It has been known for quite some time that even trace amounts of sulfuric acid can, in the presence of water vapor, homogeneously nucleate to form new particles. If, as has been suggested, nucleation of sulfuric acid/water particles in the free troposphere or boundary layer is a major particle number source, then methods for the prediction of nucleation rates are needed to assess the impacts of changing sulfur precursor gas concentrations upon global new particle formation rates. Theoretical rate expressions exist, but it is very difficult to validate these experimentally. The work of Wyslouzil et al. [1991] confirmed some aspects of binary nucleation theory, but showed that, in general, nucleation rates are stronger functions of temperature than predicted, and no simple correction factor, applicable over the range of atmospherically-relevant conditions, can be derived to bring theoretical predictions into agreement with experimentally-derived rates. The recent development of an analytical technique for measuring gas-phase sulfuric acid ( Eisele and Tanner [1993]) is a significant advancement that has made possible estimates of ambient new particle formation and growth rates for comparison with theory.
Particle nucleation is also a poorly understood feature of polar stratospheric cloud nucleation. Frozen sulfuric acid particles probably act as sites for the formation of nitric acid trihydrate crystals. Jensen et al. [1991] computed freezing rates of sulfuric acid particles under stratospheric conditions and obtained results consistent with some atmospheric observations; however, they also point out the large uncertainties in applying classical nucleation theory and the need for experimental and observational validation. Marti and Mauersberger [1993] performed laboratory experiments to condense solids from vapor mixtures of water and nictric acid under stratospheric-type conditions. Their results indicate that the composition of newly-formed PSCs may evolve with time, and the time to reach equilibrium composition in the stratosphere can be appreciable; this would have an effect upon the heterogeneous chemistry that such particles participate in.