A chronology of annual mean effective radii of stratospheric aerosols from volcanic eruptions during the twentieth century as derived from ground-based spectral extinction measurements

Abstract

Stratospheric extinction can be derived from ground-based spectral photometric observations of the Sun and other stars (as well as from satellite and aircraft measurements available since 1979) and is found to increase after large volcanic eruptions. This increased extinction shows a characteristic wavelength dependence that gives information about the chemical composition and the effective (or area-weighted mean) radius of the particles responsible for it. Known to be tiny aerosols constituted of sulfuric acid in a water solution, the stratospheric particles at midlatitudes exhibit a remarkable uniformity of their column-averaged effective radii reff in the first few months after the eruption. Considering the seven largest aerosol-producing eruptions of the twentieth century, reff at this phase of peak aerosol abundance is ∼0.3 μm in all cases. A year later, reff either has remained about the same size (almost certainly in the case of the Katmai eruption of 1912) or has increased to ∼0.5 μm (definitely so for the Pinatubo eruption of 1991). The reasons for this divergence in aerosol growth are unknown.