Gas phase reactions of the hydroperoxyl radical HO_2 with carbonyl compounds

Abstract

The hydroperoxyl radical HO2 is a key member of the hydrogen (HOx) radical family and is present throughout the environment. It is often the dominant HOx species throughout the atm., where its chem. influences ozone concns. and air pollution. Simple carbonyl compds. such as formaldehyde, acetaldehyde, and acetone are among the most abundant reactive orgs. in the free troposphere. Their primary degrdn. paths; reaction with OH and photolysis. Reactions of HO2 with carbonyls in the gas phase are believed to proceed through a hydrogen bonded adduct. The adduct can then undergo an intramol. rearrangement to form a peroxy radical (see figure). The reaction of HO2 with formaldehyde, HCHO, has been well studied, yet the spectroscopic evidence for the rearrangement lies in a broad, featureless UV spectrum. The reaction of HO2 with acetone, however, was found not to proceed at room temp., and its significance in the atm. was thus not considered. Recent theor. calcns., however, predict that HO2 may react at lower temps. relevant to the free troposphere, conditions which favor adduct formation. The computations suggest that this reaction may in fact be another important sink for acetone near the tropopause. Here we report spectroscopic evidence for formation of the hydroxy-Me peroxyl radical, the intermediate formed from reaction of HO2 with HCHO. We observe both the mid-IR OH stretch band, and the characteristic A-X near IR electronic band of the peroxyl radical, and compare to calcns. In a second set of expts., we obtain equil. data on the HO2 + acetone reaction using the IR-Kinetic Spectroscopy (IRKS) instrument at the NASA Jet Propulsion Lab. Using diode laser spectroscopy to detect HO2, we det. the DrH and DrS. These results shed light on the atm. relevance of this reaction for carbonyl loss in the atm.

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