Pre-melting and the adsorption of formic acid at the air–ice interface at 253 K as seen by NEXAFS and XPS

Abstract

Interactions between trace gases and ice are important in environmental chemistry and for Earth's climate. In particular, the adsorption of trace gases to ice surfaces at temperatures approaching the melting point has raised interest in the past, because of the prevailing pre-melting. Here, we present Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy data at ambient partial pressure of water to better define the onset temperature of pre-melting at the interfacial region of ice. Further, this study directly compares the interaction between an organic acid common in the atmosphere, formic acid, and that of an aliphatic carbon with ice at 253 K. It makes use of X-ray Photoelectron Spectroscopy (XPS) with its inherent narrow probing depth covering both the surface and near-surface bulk region when detecting electrons. We use the tender X-ray range for excitation to locate the organic species within the interfacial region with an extended probing depth compared to published XPS work. Electron kinetic energy dependent C1s photoemission data indicate that, at low coverage of a few 10 molecules cm, the presence of formic acid is restricted to the upper ice layers of the interfacial region. Increasing the dosage, formic acid penetrates 6-7 nm into the air-ice interface. The presence of the more hydrophobic aliphatic carbon is restricted to the upper ice monolayers. This direct comparison of an organic acid with an aliphatic compound confirms the emerging picture where solutes enter the interfacial region of ice at a depth related to their specific tendency to form solvation shells.

Structure and water attachment rates of ice in the atmosphere: role of nitrogen

“…This behavior is similar to that observed for hydrocarbons and even far more soluble small organic molecules such as glyoxal, which have been observed to remains on the ice surface rather than sink into the premelting layer as ions do. [21,22] Also notice that only occasionally do we find finite nitrogen densities at negative z, whence, only a small fraction of all adsorbed nitrogen is dissolved into the premelting layer. A finite but very small amount of nitrogen may be seen to penetrate the premelting film for the pI plane at the two highest temperatures, but is negligible in most other cases.…”

MIMiX: A Multipurpose In-situ Microreactor system for X-ray microspectroscopy to mimic atmospheric aerosol processing

“…We therefore assume that the observed droplet is highly diluted with water and that the characteristic reversal of the intensity ratios of the spectral features II and III (compare blueish spectra in Fig. 7c1 with c2), as a result of a change in coordination geometry and bond strength as observed by, e.g., Sellberg et al (2014); Nilsson et al (2010), and, Waldner et al (2018, can to a large extent be attributed to 235 H 2 O molecules only. The observed ice crystal remained stable in shape over many hours.…”

“…A comparison of the corresponding spectra ( Fig. 7c1 & c2) with reference spectra of liquid water (Sellberg et al, 2014;Nilsson et al, 2010) and ice (Sellberg et al, 2014;Nilsson et al, 2010;Waldner et al, 2018), suggests that the spectral components represent https://doi.org/10.5194/amt-2019-507 Preprint. Discussion started: January 2020 c Author(s) 2020.…”

“…CC BY 4.0 License. NNMA component (b2) Dry ammonium sulfate (Zelenay et al, 2011) Bulk ice, XAS, 232 K (Sellberg et al, 2014) Bulk ice, XAS, 100 K (Nilsson et al, 2010) Bulk ice, PAEY, 251 K (Waldner et al, 2018) NNMA component (b1) Isoprene SOA H2O (liquid), XAS, 264 K (Sellberg et al, 2014) H2O (liquid), XAS (Nilsson et al, 2010) NNMA cluster oxygen NEXAFS spectra at 261 K + references…”

“…Photon energy (eV) the corresponding spectra along with reference spectra. Accordingly, the three main spectral features are labeled I, II, III, and emphasized by a grey background in analogy to Bartels-Rausch et al (2017) and Waldner et al (2018). All spectra were shifted by 3.2 eV to larger energy values with respect to the reference spectra.…”