Thorsten Bartels-Rausch (Paul Scherrer Institut, Switzerland)
Wednesday 15 May
10:00 - 11:00
Field studies and satellite observations over the last decades have provided impressive evidence of how chemistry and physical processes in Earth’s ice and snow cover change the composition of the atmosphere and the fate of toxins. The direct impact on geochemical cycles, air quality, the food-chain, and the climate system is well established. Yet, our ability to predict the fate of chemicals in snow or air masses in exchange with the cryosphere on a regional scale or to capture those processes in snow chemistry models is still hampered by our limited understanding of the underlying mechanisms. Investigating the mechanism of chemistry in ice might be of further general interest as it tackles fundamental concepts in chemistry such as interfacial acid-base equilibria.
In this presentation, I will discuss how data from well-controlled laboratory-based experiments has and will deepen our understanding of multiphase chemistry in snow. The presentation will focus on apparently simple questions such as
- Where do chemical reactions occur in snow?
- When do salt solutions freeze?
- To which degree do acids dissociate at surfaces?
In detail, snow is a complex multi-component mixture. Impurities at environmentally relevant levels can be hosted at the air-ice interface, at grain boundaries, as solid solution in the ice crystal, as aerosol deposits, and in small liquid inclusions and patches. Identifying these compartments and characterising the chemical environment that they provide is an ongoing research field.
Here, I will present recent work on the chemical reactivity of liquid aerosol deposits in snow. This study investigated the oxidation of bromide by ozone. Our results quantitatively show how the presence of organics significantly slows chemical reactivity due to effects on the viscosity of the medium and solubility of ozone in the medium.
Below the eutectic, salt in solution precipitates. Recently, the presence of liquid-like features well below the eutectic temperature has been described. This finding raised quite some interest, as the presence of liquid would imply enhanced reactivity -over a wide temperature range- compared to the salt deposits. Here, I will present O K-edge and Cl K-edge partial electron yield X-ray absorption (NEXAFS) data of sodium chloride – water mixtures at, above, and below the eutectic. The results clearly give no evidence for the presence of quasi-liquid at the air-ice interface below the eutectic in thermodynamic equilibrium. Differences to previous studies are discussed.
Last, I will touch the adsorption of trace gases to the air-ice interface. The approach of this study is to combine two surface sensitive spectroscopic methods, core-electron photoemission (XPS) and partial electron yield X-ray absorption (NEXAFS) measurements, to directly probe the hydrogen-bonding network and the concentration, depth profile, and dissociation degree of the adsorbate. The interfacial dissociation of acids is of interest because it directly impacts their partitioning equilibrium, photochemistry, and general reactivity. We observed a co-existence of protonated and dissociated acid even at -10°C. These study gives clear evidence for nonuniformity across the air−ice interface and questions the use of acid−base concepts in interfacial processes.