Collision dynamics and uptake of water on alcohol-covered ice

Abstract

Molecular scattering experiments are used to investigate water interactions with methanol and n-butanol covered ice between 155 K and 200 K. The inelastically scattered and desorbed products of an incident molecular beam are measured and analyzed to illuminate molecular scale processes. The residence time and uptake coefficients of water impinging on alcohol-covered ice are calculated. The surfactant molecules are observed to affect water transport to and from the ice surface in a manner that is related to the number of carbon atoms they contain. Butanol films on ice are observed to reduce water uptake by 20%, whereas methanol monolayers pose no significant barrier to water transport. Water colliding with methanol covered ice rapidly permeates the alcohol layer, but on butanol water molecules have mean surface lifetimes of ≲ 0.6 ms, enabling some molecules to thermally desorb before reaching the water ice underlying the butanol. These observations are put into the context of cloud and atmospheric scale processes, where such surfactant layers may affect a range of aerosol processes, and thus have implications for cloud evolution, the global water cycle, and long term climate

Water Accommodation and Desorption Kinetics on Ice

“…Absolute values are obtained by normalizing each integral by comparison with a contiguously measured case of D 2 O scattering from bare graphite, for which the trapping-desorption probability is wellknown under the present conditions. 15 No desorbing HDO is observed, eliminating the possibility of isotopic exchange between trapped D 2 O and surface H 2 O molecules as a major sink of heavy water. Figure 4 shows the calculated accommodation coefficients.…”

“…15 Briefly, the integral of the TOF distribution represents the thermally desorbed D 2 O molecules, while the remaining fraction of the incident molecules accommodate on the ice. Absolute values are obtained by normalizing each integral by comparison with a contiguously measured case of D 2 O scattering from bare graphite, for which the trapping-desorption probability is wellknown under the present conditions.…”

“…The H 2 O−ice layer is maintained at constant thickness of about 1 μm during experiments and is monitored using the interference signal produced by a low power laser (860 μW) with a wavelength of 670 nm directed at the surface. 12,13,15 The procedure produces polycrystalline ice surfaces that have been characterized in earlier work. 13 The incident D 2 O/He beam enters the inner chamber through a circular opening with a diameter of 5 mm and collides with the surface at an angle of 45°.…”

Water Accommodation on Ice and Organic Surfaces: Insights from Environmental Molecular Beam Experiments

“…14,32 Water interactions with organic phases are no better understood, and changes in hydrophilicity with oxidation state also need to be considered under atmospheric conditions. 2,33 To begin to address these questions, we have recently investigated the effect of adsorbed alcohols 34 and AcOH 32 on water uptake on ice, as well as water accommodation on the bulk phases of these compounds. 32,35 Experimental tools for exploring microphysical and chemical processes taking place on volatile surfaces are limited.…”

“…36 In the past few years, we have developed a molecular beam technique that allows for experiments at pressures up to 1 Pa, which makes it possible to perform experiments with ice and other volatile materials under conditions relevant for atmospheric processes. 4,7,32,34,35,37,38 The method has been termed an environmental molecular beam (EMB) method in analogy with environmental scanning electron microscopy (ESEM) that is carried out at elevated pressures.…”