Measurements of Immersion Freezing and Heterogeneous Chemistry of Atmospherically Relevant Single Particles with Micro-Raman Spectroscopy.

In the atmosphere, there are several different trajectories by which particles can nucleate ice; two of the major pathways are deposition and immersion freezing. Single particle depositional freezing has been widely studied with spectroscopic methods while immersion freezing has been predominantly studied either for particles within bulk aqueous solutions or using optical imaging of single particles. Of the few existing spectroscopic methods that monitor immersion freezing, there are limited opportunities for investigating the impact of heterogeneous chemistry on freezing. Herein, we describe a method that couples a confocal Raman spectrometer with an environmental cell to investigate single particle immersion freezing along with the capability to investigate in situ the impact of heterogeneous reactions with ozone and other trace gases on ice nucleation. This system, which has been rigorously calibrated (temperature and relative humidity) across a large dynamic range, is used to investigate low temperature water uptake and heterogeneous ice nucleation of atmospherically relevant single particles deposited on a substrate. The use of Raman spectroscopy provides important insights into the phase state and chemical composition of ice nuclei and, thus, insights into cloud formation.