Influence of Particle Aspect Ratio on the Midinfrared Extinction Spectra of Wavelength-Sized Ice Crystals

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• 作者：Robert Wagner ; Stefan Benz ; Ottmar M&ouml ; hler ; Harald Saathoff ; Martin Schnaiter ; Thomas Leisner
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：December 20, 2007
• 年：2007
• 卷：111
• 期：50
• 页码：13003 - 13022
• 全文大小：2553K
• 年卷期：v.111,no.50(December 20, 2007)
• ISSN：1520-5215

文摘

We have used the T-matrix method and the discrete dipole approximation to compute the midinfrared extinctioncross-sections (4500-800 cm^-1) of randomly oriented circular ice cylinders for aspect ratios extending up to10 for oblate and down to ¹/₆ for prolate particle shapes. Equal-volume sphere diameters ranged from 0.1 to10 m for both particle classes. A high degree of particle asphericity provokes a strong distortion of thespectral habitus compared to the extinction spectrum of compactly shaped ice crystals with an aspect ratioaround 1. The magnitude and the sign (increase or diminution) of the shape-related changes in both theabsorption and the scattering cross-sections crucially depend on the particle size and the values for the realand imaginary part of the complex refractive index. When increasing the particle asphericity for a givenequal-volume sphere diameter, the values for the overall extinction cross-sections may change in oppositedirections for different parts of the spectrum. We have applied our calculations to the analysis of recentexpansion cooling experiments on the formation of cirrus clouds, performed in the large coolable aerosol andcloud chamber AIDA of Forschungszentrum Karlsruhe at a temperature of 210 K. Depending on the natureof the seed particles and the temperature and relative humidity characteristics during the expansion, ice crystalsof various shapes and aspect ratios could be produced. For a particular expansion experiment, using Illitemineral dust particles coated with a layer of secondary organic matter as seed aerosol, we have clearly detectedthe spectral signatures characteristic of strongly aspherical ice crystal habits in the recorded infrared extinctionspectra. We demonstrate that the number size distributions and total number concentrations of the ice particlesthat were generated in this expansion run can only be accurately derived from the recorded infrared spectrawhen employing aspect ratios as high as 10 in the retrieval approach. Remarkably, the measured spectracould also be accurately fitted when employing an aspect ratio of 1 in the retrieval. The so-deduced iceparticle number concentrations, however, exceeded the true values, determined with an optical particle counter,by more than 1 order of magnitude. Thus, the shape-induced spectral changes between the extinction spectraof platelike ice crystals of aspect ratio 10 and compactly shaped particles of aspect ratio 1 can be efficientlybalanced by deforming the true number size distribution of the ice cloud. As a result of this severe size/shapeambiguity in the spectral analysis, we consider it indispensable to cross-check the infrared retrieval results ofwavelength-sized ice particles with independent reference measurements of either the number size distributionor the particle morphology.