Activation Barriers in the Growth of Molecular Clusters Derived from Sulfuric Acid and Ammonia

Electrospray Ionization-Based Synthesis and Validation of Amine-Sulfuric Acid Clusters of Relevance to Atmospheric New Particle Formation

Atmospheric new particle formation (NPF) is the process by which atmospheric trace gases such as sulfuric acid, ammonia, and amines cluster and grow into climatically relevant particles. The mechanism by which these particles form and grow has remained unclear, in large part due to difficulties in obtaining molecular-level information about the clusters as they grow. Mass spectrometry-based methods using electrospray ionization (ESI) as a cluster source have shed light on this process, but the produced cluster distributions have not been rigorously validated against experiments performed in atmospheric conditions. Ionic clusters are produced by ESI of solutions containing the amine and bisulfate or by spraying a sulfuric acid solution and introducing trace amounts of amine gas into the ESI environment. The amine content of clusters can be altered by increasing the amount of amine introduced into the ESI environment, and certain cluster compositions can only be made by the vapor exchange method. Both approaches are found to yield clusters with the same structures. Aminium bisulfate cluster distributions produced in a controlled and isolated ESI environment can be optimized to closely resemble those observed by chemical ionization in the CLOUD chamber at CERN. These studies indicate that clusters generated by ESI are also observed in traditional atmospheric measurements, which puts ESI mass spectrometry-based studies on firmer footing and broadens the scope of traditional mass spectrometry experiments that may be applied to NPF.

Direct Link between Structure and Hydration in Ammonium and Aminium Bisulfate Clusters Implicated in Atmospheric New Particle Formation

The acid-base chemistry of amines and sulfuric acid promotes growth in the early stages of atmospheric new particle formation, with more basic amines enhancing growth rates. Hydration of these particles has been proposed to depend on acidity or basicity but is difficult to quantify; therefore, the role of water in this process is not well understood. Using tandem mass spectrometry coupled to a temperature-controlled ion trap, we show that water uptake by aminium bisulfate clusters depends on the total number of free hydrogen bond donors in the cluster and is unaffected by the interchange of amines featuring the same number of substituents but differing gas-phase basicity. Analyzing this trend reveals site-specific propensities for hydration. These results indicate that hydration is determined by structural factors and that reported dependences on acidity or basicity arise from the weaker correlation between the number of hydrogen bond donors of amines and their gas-phase basicity.

The Interplay Between Hydrogen Bonding and Coulombic Forces in Determining the Structure of Sulfuric Acid-Amine Clusters

Acid-base cluster chemistry drives atmospheric new particle formation (NPF), but the details of the growth mechanisms are difficult to experimentally probe. Clusters of ammonia, alkylamines, and sulfuric acid, species fundamental to NPF, are probed by infrared spectroscopy. These spectra show that substitution of amines for ammonia, which is linked to accelerated growth, induces profound structural rearrangement in clusters with initial compositions (NH₄⁺) _n+1(HSO₄^-) _n (1 ≤ n ≤ 3). This rearrangement is driven by the loss of N-H hydrogen bond donors, yielding direct bisulfate-bisulfate hydrogen bonds, and its onset with respect to cluster composition indicates that more substituted amines induce rearrangement at smaller sizes. A simple model counting hydrogen bond donors and acceptors explains these observations. The presence of direct hydrogen bonds between formal anions shows that hydrogen bonding can compete with Coulombic forces in determining cluster structure. These results suggest that NPF mechanisms may be highly dependent on amine identity.

Nanoparticles grown from methanesulfonic acid and methylamine: microscopic structures and formation mechanism

Mechanisms of particle formation and growth in the atmosphere are of great interest due to their impacts on climate, health and visibility. However, the microscopic structures and related properties of the smallest nanoparticles are not known. In this paper we pursue computationally a microscopic description for the formation and growth of methanesulfonic acid (MSA) and methylamine (MA) particles under dry conditions. Energetic and dynamics simulations were used to assess the stabilities of proposed model structures for these particles. Density functional theory (DFT) and semi-empirical (PM3) calculations suggest that (MSA-MA)₄ is a major intermediate in the growth process, with the dissociation energies, enthalpies and free energies indicating considerable stability for this cluster. Dynamics simulations show that this species is stable for at least 100 ps at temperatures up to 500 K, well above atmospheric temperatures. In order to reach experimentally detectable sizes (>1.4 nm), continuing growth is suggested to occur via clustering of (MSA-MA)₄. The dimer (MSA-MA)₄(MSA-MA)₄ may be one of the smaller experimentally measured particles. Step by step addition of MSA to (MSA-MA)₄, is also a likely potential growth mechanism when MSA is excess. In addition, an MSA-MA crystal is predicted to exist. These studies demonstrate that computations of particle structure and dynamics in the nano-size range can be useful for molecular level understanding of processes that grow clusters into detectable particles.

Mechanisms of Atmospherically Relevant Cluster Growth

Atmospheric aerosols impact global climate either directly by scattering solar radiation or indirectly by serving as cloud condensation nuclei, which influence cloud albedo and precipitation patterns. Our scientific understanding of these impacts is poor relative to that of, for instance, greenhouse gases, in part because it is difficult to predict particle number concentrations. One important pathway by which particles are added to the atmosphere is new particle formation, where gas phase precursors form molecular clusters that subsequently grow to the climatically relevant size range (50-100 nm diameter). It is predicted that up to 50% of atmospheric particles arise from this process, but the key initial chemical processes are poorly resolved. In general, a combination of inorganic and organic molecules are thought to contribute to new particle formation, but the chemical composition of molecular clusters and pathways by which they grow to larger sizes is unclear. Cluster growth is a key component of new particle formation, as it governs whether molecular clusters will become climatically relevant. This Account discusses our recent work to understand the mechanisms underlying new particle growth. Atmospherically relevant molecular clusters containing the likely key contributors to new particle formation (sulfuric acid, ammonia, amines, and water) were investigated experimentally by Fourier transform mass spectrometry as well as computationally by density functional theory. Our laboratory experiments investigated the molecular composition of charged clusters, the molecular pathways by which these clusters may grow, and the kinetics of base incorporation into them. Computational chemistry allowed confirmation and rationalization of the experimental results for charged clusters and extension of these principles to uncharged and hydrated clusters that are difficult to study by mass spectrometry. This combination of approaches enabled us to establish a framework for cluster growth involving sulfuric acid, ammonia, amines, and water. Charged or uncharged, cluster growth occurs primarily through an ammonium (or aminium) bisulfate coordinate. In these clusters, proton transfer is maximized between acids and bases to produce cations (ammonium, aminium) and anions (bisulfate), whereas additional molecules (water and unneutralized sulfuric acid) remain un-ionized. Experimental measurements suggest the growth of positively charged clusters occurs by successive acidification and neutralization steps. The acidification step is nearly barrierless, whereas the neutralization step exhibits a significant activation barrier in the case of ammonia. Bases are also incorporated into these clusters by displacement of one base for another. Base displacement is barrierless on the cluster surface but not within the cluster core. The favorability of amines relative to ammonia in charged clusters is governed by the trade-off between gas phase basicity and binding energetics. Computational studies indicate that water has a relatively small effect on cluster energetics. In short, amines are effective at assisting the formation and initial growth of clusters but become less important as cluster size increases, especially when hydration is considered. More generally, this work shows how experiment and computation can provide important, complementary information to address problems of environmental interest.

Particle formation and growth from oxalic acid, methanesulfonic acid, trimethylamine and water: a combined experimental and theoretical study

A combined experimental-theoretical study on the effect of oxalic acid on particle formation and growth from the reaction of MSA with trimethylamine in the absence and presence of water.