Kusiel S. Shifrin

Simple relationships for the Ångström parameter of disperse systems.

Simple relationships for calculating the Ångström parameter a of any disperse system are obtained (a) for a polycomponent system, through values of ai of individual components, and (b) for the simplest disperse systems, consisting of large, small, and soft particles. It is shown that the parameter a of a monodisperse particle system is sensitive to the exact structure of the spectral variability optical thickness of the system curve τ(λ) and that when calculating α, one should not use the van de Hulst approximate formula for τ(λ) when ripples and another important details are not accounted for. The error connected with the use of the van de Hulst formula when one is estimating a depends on the value of optical hardness of the particle. It is small when particles are soft, and it becomes noticeable as particles get harder.

Quasi-stationary scattering of electromagnetic pulses by spherical particles

The Lorentz-Mie theory is generalized for the case of a spherical particle irradiated by a pulse with a finite length L that is transferred by a carrier wavelength λ(0). Two cases should be physically distinguished, depending on radiation-receiver properties: quasi-stationary scattering (a receiver integrates the entire signal over time) and nonstationary scattering, when a receiver is capable of recording scattered signal changes with time. General formulas that allow one to calculate optical characteristics for both scattering cases and for an arbitrary ratio L/λ(0) are derived. Quasi-stationary-scattering peculiarities and limiting cases of small and large particles are studied in detail. The formulas are illustrated with calculations of spherical-particle optical characteristics for pulses of different lengths, for differently sized particles, and for a case in which a scattered pulse has a Gaussian form. The results obtained should be taken into account when one is studying the passage of a pulse through scattering media.

Spectral attenuation and aerosol particle size distribution

The problem of the reconstruction of the spectrum of a dispersed system from data on its spectral attenuation is studied. The numerical algorithm for obtaining the particle size distribution by the use of the concept of regularization is thoroughly treated. The applicability of this method to the reconstruction of the particle size distribution of a typical marine aerosol is tested. A method of choosing the regularization parameter of the solution for the inverse problem based on an objective estimate of the validity of the obtained solution is proposed. Results are presented for a set of numerical experiments in which the radius interval for which the distribution function can be obtained with a satisfactory accuracy is estimated. The validity of solutions is estimated depending on the measuring spectral range for the attenuation, the radius interval, and the number and position of points within this interval. The possibility of extending the radius interval for which the distribution function can be obtained by the use of extrapolation of the distribution function tail is discussed.

DETERMINATION OF THE AEROSOL PARTICLE-SIZE DISTRIBUTION FROM SIMULTANEOUS DATA ON SPECTRAL ATTENUATION AND THE SMALL-ANGLE PHASE FUNCTION

Retrieval of the aerosol particle distribution function in the marine atmospheric boundary layer for a radius interval as great as 30 ?m is considered with the use of simultaneous data on the spectral attenuation (in the spectral range of 0.4 -5 ?m) and small-angle phase function (in the angle interval of 0 -8 degrees ). An iterative procedure is constructed combining inversion algorithms for the spectral attenuation and phase function, each being efficient for a specific particle-size interval. Numerical experiments with two aerosol models are performed. The effect of random errors of the small-angle phase function is considered for different angular resolutions. It is shown that, given certain conditions, the inversion methodology under discussion makes it possible to retrieve the particle-size distribution function for the radius interval of 0.2 -30 ?m with a mean relative error of less than 10 % and a maximum deviation not exceeding 20 %.

An algorithm for determining the radiance reflected from the rough sea surface using MODIS-N satellite radiometer data

A new algorithm is suggested for determining the radiance B/sub r/(/spl theta/, /spl phi/) reflected from the rough sea surface. The knowledge of B/sub r/(/spl theta/, /spl phi/) is needed for excluding the reflected solar radiation from the signal at the space receiver. Information on the reflected radiance is also required when estimating the state of the sea surface from space. The information (on reflected radiance) is important for problems such as estimating the chlorophyll concentration in sea water. The suggested algorithm was constructed based on direct measurements of the radiance coefficient /spl rho/ published in V. G. Akimov ei al. (1993) and J. A Shaw et al. (1997). The experimental results show that the slope distribution of sea surface elements depends not only on the wind speed near the sea surface but also on the stability of the lower part of the marine atmospheric boundary layer. It is shown that the failure to account for this effect causes an error of /spl plusmn/30% when estimating B/sub r/(/spl theta/, /spl phi/).

The airborne identification of oil films at the Caspian sea surface using CO2 lidar

Abstract Contrast in the reflectivity between pure and contaminated sea surfaces is the result of two effects, namely: (1) the different reflectivity of sea water and oil films on sea water, and (2) the damping effect by the oil film on the sea waves. The problem is to estimate the contribution of these two effects on the total contrast, so that the substance effect can be calculated. Magnitude is related to the oil film thickness. The spectral behaviour of the contrast and its relationship with the film thickness were calculated for an undisturbed sea surface. The estimate of the damping effect was made using the Cox and Munks slope distributions for pure and contaminated surfaces. The method chosen was verified by a series of tests conducted over the Caspian Sea from an aircraft carrying a CO2 laser sensor operating at 10.6 μm.

Optical properties of the atmosphere over the ocean

Optical properties of the atmosphere over the ocean are essentially different from the optical properties of the atmosphere over the land. For a cloudless atmosphere this difference is determined by the mechanisms of aerosol formation. For cloudy atmospheres this difference results from different mechanisms of convection. Convection over the sea is essentially weaker than over the land and the cumuli-form of clouds over the sea is essentially thinner. We must take into account the differences between optical properties for the atmosphere over the ocean and over the land for the purposes of remote sensing, LIDAR investigations, and in calculations of the radiative energy transfer between ocean and atmosphere. Taking this into account, it becomes apparent that we must correct all preceding atlases and maps that do not reflect this difference. It is also important for engineers who design optical instruments for use in the atmosphere over the oceans to consider the implications of this difference.

Remark about the notation used for calculating the electromagnetic field scattered by a spherical particle

In the theory of light scattering by small particles we use two different kinds of notation for the incident plane wave. In both cases we often use the same notation for the scattered spherical wave (compare H. C. van de Hulst, Light Scattering by Small Particles (Wiley, New York, 1957), Chap. 3, p. 18 and Chap. 1, p. 8, with C. Bohren and D. Huffman, Absorption and Scattering of Light by Small Particles (Wiley, New York, 1957), Chap. 3, p. 18 and Chap. 9, p. 124). If one is careless, this will lead to confusion. We must be aware of this and, correspondingly, change the calculated electromagnetic fields.

Information content of the spectral transmittance of the marine atmospheric boundary layer

We developed a procedure for using data for attenuation ς of the marine atmosphere at λ = 0.55 μm and Ångström parameter α in the visible range for the estimation of aerosol particle size spectrum. We evaluated the aerosol microstructure in the marine atmospheric boundary layer (MABL). To eliminate the effect of the upper troposphere and stratosphere, we assumed that the optical characteristics of the microstructure are average for the typical marine atmosphere. The sought-for MABL microstructure is parameterized by the sum of two fractions, each having a log-normal distribution (the fine and large components). The problem amounts to determining six unknown parameters from two characteristics. In accordance with experimental data as well as with theoretical aerosol models, the total particle concentration n and the fraction of the large component c(2) are assumed to be constant for the central regions of the world ocean. In this way, the problem can be reduced to the determination of the acceptable value area of the remaining four parameters. For all models situated in this area, values of ς and α fall within some intervals Δς and Δα, specific for each aerosol type. Since the problem is ambiguous, the number of models comprising an acceptable ensemble is great. So this number is equal to 5972 in the example that illustrates our procedure. It is noteworthy, however, that all the models entering the ensembles have a similar microstructure within the active radius interval of 0.02-3 μm, which is the main interval that governs the transmittance in the 0.3-1 μm spectral range. The average curve that can be plotted for the entire ensemble can be used as a solution to the problem, which is the main result of this study. We are also concerned with how aerosol transmittance measurements in one of the infrared channels could be used to diminish the ambiguity of the problem. The answer depends on the specific aerosol structure. In most cases, additional IR data in one channel barely decreases the ambiguity of the problem. However, such data might be useful for some other distributions. We consider the effect of six IR channels in our example.

Efficiencies for extinction and backscattering of a microwave pulse incident on water drops

Efficiencies for extinction and backscattering of microwave radiation by water drops are calculated from general formulae describing scattering of an arbitrarily short duration electromagnetic field pulse by a spherical particle. Water drops of radius less than 2.5 mm, covering all types of clouds and precipitations, are considered. The calculations show that for large particles (the radius is compatible with the carrier wave length) scattering characteristics of very narrow pulses and monochromatic waves differ noticeably. For wave lengths shorter than 22 mm, ranges of particle size are found where pulse and monochromatic scattering characteristics are appreciatively different. For longer waves these differences disappear.