Measurements of Immersion Freezing and Heterogeneous\nChemistry of Atmospherically Relevant Single Particles with Micro-Raman\nSpectroscopy

Abstract

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