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Title: Mechanism of the hydroxy radical oxidation of methacryoyl peroxynitrate (MPAN) and its pathway toward secondary organic aerosol formation in the atmosphere

Methacryoyl peroxynitrate (MPAN), the acylperoxyl nitrate of methacrolein, has been suggested to be an important secondary organic aerosol (SOA) precursor from isoprene oxidation. Yet, the mechanism by which MPAN produces SOA via reaction with the hydroxyl radical (OH) is unclear. We systematically evaluate three proposed mechanisms in controlled chamber experiments and provide the first experimental support for the theoretically-predicted lactone formation pathway from the MPAN + OH reaction, producing hydroxymethyl-methyl-α-lactone (HMML). The decomposition of the MPAN-OH adduct yields HMML + NO3 (~ 75%) and hydroxyacetone + CO + NO3 (~ 25%), out-competing its reaction with atmospheric oxygen. The production of other proposed SOA precursors, e.g., methacrylic acid epoxide (MAE), from MPAN and methacrolein are negligible (< 2 %). Furthermore, we show that the beta-alkenyl moiety of MPAN is critical for lactone formation. Alkyl radicals formed via OH abstraction nstead of addition are thermalized; thus, even if they are structurally identical to the MPAN-OH adduct, they do not decompose to HMML. The SOA formation from HMML, via polyaddition of the lactone to organic compounds, is close to unity under dry conditions. However, the SOA yield is sensitive to particle liquid water and solvated ions. In hydrated sulfate-containing particles, HMML reacts primarilymore » with H2O and aqueous sulfate, producing monomeric 2-methylglyceric acid (2MGA) and the associated organosulfate. 2MGA, a tracer for isoprene SOA, is semivolatile and its volatility increases with decreasing pH in the aerosol water. Conditions that enhance the production of neutral 2MGA will suppress SOA mass from the HMML channel. Considering the liquid water content and pH ranges of ambient particles, MGA may exist largely as a gaseous compound in some parts of the atmosphere.« less
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Publication Date:
OSTI Identifier:
Report Number(s):
48275; KP1704020
DOE Contract Number:
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Chemistry Chemical Physics. PCCP, 17(27):17914-17926
Research Org:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org:
Country of Publication:
United States
Environmental Molecular Sciences Laboratory