A Closure Study of the Reaction between Sulfur Dioxide and the Sulfate Radical Ion from First-Principles Molecular Dynamics Simulations.

In a previous study, we applied quantum chemical methods to study the reaction between sulfur dioxide (SO2) and the sulfate radical ion (SO4(-)) at atmospheric relevant conditions and found that the most likely reaction product is SO3SO3(-). In the current study, we investigate the chemical fate of SO3SO3(-) by reaction with ozone (O3) using first-principles molecular dynamics collision simulations. This method assesses both dynamic and steric effects in the reactions and therefore provides the most likely reaction pathways. We find that the majority of the collisions between SO3SO3(-) and O3 are nonsticking and that the most frequent reactive collisions regenerate sulfate radical ions and produce sulfur trioxide (SO3) while ejecting an oxygen molecule (O2). The rate of this reaction is determined to be 2.5 × 10(-10) cm(3) s(-1). We then conclude that SO4(-) is a highly efficient catalyst in the oxidation of SO2 by O3 to SO3.

Correction: Resolving the anomalous infrared spectrum of the MeCN-HCl molecular cluster using ab Initio molecular dynamics.

Correction for 'Resolving the anomalous infrared spectrum of the MeCN-HCl molecular cluster using ab Initio molecular dynamics' by Nicolai Bork et al., Phys. Chem. Chem. Phys., 2014, 16, 24685-24690.

Structures, Hydration, and Electrical Mobilities of Bisulfate Ion-Sulfuric Acid-Ammonia/Dimethylamine Clusters: A Computational Study.

Despite the well-established role of small molecular clusters in the very first steps of atmospheric particle formation, their thermochemical data are still not completely available due to limitation of the experimental techniques to treat such small clusters. We have investigated the structures and the thermochemistry of stepwise hydration of clusters containing one bisulfate ion, sulfuric acid, base (ammonia or dimethylamine), and water molecules using quantum chemical methods. We found that water facilitates proton transfer from sulfuric acid or the bisulfate ion to the base or water molecules, and depending on the hydration level, the sulfate ion was formed in most of the base-containing clusters. The calculated hydration energies indicate that water binds more strongly to ammonia-containing clusters than to dimethylamine-containing and base-free clusters, which results in a wider hydrate distribution for ammonia-containing clusters. The electrical mobilities of all clusters were calculated using a particle dynamics model. The results indicate that the effect of humidity is negligible on the electrical mobilities of molecular clusters formed in the very first steps of atmospheric particle formation. The combination of the results of this study with those previously published on the hydration of neutral clusters by our group provides a comprehensive set of thermochemical data on neutral and negatively charged clusters containing sulfuric acid, ammonia, or dimethylamine.

Resolving the anomalous infrared spectrum of the MeCN-HCl molecular cluster using ab initio molecular dynamics.

We present a molecular dynamics (MD) based study of the acetonitrile-hydrogen chloride molecular cluster in the gas phase, aimed at resolving the anomalous features often seen in infrared spectra of hydrogen bonded complexes. We find that the infrared spectrum obtained from the Fourier transform of the electric dipole moment autocorrelation function converges very slowly due to the floppy nature of the complex. Even after 55 picoseconds of simulation, significant differences in the modelled and experimental spectrum are seen, likely due to insufficient configurational sampling. Instead, we utilize the MD trajectory for a structural based analysis. We find that the most populated values of the N-H-Cl angle are around 162°. The global minimum energy conformation at 180.0° is essentially unpopulated. We re-model the spectrum by combining population data from the MD simulations with optimizations constraining the N-H-Cl angle. This re-modelled spectrum is in excellent accordance with the experimental spectrum and we conclude that the observed spectral anomaly is due to the dynamics of the N-H-Cl angle.

Molecular understanding of sulphuric acid-amine particle nucleation in the atmosphere.

Nucleation of aerosol particles from trace atmospheric vapours is thought to provide up to half of global cloud condensation nuclei. Aerosols can cause a net cooling of climate by scattering sunlight and by leading to smaller but more numerous cloud droplets, which makes clouds brighter and extends their lifetimes. Atmospheric aerosols derived from human activities are thought to have compensated for a large fraction of the warming caused by greenhouse gases. However, despite its importance for climate, atmospheric nucleation is poorly understood. Recently, it has been shown that sulphuric acid and ammonia cannot explain particle formation rates observed in the lower atmosphere. It is thought that amines may enhance nucleation, but until now there has been no direct evidence for amine ternary nucleation under atmospheric conditions. Here we use the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN and find that dimethylamine above three parts per trillion by volume can enhance particle formation rates more than 1,000-fold compared with ammonia, sufficient to account for the particle formation rates observed in the atmosphere. Molecular analysis of the clusters reveals that the faster nucleation is explained by a base-stabilization mechanism involving acid-amine pairs, which strongly decrease evaporation. The ion-induced contribution is generally small, reflecting the high stability of sulphuric acid-dimethylamine clusters and indicating that galactic cosmic rays exert only a small influence on their formation, except at low overall formation rates. Our experimental measurements are well reproduced by a dynamical model based on quantum chemical calculations of binding energies of molecular clusters, without any fitted parameters. These results show that, in regions of the atmosphere near amine sources, both amines and sulphur dioxide should be considered when assessing the impact of anthropogenic activities on particle formation.

Energetics of atmospherically implicated clusters made of sulfuric acid, ammonia, and dimethyl amine.

The formation of atmospheric aerosol particles through clustering of condensable vapors is an important contributor to the overall concentration of these atmospheric particles. However, the details of the nucleation process are not yet well understood and are difficult to probe by experimental means. Computational chemistry is a powerful tool for gaining insights about the nucleation mechanism. Here, we report accurate electronic structure calculations of the potential energies of small clusters made from sulfuric acid, ammonia, and dimethylamine. We also assess and validate the accuracy of less expensive methods that might be used for the calculation of the binding energies of larger clusters for atmospheric modeling. The PW6B95-D3 density-functional-plus-molecular-mechanics calculation with the MG3S basis set stands out as yielding excellent accuracy while still being affordable for very large clusters.

Reactions and reaction rate of atmospheric SO2 and O3(-) (H2O)n collisions via molecular dynamics simulations.

We present an ab initio study of gaseous SO(2) and O(3)(­)(H(2)O)(n) collisions. Opposed to the usual approach to determine reaction rates via structural optimizations and transition state theory, we successfully approach this problem using ab initio molecular dynamics. We demonstrate the advantages of this approach, being the automatic and unbiased inclusion of dynamic and steric effects as well as the simultaneous assessment of all possible reactions. For this particular system, we find that only one reaction will be of atmospheric significance. Further, we identify the main geometrical parameters governing and limiting the observed reaction and suggest a new measure of the reaction rate being ca. 3/4 of the collision rate.

Comparing simulated and experimental molecular cluster distributions.

Formation of secondary atmospheric aerosol particles starts with gas phase molecules forming small molecular clusters. High-resolution mass spectrometry enables the detection and chemical characterization of electrically charged clusters from the molecular scale upward, whereas the experimental detection of electrically neutral clusters, especially as a chemical composition measurement, down to 1 nm in diameter and beyond still remains challenging. In this work we simulated a set of both electrically neutral and charged small molecular clusters, consisting of sulfuric acid and ammonia molecules, with a dynamic collision and evaporation model. Collision frequencies between the clusters were calculated according to classical kinetics, and evaporation rates were derived from first principles quantum chemical calculations with no fitting parameters. We found a good agreement between the modeled steady-state concentrations of negative cluster ions and experimental results measured with the state-of-the-art Atmospheric Pressure interface Time-Of-Flight mass spectrometer (APi-TOF) in the CLOUD chamber experiments at CERN. The model can be used to interpret experimental results and give information on neutral clusters that cannot be directly measured.