Binary condensation on soluble aerosols -: On the activation and growth of the stratospheric aerosol from coupled HNO3-H2O uptake

Combining the well-known binary nucleation theory with the Kohler approach, an equilibrium condensation theory is formulated to study binary condensation processes on soluble aerosols. Especially, an appropriate Gibbs free energy together with the corresponding Kelvin equations are derived. In the case of an ideal droplet (Raoult's law for the activities), the differences between the binary condensation, binary nucleation and the Kohler theory are investigated showing that a stable droplet's stare can be predicted only within the framework of the condensation theories. The Gibbs free energy is applied to study the conditions un der which stratospheric aerosol particles consisting of liquid sulfuric acid can be activated by coupled HNO3-H2O uptake (binary condensation). Supplementary to the recently published results, an energetic description of the relevant processes is given. In particular, the valley and the minima of the corresponding Gibbs Free energy determine the possible condensation processes and the equilibrium states of an aqueous H2SO4-HNO3-H2O droplet. The influence of the Kelvin effect on the critical temperature of activation and on the droplet's composition in the activated state is expected only for the nucleation mode of the stratospheric aerosol, i.e. for sulfuric acid droplets smaller than 10 nm in diameter and with less than 1000 H2SO4 molecules per droplet. The critical temperature may fall below the frost point and the HNO3 percentage of the activated droplets with less than 100 H2SO4 particles per droplet is limited by about 40 wt% if the size-independent composition regime (r > 50 nm) is reached during the condensation. For H2SO4-H2O droplets larger than 10 nm this limitation value amounts to about 50 wt% for r > 100 nm. In this case, the critical temperature is size-independent. (C) 1997 Elsevier Science Ltd. All rights reserved.