Important chemical and physical roles of aerosols are discussed, and properties of stratospheric aerosols as revealed by experimental data are described. In situ measurements obtained by mechanical collection and scattered-light detection yield the overall size distribution of the aerosols, and analyses of preserved aerosol precursor gases by wet chemical, cryogenic and spectroscopic techniques indicate the photochemical sources of particle mass. Aerosol chemical reactions including those of gaseous precursors, those in aqueous solution, and those on particle surfaces are discussed, in addition to aerosol microphysical processes such as nucleation, condensation/evaporation, coagulation and sedimentation. Models of aerosols incorporating such chemical and physical processes are presented, and simulations are shown to agree with measurements. Estimates are presented for the potential aerosol changes due to emission of particles and gases by aerospace operations and industrial consumption of fossil fuels, and it is demonstrated that although the climatic effects of existing levels of stratospheric aerosol pollution are negligible, potential increases in those levels might pose a future threat.

Stratospheric aerosols: Observation and theory

Particle size distributions in Saturn's rings from voyager 1 radio occultation

Physical properties of the stratospheric aerosol population are inferred from cloud-free SAGE II multiwavelength extinction measurements in the Antarctic during late summer (February/March) and spring (September/October, November). Seasonal changes in these properties are used to infer physical processes occurring in the Antarctic stratosphere over the course of the winter. The analysis suggests that the apparent springtime cleansing of the Antarctic stratosphere is the result of aerosol redistribution through subsidence of the polar vortex air mass and sedimentation of large polar stratospheric cloud particles. The analysis also suggests that vortex processes are responsible for a significant downward transport of aerosol through the tropopause.

Use of stratospheric aerosol properties as diagnostics of Antarctic vortex processes

A simple method is developed to obtain the relative atmospheric aerosol content over low-albedo surfaces (water) from observations of the Erts (now Landsat) type. The method is based upon a derived relationship between the aerosol optical thickness and the satellite-observed upward scattered solar radiation and upon the proportionality of aerosol optical thickness to aerosol column density. The method is successfully applied to Erts 1 observations of the Middle East by comparing the aerosol content on a day when the meteorological observations indicated high atmospheric dust content with that on a day on which normal meteorological conditions prevailed.

Relative atmospheric aerosol content from ERTS observations

Retrieval of size distributions from multispectral optical depth measurements requires solution of an ill-posed inverse problem. The ill-posedness causes problems such as solution ambiguity. Size distribution retrieval becomes more complicated in the presence of nonspherical particles and/or refractive index errors. A new retrieval algorithm is first developed which allows for both smoothing and nonnegativity constraint along with the L-curve method for choosing the Lagrange multiplier that controls the degree of the imposed smoothing. This new algorithm is compared to an iterative algorithm and the method of truncated singular value decomposition, demonstrating that the new algorithm outperforms the other two. With the new algorithm to perform the size distribution retrieval and with the T-matrix method to calculate optical depth for a given cloud consisting of randomly oriented finite circular cylinders, the influence of particle nonsphericity on the size distribution retrieval is investigated by use of the Mie theory for spheres as well as the anomalous diffraction theory for cylinders. The results show that spurious particles and even spurious particle modes occur for both approximations. The effect of refractive index errors are also investigated, showing that even a small perturbation of refractive indices can cause serious distortions of retrieved size distributions. A further examination reveals that either applying an approximate light-scattering theory (Mie theory or anomalous diffraction theory) to nonspherical particles or using incorrect refractive indices results in systematic errors in the model which in turn conspire with the ill-posedness inherent in the retrieval to cause the distortions of retrieved size distributions. The retrieval process essentially transforms the model error into the error in retrieved size distribution, yet improves the agreement between true and retrieved optical depth.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1998JD200122

Particle size distribution retrieval from multispectral optical depth: Influences of particle nonsphericity and refractive index

A critical survey of the literature on the use of light-scattering mechanisms in the remote monitoring of atmospheric aerosols, their geographical and spatial distribution, and temporal variations was undertaken to aid in the choice of future operational systems, both ground based and air or space borne. An evaluation, mainly qualitative and subjective, of various techniques and systems is carried out. No single system is found to be adequate for operational purposes. A combination of earth surface and space-borne systems based mainly on passive techniques involving solar radiation with active (lidar) systems to provide auxiliary or backup information is tentatively recommended.

D. Deirmendjian

Papers

A survey of light‐scattering techniques used in the remote monitoring of atmospheric aerosols