Gas-phase atmospheric chemistry by cavity ringdown spectroscopy and quantum chemistry studies

This thesis discusses gas-phase atmospheric chemistry related to trace air pollutant detection, ozonolysis reaction of alkenes, and reaction kinetics and energetics of organosulfur compounds in the atmosphere. The methods used are cavity ringdown spectroscopy (CRDS) and quantum chemistry calculations.; For trace pollutant measurements in the atmosphere, CRDS detection of HONO and NO₂ has been demonstrated. Absorption cross sections of HONO are measured, and a CRDS detection sensitivity of HONO down to ppbv level has been achieved. CRDS detection of atmospheric NO₂ is also demonstrated, with an instrumental detection limit of 0.2 ppbv. Measurements of indoor and outdoor NO₂ concentrations are performed. The CRDS method provides a simple, reliable and accurate way of measuring ambient NO₂. Possible improvements for developing compact instruments for field measurements are discussed.; OH radical production mechanisms in the ozonolysis reactions of ethene and its methyl-substitutions have been studied by the CRDS technique. HCO, CH₂CHO and CH₂C(O)CH₃ radicals, which are the proposed co-products of OH from dissociation of Criegee intermediates, are searched using CRDS. CH₂CHO is observed in the ozonolysis reaction of 2-butene, and its formation is found to decrease drastically with increasing pressure. This, along with the quantum chemistry studies, indicates that the Criegee intermediate syn-CH₃CHOO could isomerize to CH₂CHOOH and then dissociate to the CH₂CHO + OH products.; The atmospheric chemistry of organosulfur compounds is studied by quantum chemistry. In the reaction of dimethyl sulfide (DMS) with OH radical, two addition complexes, (CH₃)₂S·OH via 2-center-3-electron bonding and (CH₃)₂S·HO via dipole-dipole interaction, are identified at the DFT-B3LYP level. Further reaction of (CH₃)₂S·OH with O₂ could produce dimethyl sulfoxide (DMSO) in the atmosphere. Reaction between DMSO and OH radical/Cl atom is initialized by formation of the addition complex as well. For DMSO + OH reaction in the absence of O₂, CH₃ S(O)OH + CH₃ is the most prominent product channel with an overall negative energy barrier. To further assist the understanding of the atmospheric fate of organosulfur compounds, the enthalpies of formation of more than 200 sulfur-containing species are calculated at G3X level, using both atomization energy and isodesmic reaction procedures.