Rigorous inversion of absorption coefficient from spectral properties of particulate material

2021 ◽  
Author(s):  
Antti Penttilä ◽  
Timo Väisänen ◽  
Julia Martikainen ◽  
Cristian Carli ◽  
Fabrizio Capaccioni ◽  
...  
Keyword(s):  
Particle Size ◽  
Refractive Index ◽  
Radiative Transfer ◽  
Imaginary Part ◽  
Random Media ◽  
Near Infrared ◽  
Complex Refractive Index ◽  
Geometric Optics ◽  
Size Fractions ◽  
Spectral Behavior

<p>The optical constant of the material, meaning the complex refractive index <em>m</em>=<em>n</em>+<em>ik</em>, is an essential parameter when considering the reflection and absorption properties of that material. The refractive index is a function of wavelength of the light, and usually the imaginary part <em>k</em> is what governs the reflection or transmission spectral behavior of the material.</p> <p>The knowledge of the complex refractive index as a function of wavelength, <em>m</em>(<em>λ</em>), is needed for light scattering simulations. On the other hand, rigorous scattering simulations can be used to invert the refractive index from measured or observed reflection spectra. We will show how the combination of geometric optics and radiative transfer codes can be used in this task.</p> <p>In this work, the possible application is with the future visual-near infrared observations of Mercury by the ESA BepiColombo mission. That application in mind, we have used four particulate igneous glassy materials with varying overall albedo and in several size fractions in reflectance spectra measurements (hawaiitic basalt, two gabbronorites, anorthosite, see details from Carli et al, Icarus 266, 2016). The grounded material consist of particle with clear edges and quite flat facets, and we choose to model the particle shapes by geometries resulting from Voronoi division of random seed points in 3D space.</p> <p>The refractive index inversion is done here using first a geometric optics code SIRIS (Muinonen et al, JQSRT 110, 2009) to simulate the average Mueller matrix, albedo, and scattering efficiency for a single Voronoi particle. Then, these properties are fed into radiative transfer code RT-CB (Muinonen, Waves in Random Media 14, 2004) to produce the reflective properties of a semi-infinite slab of these particles. This procedure is repeated for a 2D grid of particle size parameters <em>x</em>=2π<em>r</em>/<em>λ</em>, where <em>r</em> is the radius of particle, and imaginary part <em>k </em>of refractive index. In Vis-NIR wavelengths, the real part <em>n</em> is quite constant and is estimated to be about 1.58 for all the four glasses. From the simulated slab reflectance data with the 2D <em>x</em>, <em>k</em> parameter grid, we can first interpolate, and then invert the <em>k</em> parameter for any reflectance value with given wavelength and particle size.</p> <p>The resulting spectral behavior of <em>k </em>for the four glasses and for all the size fractions was seems very realistic. Carli et al. (Icarus 266, 2016) inverted the <em>k </em>spectral behavior for these same samples using Hapke modeling, and the results are quite similar. Furthermore, we have measured the transmission of the material using polished slabs of varying thinkness, and will compare the results that can be dervied from these transmission results to those from relfectance measurements.</p>

scholarly journals A method to retrieve the spectral complex refractive index and single scattering optical properties of dust deposited in mountain snow

2016 ◽  
Vol 63 (237) ◽  
pp. 133-147 ◽  
Author(s):  
S. McKENZIE SKILES ◽  
THOMAS PAINTER ◽  
GREGORY S. OKIN
Keyword(s):  
Refractive Index ◽  
Optical Properties ◽  
Radiative Transfer ◽  
Near Infrared ◽  
Direct Reduction ◽  
Complex Refractive Index ◽  
Single Scattering ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
Dust Deposition

ABSTRACTDust deposition to snow can have regionally important climatic and hydrologic impacts resulting from direct reduction of surface albedo and indirectly from the initiation of snow albedo feedbacks. Modeling the radiative impacts of dust deposited in snow requires knowledge of the optical properties of both components. Here we present an inversion technique to retrieve the effective optical properties of dust deposited in mountain snow cover from measurements of hemispherical dust reflectance and particle size distributions using radiative transfer modeling. First, modeled reflectance is produced from single scattering properties modeled with Mie theory for a specified grain size distribution over a range of values for the imaginary part of the complex refractive index (k = 0.00001–0.1). Then, a multi-step look-up table process is employed to retrieve kλ and single scattering optical properties by matching measured to modeled reflectance across the shortwave and near infrared. The real part of the complex refractive index, n, for dust aerosols ranges between 1.5 and 1.6 and a sensitivity analysis shows the method is relatively insensitive to the choice of n within this range, 1.525 was used here. Using the values retrieved by this method to update dust optical properties in a snow + aerosol radiative transfer model reduces errors in springtime albedo modeling by 50–70%.

Modelling the expected observations of the Advanced Ice Giants Net Flux Radiometer (IG-NFR) instrument concept, under study for future entry probe missions to Uranus or Neptune.

2020 ◽  
Author(s):  
Patrick Irwin ◽  
Simon Calcutt ◽  
Jack Dobinson ◽  
Juan Alday ◽  
Arjuna James ◽  
...  
Keyword(s):  
Radiative Transfer ◽  
Cross Sections ◽  
Integrated Circuit ◽  
Near Infrared ◽  
Complex Refractive Index ◽  
Radiation Energy ◽  
Discrete Ordinates ◽  
Detector Response ◽  
Net Flux

<p>The NASA Ice Giants Pre-Decadal Survey Mission Report (2017) recommended the high scientific importance of sending a mission with an orbiter and a probe to one of the Ice Giants, with preferential launch dates in the 2029-2034 timeframe. Such a mission concept is equally well supported by European scientists and Mousis et al (P&SS, 155, 12, 2018) give compelling scientific rationales for the exploration of these worlds with missions carrying in situ probes.</p> <p>In this presentation we will outline the conceptual design of the Advanced Ice Giants Net Flux Radiometer (IG-NFR) instrument, currently being designed by NASA Goddard Space Flight Center to make in situ observations of the upward and downward fluxes of solar and thermal radiation in the atmospheres of Uranus and Neptune. The IG-NFR is designed to: (i) accommodate seven filter bandpass channels in the spectral range 0.25-300 µm (ii) measure up and down radiation flux in a clear unobstructed 10° FOV for each channel; (iii) use thermopile detectors that can measure a change of flux of at least 0.5 W/m<sup>2</sup> per decade of pressure; (iv) view five distinct view angles (±80°, ±45°, and 0°); (v) predict the detector response with changing  temperature environment; (vi) use application-specific integrated circuit technology for the thermopile detector readout; (vii) be able to integrate radiance for 2s or longer, and (vii) sample each view angle including calibration targets. The IG-NFR system noise equivalent power at 298 K is 73 pW in a 1 Hz electrical bandwidth.</p> <p>We present initial simulations of the anticipated observations using two radiative transfer and retrieval tools, NEMESIS (Irwin et al., JQSRT, 109, 1136, 2008) and the Planetary Spectrum Generator (PSG, Villanueva et al., 2017, https://psg.gsfc.nasa.gov). For the NEMESIS modelling the radiative fluxes observable at varying pressure levels were calculated with a Matrix-Operator plane-parallel multiple-scattering model, using between 5 and 21 zenith angle quadrature points and up to 38 Fourier components for the azimuth decomposition. We also employed PSG to further validate our flux estimates, providing an important benchmarking and comparison test between both models. PSG solves the scattering radiative transfer employing the discrete ordinates method, with the scattering phase function described in terms of an expansion in terms of Legendre Polynomials. Molecular cross-sections are solved via the correlated-k method employing the latest HITRAN database (Gordon et al., 2017), which are completed with the latest collision-induced-absorption (CIA, Karman et al., 2019), and UV/optical cross-sections from the MPI database (Keller-Rudek et al., 2013). For the nominal case the Sun was assumed to be at an altitude of 10° above the horizon. The internal radiance field was calculated at each internal level for a standard reference Uranus atmosphere (e.g., Irwin et al., 2017) with the addition of a single cloud layer, based at 3 bar and composed of particles with a mean radius of 1.0 µm (and size variance 0.1) and assumed complex refractive index of 1.4 + 0.001i at all wavelengths. The opacity and fractional scale height of this cloud were fitted in both models to match the combined near-infrared observations of HST/WFC3, IRTF/SpeX and VLT/SINFONI analyzed by Irwin et al. (2017). The internal radiance fields were calculated from 0.4 to 300 µm using this atmospheric model.</p> <p>We will show how these simulations are being used to guide the choice of spectral filter bandwidths and centres to optimize the scientific return of such an instrument. We will show that observations with such an instrument can be used to constrain effectively the radiation energy budget in the atmospheres of the Ice Giants and can also be used to determine the pressures of cloud and haze layers and broadly constrain particle size. Such modelling also allows us to simulate the visible appearance of Uranus’ atmosphere during a descent and to perform detailed validations of the simulations by comparing the two radiative transfer models (NEMESIS and PSG).</p>

scholarly journals Simultaneous retrieval of the complex refractive index and particle size distribution

2015 ◽  
Vol 23 (15) ◽  
pp. 19328 ◽  
Author(s):  
Yatao Ren ◽  
Hong Qi ◽  
Qin Chen ◽  
Liming Ruan ◽  
Heping Tan
Keyword(s):  
Particle Size ◽  
Refractive Index ◽  
Particle Size Distribution ◽  
Size Distribution ◽  
Complex Refractive Index

scholarly journals Variability in lidar-derived particle properties over West Africa due to changes in absorption: towards an understanding

2020 ◽  
Vol 20 (11) ◽  
pp. 6563-6581 ◽  
Author(s):  
Igor Veselovskii ◽  
Qiaoyun Hu ◽  
Philippe Goloub ◽  
Thierry Podvin ◽  
Mikhail Korenskiy ◽  
...  
Keyword(s):  
Refractive Index ◽  
Imaginary Part ◽  
Complex Refractive Index ◽  
Second Phase ◽  
Height Range ◽  
Good Opportunity ◽  
Western Africa ◽  
Particle Parameters ◽  
Lidar Ratio ◽  
532 Nm

Abstract. Measurements performed in western Africa (Senegal) during the SHADOW field campaign are analyzed to show that spectral dependence of the imaginary part of the complex refractive index (CRI) of dust can be revealed by lidar-measured particle parameters. Observations in April 2015 provide good opportunity for such study, because, due to high optical depth of the dust, exceeding 0.5, the extinction coefficient could be derived from lidar measurements with high accuracy and the contribution of other aerosol types, such as biomass burning, was negligible. For instance, in the second half of April 2015, AERONET observations demonstrated a temporal decrease in the imaginary part of the CRI at 440 nm from approximately 0.0045 to 0.0025. This decrease is in line with a change in the relationship between the lidar ratios (the extinction-to-backscattering ratio) at 355 and 532 nm (S355 and S532). For instance in the first half of April, S355∕S532 is as high as 1.5 and the backscatter Ångström exponent, Aβ, is as low as −0.75, while after 15 April S355/S532=1.0 and Aβ is close to zero. The aerosol depolarization ratio δ532 for the whole of April exceeded 30 % in the height range considered, implying that no other aerosol, except dust, occurred. The performed modeling confirmed that the observed S355∕S532 and Aβ values match the spectrally dependent imaginary part of the refractive index as can be expected for mineral dust containing iron oxides. The second phase of the SHADOW campaign was focused on evaluation of the lidar ratio of smoke and estimates of its dependence on relative humidity (RH). For five studied smoke episodes the lidar ratio increases from 44±5 to 66±7 sr at 532 nm and from 62±6 to 80±8 sr at 355 nm, when RH varied from 25 % to 85 %. Performed numerical simulations demonstrate that observed ratio S355∕S532, exceeding 1.0 in the smoke plumes, can indicate an increase in the imaginary part of the smoke particles in the ultraviolet (UV) range.

scholarly journals An Empirical Method for the Determination of the Complex Refractive Index of Size-Fractionated Atmospheric Aerosols for Radiative Transfer Calculations

2001 ◽  
Vol 34 (6) ◽  
pp. 535-549 ◽  
Author(s):  
Nancy A. Marley ◽  
Jeffrey S. Gaffney ◽  
J. Christopher Baird ◽  
Cherelle A. Blazer ◽  
Paul J. Drayton ◽  
...  
Keyword(s):  
Refractive Index ◽  
Radiative Transfer ◽  
Atmospheric Aerosols ◽  
Complex Refractive Index ◽  
Empirical Method
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