The effect of the ambient relative humidity (RH) on the crystallization of malonic and glutaric acid in aqueous microdroplets was investigated using an electrodynamic balance (EDB). Aqueous droplets containing a solute were injected into a chamber maintained at selected RH values, after which the RH was cycled to characterize crystallization and deliquescence behavior. Glutaric acid crystals that were formed at >40% RH deliquesced at 85 ± 2% RH, consistent with the metastable α polymorph. Glutaric acid crystals initially formed at ≥30% RH, however, deliquesced at 90 ± 2% RH, corresponding to the stable β polymorph. These polymorph identifications were confirmed using in situ Raman spectroscopy. In the case of malonic acid, the mass change measured with the EDB at deliquescence revealed the formation of a hydrate, C_3H_4O_4?1.5H_2O, when the crystals were initially formed at ≥20% RH. The EDB response for malonic acid crystals initially formed at ～5% RH, however, corresponded to an anhydrous form. These observations suggest that, contrary to expectation, metastable crystals were favored at lower supersaturation levels compared with their respective stable forms. This unanticipated behavior is attributed to nucleation within the droplet at the higher RH value but at the air-solution interface of the droplet for lower RH values.
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