OH-initiated atmospheric degradation of hydroxyalkyl hydroperoxides: mechanism, kinetics, and structure–activity relationship
<p>Hydroxyalkyl hydroperoxides (HHPs), formed in the reactions of Criegee intermediates (CIs) with water vapor, play essential roles in the formation of secondary organic aerosol (SOA) under atmospheric conditions. However, the transformation mechanisms for the OH-initiated oxidation of HHPs r...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-03-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/22/3693/2022/acp-22-3693-2022.pdf |
| Summary: | <p>Hydroxyalkyl hydroperoxides (HHPs), formed in the reactions of Criegee
intermediates (CIs) with water vapor, play essential roles in the formation
of secondary organic aerosol (SOA) under atmospheric conditions. However,
the transformation mechanisms for the OH-initiated oxidation of HHPs remain
incompletely understood. Herein, the quantum chemical and kinetics modeling
methods are applied to explore the mechanisms of the OH-initiated oxidation
of the distinct HHPs (<span class="inline-formula">HOCH<sub>2</sub>OOH</span>, <span class="inline-formula">HOCH(CH<sub>3</sub>)OOH</span>, and
<span class="inline-formula">HOC(CH<sub>3</sub>)<sub>2</sub>OOH</span>) formed from the reactions of <span class="inline-formula">CH<sub>2</sub>OO</span>,
<i>anti-</i><span class="inline-formula">CH<sub>3</sub>CHOO</span>, and
(CH<span class="inline-formula"><sub>3</sub>)<sub>2</sub></span>COO with water vapor. The calculations
show that the dominant pathway is H-abstraction from the -<span class="inline-formula">OOH</span> group in the
initiation reactions of the OH radical with <span class="inline-formula">HOCH<sub>2</sub>OOH</span> and
<span class="inline-formula">HOC(CH<sub>3</sub>)<sub>2</sub>OOH</span>. H-abstraction from the -<span class="inline-formula">CH</span> group is competitive with
that from the -<span class="inline-formula">OOH</span> group in the reaction of the OH radical with
<span class="inline-formula">HOCH(CH<sub>3</sub>)OOH</span>. The barrier of H-abstraction from the -<span class="inline-formula">OOH</span> group slightly
increases when the number of methyl groups increase. In pristine
environments, the self-reaction of the <span class="inline-formula">RO<sub>2</sub></span> radical initially produces a
tetroxide intermediate via oxygen-to-oxygen coupling, and then it decomposes
into propagation and termination products through asymmetric two-step O–O
bond scission, in which the rate-limiting step is the first O–O bond
cleavage. The barrier height of the reactions of distinct <span class="inline-formula">RO<sub>2</sub></span> radicals
with the <span class="inline-formula">HO<sub>2</sub></span> radical is not affected by the number of methyl substitutions.
In urban environments, the reaction with <span class="inline-formula">O<sub>2</sub></span> to form formic acid and the
<span class="inline-formula">HO<sub>2</sub></span> radical is the dominant removal pathway for the <span class="inline-formula">HOCH<sub>2</sub>O</span> radical
formed from the reaction of the <span class="inline-formula">HOCH<sub>2</sub>OO</span> radical with NO. The <span class="inline-formula"><i>β</i></span>-site
C–C bond scission is the dominant pathway in the dissociation of the
<span class="inline-formula">HOCH(CH<sub>3</sub>)O</span> and <span class="inline-formula">HOC(CH<sub>3</sub>)<sub>2</sub>O</span> radicals formed from the reactions
of NO with <span class="inline-formula">HOCH(CH<sub>3</sub>)OO</span> and <span class="inline-formula">HOC(CH<sub>3</sub>)<sub>2</sub>OO</span> radicals. These new
findings deepen our understanding of the photochemical oxidation of
hydroperoxides under realistic atmospheric conditions.</p> |
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| ISSN: | 1680-7316 1680-7324 |