The results are presented of reconstructing the characteristics of scattering and absorption of light by smoke aerosol from the data on the microstructure and refractive index of smoke particles. The microphysical characteristics were obtained by inverting the spectral and angular measurements of the polarization scattering phase functions and the extinction coefficients of light in smoke aerosol formed as a result of the thermal decomposition of wood materials in the big aerosol chamber of the IAO SB RAS. As a result, the optical characteristics of smoke were determined, which were not measured experimentally (microphysical extrapolation method), such as the angular and spectral dependences of the scattering phase functions, the asymmetry parameter (the mean cosine of the scattering angle), the spectral dependences of the scattering and absorption coefficients, and the single scattering albedo. The temporal variability of the reconstructed optical characteristics of the smoke is presented.
The problem is considered of determining the integral distribution function of the aerosol particle number concentration using the data of distribution function of geometrical cross-section of particles. It is assumed that the integral distribution function of the geometrical cross-section is determined from the solution of the inverse problem for the characteristics of aerosol light scattering. It is shown that the relationship between two microstructure characteristics is described by the Volterra integral equation of the 2nd kind. A finite-difference algorithm for solving the problem is proposed. The results are presented of applying the method for reconstructing the distribution function of the number concentration of smoke particles.
Measurements of polarization scattering phase functions and spectral extinction coefficients were carried out in smoke aerosols formed as a result of thermal decomposition of wood materials, in a big aerosol chamber (BAC) of the IAO SB RAS. The results are presented of reconstructing of the parameters of the microstructure and the complex refractive index of smoke aerosols based on the developed algorithm for inverting optical measurements. Temporal variability of the microphysical characteristics of smoke is studied.
Diurnal variation of the effective height of the aerosol layer HV in the region of Tomsk was investigated. The effective height was determined as the ratio HV =Vτ / Vε of the volume concentrations of aerosol particles in the atmospheric column Vτ and in the near-surface layer Vε. The volume concentration of the aerosol was determined by inverting the characteristics of the spectral transmission of light in the atmospheric column and on the horizontal path. The variability of HV was compared for the submicron and coarse particle fractions. It is shown that the effective height is minimal in the morning and is amounts to 0.20–0.21 km for both fractions of aerosol. By the evening, the ratio HV increases to 1.43 km and 0.875 km for submicron and coarse aerosols, respectively.
The peculiarities of the aerosol microstructure in the near-ground layer and in the atmospheric column when smoke inflows into the atmosphere are studied. The parameters of aerosol microstructure were determined from the solution of the inverse problem for spectral aerosol extinction coefficients measured by the path method and the aerosol optical thickness measured by means of a sun-photometer in a clean atmosphere and when it was filled with smoke in the Tomsk region. The geometrical cross-section, volume concentration, and mean radius of particles of submicron and coarse aerosol fractions were calculated. Under background conditions, the coarse fraction prevails in the total aerosol volume. When air is filled with smoke, the submicron fraction of aerosol particles becomes dominant in the total aerosol volume. As a result, the particle radius averaged over the total ensemble decreases in a smoke-contaminated atmosphere. Also, when the atmosphere is filled with smoke, the nature of the regression relations between the microstructural parameters of different aerosol fractions and the spectral characteristics of the aerosol extinction of light significantly change.
The time variation of microstructure of the near-surface aerosol in the summer period has been studied based on solving the inverse problem for spectral measurements of aerosol extinction coefficient. Experimental data were obtained in Tomsk on a horizontal path using the measurer of spectral transmittance of the atmosphere at 11 wavelengths in the range from 0.45 to 3.9 μm. The numerical algorithm, based on the method of integral distributions, was used to solve the inverse problem. The volume concentration and the mean radius of aerosol particles with separation into a submicron and coarse fraction are considered. Specific features of the aerosol microstructure during the smoke haze in the atmosphere are determined.
Results of reconstruction of the volume concentration and geometric cross section of the coarsely dispersed aerosol by solving the inverse problem for spectral measurements of the aerosol optical thickness (АОТ) and calculated from regression relations for the АОТ measured at wavelengths of 2.203 and 1.557 μm are compared. It is shown that the regression equations retrieved from the data collected during a one month period allow the microstructural parameters of coarsely dispersed particles to be estimated for a one year period practically without loss of accuracy.
Possibilities of estimating the microstructure parameters of the atmospheric aerosol from the regression equations describing their correlation with spectral measurements of the aerosol optical thickness (AOT) are considered. Special attention is given to the problem of estimating the volume concentration, total cross section, and average radius of particles of coarsely dispersed aerosol fraction. The necessity of solving this problem is caused by large errors in reconstructing these parameters using the direct AOT inversion method for insufficiently wide spectral range of measurements. In the report the coefficients of simple and multiple linear regression relations and the results of reconstruction on their basis of the parameters of coarsely dispersed aerosol from the data of solar photometry in Tomsk are presented.
Results of investigations of the time variability of microstructure aerosol parameters reconstructed by inversion of spectral measurements of the aerosol optical thickness (АОТ) are presented. The input data were obtained at the IAO SB RAS using solar photometers of SP type in the observation periods 2003–2006 and 2011–2014. During the first period of measurement, the АОТ was measured at 13 wavelengths in the range 0.37–4 μm. During the second observation period, the upper limit of the spectral range of measurements was 2.14 μm. The total volume of the data processed included over 6 thousands hour-averaged spectra. The geometric aerosol cross section, volume concentration, and average particle radius are analyzed.
Special features of determining the aerosol microstructure from spectral measurements of the aerosol optical depth (AOD) caused by low information content of the solution for large particles are considered. Such conditions are observed when sizes of aerosol particles are much larger than the maximum measurement wavelength. We propose an AOD inversion algorithm based on a hybrid model of aerosol microstructure. The model includes a description of the disperse aerosol composition in the informative range of particle sizes based on the integral distribution function supplemented with a parametric representation of the microstructure of large particles. Results of approbation of the algorithm for inversion of the data of field experiments are presented that demonstrate the feasibility of retrieving the microstructural parameters of coarsely dispersed aerosol fraction when the upper limit of the spectral range decreases from 4 to 1.56 μm.
The influence of the upper limit λmax of the spectral range in which the atmospheric transmission is measured on the results of retrieval of the aerosol microstructure parameters when solving the inverse problem of sun photometry from the data of numerical modeling and field experiments is investigated. In the numerical experiment the aerosol model comprising fine submicron (f) and coarsely dispersed (c) particle fractions is used. Here λmax is chosen from the range 1.052-3.973 μm. The method of integral distributions is used to solve the inverse problem. It is demonstrated that the limited spectral range causes underestimation of the contribution of large particles to aerosol distributions. In particular, at λmax = 1.246 μm, losses of the retrieved concentration of particles of the c fraction against the background of the decreased (to 18 % at λ = 1.246 μm) volume aerosol concentration can reach 42 %.
Results of a comparison of synchronous measurements of the aerosol optical thickness with SP-6 and CE-318 solar photometers with the microstructure parameters reconstructed by solving the inverse problem are presented. Measurements were performed in Tomsk during one year. The volume concentration and the average particle radius integrated over the entire thickness of the atmosphere were considered as aerosol microstructure parameters. These parameters were estimated both for the particle ensemble and for individual aerosol fractions, submicron and coarse ones. The satisfactory agreement of the parameters being compared is demonstrated. Possible reasons for the established discrepancies are discussed.
The progress in the technology of generation of high-power femtosecond pulses opens new possibilities for solving urgent problems in atmospheric optics. High energy density and wide spectral range of femtosecond pulses in air increase significantly the information content of laser sensing. The capabilities of spectroscopic methods harnessing the phenomena of light emission and nonlinear and incoherent light scattering also extend. Secondary effects, in particular, the occurrence of a stable supercontinuum of direct white light allows its use as a white-light lidar in remote analysis of optical and microphysical properties of the atmospheric aerosol. Based on the experience of solving inverse problems of laser sensing at the institute of Atmospheric Optics, the refine the methodology of solving this class of problems with the use of promising white-light lidars. As the first necessary stage, the results of numerical experiments on multifrequency aerosol sensing in the visible and near-IR wavelength ranges along vertical sensing path are discussed in the present report. The problem of optimal selection of a wavelength range in solved, the stability of the available reconstruction algorithms under conditions of noise background are investigated for the case wide-aperture signal reception, and the limiting sensing regimes are evaluated.
The lidar experimental data recorded for dense stratocumulus clouds above Tomsk city are presented. The measurements were performed by a lidar with changeable field of view (FOV). New algorithms for resolving a lidar equation taking into account multiple scattering have been introduced to restore extinction coefficient profiles. These algorithms are based on the original description of a lidar signal including its asymptotic properties for large FOVs. In order to solve an inverse problem only one parameters, namely the effective particle size has to be a set a priory. From experimental data inversion results it was established that extinction coefficient values in a cloud layer depth are close to the range of 30-40 km-1.
The lidar signal component caused by a scattering of the first two orders is described in detail within the framework of the theory of laser sensing taking into account a multiple scattering in the small-angle approximation. Simple formulas are obtained determining a double scattering signal with consideration for lidar geometrical parameters for the media with a strongly extended scattering phase function. The role of diffraction and geometrical optics components of the scattering phase function in a doubly scattered signal is investigated. Estimations are obtained of an accuracy of a considered approximation depending on the field-of-view angle of the receiver at different optical thickness of the scattering layer.
The lidar equation with consideration of multiple scattering in the small-angle approximation is analyzed in the present work. To describe a lidar signal, a model of the scattering phase function represented by the sum of diffraction and geometric-optics components is used. A method for separating the diffraction and geometric-optics lidar signal components is suggested. It is shown that for large field-of-view angles of the receiver, this allows the diffraction component of the scattering phase function to be replaced by one parameter -- the effective radius of particles.
A solution to the inverse problem of laser sensing of sea water is considered taking into account multiple scattering. Two forms of the lidar equation have been analyzed. The first form is based on the selection of the component caused by single scattering. The second form is based on the asymptotic signal, which would be recorded in case of unbounded field-of-view angle of the receiver. For each form of the lidar signal the iterative algorithms for the reconstruction of the extinction coefficient profile have been developed. The algorithms use physically justified functional relation between the extinction and scattering coefficients of sea water. The examples are given of the inversion of real lidar signals by the developed algorithms.
An equation for a multiply scattered lidar return from sea water is considered. The multiply scattered radiation is taken into account in the small-angle approximation, and only single scattering is considered at large angles. A relationship between the components of multiply and single scattered radiation is numerically studied depending on the receiver field-of-view angle at various optical depths of a water layer. A new representation of lidar return is suggested. It is based on the expression for a lidar return assuming unbounded field-of-view angle of the retriever. This approach allows us to consider multiple scattering as a useful signal component in solving inverse problems.
An exact analytic description of the lidar geometric factor, in the form of an integral of a product of the Bessel functions, has been analyzed for separated source and receiver.
Within the framework of the small-angle approximation, a rigorous analytic solution to the problem of determining the instrumental function has been obtained for measurements of the scattering phase functions considering finite angular sizes of a radiation source and receiver. It has been shown that the instrumental function is described by an integral of a product of the Bessel functions. The instrumental function is expressed in terms of elementary functions. The effect of the instrumental function on the accuracy of measuring small-angle scattering-phase functions for different particle sizes is discussed.
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