Not sure about this (last serious physics exposure A levels 1962) but is not the "visibility" of particles dependent on the wavelength of the illuminating beam. Which is why Radar is not seeing some of this stuff. If the particles are the wrong size to be illuminated in the visible light band, we will not see them, just like the radar.
The other issue is gases. The atmosphere is full of molecules that we cannot see. When volcanic sulphur compounds are present (mainly SO2) we cannot see them.
As I recall sulphur embrittlement of turbine blades used to be a serious problem years ago, when fuel content and additive contamination were the source.