Reactions and Reaction Rate of Atmospheric SO₂ and O₃^– (H₂O)_<i>n</i> Collisions via Molecular Dynamics Simulations

We present an ab initio study of gaseous SO₂ and O₃^–(H₂O)_<i>n</i> 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

Reactions and Reaction Rate of Atmospheric SO₂ and O₃^– (H₂O)_<i>n</i> Collisions via Molecular Dynamics Simulations

We present an ab initio study of gaseous SO₂ and O₃^–(H₂O)_<i>n</i> 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

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 cm³ s^–1. We then conclude that SO₄^– is a highly efficient catalyst in the oxidation of SO₂ by O₃ to SO₃

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

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