Retrieval of stratospheric aerosol size distributions and integral properties from simulated lidar backscatter measurements.

A new approach for retrieving aerosol properties from extinction spectra is extended to retrieve aerosol properties from lidar backscatter measurements. In this method it is assumed that aerosol properties are expressed as a linear combination of backscatters at three or fewer wavelengths commonly used in lidar measurements. The coefficients in the weighted linear combination are obtained by minimization of the retrieval error averaged for a set of testing size distributions. The formulas can be used easily by investigators to retrieve aerosol properties from lidar backscatter measurements such as the Lidar In-Space Technology Experiment and Pathfinder Instruments for Clouds and Aerosols Spaceborne Observations.

Retrieval analysis of aerosol-size distribution with simulated extinction measurements at SAGE III wavelengths.

The retrieval of aerosol-size distribution from simulated aerosol-extinction-coefficient measurements of the new satellite instrument, the Stratospheric Aerosol and Gas Experiment (SAGE) III, is investigated. A detailed discussion on the aerosol-size-distribution information content of the SAGE III aerosol-extinction-coefficient measurement is provided. Results of the investigation indicate that unimodal as well as bimodal log-normal size distributions can be inferred. In addition, it is shown that a shape-constraint-free size distribution can be derived from SAGE III aerosol measurements by use of the randomized minimization search technique and the optimal estimation theory.

SAGE II aerosol data validation based on retrieved aerosol model size distribution from SAGE II aerosol measurements.

This paper describes an investigation of the comprehensive aerosol correlative measurement experiments conducted between November 1984 and July 1986 for satellite measurement program of the Stratospheric Aerosol and Gas Experiment (SAGE II). The correlative sensors involved in the experiments consist of the NASA Ames Research Center impactor/laser probe, the University of Wyoming dustsonde, and the NASA Langley Research Center airborne 14-inch (36 cm) lidar system. The approach of the analysis is to compare the primary aerosol quantities measured by the ground-based instruments with the calculated ones based on the aerosol size distributions retrieved from the SAGE II aerosol extinction measurements. The analysis shows that the aerosol size distributions derived from the SAGE II observations agree qualitatively with the in situ measurements made by the impactor/laser probe. The SAGE II-derived vertical distributions of the ratio N0.15/N0.25 (where Nr is the cumulative aerosol concentration for particle radii greater than r, in micrometers) and the aerosol backscatter profiles at 0.532- and 0.6943-micrometer lidar wavelengths are shown to agree with the dustsonde and the 14-inch (36-cm) lidar observations, with the differences being within the respective uncertainties of the SAGE II and the other instruments.

Stratospheric Aerosol Effects from Soufriere Volcano as Measured by the SAGE Satellite System.

During its April 1979 eruption series, Soufriere Volcano produced two major stratospheric plumes that the SAGE (Stratospheric Aerosol and Gas Experiment) satellite system tracked to West Africa and the North Atlantic Ocean. The total mass of these plumes, whose movement and dispersion are in agreement with those deduced from meteorological data and dispersion theory, was less than 0.5 percent of the global stratospheric aerosol burden; no significant temperature or climate perturbation is therefore expected.

Modeling of growth and evaporation effects on the extinction of 1.0-microm solar radiation traversing stratospheric sulfuric acid aerosols.

In earlier papers we presented results of parametric studies of the separate and combined effects of aerosol microphysical processes on the time dependence of extinction of four visible and IR laser beams traversing an aerosol medium. Results of these studies can be applied to monitor the temporal changes of aerosol properties inside a cloud chamber or in an open environment in the troposphere. As a continuation of this series, the effects of growth and evaporation of sulfuric acid aerosols in the stratosphere on the extinction of solar radiation traversing such an aerosol medium are reported in this paper. Extinction of 1.0-microm solar radiation was studied since this wavelength was used to monitor the aerosol extinction properties by two recent satellite experiments: Stratospheric Aerosol Measurement II (SAM II) and Stratospheric Aerosol and Gas Experiment (SAGE). Our modeling results show that aerosol extinction is not very sensitive to the change of ambient water vapor concentration but is sensitive to the change of ambient temperature, especially at low ambient temperature and high ambient water vapor concentration. The effects of initial aerosol size distribution and composition on the change of aerosol extinction due to growth and evaporation processes are elucidated. The application of results of this parametric study is discussed.

Modeling of growth, evaporation and sedimentation effects on transmission of visible and IR laser beams in artificial fogs.

The dense polydisperse aerosol particles in a quiet chamber may spontaneously go through different microphysical processes including gravitational sedimentation, thermal coagulation, and growth or evaporation. In an earlier paper, we presented the results of a parametric study of the combined and separate effects of thermal coagulation and sedimentation on the time dependence of extinction of four visible and IR laser beams traversing an aerosol medium. As a continuation of this series of studies, the separate and combined effects of growth or evaporation and gravitational sedimentation on the time dependence of extinction of the same four visible and IR laser beams traversing in artificial fogs will be reported in this paper. The method of numerically modeling the change of water droplet size distribution with time due to growth/evaporation and the cutoff of larger aerosols due to gravitational sedimentation is described in detail. Factors governing the relative importance of these two processes are discussed Results of this study show that the relative humidity or ambient temperature is a crucial parameter in determining the optical depth of the water droplet and aerosol media undergoing microphysical processes.

Modeling of coagulation-sedimentation effects on transmission of visible/IR laser beams in aerosol media.

It has been shown that the size distribution of dense polydisperse aerosol particles settling in a quiet chamber can be retrieved from the time dependence of optical extinction of a laser beam traversing the aerosol. However, in this method, the effect of coagulation on the size distribution was completely ignored. In order to understand the effects of coagulation, a parametric study of the separate and combined effects of sedimentation and (thermal) coagulation on the time dependence of extinction of four visible and IR laser beams traversing such an aerosol medium has been carried out, and the results are presented in this paper. The method of numerically modeling the change of aerosol size distribution with time due to thermal coagulation and the cutoff of larger aerosols due to gravitational sedimentation is described in detail. Results of the study show that, whereas the coagulation effects are predominant for smaller size particles (diameter <1.4 ,microm in the particular experimental setup under stud , the sedimentation effects are predominant for particles larger than 2.0 microm in diameter.