Laboratory Studies of Atmospherically Important Gas-Phase Peroxy Radical Reactions
<p>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...
| Summary: | <p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p> |
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