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Single particle analysis of ice crystal residuals observed in orographic wave clouds over Scandinavia during INTACC experiment

by A. C. Targino, R. Krejci, K. J. Noone, P. Glantz
Atmospheric Chemistry and Physics Discussions ()

Abstract

Individual ice crystal residual particles collected over Scandinavia\nduring the INTACC (INTeraction of Aerosol and Cold Clouds) experiment in\nOctober 1999 were analyzed by Scanning Electron Microscopy (SEM)\nequipped with Energy-Dispersive X-ray Analysis (EDX). Samples were\ncollected onboard the British Met Office Hercules C-130 aircraft using a\nCounterflow Virtual Impactor (CVI). This study is based on six samples\ncollected in orographic clouds. The main aim of this study is to\ncharacterize cloud residual elemental composition in conditions affected\nby different airmasses. In total 609 particles larger than 0.1 mu m\ndiameter were analyzed and their elemental composition and morphology\nwere determined. Thereafter a hierarchical cluster analysis was\nperformed on the signal detected with SEM-EDX in order to identify the\nmajor particle classes and their abundance. A cluster containing mineral\ndust, represented by aluminosilicates, Fe-rich and Si-rich particles,\nwas the dominating class of particles, accounting for about 57.5% of\nthe particles analyzed, followed by low-Z particles, 23.3% (presumably\norganic material) and sea salt (6.7%). Sulfur was detected often across\nall groups, indicating ageing and in-cloud processing of particles. A\ndetailed inspection of samples individually unveiled a relationship\nbetween ice crystal residual composition and airmass origin. Cloud\nresidual samples from clean airmasses (that is, trajectories confined to\nthe Atlantic and Arctic Oceans and/or with source altitude in the free\ntroposphere) were dominated primarily by low-Z and sea salt particles,\nwhile continentally-influenced airmasses (with trajectories that\noriginated or traveled over continental areas and with source altitude\nin the continental boundary layer) contained mainly mineral dust\nresiduals. Comparison of residual composition for similar cloud ambient\ntemperatures around -27 degrees C revealed that supercooled clouds are\nmore likely to persist in conditions where low-Z particles represent\nsignificant part of the analyzed cloud residual particles. This\nindicates that organic material may be poor ice nuclei, in contrast to\npolluted cases when ice crystal formation was observed at the same\nenvironmental conditions and when the cloud residual composition was\ndominated by mineral dust. The presented results suggest that the\nchemical composition of cloud nuclei and airmass origin have a strong\nimpact on the ice formation through heterogeneous nucleation in\nsupercooled clouds.

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