Acoustic Levitation: A novel method to simulate raindrop-atmosphere isotope exchange

Isotope exchange between rain drops and the surrounding atmosphere is affected by raindrop size and shape, atmospheric humidity and temperature profiles. The isotopic ratio in precipitation can thus provide an integrated proxy for atmospheric conditions. However, there are limited data on rain drop-atmosphere exchange due to practical challenges in making in-situ measurements on rain drops. To address this logistical limitation, we present initial results to test the feasibility of acoustic levitation, which uses sound at inaudible decibel levels, as a method to suspend water droplets in air to allow for isotopic measurements of in situ droplets of various size and shapes. We suspended different types of water drops with known isotope values for specific time durations and extracted the drops to measure changes in water isotopologues during droplet suspension. A Picarro L2140-I laser isotope analyzer was used to measure background humidity and isotope concentrations during the droplet suspension experiments. After the droplets were extracted from acoustic suspension, they were individually measured. A thermal imaging video camera was used to capture skin temperatures of the water droplets during suspension and constrain the droplet boundary vapor pressure. We will present the first results of these experiments under conditions where droplets were suspended in ambient unsaturated laboratory background humidity and temperatures. As expected, water droplets become more enriched in δ¹⁸O, δ¹⁷O and δ²H with time as evaporation occurs and we will use models for droplet evaporation to study the exchange processes. Future designs of the acoustic levitation equipment will incorporate a chamber that can control humidity, wind and temperature around the suspended drops. As further advancements are made, acoustic levitation may be a viable method to better study precipitation-atmosphere isotope exchange including the effects of aerosols.