Gorden Videen

Effect of black carbon on the optical properties and climate forcing of sulfate aerosols

We study the optical properties of anthropogenic sulfate aerosols containing black carbon using a recently developed exact solution of the scattering problem for a spherical particle (sulfate aerosol) containing an eccentrically located spherical inclusion (black carbon). We present the expression for the change of planetary albedo due to addition of an absorbing, but optically thin aerosol layer and estimate the effect of the black carbon within the sulfate aerosol layer on the aerosol direct radiative forcing. The black carbon within the sulfate aerosol reduces the expected sulfate direct cooling effect by about 0.034 W/m 2 for each 1% of the black carbon to sulfate mass mixing ratio. Thus the presence of black carbon within sulfate in the background aerosol does not significantly change the previous estimates of the global aerosol direct cooling effect. However, in regions where the black carbon in sulfate concentrations are of the order of 5% or more, the local and regional effects are significant. Briegleb, 1993) suggest that the effect of anthropogenic sulfates and aerosols produced by tropical biomass burning may be sufficient to counteract any possible warming trend due to an increase of atmospheric greenhouse gases generated since the beginning of the industrial revolution. The direct cooling effect of aerosols will be reduced by the presence of black carbon. Black carbon (soot) within aerosol particles increases the absorption of solar radiation and de- creases the reflection (albedo). In this paper we (1) calculate the optical properties of sul- fate aerosol containing black carbon using a recently devel- oped exact solution of the scattering problem by a spherical particle (sulfate aerosol) containing an eccentrically located spherical inclusion (black carbon), (2) present the expression for the change of planetary albedo due to the addition of an absorbing aerosol layer, and (3) estimate the effect of the black carbon within a sulfate aerosol layer on the aerosol direct radiative forcing.

Black carbon and absorption of solar radiation by clouds

The exact solution of the scattered electromagnetic field from a water droplet containing an arbitrarily located spherical black carbon particle is used to investigate the effect of black carbon on the absorption of solar radiation by clouds. When droplet absorption is averaged over all possible locations of black carbon within a droplet, the averaged absorption is close to the value calculated using the effective medium approximation. The preferential black carbon location on the top or close to the bottom of the droplet leads to an increased absorption. The estimated upper bound on the increased absorption of solar radiation (global and annual average) is 1–3 W/m2 over the absorption of pure water clouds.

Multiple scattering by random particulate media: exact 3D results

We use the numerically exact superposition T-matrix method to perform extensive computations of electromagnetic scattering by a 3D volume filled with randomly distributed wavelength-sized particles. These computations are used to simulate and analyze the effect of randomness of particle positions as well as the onset and evolution of various multiple-scattering effects with increasing number of particles in a statistically homogeneous volume of discrete random medium. Our exact results illustrate and substantiate the methodology underlying the microphysical theories of radiative transfer and coherent backscattering. Furthermore, we show that even in densely packed media, the light multiply scattered along strings of widely separated particles still provides a significant contribution to the total scattered signal and thereby makes quite pronounced the classical radiative transfer and coherent backscattering effects.

Shape matters: plasmonic nanoparticle shape enhances interaction with dielectric substrate.

Numerical analyses of the ultraviolet and visible plasmonic spectra measured from hemispherical gallium nanostructures on dielectric substrates reveal that resonance frequencies are quite sensitive to illumination angle and polarization in a way that depends on nanostructure size, shape, and substrate. Large, polarization-dependent splittings arise from the broken symmetry of hemispherical gallium nanoparticles on sapphire substrates, inducing strong interactions with the substrate that depend sensitively on the angle of illumination and the nanoparticle diameter.

Aerosol Fluorescence Spectrum Analyzer for Rapid Measurement of Single Micrometer-Sized Airborne Biological Particles

ABSTRACT We report the operation of an aerosol analyzer capable of measuring the fluorescence spectra of single micrometer-sized bioaerosol particles as they flow through the instrument. Particles entrained in an airstream initially traverse a cw (continuous wave) (488 nm) “trigger” laser beam where their elastic scattering and total fluorescence is measured with photomultipliers. When the elastic scattering and/or fluorescence signals exceed preset levels, a 266 nm UV “probe” laser is triggered to fire and illuminate preselected particles downstream from the trigger laser. The UV laser-excited spectra of particles are measured with the instruments image-intensified CCD detector that is gated to be on when the probe laser fires. We demonstrate the ability of the instrument to capture the fluorescence spectra of single micrometer-sized airborne biological particles. Such spectra should be useful in differentiating between biological and nonbiological aerosols and in partially characterizing airborne biolo...

Validity criteria of the discrete dipole approximation.

There are two widely accepted restrictions on the application of the discrete dipole approximation (DDA) in the study of light scattering by particles comparable to the wavelength: (1) when considering dielectric particles, the size of the cells must satisfy the condition kd|m|<0.5, where k is the wavenumber, d is the size of the cells, and m is the complex refractive index of the constituent material and (2) when considering conductive particles, the size of the cells must be small enough to reproduce sufficiently the evolution of the electromagnetic field in the skin layer. We examine both restrictions when the DDA is applied to irregularly shaped particles and show that its restrictions are not as strong as is widely accepted. For instance, when studying irregularly shaped particles averaged over orientations, even at kd|m|=1, the DDA provides highly accurate numerical results. Moreover, we show that the impact of using large constituent cells is similar to that produced by surface roughness; therefore, the replacement of the target particle by an array of large constituent cells has the same effect, qualitatively, as incorporating additional small-scale surface roughness on the particle. Such a modification of the target particle can be desirable in many practical applications of DDA when irregularly shaped particles are considered. When applying DDA to conductive, nonspherical particles, the insufficient description of the electromagnetic field in the skin layer does not lead to a violation of the Maxwell equations, although it has a visible but nonmajor influence on the light-scattering properties of the target.

COHERENT BACKSCATTERING VERIFIED NUMERICALLY FOR A FINITE VOLUME OF SPHERICAL PARTICLES

We consider electromagnetic scattering by a spherical volume sparsely and randomly populated by spherical particles of equal size and optical properties. The far-field scattering matrix of the entire volume is computed using an exact method and an approximate method. The former is a direct computer solver of the Maxwell equations called the superposition T-matrix method (STMM). The latter is a solver based on numerical Monte Carlo integration of the ladder and cyclical diagrams appearing in the microphysical theory of radiative transfer and coherent backscattering (RT-CB). The quantitative agreement between the STMM and RT-CB computations provides verification of the RT-CB theory. Prominent backscattering features exhibited by the STMM data cannot be reproduced by keeping only the ladder diagrams of RT. Our results strongly support the CB explanation of opposition brightness and polarization phenomena observed for a class of atmosphereless solar-system objects. Further research is necessary to determine the range of quantitative applicability of the RT-CB theory to densely packed particulate media.

Light scattering from deformed droplets and droplets with inclusions. I. Experimental results

We provide experimental results from the scattering of light by deformed liquid droplets and droplets with inclusions. The characterization of droplet deformation could lead to improved measurement of droplet size as measured by commercial aerodynamic particle-sizing instruments. The characterization of droplets with inclusions can be of importance in some industrial, occupational, and military aerosol monitoring situations. The nozzle assembly from a TSI Aerodynamic Particle Sizer was used to provide the accelerating flow conditions in which experimental data were recorded. A helium-neon laser was employed to generate the light-scattering data, and an externally triggered, pulsed copper vapor laser provided illumination for a droplet imaging system arranged orthogonal to the He-Ne scattering axis. The observed droplet deformation correlates well over a limited acceleration range with theoretical predictions derived from an analytical solution of the Navier-Stokes equation.

Effective medium theories for irregular fluffy structures: aggregation of small particles

The extinction efficiencies as well as the scattering properties of particles of different porosity are studied. Calculations are performed for porous pseudospheres with small size (Rayleigh) inclusions using the discrete dipole approximation. Five refractive indices of materials covering the range from 1.20+0.00i to 1.75+0.58i were selected. They correspond to biological particles, dirty ice, silicate, and amorphous carbon and soot in the visual part of the spectrum. We attempt to describe the optical properties of such particles using Lorenz-Mie theory and a refractive index found from some effective medium theory (EMT) assuming the particle is homogeneous. We refer to this as the effective model. It is found that the deviations are minimal when utilizing the EMT based on the Bruggeman mixing rule. Usually the deviations in the extinction factor do not exceed approximately 5% for particle porosity P = 0 - 0.9 and size parameters x(porous) = 2 pi r(s,porous)/lambda < or approximately = 25. The deviations are larger for scattering and absorption efficiencies and smaller for particle albedo and the asymmetry parameter. Our calculations made for spheroids confirm these conclusions. Preliminary consideration shows that the effective model represents the intensity and polarization of radiation scattered by fluffy aggregates quite well. Thus the effective models of spherical and nonspherical particles can be used to significantly simplify the computations of the optical properties of aggregates containing only Rayleigh inclusions.

Scattering by a composite sphere with an absorbing inclusion and effective medium approximations

We investigate the accuracy of three effective medium approximations when applied to the scattering by inhomogeneous spherical particles. We compare gross scattering properties (extinction, scattering, and absorption cross sections, single scattering albedo, and the asymmetry parameter) and the scattering phase functions calculated using the effective medium approximations to those scattering properties obtained using an exact solution of the boundary value problem for a water droplet (sphere) containing an arbitrarily located spherical carbon inclusion. For small inclusion size parameters (xinc<0.5), all effective medium approximations predict gross scattering properties to within 1% accuracy. As the inclusion size increases, the extended effective medium approximation provides greater accuracy than the other approximations. In addition, the extended effective medium approximation more accurately reproduces the structure of the scattering phase function.