one study in Australia, concentrations of 10 mg/m3 for just the breathable particles were reported. If anyone wants to help convert this to mass fraction (mass ratio solids:vapor), help is welcomed!
It is very difficult to determine particle loading in volcanic eruptions. Any sensors in the path of advancing gas/particle clouds are destroyed, but remote sensing techniques are improving. The bottom line is that particle loading in destructive emissions are high compared to even desert dust storms, thus their destructiveness. These flows can be "internally supersonic," that is, supersonic inside themselves, but subsonic compared to atmospheric sound speeds (~340 m/s). Thus, they don't generate shock waves. The lateral blast at Mount St. Helens did not generate atmospheric shock waves. It did generate compression waves as the blast pushed on the atmosphere, and due to the complex structure of the atmosphere, these waves steepened into audible acoustic waves to the north, e.g., hear in Vancouver B.C.
Bottom line: there are shocks and there are shocks. Some are inside these wierd geological flows, and others are in the domain of atmospheric sciences.