AbstractsChemistry

First-principles studies of ionic oxidation of sulfur dioxide and molecular clustering

by Tchinda Narcisse Tsona




Institution: University of Helsinki
Department:
Year: 2016
Keywords: physics
Posted: 02/05/2017
Record ID: 2064183
Full text PDF: http://hdl.handle.net/10138/161924


Abstract

Sulfur oxidation products are involved in the formation of acid rain and atmospheric aerosol particles. The formation mechanism of these sulfur-containing species is often complex, especially when ions are involved. The work of this thesis uses computational methods to explore reactions of sulfur dioxide with some atmospheric ions, and to examine the effect of humidity on the stability and electrical mobilities of sulfuric acid-based clusters formed in the first steps of atmospheric particle formation. Quantum chemical calculations are performed to provide insights into the mechanism of the reaction between sulfur dioxide (SO2) and the superoxide ions (O2-) in the gas phase. This reaction was investigated in various experimental studies based on mass spectrometry, but discrepancies on the structure of the product remained disputed. The performed calculations indicate that the peroxy SO2O2- molecular complex is formed upon collision of SO2 and O2-. Due to a high energy barrier, SO2O2- is unable to isomerize to the sulfate radical ion (SO4-), the most stable form of the singly charged tetraoxysulfurous ion. It is suggested that SO2O2- is the major product of SO2 and O2- collision. The gas-phase reaction between SO2 and SO4- is further explored. From quantum chemical calculations and transition state theory, it is found that SO2 and SO4- cluster effectively to form SO2SO4-, which reacts fast at low relative humidity to form SO3SO3-. This species has never been observed in the atmosphere and its decomposition upon collision with other atmospheric species is most likely. First-principles molecular dynamics simulations are used to probe the decomposition by collisions with ozone (O3). The most frequent reactive collisions lead to the formation of SO4-, SO3, and O2. This implies that SO4- acts as a good catalyst in the SO2 oxidation by O3 to SO3. The best structures and the thermochemistry of the stepwise hydration of bisulfate ion, sulfuric acid, base (ammonia or dimethylamine) clusters are determined using quantum chemical calculations. The results indicate that ammonia-containing clusters are more hydrated than dimethylamine-containing ones. The effect of humidity on the mobilities of different clusters is further examined and it is finally found that the effect of humidity is negligible on the electrical mobilities of bisulfate ion, sulfuric acid, ammonia or dimethylamine clusters. Rikin hapettumistuotteet osallistuvat happosateiden ja ilmakehän aerosolien muodostumiseen. Näiden muodostumisreaktiot ovat usein monimutkaisia, varsinkin jos muodostumiseen osallistuu ionisia yhdisteitä. Tässä työssä on käytetty laskennallisia menetelmiä tutkittaessa ikkidioksidin reaktioita joidenkin ilmakehässä esiintyvien ionien kanssa. Lisäksi on tarkasteltu suhteellisen kosteuden vaikutusta rikkihappoklustereiden vakauteen ja elektroniseen liikkuvuuteen osana uusien ilmakehön aerosolihiukkasten muodostumista. Kvanttikemiallisilla laskulla pyrittään ymmärtämään paremmin rikkidioksidin (SO2) ja superoksidi-ionin (O2-) välisen…