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Published July 21, 2015 | Published + Supplemental Material
Journal Article Open

Mechanism of the hydroxyl radical oxidation of methacryloyl peroxynitrate (MPAN) and its pathway toward secondary organic aerosol formation in the atmosphere


Methacryloyl peroxynitrate (MPAN), the acyl peroxynitrate 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 through 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 + NO_3 ( 75%) and hydroxyacetone + CO + NO_3 ( 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 cold via H-abstraction by OH do not decompose to HMML, even if they are structurally identical to the MPAN–OH adduct. The SOA formation from HMML, from polyaddition of the lactone to organic compounds at the particle interface or in the condensed phase, is close to unity under dry conditions. However, the SOA yield is sensitive to particle liquid water and solvated ions. In hydrated inorganic particles, HMML reacts primarily with H¬_2O to produce the monomeric 2-methylglyceric acid (2MGA) or with aqueous sulfate and nitrate to produce the associated organosulfate and organonitrate, respectively. 2MGA, a tracer for isoprene SOA, is semivolatile and its accommodation in aerosol water decreases with decreasing pH. Conditions that enhance the production of neutral 2MGA suppress SOA mass from the HMML channel. Considering the liquid water content and pH ranges of ambient particles, 2MGA will exist largely as a gaseous compound in some parts of the atmosphere.

Additional Information

© 2015 the Owner Societies. Received 6th April 2015; Accepted 11th June 2015. First published online 12 Jun 2015. We acknowledge funding from the U.S. National Science Foundation (NSF) Division of Atmospheric and Geospace Sciences (AGS) grant AGS-1240604 and the U.S. Department of Energy (DOE) grant DE-SC0006626. TBN is supported by the NSF postdoctoral research fellowship (PRF) award AGS-1331360. We thank Dr. Dennis Fitz (UC Riverside) for assistance with the Fitz Aerometric NO2/APN instrument, Matthew Coggon (Caltech) for AMS data collection and processing, Dr. Nathan F. Dalleska (Caltech Global Environmental Center) for help with the HR-MS analysis software and for use of the pH probe, and Dr. Avram Gold and Dr. Zhenfa Zhang (University of North Carolina) for assistance in synthesizing MAE. The HPLC-UV/Vis-ESI/HRMS analysis was performed at the W. R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE's Office of Biological and Environmental Research and located at PNNL. PNNL is operated for the DOE by Battelle Memorial Institute under contract #DE-AC06-76RL0 1830.

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Published - c5cp02001h.pdf

Supplemental Material - c5cp02001h1.pdf


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