Abstract
Abstract. During an intensive campaign at the high alpine research station Jungfraujoch, Switzerland, in February/March 2006 ice particle residuals within mixed-phase clouds were sampled using the Ice-counterflow virtual impactor (Ice-CVI). Size, morphology, chemical composition, mineralogy and mixing state of the ice residual and the interstitial (i.e., non-activated) aerosol particles were analyzed by scanning and transmission electron microscopy. Ice nuclei (IN) were identified from the significant enrichment of particle groups in the ice residual (IR) samples relative to the interstitial aerosol. In terms of number lead-bearing particles are enriched by a factor of approximately 25, complex internal mixtures with silicates or metal oxides as major components by a factor of 11, and mixtures of secondary aerosol and carbonaceous material (C-O-S particles) by a factor of 2. Other particle groups (sulfates, sea salt, Ca-rich particles, external silicates) observed in the ice-residual samples cannot be assigned unambiguously as IN. Between 9 and 24% of all IR are Pb-bearing particles. Pb was found as major component in around 10% of these particles (PbO, PbCl2). In the other particles, Pb was found as some 100 nm sized agglomerates consisting of 3–8 nm sized primary particles (PbS, elemental Pb). C-O-S particles are present in the IR at an abundance of 17–27%. The soot component within these particles is strongly aged. Complex internal mixtures occur in the IR at an abundance of 9–15%. Most IN identified at the Jungfraujoch station are internal mixtures containing anthropogenic components (either as main or minor constituent), and it is concluded that admixture of the anthropogenic component is responsible for the increased IN efficiency within mixed phase clouds. The mixing state appears to be a key parameter for the ice nucleation behaviour that cannot be predicted from the sole knowledge of the main component of an individual particle.
Highlights
The properties and formation processes of mixed phase clouds are poorly understood (Lohmann and Feichter, 2005)
Ice nuclei (IN) are needed which initiate the nucleation of ice at temperatures or vapour pressures closer to the equilibrium conditions than that of homogeneous nucleation
Reliable IN identification can be reached by determination of the enrichment of a given particle group in ice residual (IR) samples relative to the interstitial aerosol
Summary
The properties and formation processes of mixed phase clouds are poorly understood (Lohmann and Feichter, 2005). Increased ice nuclei concentrations appear to decrease cloud lifetime and cloud cover leading to a warming of the atmosphere (Lohmann and Diehl, 2006). The theoretical treatment of ice nucleation is complicated by the fact that this process takes place under non-equilibrium conditions. Ice formation can be initiated either by homogeneous or heterogeneous nucleation. Under ambient atmospheric conditions homogeneous nucleation takes place only in high clouds at temperatures below −38 ◦C. In lower mixed phase clouds, heterogeneous nucleation dominates. In this process, ice nuclei (IN) are needed which initiate the nucleation of ice at temperatures or vapour pressures closer to the equilibrium conditions than that of homogeneous nucleation. For a special IN (e.g. kaolinite), each mode is characterized by a specific threshold temperature and supersaturation value frequently plotted as temperature/supersaturation curve (e.g., Schaller and Fukuta, 1979; Zimmermann et al, 2007, 2008)
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