Aerosol Refractive Index Research Articles

Simultaneous determination of aerosol size distribution and refractive index and surface albedo from radiance—Part I: Theory

Simultaneous determination of aerosol size distribution and refractive index and surface albedo from radiance—Part I: Theory

大気のエーロゾルの有効複素屈折率の推定とその有用性。Part I.理論

A method of estimating the effective absorption property of atmospheric aerosols is proposed. This method is based on the dependence of reflectance of solar radiation at some altitudes on aerosol refractive index. Sensitivity studies of the monochromatic and entire solar spectrum reflectance to various radiative parameters show that this method has enough sensitivity to estimate the absorption property. This effective value may be different from the real value for an individual particle. To assess the validity of the effective value in radiative transfer calculation, modeling studies are performed, that is, some uncertainties due to complicated features, such as refractive index depending on radius and wavelength, non-Lambertian surface and wavelength dependence of the surface reflectivity are examined with a numerical model. The results indicate that these are small effects on the estimation and the effect of aerosols on earth-atmosphere radiation field can adequately be described by the effective values.

Investigation of a 1000 MW smoke plume by means of a 1.064 μm lidar—I. Lidar calibration procedure from in situ aerosol measurements and vertical lidar shots

From airborne measurements on various days, average size distributions of natural aerosol particles for different heights were obtained at our rural experimental site. Using different aerosol refractive indices and a mean particle size distribution, the backscattering and extinction coefficients β and α were computed for a 1.064 μm wavelength. Assuming the mean refractive index of the aerosol particles over a rural site as equal to 1.5−0.005 i and taking into account size and concentration distributionsof aerosol particles with height, a vertical profile of atmospheric returns from the 1.064 μm laser radiation from the ground has been computed as characteristic of the site. From numerous lidar shots on various days, an average vertical profile of lidar returns was also obtained as characteristic of both the experimental site and the whole lidar system using a laser wavelength of 1.064 μm. The relationship between the atmospheric returns and lidar signals could then be set up and used as the calibration function of the lidar. In this way, distributionsof particle concentration could be monitored from lidar shots.

Characteristics of atmospheric extinction-to-backscattering ratio in ruby lidar measurements

In this paper the dependence of the ratiok (the atmospheric extinction-to-backscatering ratio) upon the aerosol refractive index and size distribution is theoretically studied in ruby lidar measurements. An empirical expression for the ratiok is then established. Moreover the effect of molecular scattering on the ratiok is discussed.

Aircraft observations of solar radiation in cloud-free atmospheres

Aircraft observations of solar radiative fluxes (including the downward diffuse component) made in cloud-free conditions in the lowest 8 km of the atmosphere, have been interpreted using simultaneous measurements of aerosol characteristics. Measured flux profiles have been compared with those derived from a two-stream model of radiative transfer which can incorporate both gaseous and aerosol effects. In one of the four cases examined, sufficient aerosol data were available to estimate the radiative properties of the aerosol which could then be included in the model calculations. The findings are shown to be broadly consistent with the observations when a realistic aerosol refractive index is used. The measurement of the downward diffuse radiation enabled the radiative properties of the aerosol to be calculated directly from the flux measurements. In particular, the average single-scattering albedo of aerosol within the boundary layer in continental air masses was estimated to be about 0.7.

Spatial Variation of Stratospheric Aerosol Acidity and Model Refractive Index: Implications of Recent Results

Recent experimental results indicate that little or no solid ammonium sulfate is present in background stratospheric aerosols. Other results allow straightforward calculation of sulfuric acid/water droplet properties (acidity, specific gravity, refractive index) as functions of stratospheric temperature and humidity. These results are combined with a variety of latitudinal and seasonal temperature and humidity profiles to obtain corresponding profiles of droplet properties. These profiles are used to update a previous model of stratospheric aerosol refractive index. The new model retains the simplifying approximation of vertically constant refractive index in the inner stratosphere, but has sulfuric acid/water refractive index values that significantly exceed the previously used room temperature values. Mean conversion ratios (e.g., extinction-to-number, backscatter-to-volume) obtained using Mie scattering calculations with the new refractive indices are very similar to those obtained for the old indices, because the effects of deleting ammonium sulfate and increasing acid indices tend to cancel each other.

Lower-stratospheric aerosol microstructure investigation with multifrequency lidar

The results of investigation of the lower-stratospheric aerosol microstructure by the inverse-problem method based on double-wave lidar sounding data at λ = 0.53 and 0.694 μm are discussed. Assuming the aerosol refractive index in the lower stratosphere to be close to the model values m = 1.43 and κ = 0, the altitude profiles of aerosol microstructure parameters at 14–25-km altitude were determined. An analysis of accuracy characteristics of aerosol microstructure laser sounding of the lower stratosphere is presented.

Optical studies of smoke aerosols: an inversion method and its applications

An optical method of investigating atmospheric aerosol is described. The numerical scheme for determining the microstructure and the refractive index of an aerosol from angular measurements of the polarization-scattering phase functions is described. Studies of the refractive index and microstructure of woody-smoke aerosols that depend on the relative humidity of the air have been carried out on the basis of the method described.

Information content of sky intensity and polarization measurements at right angles to the solar direction

This paper presents the results of a Monte Carlo simulation study of the brightness and polarization at right angles to the solar direction both for ground-based observations (looking up) and for satellite-based systems (looking down). Calculations have been made for a solar zenith angle whose cosine was 0.6 and wavelengths ranging from 3500 A to 9500 A. We have succeeded in demonstrating a sensitivity of signatures to total aerosol loading, aerosol particle size distribution and refractive index, and the surface reflectance albedo. For Lambertian type surface reflection the albedo effects enters solely through the intensity sensitivity, and we have found very high correlations between the polarization term signatures for the ground-based and satellite-based systems. Potential applications of these results for local albedo predictions and satellite imaging systems recalibrations are discussed.

Solar Radiative Heating in the Presence of Aerosols

A theoretical study is carried out to evaluate the effects of aerosols on the shortwave flux divergence in the lower troposphere (0–2 km) by using four computational methods: Gauss-Seidel iteration, a reference method by which all orders of scattering are accounted for, and three approximations, primary scattering, no-scattering and median wavelength. By using these procedures, the radiative transfer equation is solved for a cloudless plane parallel atmosphere of infinite extent in the horizontal, but of finite extent in the vertical. The Gauss-Seidel procedure is taken as a standard and comparisons of the flux divergence are made by using various combinations of solar zenith angle, aerosol size frequency distribution and aerosol refractive index. In many of the simulations the median wavelength approximation gives accuracies comparable to those of the no-scattering approximation, in which case the choice of method is based on the required computational time. However, inaccuracies appear with all the approximations and the degree of superiority of one method over another depends on the aerosol and gaseous constituents of the model atmosphere.

Results of a Comprehensive Atmospheric Aerosol-Radiation Experiment in the Southwestern United States. Part II: Radiation Flux Measurements and Theoretical Interpretation

Abstract The experimental results in Part I are used in the theoretical interpretation of the radiation flux measurements which were taken with an aircraft. The absorption term of the complex refractive index of aerosols is estimated to be approximately 0.01 for a real part of 1.5 for the wavelength bandwidth 0.32–0.68 μm. A regional variation in the refractive index is noted. Atmospheric heating and cooling rates due to aerosol and molecular absorption in the solar and terrestrial wavelengths are determined from the radiation flux measurements. The magnitudes of these rates are compared and their relative importance is discussed.

Results of a Comprehensive Atmospheric Aerosol-Radiation Experiment in the Southwestern United States Part I: Size Distribution, Extinction Optical Depth and Vertical Profiles of Aerosols Suspended in the Atmosphere

Abstract An exploratory field experiment was undertaken to determine the practicality of a method specifically designed to obtain the optical properties of aerosols as they relate to the earth's radiation balance. The method requires a basic set of data consisting of the vertical distribution of aerosol concentrations, size distribution, optical depth, and net radiation fluxes. From these data radiation absorptions are determined, and effective aerosol refractive indices consistent with the actual absorption are deduced through the application of precision radiative transfer calculations. The results of 11 experiment episodes involving a combined aircraft and surface-based measurement system are described. The episodes took place in an arid desert region located near Blythe, California, and in a semiarid agricultural region located near Big Spring, Texas. Part I deals with the physical-numerical depiction of such aerosol properties as optical depth, size distribution, and vertical profiles of concentratio...

Atmospheric Aerosol Index of Refraction and Size-Altitude Distribution from Bistatic Laser Scattering and Solar Aureole Measurements

The polarization and intensity of laser light scattered from a 500-m horizontal column of air was measured for angles from 8 degrees to 172 degrees . By photographing the sun's aureole, the intensity of sunlight scattered from a column of air between the earth and the sun was measured from 2 degrees to 12 degrees . These three simultaneous independent data sets were analyzed assuming single Mie scattering corrected for Rayleigh scattering. A regularized power law oversize distribution N(r) = N(O)/[1 + (r/a)(nu)] was used in the analyses. Typical parameters values obtained are a = 0.05 microm, nu = 3.5, and N(0) ~ 10(4) cm(-3). Although the laser and aureole techniques look at somewhat different volumes of air, under good vertical mixing conditions they are in quantitative agreement. The complex index of refraction of undisturbed aerosols, particles complete with liquid coatings, is found to be typically m(r) = 1.50 +/- 0.05, m(i) ~ 0.005.

Absorption Coefficient of Atmospheric Aerosol: a Method for Measurement

Recent models predict that the effect of atmospheric aerosol particles on global temperature depends in part on b(abs,)or on n(2), where b(abs) is the aerosol absorption coefficient, and n(2) is the imaginary part of the aerosol refractive index for radiation in visible wavelengths. Satisfactory techniques for measuring b(abs) have yet to be developed. A method for measurement of b(abs) is described and discussed. The apparatus uses a piece of opal glass to integrate light scattered by particles collected on Nuclepore filters. It is quite mobile, inexpensive, and can be calibrated. Analysis of the errors inherent in the measurement shows it is accurate to well within an order of magnitude for particles of radius larger than 0.1 microm.

Degree and Direction of Polarization of Multiple Scattered Light 2: Earth’s Atmosphere with Aerosols

The degree of polarization as well as the direction of the polarization are calculated by a Monte Carlo method for the reflected and transmitted photons from the earth's atmosphere. The solar photons are followed through multiple collisions with the aerosols and the Rayleigh scattering centers in the atmosphere. The aerosol number density as well as the ratio of aerosol to Rayleigh scattering vary with height. The aerosol index of refraction is assumed to be 1.55. The proportion of aerosol to Rayleigh scattering is appropriately chosen at each wavelength (lambda = 0.4 micro and 0.7 micro); ozone absorption is included where appropriate. Three different aerosol number densities are used to study the effects of aerosol variations. Results are given for a solar zenith angle of 81.37 degrees and various surface albedos. The radiance and polarization of the reflected and transmitted photons is particularly sensitive to the amount of aerosols in the atmosphere at certain angles of observation. The direction of pola ization shows little dependence on the surface albedo.

Determination of Aerosol Size Distribution from Spectral Attenuation Measurements

A method of evaluating the aerosol size distribution from the spectral attenuation measurements is shown. The process consists of solving the simultaneous integral equations, and examples are given of solutions based on the attenuation measurements made by Knestrick et al. over the Chesapeake Bay. It is found that the evaluated individual size distributions do not necessarily follow the power law, although departures from it are mostly small. If the power law is to be adopted neglecting small departures, the evaluated results are in average expressed by r(-3.57), where r is the radius of aerosols. In this study, the refractive index of aerosols is assumed to be 1.50, and some discussion is made of the effect of adopting a different refractive index value on estimation of the size distribution.