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.

Herein, we measure the water structure for individual micron-sized droplets of water, salt water, and water containing biologically and marine relevant atmospheric inclusions as a function of temperature. Individual droplets, formed on a hydrophobic substrate, are analyzed with micro-Raman spectroscopy. Analysis of the Raman spectra in the O–H stretching region shows that the equilibrium of partially and fully hydrogen-bonding water interactions change as temperature decreases up until there is a phase transition to form ice. Using these temperature-dependent measurements, the thermodynamic parameters for the interchange between partially and fully hydrogen-bonded water (PHW ⇄ FHW) for different supercooled droplets (water, salt water, and water containing biologically and marine relevant atmospheric inclusions) have been determined.

The interaction of water vapor and the water uptake behavior of atmospheric particles are often investigated as a function of relative humidity (0−100% RH) at ambient temperature. However, lower temperature studies are important to understand how atmospheric particles nucleate ice through various mechanisms including immersion freezing. Immersion freezing requires the formation of a condensed water droplet at lower temperatures prior to freezing. To better understand low-temperature water uptake behavior of marine and biologically relevant atmospheric particles, we have investigated water uptake of single atmospheric particles using a micro-Raman spectrometer coupled to an environmental cell for measurements at lower temperatures and as a function of relative humidity. These particles include sodium chloride, sucrose, Snomax, lipopolysaccharide, and laminarin. Particles range in size from 2 to 3 μm in diameter and can be monitored by using optical microscopy and Raman spectroscopy as a function of relative humidity at temperatures between 253 and 298 K. From the Raman spectra collected, we can determine a Raman growth factor defined as an increase in the intensity of the O−H stretch as a measure of the integrated water content of a particle compared to the dry particle. These data show that for lipopolysaccharide, laminarin, and Snomax, unlike simple saccharides such as sucrose and other soluble organics, as temperature decreases, water uptake begins at lower relative humidity and does not follow a solubility temperature dependence. This suggests that at lower temperatures the particles are adsorbing water on the surface rather than dissolving and absorbing water. Furthermore, repeated water uptake cycles cause a change in the morphology of some of these particles, which is shown to promote water uptake at lower relative humidity. These results give new insights into water uptake of these different marine and biologically relevant particles at low temperature at subsaturation relative humidity prior to droplet formation and immersion freezing.

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Das Phasendiagramm des Titelsystems, das mittels DSC, flammenloser Atom‐Absorptionsspektroskopie und "Röntgenstrukturanalyse (auch bei höheren Temp. mit einer HT‐Kamera nach Seeman) bestimmt wird, zeigt ein Eutektikum bei 202°C und 73.6 Atom‐% Se.