Structure of Single Aqueous MgSO₄ Droplets Ultrasonically Levitated in the Air: Supersaturation and Crystallization

Aerosol droplets are fully involved in forming clouds and play a critical role in global climate change. The investigation of the properties, speciation, and structure of aerosol droplets at a molecular level is highly needed for understanding the physicochemical behaviors and processes of aerosol droplets. We investigate the composition, speciation, and structure of a single aqueous MgSO4 droplet of 1 mm size ultrasonically levitated in the air and bulk solutions under ambient conditions by in situ Raman and synchrotron X-ray scattering. The composition and speciation are determined by measuring Raman spectra of the sulfate ion's S-O band and water's O-H bands. The X-ray data are subjected to empirical potential structure refinement modeling to extract the site-site pair correlation function, coordination number distribution, and 3-D structure for the ion solvation, association, and solvent water. In situ X-ray scattering measurements observe the crystallization of MgSO4 hydrates from a droplet.