Understanding the oxidants transition and SOA property in limonene ozonolysis: Role of different double bonds, radical chemistry, and water

Abstract

<p><strong>Abstract.</strong> Volatile organic compounds (VOCs) play an important role in air quality and climate change, largely because of their contribution to atmospheric oxidation capacity and secondary organic aerosol (SOA) formation through their oxidation. In this study, a series of products including peroxides and carbonyl compounds in both gaseous and particulate phases were simultaneously detected to help us investigate the oxidants transition and SOA property in limonene ozonolysis. Reactants ratio, OH radical scavenger and relative humidity (RH) were controlled to discuss the effect of endocyclic and exocyclic double bonds (DBs), radial chemistry and water. Alkene ozonolysis not only consumed but also regenerated oxidants, which made a great impact on atmospheric chemical processes. For this issue, we first paid attention to the generation of stabilized Criegee intermediates (SCIs) and OH radical. The variation of H<sub>2</sub>O<sub>2</sub> and hydroxymethyl hydroperoxide (HMHP) formation with RH showed the importance of the reaction with water for limonene SCIs, and the estimated SCIs yields of endocyclic and exocyclic DBs were ~ 0.24 and ~ 0.43, respectively. OH yield was determined by adding sufficient OH scavenger, and the OH yields of endocyclic and exocyclic DBs were ~ 0.65 and ~ 0.24, respectively. The results indicated that in limonene ozonolysis the endocyclic DB was inclined to generate OH radical through hydroperoxide channel, while the exocyclic DB had higher fraction of forming SCIs. Besides, other gas-phase and particle-phase peroxides were also studied. The formation of peroxyformic acid (PFA) and peroxyacetic acid (PAA) were promoted significantly by the increasing RH and the oxidation degree, and the discrepancy between the experimental and model results suggested some missing formation pathways. Considerable H<sub>2</sub>O<sub>2</sub> generation from SOA in aqueous phase was observed especially at high [O<sub>3</sub>]/[limonene], which was mainly attributed to the hydration and decomposition of particulate unstable peroxides such as peroxycarboxylic acids and peroxyhemiacetals. Different DBs and radical chemistry revealed their influence on aerosol property through affecting the behavior of SOA on generating H<sub>2</sub>O<sub>2</sub>. As a species owning high SOA formation potential, another key issue we investigated in limonene ozonolysis was SOA property, for which peroxides and carbonyls were chosen as representatives. The results showed that in limonene SOA, peroxides could account for 0.07–0.19 at low [O<sub>3</sub>]/[limonene] and 0.40–0.58 at high [O<sub>3</sub>]/[limonene], which confirmed the important contribution of peroxides to aerosol formation. The partitioning behavior of peroxides showed that multigeneration oxidation helped produce more low-volatility peroxides, which provided some explanation for higher SOA yield. The partitioning behavior of carbonyls was also discussed and the experimental partitioning coefficients (<i>K</i><sub><i>p</i></sub>) were usually several orders of magnitude higher than theoretical values, yet the relationship of <i>K</i><sub><i>p</i></sub> observed in laboratory with vapor pressure offered some reference for predicting the contribution of carbonyls to SOA formation. This study provided new insights into the oxidants transition and SOA property in limonene ozonolysis, and limonene showed its specificity in many aspects when both endocyclic and exocyclic DBs were ozonated. We suggested that the atmospheric implications of terpenes containing more than one DB and the properties of particulate products especially peroxides still needed further study.</p>