| Abstract
| - We demonstrate that the thermodynamic properties of a single liquid aerosol droplet can be explored throughthe combination of a single-beam gradient force optical trap with Raman spectroscopy. A single aqueousdroplet, 2−6 μm in radius, can be trapped in air indefinitely and the response of the particle to variations inrelative humidity investigated. The Raman spectrum provides a unique fingerprint of droplet composition,temperature, and size. Spontaneous Raman scattering is shown to be consistent with that from a bulk phasesample, with the shape of the OH stretching band dependent on the concentration of sodium chloride in theaqueous phase and on the polarization of the scattered light. Stimulated Raman scattering at wavelengthscommensurate with whispering gallery modes is demonstrated to provide a method for determining the sizeof the trapped droplet with nanometer precision and with a time resolution of 1 s. The polarization dependenceof the stimulated scatter is consistent with the dependence observed for the spontaneous scatter from thedroplet. By characterizing the spontaneous and stimulated Raman scattering from the droplet, we demonstratethat it is possible to measure the equilibrium size and composition of an aqueous droplet with variation inrelative humidity. For this benchmark study we investigate the variation in equilibrium size with relativehumidity for a simple binary sodium chloride/aqueous aerosol, a typical representative inorganic/aqueousaerosol that has been studied extensively in the literature. The measured equilibrium sizes are shown to be inexcellent agreement with the predictions of Köhler theory. We suggest that this approach could provide animportant new strategy for characterizing the thermodynamic properties and kinetics of transformation ofaerosol particles.
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