Scattering properties of needlelike and platelike ice spheroids with moderate size parameters

Zakharova, Nadia T.; Mishchenko, Michael I.

doi:10.1364/ao.39.005052

Cited by 55 publications

(25 citation statements)

References 17 publications

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“…This technique might therefore not be sensitive enough to clearly evidence the absence of crystallisation. Furthermore, the magnitude of the depolarisation level might also be low for particular habits of the crystallised particles (Zakharova and Mishchenko, 2000). A series of other more substantial arguments also support our conclusion that the solute crystallisation mechanism almost certainly does not account for the experimental observations.…”

Section: Evidence Against Ice Catalysed Solute Crystallisationsupporting

confidence: 73%

“…The depolarisation ratio, δ, is then defined as δ = I back,per /I back,par . Apart from very small particles or particles with extreme aspect ratios, δ is different from zero for light scattering by aspherical particles (Mishchenko et al, 1996;Zakharova and Mishchenko, 2000). The formation of aspherical ice crystals by homogeneous freezing of liquid aqueous organic aerosol particles or by heterogeneous nucleation on glassy aqueous particles is therefore evidenced by an increase of the depolarisation ratio above the background level of about 0.02-0.03 that is observed in the presence of spherical seed aerosol particles.…”

Section: Ice Nucleation Studies In the Aida Expansion Cloud Chambermentioning

confidence: 99%

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Ice cloud processing of ultra-viscous/glassy aerosol particles leads to enhanced ice nucleation ability

et al. 2012

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Abstract. The ice nucleation potential of airborne glassy aqueous aerosol particles has been investigated by controlled expansion cooling cycles in the AIDA aerosol and cloud chamber of the Karlsruhe Institute of Technology at temperatures between 247 and 216 K. Four different solutes were used as proxies for oxygenated organic matter found in the atmosphere: raffinose, 4-hydroxy-3-methoxy-DL-mandelic acid (HMMA), levoglucosan, and a multi-component mixture of raffinose with five dicarboxylic acids and ammonium sulphate. Similar to previous experiments with citric acid aerosols, all particles were found to nucleate ice heterogeneously before reaching the homogeneous freezing threshold provided that the freezing cycles were started well below the respective glass transition temperatures of the compounds; this is discussed in detail in a separate article. In this contribution, we identify a further mechanism by which glassy aerosols can promote ice nucleation below the homogeneous freezing limit. If the glassy aerosol particles are probed in freezing cycles started only a few degrees below their respective glass transition temperatures, they enter the liquid regime of the state diagram upon increasing relative humidity (moisture-induced glass-to-liquid transition) before being able to act as heterogeneous ice nuclei. Ice formation then only occurs by homogeneous freezing at elevated supersaturation levels. When ice forms the remaining solution freeze concentrates and re-vitrifies. If these ice cloud processed glassy aerosol particles are then probed in a second freezing cycle at the same temperature, they catalyse ice formation at a supersaturation threshold between 5 and 30 % with respect to ice. By analogy with the enhanced ice nucleation ability of insoluble ice nuclei like mineral dusts after they nucleate ice once, we refer to this phenomenon as pre-activation. We propose a number of possible explanations for why glassy aerosol particles that have re-vitrified in contact with the ice crystals during the preceding homogeneous freezing cycle exhibit pre-activation: they may retain small ice embryos in pores, have footprints on their surface which match the ice lattice, or simply have a much greater surface area or different surface microstructure compared to the unprocessed glassy aerosol particles. Pre-activation must be considered for the correct interpretation of experimental results on the heterogeneous ice nucleation ability of glassy aerosol particles and may provide a mechanism of producing a population of extremely efficient ice nuclei in the upper troposphere.

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Section: Evidence Against Ice Catalysed Solute Crystallisationsupporting

confidence: 73%

Section: Ice Nucleation Studies In the Aida Expansion Cloud Chambermentioning

confidence: 99%

Ice cloud processing of ultra-viscous/glassy aerosol particles leads to enhanced ice nucleation ability

et al. 2012

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“…Larger deviations from the results for spheres in this size regime are only expected for particles with extreme aspect ratios, e.g. needle-and plate-like particles (Zakharova and Mishchenko, 2000). Above 0.7 µm, the results for the extinction cross sections of the various spheroidal particle shapes start to diverge because we enter the size range where extinction is governed by the interference structure, i.e.…”

Section: A2 Shape Dependency Of the Computed Extinction And Absorptiomentioning

confidence: 74%

Complex refractive indices of Saharan dust samples at visible and near UV wavelengths: a laboratory study

et al. 2012

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Abstract.We have retrieved the wavelength-dependent imaginary parts of the complex refractive index for five different Saharan dust aerosol particles of variable mineralogical composition at wavelengths between 305 and 955 nm. The dust particles were generated by dispersing soil samples into a laboratory aerosol chamber, typically yielding particle sizes with mean diameters ranging from 0.3 to 0.4 µm and maximum diameters from 2 to 4 µm. The extinction and absorption coefficients as well as the number size distribution of the dust particles were simultaneously measured by various established techniques. An inversion scheme based on a spheroidal dust model was employed to deduce the refractive indices. The retrieved imaginary parts of the complex refractive index were in the range from 0.003 to 0.005, 0.005 to 0.011, and 0.016 to 0.050 at the wavelengths 955, 505, and 305 nm. The hematite content of the dust particles was determined by electron-microscopical single particle analysis. Hematite volume fractions in the range from 1.1 to 2.7 % were found for the different dusts, a range typical for atmospheric mineral dust. We have performed a sensitivity study to assess how accurately the retrieved imaginary refractive indices could be reproduced by calculations with mixing rule approximations using the experimentally determined hematite contents as input.

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“…Notable exceptions are aspherical particles with small size parameters and/or extreme aspect ratios. These particle types may reveal only a very weak backscattering linear depolarisation ratio (Zakharova and Mishchenko, 2000).…”

Section: Methodsmentioning

confidence: 99%

High variability of the heterogeneous ice nucleation potential of oxalic acid dihydrate and sodium oxalate

et al. 2010

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Abstract. The heterogeneous ice nucleation potential of airborne oxalic acid dihydrate and sodium oxalate particles in the deposition and condensation mode has been investigated by controlled expansion cooling cycles in the AIDA aerosol and cloud chamber of the Karlsruhe Institute of Technology at temperatures between 244 and 228 K. Previous laboratory studies have highlighted the particular role of oxalic acid dihydrate as the only species amongst a variety of other investigated dicarboxylic acids to be capable of acting as a heterogeneous ice nucleus in both the deposition and immersion mode. We could confirm a high deposition mode ice activity for 0.03 to 0.8 µm sized oxalic acid dihydrate particles that were either formed by nucleation from a gaseous oxalic acid/air mixture or by rapid crystallisation of highly supersaturated aqueous oxalic acid solution droplets. The critical saturation ratio with respect to ice required for deposition nucleation was found to be less than 1.1 and the size-dependent ice-active fraction of the aerosol population was in the range from 0.1 to 22%. In contrast, oxalic acid dihydrate particles that had crystallised from less supersaturated solution droplets and had been allowed to slowly grow in a supersaturated environment from still unfrozen oxalic acid solution droplets over a time period of several hours were found to be much poorer heterogeneous ice nuclei. We speculate that under these conditions a crystal surface structure with less-active sites for the initiation of ice nucleation was generated. Such particles partially proved to be almost ice-inactive in both the deposition and condensation mode. At times, the heterogeneous ice nucleation ability of oxalic acid dihydrate significantly changed when the particles had been processed in preceding cloud droplet activation steps. Such behaviour was also observed for the second investiCorrespondence to: R. Wagner (robert.wagner2@kit.edu) gated species, namely sodium oxalate. Our experiments address the atmospheric scenario that coating layers of oxalic acid or its salts may be formed by physical and chemical processing on pre-existing particulates such as mineral dust and soot. Given the broad diversity of the observed heterogeneous ice nucleability of the oxalate species, it is not straightforward to predict whether an oxalate coating layer will improve or reduce the ice nucleation ability of the seed aerosol particles.

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Scattering properties of needlelike and platelike ice spheroids with moderate size parameters

Cited by 55 publications

References 17 publications

Ice cloud processing of ultra-viscous/glassy aerosol particles leads to enhanced ice nucleation ability

Ice cloud processing of ultra-viscous/glassy aerosol particles leads to enhanced ice nucleation ability

Complex refractive indices of Saharan dust samples at visible and near UV wavelengths: a laboratory study

High variability of the heterogeneous ice nucleation potential of oxalic acid dihydrate and sodium oxalate

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