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The reaction of methyl peroxy and hydroxyl radicals as a major source of atmospheric methanol.

ABSTRACT: Methyl peroxy, a key radical in tropospheric chemistry, was recently shown to react with the hydroxyl radical at an unexpectedly high rate. Here, the molecular reaction mechanisms are elucidated using high-level quantum chemical methodologies and statistical rate theory. Formation of activated methylhydrotrioxide, followed by dissociation into methoxy and hydroperoxy radicals, is found to be the main reaction pathway, whereas methylhydrotrioxide stabilization and methanol formation (from activated and stabilized methylhydrotrioxide) are viable minor channels. Criegee intermediate formation is found to be negligible. Given the theoretical uncertainties, useful constraints on the yields are provided by atmospheric methanol measurements. Using a global chemistry-transport model, we show that the only explanation for the high observed methanol abundances over remote oceans is the title reaction with an overall methanol yield of ?30%, consistent with the theoretical estimates given their uncertainties. This makes the title reaction a major methanol source (115?Tg per year), comparable to global terrestrial emissions.

SUBMITTER: Muller JF

PROVIDER: S-EPMC5071643 | biostudies-literature | 2016 Oct

REPOSITORIES: biostudies-literature

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The reaction of methyl peroxy and hydroxyl radicals as a major source of atmospheric methanol.

Müller Jean-François JF Liu Zhen Z Nguyen Vinh Son VS Stavrakou Trissevgeni T Harvey Jeremy N JN Peeters Jozef J

Nature communications 20161017

Methyl peroxy, a key radical in tropospheric chemistry, was recently shown to react with the hydroxyl radical at an unexpectedly high rate. Here, the molecular reaction mechanisms are elucidated using high-level quantum chemical methodologies and statistical rate theory. Formation of activated methylhydrotrioxide, followed by dissociation into methoxy and hydroperoxy radicals, is found to be the main reaction pathway, whereas methylhydrotrioxide stabilization and methanol formation (from activat ...[more]

PMID: 27748363

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Project description:Understanding the mechanism of hydrazine oxidation reaction by OH radical along with the rate constants of all possible pathways leads to explain the fate of hydrazine in the atmosphere. In this article, the comprehensive mechanisms and kinetics of the hydrazine plus hydroxyl radical reaction have been investigated theoretically at different temperatures and pressures. To achieve the main goals, a series of high levels of quantum chemical calculations have been widely implemented in reliable channels of the H-abstraction, SN2, and addition/elimination reactions. The energy profile of all pathways accompanied by the molecular properties of the involved stationary points has been characterized at the MP2, M06-2X, and CCSD(T)/CBS levels. To estimate accurate barrier energies of the H-abstraction channels, large numbers of the CCSD (T) calculations in conjunction with various augmented basis sets have been implemented. The direct dynamic calculations have been carried out using the validated M06-2X/maug-cc-pVTZ level, and also by the CCSD(T) (energies) + MP2 (partition functions) level. The pressure-dependent rate constants of the barrierless pathways have been investigated by the strong collision approach. Therefore, the main behaviors of the N2H4 + OH reaction have been explored according to the influences of temperature and pressure on the computed rate coefficients within the well-behaved theoretical frameworks of the TST, VTST, and RRKM theories. It has been found that the H-abstraction mechanism (to form N2H3) is dominant relative to the SN2 reaction and OH-addition to the N center of N2H4 moiety (to form H2NOH + NH2). The computed high pressure limit rate constant of the main reaction pathway, k(298.15) = 7.31 × 10-11 cm3 molecule-1 s-1, has an excellent agreement with the experimental value (k (298.15) = (6.50 ± 1.3) × 10-11 cm3 molecule-1 s-1) recommended by Vaghjiani. Also, the atmospheric lifetime of hydrazine degradation by OH radicals has been demonstrated to be 32.80 to 1161.11 h at the altitudes of 0-50 km. Finally, the disagreement in the calculated rate constants between the previous theoretical study and experimental results has been rectified.

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The reaction of methyl peroxy and hydroxyl radicals as a major source of atmospheric methanol.

Publications

The reaction of methyl peroxy and hydroxyl radicals as a major source of atmospheric methanol.

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