boundaries shown on the HYSPLIT maps. It is based on correlations between ash seen in visible satellite imagery from eruptions of a few volcanoes, and further refined based on the extent of the observed ash. More research is needed here, but little satellite or other observational verification data exists.
The ash concentration threshold that is dangerous to aircraft is not known. From what we know, pilots simply want to avoid the plume.