I'm far from an expert on volcanic aerosols, but basing it on the opacity of interstellar dust, it's likely to be very complicated.
Smaller particles tend to scatter and absorb more light per unit mass of ash.
Fluffier - more snowflake-like and less-spherical - grains tend to scatter more light per unit mass.
Scattering and absorption properties will depend differently on wavelength, grain size, grain shape and grain material.
The ash might be associated with SO2? This also casts a brown haze that likely interferes with the signal from the solids. Ice mantles/coatings can also modify the optical properties of ash grains.