Improved quasi-unary nucleation model for binary H2 S O4 - H2 O homogeneous nucleation

Aerosol nucleation events have been observed at a variety of locations worldwide, and may have significant climatic and health implications. Binary homogeneous nucleation (BHN) of H2 S O4 and H2 O is the foundation of recently proposed nucleation mechanisms involving additional species such as ammonia, ions, and organic compounds, and it may dominate atmospheric nucleation under certain conditions. We have shown in previous work that H2 S O4 - H2 O BHN can be treated as a quasi-unary nucleation (QUN) process involving H2 S O4 in equilibrium with H2 O vapor, and we have developed a self-consistent kinetic model for H2 S O4 - H2 O nucleation. Here, the QUN approach is improved, and an analytical expression yielding H2 S O4 - H2 O QUN rates is derived. Two independent measurements related to monomer hydration are used to constrain the equilibrium constants for this process, which reduces a major source of uncertainty. It is also shown that the capillarity approximation may lead to a large error in the calculated Gibbs free energy change for the evaporation of H2 S O4 molecules from small H2 S O4 - H2 O clusters, which affects the accuracy of predicted BHN nucleation rates. The improved QUN model-taking into account the recently measured energetics of small clusters-is thermodynamically more robust. Moreover, predicted QUN nucleation rates are in better agreement with available experimental data than rates calculated using classical H2 S O4 - H2 O BHN theory.