Journal article

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

Targino A, Krejci R, Noone K, Glantz P ...see all

Atmospheric Chemistry and Physics, vol. 6, issue 7 (2006) pp. 1977-1990

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Abstract

Individual ice crystal residual particles collected over Scandinavia
during the INTACC (INTeraction of Aerosol and Cold Clouds) experiment in
October 1999 were analyzed by Scanning Electron Microscopy (SEM)
equipped with Energy-Dispersive X-ray Analysis (EDX). Samples were
collected onboard the British Met Office Hercules C-130 aircraft using a
Counterflow Virtual Impactor (CVI). This study is based on six samples
collected in orographic clouds. The main aim of this study is to
characterize cloud residual elemental composition in conditions affected
by different airmasses. In total 609 particles larger than 0.1 mu m
diameter were analyzed and their elemental composition and morphology
were determined. Thereafter a hierarchical cluster analysis was
performed on the signal detected with SEM-EDX in order to identify the
major particle classes and their abundance. A cluster containing mineral
dust, represented by aluminosilicates, Fe-rich and Si-rich particles,
was the dominating class of particles, accounting for about 57.5% of
the particles analyzed, followed by low-Z particles, 23.3% (presumably
organic material) and sea salt (6.7%). Sulfur was detected often across
all groups, indicating ageing and in-cloud processing of particles. A
detailed inspection of samples individually unveiled a relationship
between ice crystal residual composition and airmass origin. Cloud
residual samples from clean airmasses (that is, trajectories confined to
the Atlantic and Arctic Oceans and/or with source altitude in the free
troposphere) were dominated primarily by low-Z and sea salt particles,
while continentally-influenced airmasses (with trajectories that
originated or traveled over continental areas and with source altitude
in the continental boundary layer) contained mainly mineral dust
residuals. Comparison of residual composition for similar cloud ambient
temperatures around -27 degrees C revealed that supercooled clouds are
more likely to persist in conditions where low-Z particles represent
significant part of the analyzed cloud residual particles. This
indicates that organic material may be poor ice nuclei, in contrast to
polluted cases when ice crystal formation was observed at the same
environmental conditions and when the cloud residual composition was
dominated by mineral dust. The presented results suggest that the
chemical composition of cloud nuclei and airmass origin have a strong
impact on the ice formation through heterogeneous nucleation in
supercooled clouds.

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Authors

  • A. C. Targino

  • R. Krejci

  • K. J. Noone

  • P. Glantz

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