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.
Where have I assumed that ash is the only pollutant in the air? I do assume that any pollutant which is dense enough to cause a problem will be visible in clear air.
If you are flying in an area with known volcanic ash I would assume the worst and that any pockets of mist or thin cloud especially with a pollution colouring is likely to contain ash and would avoid flying for prolonged periods in such pockets.
Equally a pilot will avoid flying in visible moisture ie cloud at temperatures of zero or below when that cloud is icing up his aircraft.
He will climb descend or avoid to get out of the icing situation.
As long as the pilot is visual his best means of detection are his eyes. most of this week above 5000 feet there has been very little cloud over a large portion of the UK and below 5000 feet well broken cumulus with suoerb visibility above.
What's the difference between ash and ice
One is a solid particle the other is formed from a liquid and is accumlative they are very different in form and how they react with the airframe.
Pace