Any solid aerosol injected directly into the stratosphere for geoengineering purposes would be subject to coagulation with itself and with the natural background or volcanic sulfate aerosol. Aggregates of solid aerosols have very different physical structure and scattering properties than liquid sulfate aerosol particles do. The lifetime and scattering properties of a solid aerosol are strongly dependent on these dynamical interactions, and the chemical properties of the aerosol depend on the extent to which it becomes coated by the ambient sulfate.