Stratospheric aerosol particles and their optical properties

Abstract

This review emphasizes those aspects of aerosol particles in the stratosphere that have not been covered by other reviews. This approach is not severely limiting because of the almost explosive increase in our knowledge of such particulate matter during the last few years. Furthermore, this review places special emphasis on the optical properties of the particles. Stratospheric particles have been investigated by remote optical means for at least 80 years, but not until about 1960 were such particles collected and examined. The particles consist largely of volcanic ash for periods of possibly a few months following major explosive volcanic eruptions, but at other times they consist largely of impure sulfuric acid which may be in the form of droplets or crystals. The sulfuric acid is believed to be formed largely by the oxidation and hydration of sulfur dioxide, much of which is introduced into the stratosphere by volcanic eruptions. There is considerable uncertainty as to whether the particle size distributions are unimodal or polymodal or whether the nature of the distribution varies markedly with time and space. The mixing ratio of the particles, at least of the larger ones in the 0.1‐ to 1µm‐radius interval, is almost always greatest a few kilometers above the tropopause. However, this ‘layer’ is often highly structured. The particles may serve as catalysts for, or otherwise participate in, stratospheric chemical reactions. Stratospheric particles, by absorbing and scattering radiation from the sun and earth, can affect the atmosphere's radiation balance, thereby affecting the climate. Some numerical results concerning the amount of energy lost from an incident light beam as it propagates through a layer of uniformly distributed particles are presented. The results are shown as contours of constant values of the total percentage loss of energy due to extinction, scattering, and absorption for a vertically incident solar beam over a wide range of particle radii and imaginary refractive indices for real refractive indices of 1.4 and 1.5. The application of Mie scattering theory to the use of laser radar (lidar) is also described. Another major effect of the stratospheric aerosols is the denial of radiation to the region below the stratosphere. At the same time, they add energy to the stratosphere by the absorption of radiation. Various types of analytical models of climate change due to aerosols are discussed, and the energetic equilibrium of small particles in the atmosphere is also considered in some detail.