Articles | Volume 8, issue 8
Atmos. Meas. Tech., 8, 3519–3526, 2015
Atmos. Meas. Tech., 8, 3519–3526, 2015

Research article 27 Aug 2015

Research article | 27 Aug 2015

Probing ice-nucleation processes on the molecular level using second harmonic generation spectroscopy

A. Abdelmonem1, J. Lützenkirchen2, and T. Leisner1 A. Abdelmonem et al.
  • 1Institute of Meteorology and Climate Research – Atmospheric Aerosol Research (IMKAAF), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany
  • 2Institute of Nuclear Waste Disposal (INE), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany

Abstract. We present and characterize a novel setup to apply second harmonic generation (SHG) spectroscopy in total internal reflection geometry (TIR) to heterogeneous freezing research. It allows to monitor the evolution of water structuring at solid surfaces at low temperatures prior to heterogeneous ice nucleation. Apart from the possibility of investigating temperature dependence, a major novelty in our setup is the ability of measuring sheet-like samples in TIR geometry in a direct way. As a main experimental result, we find that our method can discriminate between good and poor ice nucleating surfaces. While at the sapphire basal plane, which is known to be a poor ice nucleator, no structural rearrangement of the water molecules is found prior to freezing, the basal plane surface of mica, an analogue to ice active mineral dust surfaces, exhibits a strong change in the nonlinear optical properties at temperatures well above the freezing transition. This is interpreted as a pre-activation, i.e. an increase in the local ordering of the interfacial water which is expected to facilitate the crystallization of ice at the surface. The results are in line with recent predictions by molecular dynamics simulations on a similar system.

Short summary
This manuscript belongs and is important to the environmental and atmospheric science, particularly cloud formation and cloud seeding, and presents a setup to apply Second Harmonic Generation spectroscopy to heterogeneous freezing research. We describe the setup and provide first results on temperature-dependent structural changes of water on the surfaces of two relevant atmospheric aerosol substances (sapphire and mica as poor and good ice nucleators, respectively).