Single particle analysis of ice crystal residuals observed in orographic wave clouds over Scandinavia during INTACC experiment A. C. Targino1,*, R. Krejci1, K. J. Noone2, and P. Glantz1,** 1Department of Meteorology, Stockholm University, 106 91, Stockholm, Sweden 2IGBP Secretariat Royal Swedish Academy of Sciences, 104 05, Stockholm, Sweden *now at: Department of Chemistry, University of York, YO10 5DD, York, UK **now at: Department of Applied Environmental Science, Stockholm University, 106 91, Stockholm, Sweden
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 μm 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°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.
Citation: Targino, A. C., Krejci, R., Noone, K. J., and Glantz, P.: Single particle analysis of ice crystal residuals observed in orographic wave clouds over Scandinavia during INTACC experiment, Atmos. Chem. Phys., 6, 1977-1990, doi:10.5194/acp-6-1977-2006, 2006.