On the Formation of Stratospheric Aerosols

Abstract

Laboratory observations were made of the formation of Aitken nuclei and large particles in systems of air containing trace quantities of H₂O, S₀2, NH₃ and O₃, in varying proportions and under varying conditions of radiation with ultraviolet light. These studies yielded information concerning the photooxidation of SO₂, the mechanism of formation of stratospheric aerosols, and the possible role of trace organic gases in atmospheric nucleation processes. Specific results were: The proposed chemical mechanism for the formation of stratospheric aerosols based on the laboratory experiments is:which after n − 1 steps becomesReactions (4a)-(4n) produce sulfuric acid embryonic nuclei:while reaction (5) and (6) produce salt embryos:Reaction (7) is a rapid oxidation of SO₂ in solution. Continued addition of NH₃ neutralizes the acid providing more NH₄⁺ catalyst. The mechanism explains many of the observed characteristics of stratospheric aerosols. Hypothetical considerations show how large volcanic injections of SO₂ into the stratosphere can produce a long-lived H₂SO₄ aerosol which eventually becomes neutralized as NH₃ from the troposphere mixes into the stratosphere.