Suspended in sound: New constraints on isotopic fractionation of falling hydrometeors using acoustically levitated water droplets

The isotopic composition of precipitation (delta O-18, delta H-2, and delta O-17) is affected by evaporation and exchange as hydrometeors descend. These processes can significantly alter the isotopic ratio of precipitation relative to its initial condensation state in the cloud yet are exceedingly difficult to study in situ. The most widely utilized model for droplet-atmosphere exchange was derived from controlled experiments where droplets were suspended by forced air in a narrow glass tube- a design that manipulated the structure of the boundary layer around the droplet. Here, we provide a novel experimental test of atmosphere-hydrometeor isotopic exchange using the mechanism of acoustic levitation, where sound waves are projected vertically to levitate droplets in free-flowing ambient air. We present results from a series of droplet levitation experiments where the droplets' surface temperatures were measured by a thermal camera and the background atmospheric isotope concentration was measured via cavity-ringdown spectroscopy, providing a high degree of constraint on the fractionation conditions. We show that isotope enrichment of the suspended droplets met first order expectations based on existing models. However, to account for the slope of delta O-18 versus delta H-2 (i.e., the meteoric water line) and deuterium excess of the droplets as they evolved, we had to modify the existing model for droplet-atmosphere exchange to account for the fact that some portion of the evaporative flux from the droplet remained present in the boundary layer around the droplet leading to an evolving feedback between droplet and the atmosphere-that is, a quasi closed-system effect. The isotopic enrichment of the boundary layer surrounding the droplet as a consequence of the closed-system dynamics, drives more rapid delta O-18 isotope enrichment relative to delta H-2 of the droplet compared to what is predicted using an open system model. Although these were controlled experiments, they illustrate important dynamics regarding the isotopic signature of feedbacks between droplet evaporation and atmospheric humidity.