Effects of multiple scattering on radiative properties of soot fractal aggregates

2014 ◽  
Vol 133 ◽  
pp. 374-381 ◽  
Author(s):  
Jérôme Yon ◽  
Fengshan Liu ◽  
Alexandre Bescond ◽  
Chloé Caumont-Prim ◽  
Claude Rozé ◽  
...  
Keyword(s):  
Multiple Scattering ◽  
Radiative Properties ◽  
Fractal Aggregates

Optical Diagnostics and Radiative Properties of Simulated Soot Agglomerates

1991 ◽  
Vol 113 (4) ◽  
pp. 953-958 ◽  
Author(s):  
J. C. Ku ◽  
K.-H. Shim
Keyword(s):  
Optical Diagnostics ◽  
Forward Scattering ◽  
Radiative Properties ◽  
Weak Effect ◽  
Radiative Transport ◽  
Fractal Aggregates ◽  
Scattering Coefficients ◽  
Primary Particles ◽  
Accurate Relation ◽  
Number Of Particles

The effect of agglomeration on the optical diagnostics and radiative properties of simulated soot agglomerates is investigated, using results from the Jones solution. It is found that agglomeration has a very strong effect on scattering, but only a weak effect on extinction (≅ absorption). An accurate relation has been developed, based on near-forward scattering coefficients, for inferring the number of primary particles in soot agglomerates. General models for both total and differential scattering coefficients have also been established. These results are in general agreement with those predicted for fractal aggregates having a large number of particles. Because of the effect of agglomeration, scattering may not be negligible in treating radiative transport from soot agglomerates.

Independent Evaluation of the Ability of Spaceborne Radar and Lidar to Retrieve the Microphysical and Radiative Properties of Ice Clouds

2006 ◽  
Vol 23 (2) ◽  
pp. 211-227 ◽  
Author(s):  
Robin J. Hogan ◽  
Malcolm E. Brooks ◽  
Anthony J. Illingworth ◽  
David P. Donovan ◽  
Claire Tinel ◽  
...  
Keyword(s):  
Particle Size ◽  
Water Content ◽  
Multiple Scattering ◽  
Optical Depth ◽  
Radiative Properties ◽  
Molecular Scattering ◽  
Retrieval Algorithms ◽  
Ice Water Content ◽  
Ice Water ◽  
Better Than

Abstract The combination of radar and lidar in space offers the unique potential to retrieve vertical profiles of ice water content and particle size globally, and two algorithms developed recently claim to have overcome the principal difficulty with this approach—that of correcting the lidar signal for extinction. In this paper “blind tests” of these algorithms are carried out, using realistic 94-GHz radar and 355-nm lidar backscatter profiles simulated from aircraft-measured size spectra, and including the effects of molecular scattering, multiple scattering, and instrument noise. Radiation calculations are performed on the true and retrieved microphysical profiles to estimate the accuracy with which radiative flux profiles could be inferred remotely. It is found that the visible extinction profile can be retrieved independent of assumptions on the nature of the size distribution, the habit of the particles, the mean extinction-to-backscatter ratio, or errors in instrument calibration. Local errors in retrieved extinction can occur in proportion to local fluctuations in the extinction-to-backscatter ratio, but down to 400 m above the height of the lowest lidar return, optical depth is typically retrieved to better than 0.2. Retrieval uncertainties are greater at the far end of the profile, and errors in total optical depth can exceed 1, which changes the shortwave radiative effect of the cloud by around 20%. Longwave fluxes are much less sensitive to errors in total optical depth, and may generally be calculated to better than 2 W m−2 throughout the profile. It is important for retrieval algorithms to account for the effects of lidar multiple scattering, because if this is neglected, then optical depth is underestimated by approximately 35%, resulting in cloud radiative effects being underestimated by around 30% in the shortwave and 15% in the longwave. Unlike the extinction coefficient, the inferred ice water content and particle size can vary by 30%, depending on the assumed mass–size relationship (a problem common to all remote retrieval algorithms). However, radiative fluxes are almost completely determined by the extinction profile, and if this is correct, then errors in these other parameters have only a small effect in the shortwave (around 6%, compared to that of clear sky) and a negligible effect in the longwave.

Predicting the Optical Properties of Arbitrarily Shaped Black Carbon Aerosols with Graph Neural Networks

2021 ◽  
Author(s):  
Kara D. Lamb ◽  
Pierre Gentine
Keyword(s):  
Remote Sensing ◽  
Optical Properties ◽  
Black Carbon ◽  
Radiative Forcing ◽  
Training Data ◽  
Radiative Properties ◽  
Training Dataset ◽  
Small Scale ◽  
Atmospheric Models ◽  
Fractal Aggregates

<p>Aerosols sourced from combustion such as black carbon (BC) are important short-lived climate forcers whose direct radiative forcing and atmospheric lifetime depend on their morphology. These aerosols are typically fractal aggregates consisting of ~20-80 nm spheres. This complex morphology makes modeling their optical properties difficult, contributing to uncertainty in both their direct and indirect climate effects. Accurate and fast calculations of BC optical properties are needed for remote sensing inversions and for radiative forcing calculations in atmospheric models, but current methods to accurately calculate the optical properties of these aerosols such as the multi-sphere T-matrix method or generalized multiple-particle Mie Theory are computationally expensive and must be compiled in extensive data-bases off-line and then used as a look-up table. Recent advances in machine learning approaches have applied the graph convolutional neural network (GCN) to various physical science applications, demonstrating skill in generalizing beyond initial training data by exploiting and learning internal properties and interactions inherent to the larger system. Here we demonstrate for the first time that a GCN trained to predict the optical properties of numerically-generated BC fractal aggregates can accurately generalize to arbitrarily shaped aerosol particles, even over much larger aggregates than in the training dataset, providing a fast and accurate method to calculate aerosol optical properties in atmospheric models and for observational retrievals. This approach could be integrated into atmospheric models or remote sensing inversions to more realistically predict the physical properties of arbitrarily-shaped aerosol and cloud particles. In addition, GCN’s can be used to gain physical intuition on the relationship between large-scale properties (here of the radiative properties of aerosols) and small-scale interactions (here of the spheres’ positions and their interactions).</p>

Radiative Properties of Flame-Generated Soot

1993 ◽  
Vol 115 (2) ◽  
pp. 409-417 ◽  
Author(s):  
U¨. O¨. Ko¨ylu¨ ◽  
G. M. Faeth
Keyword(s):  
Optical Properties ◽  
Multiple Scattering ◽  
Computer Simulations ◽  
Rayleigh Scattering ◽  
Mie Scattering ◽  
Mean Field Approximation ◽  
Fractal Aggregates ◽  
Approximate Methods ◽  
Soot Aggregate ◽  
Equivalent Sphere

Approximate methods for estimating the optical properties of flame-generated soot aggregates were evaluated using existing computer simulations and measurements in the visible and near-infrared portions of the spectrum. The following approximate methods were evaluated for both individual aggregates and polydisperse aggregate populations: the Rayleigh scattering approximation, Mie scattering for an equivalent sphere, and Rayleigh-Debye-Gans (R-D-G) scattering for both given and fractal aggregates. Results of computer simulations involved both prescribed aggregate geometry and numerically generated aggregates by cluster-cluster aggregation; multiple scattering was considered exactly, considered using the mean-field approximation, and ignored using the R-D-G approximation. Measurements involved the angular scattering properties of soot in the postflame regions of both premixed and nonpremixed flames. The results show that available computer simulations and measurements of soot aggregate optical properties are not adequate to provide a definitive evaluation of the approximate prediction methods. The simulations involve either exact solutions for small aggregates where effects of multiple scattering are small, or approximate solutions of uncertain accuracy for the large aggregates of interest for practical flames. The measurements are limited to conditions where soot aggregate structure is not known, and for relatively large scattering angles where the various approximations yield similar results. Within these limitations (for aggregates larger than the Rayleigh scattering regime) the approximate theories performed as follows: Rayleigh scattering generally understimated scattering, Mie scattering for an equivalent sphere yielded unreliable results, while basic and fractal aggregate R-D-G scattering yielded best results for given and fractal aggregates, respectively. However, existing simulations suggest significant effects of multiple scattering for soot aggregates (except near soot inception conditions) that are not included in R-D-G scattering so that improved approximate optical theories for soot aggregates should be sought.

Radiative Properties of Numerically Generated Fractal Soot Aggregates: The Importance of Configuration Averaging

2009 ◽  
Author(s):  
Fengshan Liu ◽  
Gregory J. Smallwood
Keyword(s):  
Primary Particle ◽  
Particle Diameter ◽  
Practical Interest ◽  
Aggregate Size ◽  
Small Variation ◽  
Radiative Properties ◽  
Fractal Aggregates ◽  
Cluster Aggregation ◽  
Fractal Parameters ◽  
Primary Particles

The radiative properties of numerically generated fractal soot aggregates were studied using the numerically accurate generalized multi-sphere Mie-solution method. The fractal aggregates investigated in this study contain from 10 to 600 primary particles of 30 nm in diameter. These fractal aggregates were numerically generated using a combination of the particle-cluster and cluster-cluster aggregation algorithms with fractal parameters representing flame generated soot. Ten different realizations were obtained for a given aggregate size measured by the number of primary particles. The wavelength considered is 532 nm and the corresponding size parameter of primary particle is 0.177. Attention is paid to the effect of different realizations of a fractal aggregate with identical fractal dimension, prefactor, primary particle diameter, and the number of primary particles on its orientation-averaged radiative properties. Most properties of practical interest exhibit relatively small variation with aggregate realization. However, other scattering properties, especially the vertical-horizontal differential scattering cross section, are very sensitive to the variation in geometrical configuration of primary particles. Orientation-averaged radiative properties of a single aggregate realization are not always sufficient to represent the properties of random-oriented ensemble of fractal aggregates.

Scaling condition for multiple scattering in fractal aggregates

2003 ◽  
Vol 338 (1-4) ◽  
pp. 103-106 ◽  
Author(s):  
Guillermo P. Ortiz ◽  
W.Luis Mochán
Keyword(s):  
Multiple Scattering ◽  
Fractal Aggregates

scholarly journals Radiative Properties of Numerically Generated Fractal Soot Aggregates: The Importance of Configuration Averaging

2009 ◽  
Vol 132 (2) ◽  
Author(s):  
Fengshan Liu ◽  
Gregory J. Smallwood
Keyword(s):  
Primary Particle ◽  
Particle Diameter ◽  
Practical Interest ◽  
Aggregate Size ◽  
Small Variation ◽  
Radiative Properties ◽  
Fractal Aggregates ◽  
Cluster Aggregation ◽  
Fractal Parameters ◽  
Primary Particles

The radiative properties of numerically generated fractal soot aggregates were studied using the numerically accurate generalized multisphere Mie-solution method. The fractal aggregates investigated in this study contain 10–600 primary particles of 30 nm in diameter. These fractal aggregates were numerically generated using a combination of the particle-cluster and cluster-cluster aggregation algorithms with fractal parameters representing flame-generated soot. Ten different realizations were obtained for a given aggregate size measured by the number of primary particles. The wavelength considered is 532 nm, and the corresponding size parameter of primary particle is 0.177. Attention is paid to the effect of different realizations of a fractal aggregate with identical fractal dimension, prefactor, primary particle diameter, and the number of primary particles on its orientation-averaged radiative properties. Most properties of practical interest exhibit relatively small variation with aggregate realization. However, other scattering properties, especially the vertical-horizontal differential scattering cross section, are very sensitive to the variation in geometrical configuration of primary particles. Orientation-averaged radiative properties of a single aggregate realization are not always sufficient to represent the properties of random-oriented ensemble of fractal aggregates.

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