Laboratory Kinetics

Photolysis Quantum Yields and Reaction Kinetics of Chlorine Nitrate

Chlorine nitrate, ClONO2, is an important reservoir for chlorine throughout the stratosphere. The major loss processes for chlorine nitrate in the stratosphere are photolysis and reaction with O(3P) atoms, and a quantitative understanding of these processes is required to determine the effectiveness of chlorine nitrate as a reservoir species. Experiments have shown that photolysis of ClONO2 at 308 nm leads to production of both Cl-atoms (quantum yield ~ 0.7) and ClO radicals (quantum yield ~ 0.3). The quantum yield for O-atom production has also been shown to be < 0.05. In addition, the rate coefficient for the reaction of O(3P) with ClONO2 has been measured to assess the impact of this reaction on the partitioning of chlorine in the stratosphere. This work was sponsored by the NASA Upper Atmospheric Research Program.

UV/Visible Absorption Spectrum of Hypobromous Acid, HOBr

The production and subsequent photolysis of HOBr in the lower stratosphere leads to an important catalytic cycle for the destruction of ozone. While photolysis of HOBr was assumed in atmospheric models to be quite rapid, quantification of this process has until now been impossible due to difficulty in synthesizing HOBr in the gas phase so that its UV/visible absorption spectrum could be measured. In collaboration with James Burkholder (NOAA Aeronomy Lab), HOBr was synthesized from the reaction of Br2O and H2O and its UV/visible spectrum was measured. These data were used to show that photolysis of HOBr is not as rapid as previously assumed and that HOBr levels in the stratosphere are higher than those indicated by previous model calculations. This work was partially supported by the NASA Upper Atmospheric Research Program.

Reaction Rate Coefficients for O(3P) with Biogenic Hydrocarbons

Hydrocarbons of biogenic origin, such as isoprene (C5H8) and the terpenes (C10H16) are oxidized in the atmosphere mainly by their reaction with OH and NO3 radicals. However, reactions of O(3P) atoms with these compounds can also be of some significance particularly near sunrise or sunset. These reactions also occur in smog chamber studies of alkene oxidation where O(3P) concentrations are higher than those found in the atmosphere. Thus, rate constant data for these reactions are of importance in fully understanding the atmospheric oxidation of these biogenic compounds. We have determined the rate constant for reaction of O(3P) with isoprene, alpha-pinene, and Delta3-carene. The reaction of alpha-pinene with O(3P) is faster than expected when compared with its reaction with OH and will represent a non-negligible atmospheric loss for this molecule.

Reactions of Alkoxy Radicals

Alkoxy radicals (RO*, where R is an alkyl group) are reactive intermediates formed in the atmospheric oxidation of all hydrocarbons. The alkoxy radicals are very short-lived, typically less than one microsecond, and enjoy a very rich chemistry. They can react with O2 to produce stable carbonyl compounds, or can undergo a number of different types of unimolecular reactions. The branching ratios for these various reaction pathways will ultimately determine the products obtained from the oxidation of the parent hydrocarbon. During this fiscal year, we studied the reactivity of a number of alkoxy radicals:

1) The CH3(C=O)CH2* radical, which is obtained from acetone oxidation, was shown to decompose to CH3CO and CH2O even at low temperatures, contrary to theoretical predictions.

2) Analysis of organic nitrate yields was carried to determine the chemistry of the numerous alkoxy radicals obtained from the OH-initiated oxidation of isopentane (C5H12).

3) The CH2ClO* radical, obtained in the atmospheric oxidation of methyl chloride (CH3Cl), was shown to react with O2 to form formyl chloride (HCOCl) or to undergo a novel unimolecular decomposition to HCl and HCO. A study of this chemistry as a function of temperature and pressure shows that the reaction with O2 will be dominant under conditions of relevance to the troposphere and lower stratosphere.

Collaborations

Carla Kegley-Owen (graduate student, University of Colorado) has been working in our laboratory studying the photochemistry of ClONO2. Collaboration on the measurement of the UV/visible absorption spectra of HOBr and Br2O continues with James Burkholder (NOAA Aeronomy Laboratory and University of Colorado). Suzanne Paulson (University of California, Los Angeles) is collaborating with us on the study of the rate coefficients for reaction of O(3P) with the biogenic compounds isoprene, alpha-pinene, and Delta3-carene. Jonathan Williams (graduate student, University of East Anglia, England) is working with us on a study of acetone oxidation as a function of temperature. Collaborations with Timothy Wallington (Ford Motor Company, Dearborn, MI) continue with a study of the atmospheric oxidation mechanism of CH3Cl as a function of temperature, and on the study of the rate coefficients for the reaction of Cl-atoms with CH3OH and CH3CHO. Frank Flocke (KFA, Juelich, Germany) is working with us on a study of the production of organic nitrates in the oxidation of branched-chain alkanes of importance in the atmosphere.