Title: Importance of Unimolecular HO₂ Elimination in the Heterogeneous OH Reaction of Highly Oxygenated Tartaric Acid Aerosol

Abstract

Oxygenated organic molecules are abundant in atmospheric aerosols and are transformed by oxidation reactions near the aerosol surface by gas-phase oxidants such as hydroxyl (OH) radicals. To gain better insights into how the structure of an organic molecule, particularly in the presence of hydroxyl groups, controls the heterogeneous reaction mechanisms of oxygenated organic compounds, this paper investigates the OH-radical initiated oxidation of aqueous tartaric acid (C₄H₆O₆) droplets using an aerosol flow tube reactor. The molecular composition of the aerosols before and after reaction is characterized by a soft atmospheric pressure ionization source (Direct Analysis in Real Time) coupled with a high-resolution mass spectrometer. The aerosol mass spectra reveal that four major reaction products are formed: a single C₄ functionalization product (C₄H₄O₆) and three C₃ fragmentation products (C₃H₄O₄, C₃H₂O₄, and C₃H₂O₅). The C₄ functionalization product does not appear to originate from peroxy radical self-reactions but instead forms via an α-hydroxylperoxy radical produced by a hydrogen atom abstraction by OH at the tertiary carbon site. The proximity of a hydroxyl group to peroxy group enhances the unimolecular HO₂ elimination from the α-hydroxylperoxy intermediate. This alcohol-to-ketone conversion yields 2-hydroxy-3-oxosuccinic acid (C₄H₄O₆), the major reaction product. While in general, C–C bond scission reactions aremore » expected to dominate the chemistry of organic compounds with high average carbon oxidation states (OS_C), our results show that molecular structure can play a larger role in the heterogeneous transformation of tartaric acid (OS_C = 1.5). Finally, these results are also compared with two structurally related dicarboxylic acids (succinic acid and 2,3-dimethylsuccinic acid) to elucidate how the identity and location of functional groups (methyl and hydroxyl groups) alter heterogeneous reaction mechanisms.« less

Authors:

Cheng, Chiu Tung ^[1]; Chan, Man Nin ^[2]; Wilson, Kevin R. ^[3]

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+ Show Author Affiliations

1. The Chinese Univ. of Hong Kong (China). Earth System Science Programme. Faculty of Science
2. The Chinese Univ. of Hong Kong (China). Earth System Science Programme. Faculty of Science. The Inst. of Environment, Energy and Sustainability
3. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Chemical Sciences Division

Publication Date:

Sat Jul 09 00:00:00 EDT 2016

Research Org.:

Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); The Chinese Univ. of Hong Kong (China)

OSTI Identifier:

1454478

Grant/Contract Number:  

AC02-05CH11231; 4053089; 3132765

Resource Type:

Accepted Manuscript

Journal Name:

Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory

Additional Journal Information:

Journal Volume: 120; Journal Issue: 29; Journal ID: ISSN 1089-5639

Publisher:

American Chemical Society

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY