Near-Barrierless Ammonium Bisulfate Formation via a Loop-Structure Promoted Proton-Transfer Mechanism on the Surface of Water

Authors

Lei Li, University of Nebraska–Lincoln
Manoj Kumar, University of Nebraska–LincolnFollow
Chongqin Zhu, University of Nebraska–LincolnFollow
Jie Zhong, University of Nebraska–LincolnFollow
Joseph S. Francisco, University of Nebraska-LincolnFollow
Xiao Cheng Zeng, University of Nebraska-LincolnFollow

Date of this Version

1-2016

Citation

Published in Journal of the American Chemical Society 138 (2016), pp 1816−1819. doi 10.1021/jacs.5b13048

Comments

Copyright © 2016 American Chemical Society. Used by permission.

Abstract

In the atmosphere, a well-known and conventional pathway toward the formation of ammonium sulfate is through the neutralization of sulfuric acid with ammonia (NH₃) in water droplets. Here, we present direct ab initio molecular dynamics simulation evidence of the formation of ammonium bisulfate (NH₄HSO₄) from the hydrated NH₃ and SO₃ molecules in a water trimer as well as on the surface of a water droplet. This reaction suggests a new mechanism for the formation of ammonium sulfate in the atmosphere, especially when the concentration of NH₃ is high (e.g., ∼10 μg m⁻³) in the air. Contrary to the water monomer and dimer, the water trimer enables near-barrierless proton transfer via the formation of a unique loop structure around the reaction center. The formation of the loop structure promotes the splitting of a water molecule in the proton-transfer center, resulting in the generation a NH₄⁺/HSO₄⁻ ion pair. The loop-structure promoted proton-transfer mechanism is expected to be ubiquitous on the surface of cloud droplets with adsorbed NH₃ and SO₃ molecules and, thus, may play an important role in the nucleation of aerosol particles (e.g., fine particles PM_2.5) in water droplets.