Adsorption of acetic acid on ice studied by ambient-pressure XPS and partial-electron-yield NEXAFS spectroscopy at 230-240 K.

Ice plays a key role in the environment, and the ice-air interface influences heterogeneous chemical reactions between snowpack or cirrus clouds and the surrounding air. Soluble gases have been suspected to affect the topmost, disordered layer on ice (often referred to as a quasiliquid layer, QLL). Changes are especially expected in the hydrogen-bonding structure of water in the presence of solutes at the ice surface. Here, we used ambient-pressure X-ray photoelectron spectroscopy (XPS) to detect acetic acid at the ice surface at 230-240 K under atmospheric conditions for the first time. Electron-kinetic-energy-dependent C 1s spectra indicate that acetic acid remains confined to the topmost ice surface layers. Spectral analysis provides information about the protonation state of acetate at the ice surface. Surface-sensitive Auger-electron-yield C-edge near-edge X-ray absorption fine structure (NEXAFS) spectra were recorded to probe the molecular state of the adsorbed species. The O-edge NEXAFS spectra show only minor differences between clean ice and ice with adsorbed acetic acid and thus indicate that acetic acid does not lead to an extended disordered layer on the ice surface between 230 and 240 K.