Laser flash photolysis studies of some gas phase reactions of atmospheric interest.

摘要

The first part of my work involved temperature-dependent kinetics studies of chlorine atom reactions with a number of atmospheric trace gases including acetone, butanone, 3-pentanone, dimethyl selenide (DMSe) and pyridine. Atomic Cl was detected by atomic resonance fluorescence (RF) spectroscopy following Cl generation by LFP. The temporal profiles of Cl atoms at different temperatures provided important kinetic and mechanistic information for these reactions. For most of the reactions, temperature-dependent rate coefficients were established for the first time, which were used to evaluate the importance of these reactions as potential sinks for the investigated compounds as well as the relevant atmospheric chemistry. The studies of Cl reactions with DMSe represent the initial stages of the investigation of the atmospheric component of the biogeochemical cycling of selenium. For the Cl + pyridine reaction, both reversible addition and H-abstraction pathways were characterized, and the first experimental determination of bond strength of the gas-phase Cl-pyridine adduct was obtained.In an extension of the study of the Cl reaction with DMSe, we also investigated the possible adducts formed via reactions of Cl and Br atoms with DMSe using time-resolved UV-visible absorption spectroscopy (TRUVVAS) in conjunction with LFP. Significant transient absorptions were observed following photolysis of Cl2CO/DMSe/N2 or CF2Br2/DMSe/N 2 mixtures, which had the spectroscopic and chemical kinetic signatures expected for adducts. Hence, our results strongly suggest that these adducts, like their DMS analogues, are relatively stable (at 298 K) against decomposition on the experimental time scale (microseconds to milliseconds) of LFP experiments. Observation of a stable DMSe--Cl adduct is contrary to a theoretical prediction that rapid fragmentation to CH3SeCl and CH3 occurs.The third experiment focused on weakly-bound HO2 complexes. Theoretical calculations suggest that HO2 is able to form hydrogen bonds with some molecules that may be of atmospheric importance particularly under the cold conditions of the upper troposphere. The formation of these relatively stable complexes will potentially influence our understanding of HO2 chemistry at high altitudes. Experimental investigations of these complexes are scarce. We used LFP to produce HO2 and time-resolved infrared tunable diode laser absorption spectroscopy (TDLAS) to monitor HO 2 in the presence of formic and acetic acids. Equilibration kinetics were observed at low temperatures, which are consistent with HO2 complex formation. The bond strengths were determined using the equilibrium constants obtained in the experiment coupled with some structural information calculated from quantum chemical approaches. This is the first experimental measurement of the binding energies of doubly-hydrogen-bonded HO2-carboxylic acid complexes. The results agree well with the most recent theoretical predictions.The fourth component of the research involved application of the LFP--RF technique to investigate reactive and non-reactive quenching of O( 1D) by the long-lived greenhouse gases nitrogen trifluoride (NF 3), sulfuryl fluoride (SO2F2), and trifluoromethyl sulfur pentafluoride (SF5CF3). This study attempted to quantify the reactivity of the above compounds toward O(1D), and to determine the importance of these reactions as loss processes for these stable greenhouse gases in the atmosphere. The reaction of NF3 with O(1D) is fast and proceeds almost exclusively via reactive quenching, i.e., NF3 is consumed in the reaction. This reaction appears to be the most important known atmospheric sink for NF3. Our results suggest that the rate coefficient for reactive quenching is more than a factor of two faster than suggested in one previously published study. The removal of SO2F2 by O(1D) reaction is even more efficient than removal of NF3. However, it still appears to be a relatively minor sink for atmospheric SO2F2 compared to ocean uptake. The O(1D) + SF5CF 3 reaction was found to be very slow, and should not contribute significantly to the removal of SF5CF3 from the atmosphere. This research has made an important contribution to accurate evaluation of the atmospheric residence times and global warming potentials of the above species. (Abstract shortened by UMI.)