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

Abstract

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-a-lactone (HMML). The decomposition of the MPAN-OH adduct yields HMML + NO 3 (B75%) and hydroxyacetone + CO + NO 3 (B25%), 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 (o2%). 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 2 O 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.

Influence of urban pollution on the production of organic particulate matter from isoprene epoxydiols in central Amazonia

“…The large dashed black arrow represents the analytical methods that use different types of molecular and statistical tracers (listed in the boxes) to quantify the IEPOX-derived PM mass concentrations. For simplicity, the figure omits the many routes leading to the production of glyoxal (Fu et al, 2008), possible ISOPOO isomerization when NO and HO 2 concentrations are sufficiently low (Crounse et al, 2011;Liu et al, 2016a), second-generation production of peroxymethacrylic nitric anhydride (Lin et al, 2013;Nguyen et al, 2015), and particle water and other inorganic components. 3-methyltetrahydrofuran-3,4-diols are abbreviated as 3-MeTHF-3,4-diols.…”

“…MACR can be oxidized to produce peroxymethacrylic nitric anhydride (MPAN), which is a precursor to methacrylic acid epoxide (MAE) and hydroxymethylmethyl-α-lactone (HMML), and these compounds can undergo reactive uptake to produce PM (Kjaergaard et al, 2012;Lin et al, 2013;Worton et al, 2013;Nguyen et al, 2015). Glyoxal produced from isoprene oxidation can contribute to PM production (Volkamer et al, 2007;Ervens and Volkamer, 2010;McNeill et al, 2012;Marais et al, 2016).…”

The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization

“…The CPOT has been constructed as a complement to the Caltech 24 m 3 batch chambers (Bates et al, , 2016Schilling et al, 2015;Hodas et al, 2015;Loza et al, 2013Loza et al, , 2014McVay et al, 2014McVay et al, , 2016Nguyen et al, 2014Nguyen et al, , 2015Schwantes et al, 2015;Yee et al, 2013;Zhang et al, 2014; in carrying out studies of SOA formation resulting from the oxidation of volatile organic compounds (VOCs) by oxidants OH, O 3 and NO 3 over timescales not accessible in a batch chamber. Due to its steady-state operation, the CPOT also affords the capability to collect sufficient quantities of SOA generated in the reactor for comprehensive composition determination by offline mass spectrometry.…”

Different roles of water in secondary organic aerosol formation from toluene and isoprene

“…MPAN is one of key precursors of SOA from isoprene under high NOx conditions (Surratt et al, 2010). With additional OH sources, MPAN can be oxidized by OH to form SOA products (such as 2-methyltetrols) (Surratt et al, 2010;Lin et al, 2013;Nguyen et al, 2015). Both the results from Nguyen et al (2014) On the other hand, similar to the isoprene-O3 system, SCI-related reactions in the isoprene-NO2 system were probably key pathways.…”