Molecular Specificity and Proton Transfer Mechanisms in Aerosol Prenucleation Clusters Relevant to New Particle Formation

Abstract

Conspectus Atmospheric aerosol particles influence the Earth’s radiative energy balance and cloud properties, thus impacting the air quality, human health, and Earth’s climate change. Because of the important scientific and overarching practical implications of aerosols, the past two decades have seen extensive research efforts, with emphasis on the chemical compositions and underlying mechanisms of aerosol formation. It has been recognized that new particle formation (NPF) contributes up to 50% of atmospheric aerosols. Nowadays, the general consensus is that NPF proceeds via two distinct stages: the nucleation from gaseous precursors to form critical nuclei of sub-1–2 nm size, and the subsequent growth into large particles. However, a fundamental understanding of both the NPF process and molecular-level characterization of the critical size aerosol clusters is still largely missing, hampering the efforts in developing reliable and predictive aerosol nucleation and climate models. Both field measurements and laboratory experiments have gathered convincing evidence about the importance of volatile organic compounds (VOCs) in enhancing the nucleation and growth of aerosol particles. Numerous and abundant small clusters composed of sulfuric acid or bisulfate ion and organic molecules have been shown to exist in ∼2 nm sized aerosol particles. In particular, kinetic studies indicated the formation of clusters with one H2SO4 and one or two organics being the rate-limiting step. This Account discusses our effort in developing an integrated approach, which involves the laboratory cluster synthesis via electrospray ionization, size and composition analysis via mass spectrometry, photoelectron spectroscopic characterization, and quantum mechanics based theoretical modeling, to investigate the structures, energetics, and thermodynamics of the aerosol prenucleation clusters relevant to NPF. We have been focusing on the clusters formed between H2SO4 or HSO4 – and the organics from oxidation of both biogenic and anthropogenic emissions. We illustrated the significant thermodynamic advantage by involving organic acids in the formation and growth of aerosol clusters. We revealed that the functional groups in the organics play critical roles in promoting NPF process. The enhanced roles were quantified explicitly for specific functional groups, establishing a Molecular Scale that ranks highly hierarchic intermolecular interactions critical to aerosol formation. The different cluster formation pathways, probably mimicking the various polluted industrial environments, that involve cis-pinonic and cis-pinic acids were unveiled as well. Furthermore, one intriguing fundamental phenomenon on the unusual protonation pattern, which violates the gas-phase acidity (proton affinity) prediction, was discovered to be common in sulfuric acid–organic clusters. The mechanism underlying the phenomenon has been rationalized by employing the temperature-dependent experiments of sulfuric acid–formate/halide model clusters, which could explain the hig...

Stabilizing the Exotic Carbonic Acid by Bisulfate Ion

“…While isomers 1 , 3 , and 4 may be regarded as HSO4 – ·H 2 CO 3 , isomer 2 is better recognized as H 2 SO4·HCO 3 − , which is only about 2 kJ/mol higher in energy than isomer 1 . Isomer 2 is unexpected solely from gas-phase proton affinity prediction and its stability can be attributed to the formation of two strong hydrogen bonds and highly delocalized extra electrons according to previous findings by Hou et al [ 24 , 27 , 36 , 37 , 38 ]. Such electron delocalization can be partly reflected by the highest occupied molecular orbitals (HOMOs) as presented in Figure S2 .…”

“…Geometry optimizations of all minima and transition states on the potential energy surface of the anionic molecular complex [H 2 CO 3 •HSO 4 ] − were performed by using DFT calculations. Specifically, M06-2X functional and aug-cc-pVTZ basis set were employed, as previous studies showed that this combination could provide reliable results for hydrogen bonded, bisulfate ion-containing complexes [24,27,35,36]. Harmonic frequency analysis was conducted at the same level of theory.…”

“…Hydrogen bonding is an effective way to stabilize otherwise unstable and uncommon structures [24][25][26][27][28][29][30][31][32][33][34]. For example, Hou et al have performed a series of studies on bisulfate ion-containing complexes, showing that hydrogen bonding interactions can alter the protonation pattern in the complex, which violates the gas-phase proton affinity prediction [24,27,[35][36][37][38]. This motivated us to wonder whether an appropriate partner such as bisulfate ion could also stabilize the exotic conformers of H 2 CO 3 molecule.…”

“…Hydrogen bonding is an effective way to stabilize otherwise unstable and uncommon structures [ 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 ]. For example, Hou et al have performed a series of studies on bisulfate ion-containing complexes, showing that hydrogen bonding interactions can alter the protonation pattern in the complex, which violates the gas-phase proton affinity prediction [ 24 , 27 , 35 , 36 , 37 , 38 ].…”

Helium Nanodroplet Infrared Action Spectroscopy of the Proton-Bound Dimer of Hydrogen Sulfate and Formate: Examining Nuclear Quantum Effects

“… 12 , 22 This technique has been applied to the study of deprotonated water clusters, 17 , 19 model carboxylate proton-bound dimers, 26 − 28 and putative aerosol nucleation clusters. 12 , 13 The structure of AHA – systems has also been investigated utilizing photoelectron spectroscopy 9 − 11 , 29 and room-temperature IR action spectroscopy. 24 , 25 …”