The Role of Organic Heterogeneous Nuclei in Contact Crystallization of Atmospherically Relevant Salts

Kyle McMillan,
ANYL 3rd year, Tolbert group

The overall radiative effect of atmospheric aerosols on climate is currently a major source of uncertainty in climate models. Essential in resolving this uncertainty is a complete understanding of the particle phase state in the atmosphere. Often overlooked in current studies on the particle phase is the role of collisions in inducing contact nucleation. Thus, more research that focuses on determining which atmospherically relevant compounds are most likely to be involved in contact nucleation under varying conditions is warranted. Of interest here was evaluating the effectiveness of organic-containing contact nuclei (CN) in inducing contact efflorescence of aqueous salt-containing droplets. To do this, contact efflorescence was probed on freely floating aqueous ammonium sulfate (AS) droplets using a long-working distance optical trap. In this setup, optically levitated droplets were contacted with both glassy (D-(+)-raffinose and citric acid) and surface-active (stearic acid and cis-pinonic acid) organic CN over a range of relative humidity (RH) values above the homogeneous efflorescence RH of AS. The purpose of these experiments was to determine if certain organic-containing CN could induce contact efflorescence of aqueous droplets at an RH significantly above the homogeneous efflorescence RH of AS. Far-field imaging was employed as a way of probing crystallization either in the contact or homogeneous modes, and was supplemented with near-field imaging, which was used to track contact events. Droplet compositions before and after crystal nucleation were characterized using Raman spectrometry to track the liquid water content of the droplet. Promising in these experiments is the effectiveness of surface-active organics—in particular, cis- pinonic acid—in inducing contact efflorescence of AS, which has major implications for the particle phase state of the troposphere. Continued experiments in this area will provide key insight into the interactions that cause contact to be effective at initiating nucleation and will ultimately lay the groundwork for analogous contact ice nucleation studies.