Stratospheric Ion and Aerosol Chemistry and Possible Links with Cirrus Cloud Microphysics—A Critical Assessment

Abstract

It has been postulated that variations in galactic cosmic rays could affect production of stratospheric aerosols which, after transport to the upper troposphere, could affect latent heat release in convective and cyclonic systems. This hypothesis is based on the fact that low energy cosmic rays are carriers of solar variability to the lower atmosphere where consequent changes of orders of tens percent in lower stratospheric ion production on the 11-year time scale have been observed. The purpose of this work was to assess stratospheric ion chemistry, its potential link to aerosol formation and the subsequent transport of stratospheric aerosols to upper tropospheric regions favorable for cirrus cloud formation. It was found after reviewing the various gas-to-particle conversion processes, that ion induced nucleation and any other known phase transitions involving ions and sulfuric acid vapor are not likely to be efficient processes for stratospheric aerosol formation. They cannot compete with condensation of sulfuric acid on preexisting particles larger than about 0.15 micrometer radius of surface (volcanos) or meteoritic origin. Hence, galactic cosmic rays can not have a significant impact on the stratospheric aerosol population. Changes in the stratospheric aerosol burden due to volcanic activities (injection of both particles and sulfur bearing molecules which eventually are oxidized to sulfuric acid) are by up to two orders of magnitude larger than changes in ion densities. Consequently, this assessment leads to the conclusion that there exists a possibility for a modulation of cirrus cloud radiative properties (size distribution, number density and chemical composition). Since the stratospheric aerosol has lifetimes of one-half to two years (depending on altitude), such modulation could extend over long time periods and would dwarf any possible influence of solar induced events that involve stratospheric ion chemistry.