AbstractsPhysics

Modelling the Role of Charge in Atmospheric Particle Formation Using Quantum Chemical Methods

by Kai Ruusuvuori




Institution: University of Helsinki
Department: Department of Physics, Ilmakehätieteiden osasto
Year: 2015
Keywords: teoreettinen fysiikka
Record ID: 1140861
Full text PDF: http://hdl.handle.net/10138/153056


Abstract

New particle formation is an important process in the atmosphere. As ions are constantly produced in the atmosphere, the behaviour and role of charged particles in atmospheric processes needs to be understood. In order to gain insight on the role of charge in atmospheric new particle formation, the electron structure of the molecules taking part in this process needs to be taken into account using quantum chemical methods. Quantum chemical density functional theory was employed in an effort to reproduce an experimentally observed sign preference. While computational results on molecular structures agreed well with results obtained by other groups, the computationally obtained sign preference was opposite to the experimentally observed. Possible reasons for this discrepancy were found in both computational results and experiments. Simulations of clusters containing water, pyridine, ammonia and a proton were performed using density functional theory. The clusters were found to form a core consisting of ammonium ion and water with the pyridine molecule bonding to the ammonium ion. However, the solvation of the ammonium ion was observed to affect the possibility of proton transfer. Calculations of proton affinities and gas phase basicities of several compounds, which can be considered as candidates to form atmospheric ions in the boreal forest, were performed. The generally small differences between the calculated gas phase basicites and proton affinities implied only small entropy changes in the protonation reaction. Comparison with experiments resulted in the conclusion that the largest experimentally observed peaks of atmospheric ions most likely corresponded to pyridine and substituted pyridines. Furthermore, a combination of low proton affinity and high observed cation concentration was concluded to imply a high concentration of neutral parent molecules in the atmosphere. A combination of quantum chemistry and a code for modelling cluster dynamics was employed to study the use of protonated acetone monomers and dimers as the ionization reagent in a chemical ionization atmospheric pressure interface time-of-flight mass spectrometer (CI-APi-TOF). The results showed that the ionization reagents successfully charged dimethylamine monomers. However, there were discrepancies between the simulated and measured cluster distributions. Possible reasons for this discrepancy were found in both measurements and the modelling parameters. Pienhiukkasten muodostuminen on tärkeä ilmakehän prosessi, jonka ensiaskeleet otetaan molekyylitasolla. Molekyylien sähköisen varaustilan roolin selvittäminen tässä - ja muissa ilmakehän prosesseissa - on tärkeää, sillä sähköisesti varattuja molekyylejä eli ioneja syntyy ilmakehässä jatkuvasti. Jotta saataisiin tietoa varauksen roolista ilmakehässä tapahtuvassa pienhiukkasten muodostumisessa, täytyy tähän prosessiin osallistuvien molekyylien elektronirakenne ottaa huomioon. Tämä pystytään tekemään niin kutsuttujen kvanttikemiallisten menetelmien avulla. Tutkimusartikkeleissamme on…