Following the recent furore about volcanic ash in the atmosphere, the UK’s Civil Aviation Authority has decided that 2mg/ m^3 is the airborne ash density below which it’s ok to fly.
Military aviators are a lot less cautious about air quality. They also have a range of countermeasures they can employ when being pursued by their opposition. Having seen the damage which ash can cause in a jet engine, today’s invention is a countermeasure based on this.
When followed by a jet aircraft, the target plane would start to inject some of its own ceramic engine outlet components into its exhaust stream, so that they burned (like an ablation shield on a space capsule).
This would periodically cause visible, dense puffs of silica ash (locally >>2mg/ m^3) to be ejected and cause any pursuing aircraft to avoid the clouds. The clouds themselves would remain airborne, a little like barrage balloons, until normal turbulence dispersed them.
Probably better to feed the jet exhaust with eg glass from some other part of the plane, in order not to undermine whatever directional control the exhaust vents may provide the pilot when being chased.