On the Background Stratospheric Aerosol Layer

Abstract

Balloon-borne aerosol particle counter measurements are used in investigating the stratospheric sulfate layer in Wyoming in 1978 and 1979, a two-year volcanically quiescent period during which the layer appears to have been in a near equilibrium background state. Subtracting the background aerosol concentration from data obtained during an earlier volcanically active period suggests that the actual decay rate of volcanic aerosol is more than 30% faster than the rate that would be obtained without this correction. At background, the aerosol size distribution is found to remain remarkably constant between the tropopause and an altitude of approximately 25 km, with an abrupt transition to a distribution dominated by smaller particles above this altitude. In some respects, the observations compare favorably with equilibrium one-dimensional stratospheric aerosol models and thus to some extent support the concept of relatively inert tropospheric sulfurous gases, such as carbonyl sulfide and carbon disulfide, as the principal background stratospheric aerosol sulfur source. It is pointed out that a comparison of the 1978-79 observations with measurements made some 20 years ago during a period void of volcanic perturbations does not preclude a long-term increase in the background stratospheric aerosol level.