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.