In Fig , we report the evaporation kinetics of deep lung fluid into an environment of varying RH using the same instrument described for our hygroscopicity measurements. As reported earlier, the droplets rapidly lose water to redress the imbalance between the starting water activity in the droplet and the surrounding environmental RH, equilibrating to a size and composition where the water vapour pressure above the droplet solution equals the partial pressure of water at a large distance from the droplet. A reduction in the RH leads to an increasing rate of evaporation and a decrease in the equilibrated size of the solution droplet, with retention of diminishing amounts of water. Once the RH is below 50%, the light scattering pattern used to estimate the droplet size becomes extremely irregular, indicating the droplet is no longer a homogenous sphere, and the reported size becomes noisy and unreliable. In Fig the onset of the disruption to the scattering pattern is identified with a star and the noisy datapoints removed. We previously demonstrated that the phase can be identified and, in this case, the particle morphology can be assigned to be non-spherical, potentially crystalline. The morphological differences observed in the SEM images between the dried particles of the different compositions will lead to different aerodynamic properties during sedimentation, which are not explicitly included our modelling.