Atmospheric
aerosols can exist as supersaturated (metastable) liquid
or glassy states, with physical and chemical properties that are distinct
from the solid or liquid phases. These unique properties of aerosols
have substantial implications on climate and health effects. Direct
investigations on metastable aerosols remain a challenge because any
interfacial contact can cause heterogeneous nucleation. In this study,
in situ Raman spectroscopic and Mie scattering imaging analysis is
applied to metastable aerosols in the absence of physical contact
using an environment-controlled electrodynamic balance (EDB). This
has allowed a detailed study of the O–H stretching regions
of the Raman spectrum, revealing evidence for the rearrangement of
hydrogen-bonding structures of levitated aqueous citric acid (CA)
and aqueous sucrose droplets at metastable liquid states. We found
that carboxyl groups in a CA droplet yield distinctive dynamics of
strong and weak hydrogen bonds, whereas hydroxyl groups in a sucrose
droplet show correlated strong and weak interactions. Such effects
are particularly important in a supersaturated solution. These results
indicate that metastable liquid aerosols from different sources may
exhibit distinct physical and chemical behavior.