Collision-sticking kinetics of acid-base clusters and its influence on atmospheric new particle formation

Kinetics of collision-sticking processes between vapor molecules and molecular clusters of low volatile compounds facilitates the initial steps of atmospheric clustering. Conventional theoretical models are quite inaccurate due to the neglection of long-range interactions that essentially govern the kinetics of these microscopic phenomena. Here, we present a consistent and generic theoretical model for evaluating collision rates between molecules and molecular clusters with intermolecular potentials properly incorporated. The model requires solely the elementary molecule-molecule potential as a-priori information but predicts collision rates of molecular clusters at arbitrary sizes, with an accuracy comparable to all-atom molecular dynamics simulations we performed for sulfuric acid-dimethylamine clusters, a typical example of acid-base induced clustering. The carefully devised simulations validate the theoretical model and elucidate the kinetics of the molecular collision-sticking process. It is found that the vibrational coupling after the collision between a sulfuric acid molecule and a sulfuric acid cluster can be occasionally unsuccessful, i.e., no stable bond is formed after the collision. However, introducing dimethylamine molecules to the sulfuric acid cluster can notably increase the probability of forming stable bonds and hence stable product clusters. The results offer fundamental insights into the initial steps of molecular clustering and will facilitate the development of efficient kinetic approach-based nucleation models.