Gas-phase collision rate enhancement factors for acid-base clusters up to 2 nm in diameter from atomistic simulation and the interacting hard sphere model

Collisions of neutral molecules and clusters is the most prevalent pathway in atmospheric new particle formation (NPF), and therefore such collisions have direct implications on air quality and climate. Until recently, these collisions have been modeled mainly using non-interacting hard-sphere (NHS) models, which systematically underestimate collision and particle formation rates, due to omission of long-range interactions. Lately, atomistic simulations have been used to study neutral molecule-molecule and molecule-cluster collisions (Halonen et al., 2019; Yang et al., 2023), but studies on cluster-cluster collisions are still lacking despite the relevant role they can play e.g. in haze formation in polluted urban areas (Guo et al., 2014). To calculate more realistic collision rates between clusters of acid-base pairs, we have studied collisions between neutral clusters of N bisulphate and N dimethylammonium ions at T = 300 K up to N = 32 using atomistic molecular dynamics (MD) simulations. Direct simulation results are then compared against both the traditional NHS model and the newly proposed interacting hard-sphere (IHS) variant (Yang et al., 2023), respectively. We find the collision rates in the atomistic MD simulations to be enhanced by factors of 2.2 - 5.6 over the NHS results, with enhancement slowly decreasing with increasing cluster size. In contrast, the IHS model yields a constant enhancement factor of 3.4 for all collisions between same-sized clusters, which decreases with increasing cluster size ratio. Our results demonstrate how even collisions between clusters of tens of acid-base pairs at a relatively high temperature cannot be accurately modeled when long-range interactions are neglected, as convergence towards the non-interacting limit is slow when cluster radius grows. Nor can the results be explained by simple point-particle models, highlighting the importance of atomistic details of intermolecular interactions.

Guo, S., Hu, M., Zamora, M. L., Peng, J., Shang, D., Zheng, J., Du, Z., Wu, Z., Shao, M., Zeng, L., et al.: Elucidating severe urban haze formation in China, Proc. Nat. Acad. Sci. USA, 111, 17373-17378, 2014.

Halonen, R., Zapadinsky, E., Kurtén, T., Vehkamäki, H., and Reischl, B.: Rate enhancement in collisions of sulfuric acid molecules due to long-range intermolecular forces, Atmos. Chem. Phys., 19, 13355-13366, 2019.

Yang, H., Neefjes, I., Tikkanen, V., Kubečka, J., Kurtén, T., Vehkamäki, H., and Reischl, B.: Collision-sticking rates of acid–base clusters in the gas phase determined from atomistic simulation and a novel analytical interacting hard-sphere model, Atmos. Chem. Phys., 23, 5993-6009, 2023.