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

Complex refractive index of ammonium nitrate in the 2-20-microm spectral range

Using high-resolution Fourier-transform infrared absorbance and transmittance spectral data for ammonium sulfate (AMS), calcium carbonate (CAC), and ammonium nitrate (AMN), we made comparisons with previously published complex reactive-index data for AMS and CAC to infer experimental parameters to determine the imaginary refractive index for AMN in the infrared wavelength range from 2 to 20 microm. Subtractive Kramers-Kronig mathematical relations were applied to calculate the real refractive index for the three compositions. Excellent agreement for AMS and CAC with the published values was found, validating the complex refractive index obtained for AMN. We performed backscatter calculations using a log-normal size distribution for AMS, AMN, and CAC aerosols to show differences in their backscattered spectra.

Backscattering measurements of atmospheric aerosols at CO2 laser wavelengths: implications of aerosol spectral structure on differential-absorption lidar retrievals of molecular species

The volume backscattering coefficients of atmospheric aerosol were measured with a tunable CO2 lidar system at various wavelengths in Utah (a desert environment) along a horizontal path a few meters above the ground. In deducing the aerosol backscattering, a deconvolution (to remove the smearing effect of the long CO2 lidar pulse and the lidar limited bandwidth) and a constrained-slope method were employed. The spectral shape beta(lambda) was similar for all the 13 measurements during a 3-day period. A mean aerosol backscattering-wavelength dependence beta(lambda) was computed from the measurements and used to estimate the error Delta(CL) (concentration-path-length product) in differential-absorption lidar measurements for various gases caused by the systematic aerosol differential backscattering and the error that is due to fluctuations in the aerosol backscattering. The water-vapor concentration-path-length product CL and the average concentration C = <CL>/L for a path length L computed from the range-resolved lidar measurements is consistently in good agreement with the water-vapor concentration measured by a meteorological station. However, I was unable to deduce, reliably, the range-resolved water-vapor concentration C(r), which is the derivative of the range-dependent product CL, because of the effect of residual noise caused mainly by errors in the deconvolved lidar measurements.

Multiple-scattering effects on differential absorption for the transmission of a plane-parallel beam in a homogeneous medium

The transmission, including all scattering orders, of a plane-parallel beam in a homogeneous scattering medium containing aerosols (e.g., water cloud) mixed with an absorbing gas (e.g., ozone) is computed with a two-stream radiative transfer model. From differential transmission the concentration of the gas is deduced. The effect of multiple-scattering on the deduced concentration is shown for conservative scattering aerosols for which the multiple scattering by the aerosols is differentially absorbed by the gas and for nonconservative scattering aerosols for which the multiple scattering is differentially absorbed by the aerosols as well as differentially absorbed by the gas. The two-stream analytical model (with no dependence on the field of view) shows good qualitative agreement (especially for a small field of view) with a numerical radiative transfer model in which the trace gas concentration is computed for the different detector's field of view.

Multiple-scattering transmission and an effective average photon path length of a plane-parallel beam in a homogeneous medium

A two-stream radiative transfer model is used to derive expressions for the multiple-scattered transmitted flux (including single-scattering contributions) and the total effective average photon path length on transmission of a normally incident plane-parallel beam on a homogeneous layer characterized by the optical depth, the single-scattering albedo, and the asymmetry parameter of the scatterers. The results are simple analytical expressions that are useful for modifying the Beer-Lambert transmission law for a thick scattering medium in which the multiple-scattering contribution to the transmission is not negligible.

Backscattering and polarization ratio for cylindrical graphite fibers at CO(2) laser wavelengths

Measurements are presented of backscattering from an artificial cloud composed of cylindrical graphite fibers at a distance of 730 m with a CO(2) lidar system at 20 wavelengths and two incident polarizations (horizontal and vertical). The backscatter measurements and the polarization ratio are in good agreement with single-scattering calculations for an infinitely long cylinder at normal incidence. The measurements give experimental evidence of the predominant orientation of graphite fibers with their long axes in a random position in the x-y plane (i.e., parallel to the ground).