Abstract
Abstract. Sesquiterpenes are an important class of biogenic volatile organic compounds (BVOCs) and have a high secondary organic aerosol (SOA) forming potential. However, SOA formation from sesquiterpene oxidation has received less attention compared to other BVOCs such as monoterpenes, and the underlying mechanisms remain poorly understood. In this work, we present a comprehensive experimental investigation of the ozonolysis of α-cedrene both in a glass flow reactor (27–44 s reaction times) and in static Teflon chambers (30–60 min reaction times). The SOA was collected by impaction or filters, followed by analysis using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and electrospray ionization mass spectrometry (ESI-MS), or measured online using direct analysis in real-time mass spectrometry (DART-MS) and aerosol mass spectrometry (AMS). The slow evaporation of 2-ethylhexyl nitrate that was incorporated into the SOA during its formation and growth gives an estimated diffusion coefficient of 3 × 10−15 cm2 s−1 and shows that SOA is a highly viscous semisolid. Possible structures of four newly observed low molecular weight (MW ≤ 300 Da) reaction products with higher oxygen content than those previously reported were identified. High molecular weight (HMW) products formed in the early stages of the oxidation have structures consistent with aldol condensation products, peroxyhemiacetals, and esters. The size-dependent distributions of HMW products in the SOA, as well as the effects of stabilized Criegee intermediate (SCI) scavengers on HMW products and particle formation, confirm that HMW products and reactions of SCI play a crucial role in early stages of particle formation. Our studies provide new insights into mechanisms of SOA formation and growth in α-cedrene ozonolysis and the important role of sesquiterpenes in new particle formation as suggested by field measurements.
Highlights
Organic aerosol is ubiquitous in the atmosphere and has an important influence on air quality
A shoulder at 1762 cm−1 may indicate the presence of carboxylic acids, esters, or other species containing C=O groups, with a more electronegative atom such as oxygen being attached to the carbonyl carbon (Socrates, 2001; Kidd et al, 2014a)
The decomposition of oligomers in Secondary organic aerosol (SOA) was supported by electrospray ionization mass spectrometry (ESI-MS) measurements of SOA collected on Teflon filters and exposed to a flow of clean dry air; the relative ion intensity of oligomers to low molecular weight (LMW) products in the mass spectrum of SOA extracted after 20 h of air exposure is ∼ 15 % lower compared to that of SOA extracted immediately following collection
Summary
Organic aerosol is ubiquitous in the atmosphere and has an important influence on air quality Secondary organic aerosol (SOA) formed from the oxidation of volatile organic compounds (VOCs) contributes a substantial fraction (up to 90 %) of organic aerosol (Zhang et al, 2007). Biogenic VOCs (BVOCs) such as isoprene (C5H8), monoterpenes (C10H16), and sesquiterpenes (C15H24) account for ∼ 90 % of global VOC emissions (Guenther et al, 1995; Goldstein and Galbally, 2007) and are the dominant contributors to global SOA formation upon reaction with oxidants that are mainly anthropogenically derived (Kanakidou et al, 2005; Hallquist et al, 2009). Sesquiterpenes are an important class of BVOCs, with emissions being estimated as 9–29 % of those of monoterpenes (Helmig et al, 2007; Sakulyanontvittaya et al, 2008a).
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