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 quantum yield for the homogeneous oxidation of SO2, with irradiation in the first excited band is less than 1.0×10−9.
The addition compounds of NH3 and SO2, if they form, are not precursors to the formation of ammonium sulfate in the atmosphere.
Aitken nuclei are formed by oxidation of SO2, by O atoms with traces of water vapor present.
The addition of NH3 to the system of SO2 with O atoms gives very rapid production of high concentrations of Aitken nuclei and large particles, probably of ammonium sulfate.
Irradiation of air with H2O vapor and trace organic gases at wavelengths <2500 Å produces Aitken nuclei.
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.
