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  • 1.
    Kahnert, Michael
    SMHI, Research Department, Air quality.
    Optical properties of black carbon aerosols encapsulated in a shell of sulfate: comparison of the closed cell model with a coated aggregate model2017In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 25, no 20, p. 24579-24593Article in journal (Refereed)
  • 2.
    Kahnert, Michael
    et al.
    SMHI, Research Department, Air quality.
    Nousiainen, Timo
    Lindqvist, Hannakaisa
    Models for integrated and differential scattering optical properties of encapsulated light absorbing carbon aggregates2013In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 21, no 7, p. 7974-7993Article in journal (Refereed)
    Abstract [en]

    Optical properties of light absorbing carbon (LAC) aggregates encapsulated in a shell of sulfate are computed for realistic model geometries based on field measurements. Computations are performed for wavelengths from the UV-C to the mid-IR. Both climate- and remote sensing-relevant optical properties are considered. The results are compared to commonly used simplified model geometries, none of which gives a realistic representation of the distribution of the LAC mass within the host material and, as a consequence, fail to predict the optical properties accurately. A new core-gray shell model is introduced, which accurately reproduces the size- and wavelength dependence of the integrated and differential optical properties. (C) 2013 Optical Society of America

  • 3.
    Kahnert, Michael
    et al.
    SMHI, Research Department, Air quality.
    Nousiainen, Timo
    Lindqvist, Hannakaisa
    Ebert, Martin
    Optical properties of light absorbing carbon aggregates mixed with sulfate: assessment of different model geometries for climate forcing calculations2012In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 20, no 9Article in journal (Refereed)
    Abstract [en]

    Light scattering by light absorbing carbon (LAC) aggregates encapsulated into sulfate shells is computed by use of the discrete dipole method. Computations are performed for a UV, visible, and IR wavelength, different particle sizes, and volume fractions. Reference computations are compared to three classes of simplified model particles that have been proposed for climate modeling purposes. Neither model matches the reference results sufficiently well. Remarkably, more realistic core-shell geometries fall behind homogeneous mixture models. An extended model based on a core-shell-shell geometry is proposed and tested. Good agreement is found for total optical cross sections and the asymmetry parameter. (C) 2012 Optical Society of America

  • 4.
    Kahnert, Michael
    et al.
    SMHI, Research Department, Air quality.
    Rother, Tom
    Modeling optical properties of particles with small-scale surface roughness: combination of group theory with a perturbation approach2011In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 19, no 12, p. 11138-11151Article in journal (Refereed)
    Abstract [en]

    A T-matrix method for scattering by particles with small-scale surface roughness is presented. The method combines group theory with a perturbation expansion approach. Group theory is found to reduce CPU-time by 4-6 orders of magnitude. The perturbation expansion extends the range of size parameters by a factor of 5 compared to non-perturbative methods. An application to optically hard particles shows that small-scale surface roughness changes scattering in side-and backscattering directions, and it impacts the single-scattering albedo. This can have important implications for interpreting remote sensing observations, and for the climate impact of mineral aerosols. (C) 2011 Optical Society of America

  • 5. Merikallio, S.
    et al.
    Nousiainen, T.
    Kahnert, Michael
    SMHI, Research Department, Air quality.
    Harri, A. -M
    Light scattering by the Martian dust analog, palagonite, modeled with ellipsoids2013In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 21, no 15, p. 17972-17985Article in journal (Refereed)
    Abstract [en]

    We have investigated the suitability of the ellipsoidal model particles to mimic scattering by Martian dust particles by comparing simulations against laboratory data for palagonite, a Mars analog sample. By optimizing the shape distribution of ellipsoids, a very good match with a laboratory-measured scattering matrix was obtained. Even an equiprobable distribution of ellipsoids performed well. The asymmetry parameter and single-scattering albedo were found to depend on the assumed shape distribution as much as on the typical uncertainties associated with refractive indices and size, suggesting that shape is an important parameter that potentially influences remote retrievals of dust particle properties. (C) 2013 Optical Society of America

  • 6. Schmidt, Karsten
    et al.
    Yurkin, Maxim A.
    Kahnert, Michael
    SMHI, Research Department, Air quality.
    A case study on the reciprocity in light scattering computations2012In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 20, no 21, p. 23253-23274Article in journal (Refereed)
    Abstract [en]

    The fulfillment of the reciprocity by five publicly available scattering programs is investigated for a number of different particles. Reciprocity means that the source and the observation point of a given scattering configuration can be interchanged without changing the result. The programs under consideration are either implementations of T-matrix methods or of the discrete dipole approximation. Similarities and differences concerning their reciprocity behavior are discussed. In particular, it is investigated whether and under which conditions reciprocity tests can be used to evaluate the scattering results obtained by the different programs for the given particles. (c) 2012 Optical Society of America

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