I hope this is not a repetitive info:
you have to distinguish between ash/dust (which are hard particles and CO type and other gaseous emissions which are preceding the cloud of hard particles. Pollution of air with volcanic gases at high altitudes will inhibit normal combustion of any air breathing engine. Gas wave precedes the dust. So a mere presence of additional pollutants in the air will also be a reason to close the airspace, even before the dust cloud arrives.