Laboratory studies of atmospherically important gas-phase peroxy radical reactions

atmospheric;Peroxy;radical

Christensen, Lance Eric ; Okumura, Mitchio

University:California Institute of Technology

Department:Chemistry and Chemical Engineering

关键词: atmospheric;    Peroxy;    radical;   

Others  :  https://thesis.library.caltech.edu/1811/1/The_thesis2.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

Peroxy radicals (HO2, RO2) are important intermediates in Earth's atmosphere. They are intermediates in the oxidation of alkanes and CO in combustion and atmospheric chemical processes. In earth's atmosphere, the rates of their self and cross reactions are often the dominant loss processes when NOx concentrations fall below tens of pptv. These reactions have proven difficult to study in laboratory experiments, due to complex secondary chemistry and ambiguities in radical detection.This thesis describes a new laser-photolysis apparatus to measure the rates of peroxy radical reactions under atmospheric conditions that employs simultaneous UV direct absorption and IR wavelength-modulation spectroscopy to detect the peroxy radicals. Prior kinetic measurements of gas-phase peroxy radical reactions have typically employed flash-photolysis methods coupled with detection of the radicals only by UV absorption spectroscopy. However, uncertainties can arise because several different species often contribute to the absorption signal. The IR channel provides an independent means of monitoring HO2 radicals by detection of specific rovibrational transitions.With this apparatus, the rates of the reactions HO2 + NO2, HO2 + CH3O2, CH3O2 + CH3O2, and HO2 + HO2 were studied at temperatures from 219 K to 300 K. Our measurements have, in some cases, led to significant revision of previously accepted rate constants, mechanisms, or product yields, especially at conditions relevant to the upper atmosphere. The new rate coefficients for the HO2 + HO2 reaction are shown to account for a long-standing discrepancy in modeled vs. observed hydrogen peroxide in the stratosphere.A key finding has been the observation that many previous measurements of HO2 reactions at low temperatures have suffered from problems due to complexation between HO2 and methanol, a precursor used to generate HO2. Direct kinetic evidence is presented for the formation of the HO2?CH3OH complex; the rate coefficients, equilibrium constant, and enthalpy of reaction for HO2 + CH3OH <-> HO2?CH3OH were measured. These results are the first direct study of the chaperone effect proposed to explain the enhancement of the observed rates of the HO2 self-reaction by hydrogen-bonding species.The effects of methanol enhancement on the HO2 + NO2, HO2 + CH3O2, CH3O2 + CH3O2, and HO2 + HO2 reaction rates were measured. For the HO2 + NO2 reaction, overlapping, time-dependent signals in the UV due to the equilibrium between NO2 and N2O4 were observed that may not have been properly accounted for in previous measurements. Other studies of NO2 reactions conducted at temperatures below 250 K may be subject to similar errors. In the CH3O2 + CH3O2 reaction, detection of HO2 products has raised questions concerning the product yields and reaction mechanisms.