I suggest anyone who doubts the presence of the ash in the air goes to their local flying club or training organisation and looks at the piston engined aircraft that have, thankfully for the health of the club, still been regularly flying over the last 5 days in 20K plus visibilities at low level...
Unusual grey propwash shaped streaks and swirls are visible on leading edges and black propellors are grey. Fortunately these aircraft are cutting through the air at not much more than 100 knots and everythings at not much higher than ambient temperature. Air filters designed to cope with dusty grass runways are not presented with a problem.