WojtekSz, Pace:
The MK1 eyeball is the best way of detecting particles either water or dust. Clear air is clear air. I find it hard to understand claims of invisible.Pollution in dust form is visible to pilots either as a haze or thin cloud and usually with a slight colouration.I take your point on looking vertically but horizontally NO unless of course your in cloud or flying at night.
of course you are right: a well trained or experienced eye will see the ash in the clear air. It has been cleary demonstrated by german and british research flights where lidar measurements were taken
You both have wrong underlining assumption: That ash is only possible pollutant in the air. It is not possible to differentiate between dangerous pollutant - ash - and anything else.