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

[摘要] Individual ice crystal residual particles collected over Scandinavia duringthe INTACC (INTeraction of Aerosol and Cold Clouds) experiment in October1999 were analyzed by Scanning Electron Microscopy (SEM) equipped withEnergy-Dispersive X-ray Analysis (EDX). Samples were collected onboard theBritish Met Office Hercules C-130 aircraft using a Counterflow VirtualImpactor (CVI). This study is based on six samples collected in orographicclouds. The main aim of this study is to characterize cloud residualelemental composition in conditions affected by different airmasses. Intotal 609 particles larger than 0.1 μm diameter were analyzed and theirelemental composition and morphology were determined. Thereafter ahierarchical cluster analysis was performed on the signal detected withSEM-EDX in order to identify the major particle classes and their abundance.A cluster containing mineral dust, represented by aluminosilicates, Fe-richand Si-rich particles, was the dominating class of particles, accounting forabout 57.5% of the particles analyzed, followed by low-Z particles,23.3% (presumably organic material) and sea salt (6.7%). Sulfur wasdetected often across all groups, indicating ageing and in-cloud processingof particles. A detailed inspection of samples individually unveiled arelationship between ice crystal residual composition and airmass origin.Cloud residual samples from clean airmasses (that is, trajectories confinedto the Atlantic and Arctic Oceans and/or with source altitude in the freetroposphere) were dominated primarily by low-Z and sea salt particles, whilecontinentally-influenced airmasses (with trajectories that originated ortraveled over continental areas and with source altitude in the continentalboundary layer) contained mainly mineral dust residuals. Comparison ofresidual composition for similar cloud ambient temperatures around –27°C revealed that supercooledclouds are more likely to persist in conditions where low-Z particles represent significant part of the analyzed cloudresidual particles. This indicates that organic material may be poor icenuclei, in contrast to polluted cases when ice crystal formation wasobserved at the same environmental conditions and when the cloud residualcomposition was dominated by mineral dust.The presented results suggest that the chemical composition of cloud nucleiand airmass origin have a strong impact on the ice formation throughheterogeneous nucleation in supercooled clouds.

[学科分类] 大气科学