This work provides a new understanding of critical process parameters involved in the production of inhalation aerosol particles by ultrasonic spray freeze drying to enable precise control over particle size and aerodynamic properties. A series of highly porous mannitol, lysozyme, and bovine serum albumin (BSA) particles were produced, varying only the solute concentration in the liquid feed, cs, from 1 to 5wt%. The particle sizes of mannitol, BSA, and lysozyme powders were independent of solute concentration, and depend only on the drop size produced by atomization. Both mannitol and lysozyme formulations showed a linear relationship between the computed Fine Particle Fraction (FPF) and the square root of cs, which is proportional to the particle density, ρ, given a constant particle size dg. The FPF decreased with increasing cs from 57.0% to 16.6% for mannitol and 44.5% to 17.2% for lysozyme. Due to cohesion, the BSA powder FPF measured by cascade impaction was less than 10% and independent of cs. Ultrasonic spray freeze drying enables separate control over particle size, dg, and aerodynamic size, da which has allowed us to make the first experimental demonstration of the widely accepted rule da=dg(ρ/ρo)1/2 with particles of constant dg, but variable density, ρ (ρo is unit density).