Computer Simulation of Ion-Induced Nucleation in the Presence of Single Ions and Ion Pairs
Atmospheric aerosol particles play an important role in climate and human health. Despite their importance, the mechanisms of their formation are still poorly understood. Ion-induced nucleation may play a role in the process, but details are lacking about how the nucleation rate depends on the size, charge, and number of ions in the cluster. In order to better understand the role that ions might play in new particle formation, we have performed a series of AVUS-HR simulations of the water nucleation in the presence of both single ions and ion pairs. These simulations have shown that the location of the Gibbs free energy minimum on the free energy profile is a key factor controlling the barrier heights for different ions/pairs. When the minimum is located at larger cluster sizes, there is a smaller loss of gas phase entropy between the Gibbs free energy minimum and the critical cluster size (the cluster sizes contributing to the barrier), and the barrier height is usually lower. The location of the Gibbs free energy minimum is generally controlled by the strength of the ion-water interactions, and how quickly they decay with increasing cluster size. However, it is also possible for small differences between two different ionic clusters to persist over a wide range of cluster sizes, adding up to significant differences in the barrier height even when the minima are at similar sizes. We have also found that ion pairs are not as effective at enhancing nucleation as single ions, largely because the long range ion-water interactions are weaker for the ion pair cases, but can still significantly lower the barrier height for nucleation. Furthermore, they may still make a large contribution to atmospheric nucleation due to the much larger concentrations of potential ion-pair forming species compared to single ions.
Monte Carlo, new particle, atmospheric chemistry, aerosol
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