Review of experimental studies of secondary ice production

Secondary ice production (SIP) plays a key role in the formation of ice particles in tropospheric clouds. Future improvement of the accuracy of weather prediction and climate models relies on a proper description of SIP in numerical simulations. For now, laboratory studies remain a primary tool for developing physically based parameterizations for cloud modeling. Over the past 7 decades, six different SIP-identifying mechanisms have emerged: (1) shattering during droplet freezing, (2) the rime-splintering (Hallett–Mossop) process, (3) fragmentation due to ice–ice collision, (4) ice particle fragmentation due to thermal shock, (5) fragmentation of sublimating ice, and (6) activation of ice-nucleating particles in transient supersaturation around freezing drops. This work presents a critical review of the laboratory studies related to secondary ice production. While some of the six mechanisms have received little research attention, for others contradictory results have been obtained by different research groups. Unfortunately, despite vast investigative efforts, the lack of consistency and the gaps in the accumulated knowledge hinder the development of quantitative descriptions of any of the six SIP mechanisms. The present work aims to identify gaps in our knowledge of SIP as well as to stimulate further laboratory studies focused on obtaining a quantitative description of efficiencies for each SIP mechanism

Lässt sich die Erde künstlich kühlen?

Eigentlich ist die Mission klar: Wir müssen den weltweiten Kohlendioxidausstoß begrenzen. Doch politisch lässt sich das Ziel womöglich nicht realisieren. Könnten wir der Erderwärmung stattdessen durch gezielte Eingriffe in das Klimasystem entgegenwirken? Dass der Mensch den Strahlungshaushalt der Erde beeinflusst, ist nichts Neues. Bis vor zwei Jahrhunderten geschah das aber praktisch nur durch Veränderung der Landoberflächen, und die Auswirkungen waren so gering, dass sie sich kaum von natürlichen Klimaschwankungen abhoben. Das änderte sich jedoch mit der industriellen Revolution. Von nun an gelangte durch die zunehmende Nutzung fossiler Brennstoffe, die zur Energiegewinnung verfeuert wurden, rasch immer mehr Kohlendioxid (CO2) in die Atmosphäre. Parallel dazu kam es zu einem rapiden Bevölkerungswachstum, das mit einer Intensivierung der Landwirtschaft einherging. Durch die vermehrte Nutzviehhaltung und die künstliche Düngung stiegen vor allem die Emissionen von Methan (CH4), aber auch die von Lachgas (N2O) an. Diese Entwicklung setzt sich bis heute fort, und es gibt kaum Anzeichen für eine Trendwende. Als Folge nehmen die Konzentrationen der drei Gase in der Atmosphäre weiter zu, wo sie die Wärmestrahlung in den unteren Luftschichten zurückhalten. Dadurch heizen sie die Erde allmählich auf. Der resultierende Klimawandel ist durch Messungen der globalen Luft- und Ozeantemperatur, der Schnee- und Eisbedeckung sowie des Meeresspiegels inzwischen klar belegt

Preferential adsorption of para and ortho water molecules on charged nanoparticles in planetary ice clouds

In the Earth mesopause, nanometer-size singly charged particles form by condensation of evaporated meteorite material. Tey exhibit an enhanced water adsorption cross section due to the strong charge-dipole-interaction. In this work, we study how the nuclear spin state of water molecules afects this enhancement and whether there are conditions that could lead to the formation of spin-polarized ice. Due to symmetry constraints on the total molecular wave function, ortho (proton spins parallel) and para (spins antiparallel) water occupy diferent rotational states, resulting in a diferent average dipole orientation in electric felds. Terefore, we expect ortho and para water to exhibit distinct ad- sorption enhancement factors onto charged nanoparticles. Based on Stark-shifs of individual rotational states of water, average dipole orientations of a molecu- lar ensemble and the resulting collision cross section was calculated for various temperatures and particle sizes. We found that in the mesosphere of the Earth (T~150K) the adsorption enhancement of ortho- and para- water is approxi- mately equal while at lower temperatures prevailing around ice giant planets and their moons, signifcant spin polarizations up to 15% occur

The vapor pressure over nano-crystalline ice

The crystallization of amorphous solid water (ASW) is known to form nano-crystalline ice. The influence of the nanoscale crystallite size on physical properties like the vapor pressure is relevant for processes in which the crystallization of amorphous ices occurs, e.g., in interstellar ices or cold ice cloud formation in planetary atmospheres, but up to now is not well understood. Here, we present laboratory measurements on the saturation vapor pressure over ice crystallized from ASW between 135 and 190 K. Below 160 K, where the crystallization of ASW is known to form nano-crystalline ice, we obtain a saturation vapor pressure that is 100 to 200 % higher compared to stable hexagonal ice. This elevated vapor pressure is in striking contrast to the vapor pressure of stacking disordered ice which is expected to be the prevailing ice polymorph at these temperatures with a vapor pressure at most 18 % higher than that of hexagonal ice. This apparent discrepancy can be reconciled by assuming that nanoscale crystallites form in the crystallization process of ASW. The high curvature of the nano-crystallites results in a vapor pressure increase that can be described by the Kelvin equation. Our measurements are consistent with the assumption that ASW is the first solid form of ice deposited from the vapor phase at temperatures up to 160 K. Nano-crystalline ice with a mean diameter between 7 and 19 nm forms thereafter by crystallization within the ASW matrix. The estimated crystal sizes are in agreement with reported crystal size measurements and remain stable for hours below 160 K. Thus, this ice polymorph may be regarded as an independent phase for many atmospheric processes below 160 K and we parameterize its vapor pressure using a constant Gibbs free energy difference of 982  ±  182 J mol−1 relative to hexagonal ice

The “ideal” spectrograph for atmospheric observations

Spectroscopy of scattered sunlight in the near-UV to near-IR spectral ranges has proven to be an extremely useful tool for the analysis of atmospheric trace gas distributions. A central parameter for the achievable sensitivity and spatial resolution of spectroscopic instruments is the étendue (product of aperture angle and entrance area) of the spectrograph, which is at the heart of the instrument. The étendue of an instrument can be enhanced by (1) upscaling all instrument dimensions or (2) by changing the instrument F number, (3) by increasing the entrance area, or (4) by operating many instruments (of identical design) in parallel. The étendue can be enhanced by (in principle) arbitrary factors by options (1) and (4); the effect of options (2) and (3) is limited

Charge induced enhancement of water adsorption on nanoparticle ions

Water and other polar molecules experience an attractive force in the inhomogeneous electric field of small molecular ions or charged nanoparticles. This charge induced attractive force increases the collision cross section, and, hence, impacts the adsorption rates compared to neutral particle interactions. While ion-molecule interactions have been studied extensively, experimental data are still lacking regarding the interaction of polar molecules with nanoparticles whose radii exceed the Langevin capture radius. Precise knowledge of this effect is crucial, e.g. for describing the formation and growth of atmospheric nanoparticles and for understanding the role of charged particles in cloud formation. We present experimental results for the charge induced enhancement of the collision cross section between H2O molecules and singly charged nanoparticles with radii between 1.4 nm and 3 nm. The enhancement factor Γ with respect to the geometrical cross section increases with decreasing particle size. We also present a new model for Γ based on Stark effect adiabatic dipole orientations, which is in excellent agreement with the experimental findings