Retrieval of aerosol size distribution from angular scattering functions: effects of particle composition and shape

Use of sky brightness measurements from ground for remote sensing of particulate polydispersions

The software code SKYEAD.pack for retrieval of aerosol size distribution and optical thickness from data of direct and diffuse solar radiation is described; measurements are carried out with sky radiometers in the wavelength range 0.369-1.048 µm. The treatment of the radiative transfer problem concerning the optical quantities is mainly based on the IMS (improved multiple and single scattering) method, which uses the delta-M approximation for the truncation of the aerosol phase function and corrects the solution for the first- and second-order scattering. Both linear and nonlinear inversion methods can be used for retrieving the size distribution. Improved calibration methods for both direct and diffuse radiation, the data-analysis procedure, the results from the proposed code, and several connected problems are discussed. The results can be summarized as follows: (a) the SKYRAD.pack code can retrieve the columnar aerosol features with accuracy and efficiency in several environmental situations, provided the input parameters are correctly given; (b) when data of both direct and diffuse solar radiation are used, the detectable radius interval for aerosol particles is approximately from 0.03 to 10 µm; (c) besides the retrieval of the aerosol features, the data-analysis procedure also permits the determination of average values for three input parameters (real and imaginary aerosol refractive index, ground albedo) from the optical data; (d) absolute calibrations for the sky radiometer are not needed, and calibrations for direct and diffuse radiation can be carried out with field data; (e) the nonlinear inversion gives satisfactory results in a larger radius interval, without the unrealistic humps that occur with the linear inversion, but the results strongly depend on the first-guess spectrum; (f) aerosol features retrieved from simulated data showed a better agreement with the given data for the linear inversion than for the nonlinear inversion.

Cloud-droplet-size distribution from lidar multiple-scattering measurements

A method for calculating droplet-size distribution in atmospheric clouds is presented, based on measurement of laser backscattering and multiple scattering from water clouds. The lidar uses a Nd:YAG laser that emits short pulses at a moderate repetition rate. The backscattering, which is composed mainly of single scattering, is measured with a detector pointing along the laser beam. The multiple scattering, which is mainly double scattering, is measured with a second detector, pointing at a specified angle to the laser beam. The domain of scattering angles that contribute to the doublescattering signal increases monotonically as the pulse penetrates the cloud. The water droplets within the probed volume are assumed to have a constant size distribution. Hence, from the double-scatteringmeasured signal as a function of penetration depth within the cloud, the double-scattering phase function of the scattering volume is derived. Inverting the phase function results in a cloud-droplet-size distribution in the form of a log-normal function.

Wavelength dependence of backscattering and extinction of kaolin dust at CO(2) laser wavelengths: effect of multiple scattering

A multiple wavelength, pulsed CO(2) lidar system is used to measure spectral backscattering and extinction of kaolin dust of different optical thicknesses. The measurements show that the wavelength dependence of spectral backscattering changes with increased multiple scattering, whereas the spectral extinction remains relatively unchanged. A simple analytical two-stream radiative transfer model is used to confirm the measurements qualitatively. Several equations were derived from the model to show that in general the wavelength dependence of backscatter is less dependent on wavelength for a multiplescattering case. Therefore, the aerosol cloud becomes a diffuse target that is more flat in its spectral reflectance as multiple scattering increases. An application to differential absorption detection is discussed and shows that, in general, the effect of multiple scattering on the backscattered signal from aerosols will tend to reduce the error in deducing the average path-length concentration of the absorbing tr ce gas.

Inverse problem and the pseudoempirical orthogonal function method of solution. 2: Use

The information content about tropospheric rural aerosol size distributions (i.e., functional form, accuracy of solution, and moments of the solution) that may be obtained from measurements of the backscattered radiation with no additional information is discussed. The need for additional information in the form of constraints is demonstrated. The resulting solution depends on these constraints and is thus not entirely objective. The accuracy of the solution will depend, to some degree, on the validity of the constraints. The influence of constraints is examined for the inverse problem of determining aerosol size distributions from multiwavelength measurements of backscattered radiation. Using the method of pseudoempirical orthogonal functions, two types of constraint are employed: positivity and positivity combined with smoothing. A discussion of the effects on the solutions of the two constraints is presented.