title: Near-Barrierless Ammonium Bisulfate Formation via a Loop-Structure Promoted Proton-Transfer Mechanism on the Surface of Water.
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 PM2.5) in water droplets.
authors.affiliation: Department of Chemistry, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States.
authors.forename: Lei
authors.initials: L
authors.lastname: Li
authors.affiliation: Department of Chemistry, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States.
authors.forename: Manoj
authors.initials: M
authors.lastname: Kumar
authors.affiliation: Department of Chemistry, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States.
authors.forename: Chongqin
authors.initials: C
authors.lastname: Zhu
authors.affiliation: Department of Chemistry, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States.
authors.forename: Jie
authors.initials: J
authors.lastname: Zhong
authors.affiliation: Department of Chemistry, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States.
authors.forename: Joseph S
authors.initials: JS
authors.lastname: Francisco
authors.affiliation: Department of Chemistry, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States.|Collaborative Innovation Center of Chemistry for Energy Materials, University of Science and Technology of China , Hefei, Anhui 230026, China.
authors.forename: Xiao Cheng
authors.initials: XC
authors.lastname: Zeng