Mechanisms of Atmospherically Relevant Cluster Growth

Abstract

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...

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

“…In principle, we can also record the charged cluster distributions in positive ion mode. 33 The recorded ESI mass spectrum shows the cluster compositions and sizes that may hint to their atmospheric compositions, 13,32 for example, as illustrated in Figure 1, in which the observed ESI clusters are consistent with the findings from the CLOUD experiments showing dominant clusters with one H 2 SO 4 and one or two organic molecules. 21,23 To investigate the structures, energetics, and formation mechanisms of those clusters, they are accumulated in a 3-D cryogenic and temperature-tunable (10−300 K) ion trap, mass-selected in a time-of-flight mass spectrometer, and characterized with negative ion photoelectron spectroscopy (NIPES).…”

“…4,10,11 Significant efforts from both experiment and theory have been devoted to explore the key molecules and their specific roles in contributing to the nucleation step of NPF, and particular interest has been put forth to reveal the structures, energetics, interaction nature, and thermodynamics of the clusters formed by those molecules. 4,9,12,13 Such information lays out the fundamental basis to unveil the molecular mechanisms underlying aerosol clusters formation relevant to NPF.…”

Atmospheric implication of synergy in methanesulfonic acid–base trimers: a theoretical investigation

“…When the difference in alkalinity between X and Y is large, the significance of the synergistic effect on the substitution is greater. Recently, it was further confirmed that base displacement on the cluster surface is both thermodynamically and kinetically favorable by the experiments and computations reported by Bzdek et al 51 …”

“…When the difference in alkalinity between X and Y is large, the signicance of the synergistic effect on the substitution is greater. Recently, it was further conrmed that base displacement on the cluster surface is both thermodynamically and kinetically favorable by the experiments and computations reported by Bzdek et al 51 In addition, the values of DDG 298 K q for the clusters are presented in Tables 4-6 and S6-S8; † these values are negative for the processes of attachment and substitution. The results show that the addition of a Y molecule to the MSA-X dimers and the substitution of Y 2 (M and D) for Y 1 (A and M) are spontaneous processes from the thermodynamic point of view.…”

Solvent-Mediated Charge Transfer Dynamics of a Model Brown Carbon Aerosol Chromophore: Photophysics of 1-Phenylpyrrole Induced by Water Solvation

“…Water-mediated interactions determine the structural, dynamical, and optical properties of aqueous solutions and interfaces, 1,2 which play a key role in biological folding, [3][4][5] photosynthetic energy conversion, [6][7][8] and aerosol solar absorption. [9][10][11] Considering the ubiquity of water-mediated interactions in numerous environments, there remain longstanding questions regarding how solvation shapes the photophysics of the solute partner. Thus, a fundamental description of the intermolecular interactions between the chromophore solute and H2O solvent is still far from complete.…”