Understanding the Formation and Growth of New Atmospheric Particles at the Molecular Level through Laboratory Molecular Beam Experiments

Atmospheric new particle formation (NPF), which exerts comprehensive implications for climate, air quality and human health, has received extensive attention. From molecule to cluster is the initial and most important stage of the nucleation process of atmospheric new particles. However, due to the complexity of the nucleation process and limitations of experimental characterization techniques, there is still a great uncertainty in understanding the nucleation mechanism at the molecular level. Laboratory-based molecular beam methods can experimentally implement the generation and growth of typical atmospheric gas-phase nucleation precursors to nanoscale clusters, characterize the key physical and chemical properties of clusters such as structure and composition, and obtain a series of their physicochemical parameters, including association rate coefficients, electron binding energy, pickup cross section and pickup probability and so on. These parameters can quantitatively illustrate the physicochemical properties of the cluster, and evaluate the effect of different gas phase nucleation precursors on the formation and growth of atmospheric new particles. We review the present literatures on atmospheric cluster formation and reaction employing the experimental method of laboratory molecular beam. The experimental apparatuses were classified and summarized from three aspects of cluster generation, growth and detection processes. Focus of this review is on the properties of nucleation clusters involving different precursor molecules of water, sulfuric acid, nitric acid and NxOy, respectively. We hope this review will provide a deep insight for effects of cluster physicochemical properties on nucleation, and reveal the formation and growth mechanism of atmospheric new particle at the molecular level.

Surface or Internal Hydration - Does It Really Matter?

The precise location of an ion or electron, whether it is internally solvated or residing on the surface of a water cluster, remains an intriguing question. Subtle differences in the hydrogen bonding network may lead to a preference for one or the other. Here we discuss spectroscopic probes of the structure of gas-phase hydrated ions in combination with quantum chemistry, as well as H/D exchange as a means of structure elucidation. With the help of nanocalorimetry, we look for thermochemical signatures of surface vs internal solvation. Examples of strongly size-dependent reactivity are reviewed which illustrate the influence of surface vs internal solvation on unimolecular rearrangements of the cluster, as well as on the rate and product distribution of ion-molecule reactions.

Lamellar Lyotropic Liquid Crystal Superior to Micellar Solution for Proton Conduction in an Aqueous Solution of 1-Tetradecyl-3-methylimidazolium Hydrogen Sulfate

Humidified perfluorosulfonic acid polymers with a nanoscopic phase-separated morphology are highly proton-conductive materials for fuel cells, yet morphology tuning of the acidic materials for enhanced conduction remains a challenge. Aqueous acidic lyotropic liquid crystals (LLCs) provide a powerful platform to construct well-defined nanostructures for proton conduction. We report an aqueous LLC formed by 1-tetradecyl-3-methylimidazolium hydrogen sulfate, exhibiting a proton conductivity of 210 mS cm^-1 at 25 °C, which surpasses that formed by alkylsulfonic acid, thus demonstrating that a mobile acidic anion is more efficient than constrained sulfonic acid functionality to transport protons in LLCs. For an aqueous solution of 1-alkyl-3-methylimidazolium hydrogen sulfate, a lamellar LLC results in higher conductivity than a micellar solution under the same hydration conditions. The peak power density of the fuel cell fabricated from porous membranes filled with the lamellar LLC is four times as high as that filled with the micellar solution. The work offers an efficient way to construct highly proton-conductive LLC materials for fuel cell application.

Mass spectrometry of aerosol particle analogues in molecular beam experiments

Nanometer-size particles such as ultrafine aerosol particles, ice nanoparticles, water nanodroplets, etc, play an important, however, not yet fully understood role in the atmospheric chemistry and physics. These species are often composed of water with admixture of other atmospherically relevant molecules. To mimic and investigate such particles in laboratory experiments, mixed water clusters with atmospherically relevant molecules can be generated in molecular beams and studied by various mass spectrometric methods. The present review demonstrates that such experiments can provide unprecedented details of reaction mechanisms, and detailed insight into the photon-, electron-, and ion-induced processes relevant to the atmospheric chemistry. After a brief outline of the molecular beam preparation, cluster properties, and ionization methods, we focus on the mixed clusters with various atmospheric molecules, such as hydrated sulfuric acid and nitric acid clusters, N_x O_y and halogen-containing molecules with water. A special attention is paid to their reactivity and solvent effects of water molecules on the observed processes.

Cluster Model Studies of Anion and Molecular Specificities via Electrospray Ionization Photoelectron Spectroscopy

Ion specificity, a widely observed macroscopic phenomenon in condensed phases and at interfaces, is a fundamental chemical physics issue. Herein we report our recent studies of such effects using cluster models in an "atom-by-atom" and "molecule-by-molecule" fashion not possible with the condensed-phase methods. We use electrospray ionization (ESI) to generate molecular and ionic clusters to simulate key molecular entities involved in local binding regions and characterize them by employing negative ion photoelectron spectroscopy (NIPES). Inter- and intramolecular interactions and binding configurations are directly obtained as functions of the cluster size and composition, providing molecular-level descriptions and characterization over the local active sites that play crucial roles in determining the solution chemistry and condensed-phase phenomena. The topics covered in this article are relevant to a wide range of research fields from ion specific effects in electrolyte solutions, ion selectivity/recognition in normal functioning of life, to molecular specificity in aerosol particle formation, as well as in rational material design and synthesis.

Structures and energetics of hydrated deprotonated cis-pinonic acid anion clusters and their atmospheric relevance

Pinonic acid, a C₁₀-monocarboxylic acid with a hydrophilic –CO₂H group and a hydrophobic hydrocarbon backbone, is a key intermediate oxidation product of α-pinene – an important monoterpene compound in biogenic emission processes that influences the atmosphere.

Reactions between microhydrated superoxide anions and formic acid

Reactions between water clusters containing the superoxide anion, O₂˙⁻(H₂O)_n (n = 0–4), and formic acid, HCO₂H, were studied experimentally in vacuo and modelled using quantum chemical methods.

Oxidation of NO˙ by small oxygen species HO2− and O2˙−: the role of negative charge, electronic spin and water solvation

Oxidation of NO˙ into NO₂⁻ occurs upon reaction with HO₂⁻(H₂O)_n clusters but not with O₂˙⁻(H₂O)_n clusters.

Nucleophilic Substitution in Reactions between Partially Hydrated Superoxide Anions and Alkyl Halides

The effect of solvation by water molecules on the nucleophilicity of the superoxide anion, O2(•-), has been investigated in detail by mass spectrometric experiments and quantum chemical calculations, including direct dynamics trajectory calculations. Specifically, the SN2 reactions of O2(•-)(H2O)n clusters (n = 0-5) with CH3Cl and CH3Br were studied. It was found that the reaction rate decreases when the number of water molecules in the cluster increases; furthermore, reaction with CH3Br is in general faster than reaction with CH3Cl for clusters of the same size. In addition, key transition-state geometries were identified and probed by Born-Oppenheimer molecular dynamics, showing how a water molecule may be transferred from the nucleophile to the leaving group during the reaction. The computational models are in good agreement with the experimental observations.

Geometry of the magic number H+(H2O)21 water cluster by proxy

Substituting H₂O with tert-butanol allows for the experimental determination of the number of dangling hydrogen bonds in the H⁺(H₂O)₂₁ cluster.

CO(2) incorporation in hydroxide and hydroperoxide containing water clusters--a unifying mechanism for hydrolysis and protolysis

The reactions of CO2 with anionic water clusters containing hydroxide, OH(-)(H2O)n, and hydroperoxide, HO2(-)(H2O)n, have been studied in the isolated state using a mass spectrometric technique. The OH(-)(H2O)n clusters were found to react faster for n = 2,3, while for n >3 the HO2(-)(H2O)n clusters are more reactive. Insights from quantum chemical calculations revealed a common mechanism in which the decisive bicarbonate-forming step starts from a pre-reaction complex where OH(-) and CO2 are separated by one water molecule. Proton transfer from the water molecule to OH(-) then effectively moves the hydroxide ion motif next to the CO2 molecule. A new covalent bond is formed between CO2 and the emerging OH(-) in concert with the proton transfer. For larger clusters, successive proton transfers from H2O molecules to neighbouring OH(-) are required to effectively bring about the formation of the pre-reaction complex, upon which bicarbonate formation is accomplished according to the concerted mechanism. In this manner, a general mechanism is suggested, also applicable to bulk water and thereby to CO2 uptake in oceans. Furthermore, this mechanism avoids the intermediate H2CO3 by combining the CO2 hydrolysis step and the protolysis step into one. The general mechanistic picture is consistent with low enthalpy barriers and that the limiting factors are largely of entropic nature.

Negative Ion Photoelectron Spectroscopy Reveals Thermodynamic Advantage of Organic Acids in Facilitating Formation of Bisulfate Ion Clusters: Atmospheric Implications

Recent lab and field measurements have indicated critical roles of organic acids in enhancing new atmospheric aerosol formation. Such findings have stimulated theoretical studies with the aim of understanding the interaction of organic acids with common aerosol nucleation precursors like bisulfate (HSO4(-)). We report a combined negative ion photoelectron spectroscopic and theoretical investigation of molecular clusters formed by HSO4(-) with succinic acid (SUA, HO2C(CH2)2CO2H), HSO4(-)(SUA)n (n = 0-2), along with HSO4(-)(H2O)n and HSO4(-)(H2SO4)n. It is found that one SUA molecule can stabilize HSO4(-) by ca. 39 kcal/mol, three times the corresponding value that one water molecule is capable of (ca. 13 kcal/mol). Molecular dynamics simulations and quantum chemical calculations reveal the most plausible structures of these clusters and attribute the stability of these clusters to the formation of strong hydrogen bonds. This work provides direct experimental evidence showing significant thermodynamic advantage by involving organic acid molecules to promote formation and growth in bisulfate clusters and aerosols.

Reactions of M(+)(H2O)n, n < 40, M = V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, with D2O reveal water activation in Mn(+)(H2O)n

Reactions of M(+)(H(2)O)(n), n < 40, M = V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, with D(2)O are studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Isotopically highly enriched metals are used as applicable. Isotopic scrambling with formation of HDO is not observed for M = Cr, Fe, Co, Ni, Cu, and Zn, which indicates that these hydrated metal ions consist of a singly charged metal center and a hydration shell of intact, inactivated water molecules. In the vanadium case, HDO formation is observed in the size region where also hydroxide formation with evolution of molecular hydrogen occurs. For manganese, HDO formation occurs in the size regime n ≈ 8-20. Additional experiments show that, in this size regime, Mn(+)(H(2)O)(n) is slowly converted into HMnOH(+)(H(2)O)(n-1) under the influence of room temperature blackbody radiation. The reaction is mildly exothermic; ΔH ≈ -21 ± 10 kJ mol(-1).

Concerted proton migration along short hydrogen bonded water bridges in bipyridine-water clusters

The outcome of reactions between D(2)O and size-selected ionic clusters of the type MH(+)(H(2)O)(n) (M = bipyridine, n = 1-30) shows that H-D-exchange is significantly higher for 2,2'-bipyridines than for 4,4'-bipyridines. This gives strong support for the idea that the existence of short water wires between the two nitrogen sites is essential to proton migration in water clusters containing basic sites.

Proton mobility in water clusters

Proton mobility in water occurs quickly according to the so-called Grotthuss mechanism. This process and its elementary reaction steps can be studied in great detail by applying suitable mass spectrometric methods to ionic water clusters. Careful choice of suitable core ions in combination with analysis of cluster size trends in hydrogen/deuterium isotope exchange rates allows for detailed insights into fascinating dynamical systems. Analysis of the experiments has been promoted by extensive and systematic quantum chemical model calculations. Detailed low-energy mechanistic pathways for efficient water rearrangement and proton transfer steps, in particular cases along short preformed "wires" of hydrogen bonds, have been identified in consistency with experimental findings.