T-matrix modeling of linear depolarization by morphologically complex soot and soot-containing aerosols

• Published in: Journal of quantitative spectroscopy & radiative transfer 2013-07, Vol.123, p.135-144
• Main Authors: Mishchenko, Michael I., Liu, Li, Mackowski, Daniel W.
• Format: Article
• Language: English
• Subjects: Atmospheric radiation; Linear depolarization; Optical cross sections; Remote sensing; Soot aggregates
• ISSN: 0022-4073; 1879-1352
• DOI: 10.1016/j.jqsrt.2012.11.012

We use state-of-the-art public-domain Fortran codes based on the T-matrix method to calculate orientation and ensemble averaged scattering matrix elements for a variety of morphologically complex black carbon (BC) and BC-containing aerosol particles, with a special emphasis on the linear depolarization ratio (LDR). We explain theoretically the quasi-Rayleigh LDR peak at side-scattering angles typical of low-density soot fractals and conclude that the measurement of this feature enables one to evaluate the compactness state of BC clusters and trace the evolution of low-density fluffy fractals into densely packed aggregates. We show that small backscattering LDRs measured with ground-based, airborne, and spaceborne lidars for fresh smoke generally agree with the values predicted theoretically for fluffy BC fractals and densely packed near-spheroidal BC aggregates. To reproduce higher lidar LDRs observed for aged smoke, one needs alternative particle models such as shape mixtures of BC spheroids or cylinders. ► New superposition T-matrix code is applied to soot aerosols. ► Quasi-Rayleigh side-scattering peak in linear depolarization (LD) is explained. ► LD measurements can be used for morphological characterization of soot aerosols.