Efficiency of the deposition mode ice nucleation on mineral dust particles

The deposition mode ice nucleation efficiency of various dust aerosols was investigated at cirrus cloud temperatures between 196 and 223 K using the aerosol and cloud chamber facility AIDA (Aerosol Interaction and Dynamics in the Atmosphere). Arizona test dust (ATD) as a reference material and two dust samples from the Takla Makan desert in Asia (AD1) and the Sahara (SD2) were used for the experiments at simulated cloud conditions. The dust particle sizes were almost lognormally distributed with mode diameters between 0.3 and 0.5 μm and geometric standard deviations between 1.6 and 1.9. Deposition ice nucleation was most efficient on ATD particles with ice-active particle fractions of about 0.6 and 0.8 at an ice saturation ratio Si<1.15 and temperatures of 223 and 209 K, respectively. No significant change of the ice nucleation efficiency was found in up to three subsequent cycles of ice activation and evaporation with the same ATD aerosol. This indicates that the phenomenon of preactivation does not apply to ATD particles. The desert dust samples SD2 and AD1 showed a significantly lower fraction of active deposition nuclei, about 0.25 at 223 K and Si<1.35. For all samples the ice activated aerosol fraction could be approximated by an exponential equation as function of Si. This indicates that deposition ice nucleation on mineral particles may not be treated in the same stochastic sense as homogeneous freezing. The suggested formulation of ice activation spectra may be used to calculate the formation rate of ice crystals in models, if the number concentration of dust particles is known. More experimental work is needed to quantify the variability of the ice activation spectra as function of the temperature and dust particle properties.

Individualists

The French-speaking Swiss traveler Ella Maillart (1903–1997) was a remarkable personality. By age thirty, she had taught French in a Welsh school, sailed in the Olympics for the Swiss team, acted on the Parisian stage, captained the Swiss women’s field hockey team, assisted on an excavation in Crete, studied film production in Moscow, published a book about a north-south walk through the Caucasus, and ridden a camel across the Kizil Kum Desert in present-day Kazakhstan and Uzbekistan, southeast of the Aral Sea—in midwinter. No one knows why she had such a penchant for adventure and variety: perhaps she was rebelling against the staid and thoroughly conventional family life of her childhood. The freedom and self-assertion taken for granted by many women today could then be achieved only by being unconventional and heading off into the unknown. At the age of twenty-eight, Maillart gazed on China for the first time. “In 1932, having gone east from Moscow, I climbed a mountain nearly 17,000 feet high on foot, and succeeded in reaching the eastern frontier of Russian Turkestan. There, at least, from the heights of the Celestial Mountains I could decry, on a plain far away and still further to the east, the yellow dust of the Takla Makan desert. It was China, the fabulous country of which, since my childhood, I had dreamed. There the caravan trails that were as old as the world, still wound. Long ago, Marco Polo followed them as far as Peking.”1 But she was unable to obtain a visa to enter Chinese Turkestan, which, like Outer Mongolia, was virtually isolated from the world by political turmoil. “Sadly,” she wrote, “I retraced my steps, turning my back on the limitless unknown that beckoned.” Maillart traveled in romantic lands whose very names evoke adventure— Pingliang, Yarkand, Kashgar. For centuries, the Silk Road was synonymous with danger, mystery, and high adventure beyond the frontiers of the Western world. The men and women who explored this remote and unfamiliar realm had no illusions about the dangers and political disorder that awaited them, but they would have been quietly horrified to hear their travels described as adventures.

Efficiency of the deposition mode ice nucleation on mineral dust particles

The deposition mode ice nucleation efficiency of various dust aerosols was investigated at cirrus cloud temperatures between 196 K and 223 K using the aerosol chamber facility AIDA (Aerosol Interaction and Dynamics in the Atmosphere). Arizona test dust (ATD) as a reference material and two dust samples from the Takla Makan desert in Asia (AD1) and Sahara (SD2) were used for the experiments at simulated cloud conditions. The dust particle sizes were almost lognormally distributed with mode diameters between 0.3 µm and 0.5 µm and geometric standard deviations between 1.6 and 1.9. Deposition ice nucleation was most efficient on ATD particles with ice-active particle fractions of about 0.6 and 0.8 at an ice saturation ratio Si<1.15 and temperatures of 223 K and 209 K, respectively. No significant change of the ice nucleation efficiency was found in up to three subsequent cycles of ice activation and evaporation with the same ATD aerosol. The desert dust samples SD2 and AD1 showed a significantly lower fraction of active deposition nuclei, about 0.25 at 223 K and Si<1.35. For all samples the ice activated aerosol fraction could be approximated by an exponential equation as function of Si. This formulation of ice activation spectra may be used to calculate the formation rate of ice crystals in models, if the number concentration of dust particles is known. More experimental work is needed to quantify the variability of the ice activation spectra as function of the temperature and dust particle properties.