Determination of the complex refractive index and size distribution of atmospheric particulates from bistatic‐monostatic lidar and solar radiometer measurements

Abstract

A method is presented for inferring both the size distribution and the complex refractive index of atmospheric particulates from combined bistatic-monostatic lidar and solar radiometer observations. The basic input measurements are spectral optical depths at several visible and near-infrared wavelengths as obtained with a solar radiometer and backscatter and angular scatter coefficients as obtained from a bistatic-monostatic lidar. The spectral optical depth measurements obtained from the radiometer are mathematically inverted to infer a columnar particulate size distribution. Advantage is taken of the fact that the shape of the size distribution obtained by inverting the particulate optical depth is relatively insensitive to the particle refractive index assumed in the inversion. Bistatic-monostatic angular scatter and backscatter lidar data are then processed to extract an optimum value for the particle refractive index subject to the constraint that the shape of the particulate size distribution be the same as that inferred from the solar radiometer data. Specifically, the scattering parameters obtained from the biitatic-monostatic lidar data are compared with corresponding theoretical computations made for various assumed refractive index values. That value which yields best agreement, in a weighted least squares sense, is selected as the optimal refractive index estimate. The results of this procedure applied to a set of simulated measurements as well as to measurements collected on two separate days are presented and discussed.

Lidar and Balloon-Borne Cascade Impactor Measurements of Aerosols: A Case Study

“…Only one day, 5 April, when the extinction was very low, yielded a high real component (1.54) consistent with ammonium sulfate and silica which are so frequently named as chief aerosol constituents. The imaginary component estimates determined from the data (average of 0.004) are in agreement with values obtained for the 500-700 nm wavelength range from a number of other desert aerosol experiments ( e g , De Luisi et al, 1970;Grams et al, 1974;Lindberg and Laude, 1974;Spinhirne et al, 1980). Although imaginary indices in the range of 0.001-0.01 do not correspond to any specific substance commonly associated with atmospheric aerosols, the occurrence of values in t h s range is possibly a result of small amounts of carbon mixing with otherwise very weakly absorbing particles (Lindberg and Gillespie, 1977;Ackerman and Toon, 1981).…”

“…Table 3 lists the aerosol extinction to backscatter ratio, S,, mixing layer height, mixing layer optical depth and total optical depth (actually optical depth to a height of 19.3 km) associated with each of the extinction profiles. The majority of the extinction profile retrievals yielded fairly low extinction standard deviations ( -$-15%) for the mixing layer, which is an indication that the requirements that horizontal homogeneity in / 3, and S, be constant with height were reasonably well met (Spinhirne et al, 1980 (Shaw, et al, 1973;King et al, 1980). The solar radiometer optical depth measurements provide a cross check to verify the lidar extinction retrieval.…”

“…The required transmission value may be obtained from lidar returns obtained at different slant angles if the atmosphere is reasonably horizontally homogeneous. Furthermore, by combining lidar returns obtained for several slant angles, it is possible to retrieve S,, aa(r) and T(r) simultaneously (Spinhirne et al, 1980). In this approach, slant path lidar measurements are processed by a multiangle integral solution of the lidar equation to extract S, and vertical profiles a,(r), &(r) and T(r).…”

Airborne Measurements of Optical Properties of Tropospheric Aerosols over an Urban Area

“…Many investigators have attempted to estimate this quantity, and found that the real and imaginary in dices of refraction were typically within the range of 1.45*1.55 and 0.005*0.02, respectively, for the tropospheric aerosols (e.g. Grams et al, 1974;Patterson et al, 1977;Reagan et al, 1980;Tanaka et al, 1983). The value of refractive index, m = 1.50-0.0li was assumed in this study.…”

Characterization of atmospheric aerosols across Canada from a ground‐based sunphotometer network: AEROCAN

“…The differences observed could be attributed to the non-homogeneity of the atmosphere and possibly some residual contamination by high altitude cirrus clouds (probable in the case for the sunphotometer measurements). The AOD profile was inferrred using the assumption of a lidar ratio (extinction/backscattering) of 30 (Reagan et al, 1980;Ansmann et al, 1992). This typical value for rural aerosol (Evans, 1988) may be appropriate for the Egert site by reason of the relatively low AOD values recorded (see Fig.…”