Aerosol iron solubility is a major uncertainty in the global biogeochemical cycle of iron and, via its impact on ocean productivity, the carbon cycle and their influence on global climate. Previous studies have reported widely different values for this solubility (0.01 -- 80%). Here we show that the primary control on aerosol iron solubility is the surface area to volume ratio of mineral aerosol particles, which changes during atmospheric transport as mineral aerosol concentration decreases due to preferential removal of larger particles (assuming particle morphology to be relatively constant with particle size). This important result indicates that aerosol iron solubility is not fixed, but will change predictably as an inverse function of dust concentration on both spatial and temporal (e.g. glacial -- interglacial) scales.