Showing papers by "Harald Saathoff published in 2013"

The accommodation coefficient of water molecules on ice – cirrus cloud studies at the AIDA simulation chamber

Abstract: . Cirrus clouds and their impact on the Earth's radiative budget are subjects of current research. The processes governing the growth of cirrus ice particles are central to the radiative properties of cirrus clouds. At temperatures relevant to cirrus clouds, the growth of ice crystals smaller than a few microns in size is strongly influenced by the accommodation coefficient of water molecules on ice, αice, making this parameter relevant for cirrus cloud modeling. However, the experimentally determined magnitude of αice for cirrus temperatures is afflicted with uncertainties of almost three orders of magnitude, and values for αice derived from cirrus cloud data lack significance so far. This has motivated dedicated experiments at the cloud chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) to determine αice in the cirrus-relevant temperature interval between 190 K and 235 K under realistic cirrus ice particle growth conditions. The experimental data sets have been evaluated independently with two model approaches: the first relying on the newly developed model SIGMA (Simple Ice Growth Model for determining Alpha), the second one on an established model, ACPIM (Aerosol-Cloud-Precipitation Interaction Model). Within both approaches a careful uncertainty analysis of the obtained αice values has been carried out for each AIDA experiment. The results show no significant dependence of αice on temperature between 190 K and 235 K. In addition, we find no evidence for a dependence of αice on ice particle size or on water vapor supersaturation for ice particles smaller than 20 μm and supersaturations of up to 70%. The temperature-averaged and combined result from both models is αice = 0.7−0.5+0.3, which implies that αice may only exert a minor impact on cirrus clouds and their characteristics when compared to the assumption of αice =1. Impact on prior calculations of cirrus cloud properties, e.g., in climate models, with αice typically chosen in the range 0.2–1 is thus expected to be negligible. In any case, we provide a well-constrained αice which future cirrus model studies can rely on.

Laser-induced plasma cloud interaction and ice multiplication under cirrus cloud conditions

Abstract: Potential impacts of lightning-induced plasma on cloud ice formation and precipitation have been a subject of debate for decades. Here, we report on the interaction of laser-generated plasma channels with water and ice clouds observed in a large cloud simulation chamber. Under the conditions of a typical storm cloud, in which ice and supercooled water coexist, no direct influence of the plasma channels on ice formation or precipitation processes could be detected. Under conditions typical for thin cirrus ice clouds, however, the plasma channels induced a surprisingly strong effect of ice multiplication. Within a few minutes, the laser action led to a strong enhancement of the total ice particle number density in the chamber by up to a factor of 100, even though only a 10−9 fraction of the chamber volume was exposed to the plasma channels. The newly formed ice particles quickly reduced the water vapor pressure to ice saturation, thereby increasing the cloud optical thickness by up to three orders of magnitude. A model relying on the complete vaporization of ice particles in the laser filament and the condensation of the resulting water vapor on plasma ions reproduces our experimental findings. This surprising effect might open new perspectives for remote sensing of water vapor and ice in the upper troposphere.

Laser filament-induced aerosol formation

Abstract: . Using the aerosol and cloud simulation chamber AIDA, we investigated the laser filament induced particle formation in ambient air, humid synthetic air, humid nitrogen, argon–oxygen mixture, and pure argon in order to simulate the particle formation under realistic atmospheric conditions as well as to investigate the influence of typical gas-phase atmospheric constituents on the particle formation. Terawatt laser plasma filaments generated new particles in the size range 3 to 130 nm with particle production rates ranging from 1 × 107 to 5 × 109 cm−3 plasma s−1 for the given experimental conditions. In all cases the particle formation rates increased exponentially with the water content of the gas mixture. Furthermore, the presence of a few ppb of trace gases like SO2 and α-pinene clearly enhanced the particle yield by number, the latter also by mass. Our findings suggest that new particle formation is efficiently supported by oxidized species like acids generated by the photoionization of both major and minor components of the air, including N2, NH3, SO2 and organics.

Ice nucleation by surrogates of Martian mineral dust: What can we learn about Mars without leaving Earth?

Abstract: [1] Water and carbon dioxide ice clouds have been observed in the Martian atmosphere where they are dynamic parts of that planet's water and carbon cycles. Many Martian atmospheric models struggle to correctly predict clouds and, with insufficient data, some use untested simplifications that cloud formation occurs exactly at the saturation point of the condensed phase or at the same conditions as terrestrial cirrus clouds. To address the lack of data, we have utilized an 84 m3 cloud chamber built for studies of high altitude cirrus and polar stratospheric ice clouds in the Earth's atmosphere and adapted to Martian conditions. Using this chamber, we have been able to produce water ice clouds from aerosol in an inert and low pressure atmosphere mimicking that of Mars. At temperatures between 189 and 215 K, we investigated cloud formation by mineral dust particulates of a similar composition and size to those found on Mars. We show that these surrogate materials nucleate effectively at the higher temperatures, with minor temperature dependence at saturations ratios with respect to the ice phase of ~1.1, similar to what has been found for terrestrial cirrus. At the lower end of the temperature range, this saturation rises to ~1.9, a result consistent with previous studies.

Ice nucleation by surrogates of Martian mineral dust: What can we learn about Mars without leaving Earth?

Abstract: [1] Water and carbon dioxide ice clouds have been observed in the Martian atmosphere where they are dynamic parts of that planet's water and carbon cycles. Many Martian atmospheric models struggle to correctly predict clouds and, with insufficient data, some use untested simplifications that cloud formation occurs exactly at the saturation point of the condensed phase or at the same conditions as terrestrial cirrus clouds. To address the lack of data, we have utilized an 84 m3 cloud chamber built for studies of high altitude cirrus and polar stratospheric ice clouds in the Earth's atmosphere and adapted to Martian conditions. Using this chamber, we have been able to produce water ice clouds from aerosol in an inert and low pressure atmosphere mimicking that of Mars. At temperatures between 189 and 215 K, we investigated cloud formation by mineral dust particulates of a similar composition and size to those found on Mars. We show that these surrogate materials nucleate effectively at the higher temperatures, with minor temperature dependence at saturations ratios with respect to the ice phase of ~1.1, similar to what has been found for terrestrial cirrus. At the lower end of the temperature range, this saturation rises to ~1.9, a result consistent with previous studies.

Corrigendum to: "Studies of heterogeneous freezing by three different desert dust samples", Atmos. Chem. Phys., 9, 2805–2824, 2009

Abstract: Connolly et al.(2009) presented a model of ice nucleation based on the ideas of a “singular” process ( Vali, 1994), where the ice formation rate on a specific dust sample surrounded by supercooled water is approximated by a formulation that depends only on aerosol properties and the temperature. Connolly et al.developed a parameterisation to describe the rate of change of the number concentration of ice crystals, Nice with respect to temperature, T :