Abstract
Ozonolysis of alkenes is an important source of secondary organic aerosol (SOA) in the atmosphere. However, the mechanisms by which stabilized Criegee intermediates (SCI) react to form and grow the particles, and in particular the contributions from oligomers, are not well understood. In this study, ozonolysis of trans-3-hexene (C6H12), as a proxy for small alkenes, was investigated with an emphasis on the mechanisms of particle formation and growth. Ozonolysis experiments were carried out both in static Teflon chambers (18-20 min reaction times) and in a glass flow reactor (24 s reaction time) in the absence and presence of OH or SCI scavengers, and under different relative humidity (RH) conditions. The chemical composition of polydisperse and size-selected SOA particles was probed using different mass spectrometric techniques and infrared spectroscopy. Oligomers having SCI as the chain unit are found to be the dominant components of such SOA particles. The formation mechanism for these oligomers suggested by our results follows the sequential addition of SCI to organic peroxy (RO2) radicals, in agreement with previous studies by Moortgat and coworkers. Smaller particles are shown to have a relatively greater contribution from longer oligomers. Higher O/C ratios are observed in smaller particles and are similar to those of oligomers resulting from RO2 + nSCI, supporting a significant role for longer oligomers in particle nucleation and early growth. Under atmospherically relevant RH of 30-80%, water vapor suppresses oligomer formation through scavenging SCI, but also enhances particle nucleation. Under humid conditions, or in the presence of formic or hydrochloric acid as SCI scavengers, peroxyhemiacetals are formed by the acid-catalyzed particle phase reaction between oligomers from RO2 + nSCI and a trans-3-hexene derived carbonyl product. In contrast to the ozonolysis of trans-3-hexene, oligomerization involving RO2 + nSCI does not appear to be prevalent in the ozonolysis of α-cedrene (C15H24), indicating different particle formation mechanisms for small and large complex alkenes that need to be taken into account in atmospheric models.
Highlights
Organic aerosol makes up a substantial fraction (20–90%) of submicron particles in the atmosphere,[1,2,3] of which up to 90% is secondary organic aerosol (SOA).[2,4] SOA is produced by atmospheric oxidation of volatile organic compounds (VOCs) that can lead to the formation of products having sufficiently low vapor pressures to either nucleate to form new particles or condense onto pre-existing particles.[5,6] At present, a quantitative understanding of SOA formation from such oxidation processes remains limited, resulting in large uncertaintiesOzonolysis of alkenes is an important source of SOA in the atmosphere.[1,5,6,10,11] Ozone–alkene reactions are known to proceed via the formation of a primary ozonide which decomposes to a carbonyl product and a biradical/zwitterion known as the Criegee intermediate.[5,6,10] Over the past decades, special attention has been given to the ozonolysis of large biogenic VOCs such as monoterpenes and sesquiterpenes,[1,3,10] which have structures and large molecular masses that favor the formation of low volatility products upon atmospheric oxidation
In the mass spectrum obtained without a scavenger (Fig. 1a), an obvious feature is the presence of an ion series starting at m/z 305 with mass differences of m/z 74, suggesting the formation of oligomers with a repeat unit that has the same mass as trans-3-hexene stabilized Criegee intermediates (SCI) (CH3CH2CHOO)
These studies show that particle formation in the trans-3hexene ozonolysis is consistent with the sequential addition of SCI to RO2 radicals as previously proposed by Sadezky et al.[43]
Summary
Organic aerosol makes up a substantial fraction (20–90%) of submicron particles in the atmosphere,[1,2,3] of which up to 90% is secondary organic aerosol (SOA).[2,4] SOA is produced by atmospheric oxidation of volatile organic compounds (VOCs) that can lead to the formation of products having sufficiently low vapor pressures to either nucleate to form new particles or condense onto pre-existing particles.[5,6] At present, a quantitative understanding of SOA formation from such oxidation processes remains limited, resulting in large uncertaintiesOzonolysis of alkenes is an important source of SOA in the atmosphere.[1,5,6,10,11] Ozone–alkene reactions are known to proceed via the formation of a primary ozonide which decomposes to a carbonyl product and a biradical/zwitterion known as the Criegee intermediate.[5,6,10] Over the past decades, special attention has been given to the ozonolysis of large biogenic VOCs such as monoterpenes and sesquiterpenes,[1,3,10] which have structures and large molecular masses that favor the formation of low volatility products upon atmospheric oxidation.
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