Abstract
Abstract. Secondary organic aerosol (SOA) is an important constituent of the atmosphere where SOA particles are formed chiefly by the condensation or reactive uptake of oxidation products of volatile organic compounds (VOCs). The mass yield in SOA particle formation, as well as the chemical composition and volatility of the particles, is determined by the identity of the VOC precursor(s) and the oxidation conditions they experience. In this study, we used an oxidation flow reactor to generate biogenic SOA from the oxidation of Scots pine emissions. Mass yields, chemical composition and volatility of the SOA particles were characterized and compared with SOA particles formed from oxidation of α-pinene and from a mixture of acyclic–monocyclic sesquiterpenes (farnesenes and bisabolenes), which are significant components of the Scots pine emissions. SOA mass yields for Scots pine emissions dominated by farnesenes were lower than for α-pinene but higher than for the artificial mixture of farnesenes and bisabolenes. The reduction in the SOA yield in the farnesene- and bisabolene-dominated mixtures is due to exocyclic C=C bond scission in these acyclic–monocyclic sesquiterpenes during ozonolysis leading to smaller and generally more volatile products. SOA particles from the oxidation of Scots pine emissions had similar or lower volatility than SOA particles formed from either a single precursor or a simple mixture of VOCs. Applying physical stress to the Scots pine plants increased their monoterpene, especially monocyclic β-phellandrene, emissions, which further decreased SOA particle volatility and increased SOA mass yield. Our results highlight the need to account for the chemical complexity and structure of real-world biogenic VOC emissions and stress-induced changes to plant emissions when modelling SOA production and properties in the atmosphere. These results emphasize that a simple increase or decrease in relative monoterpene and sesquiterpene emissions should not be used as an indicator of SOA particle volatility.
Highlights
Secondary organic aerosol (SOA) formed from oxidation of volatile organic compounds (VOCs) comprises a large fraction of the total aerosol mass in the boreal forests of the Northern Hemisphere
The VOC emissions from the Scots pine sapling were mainly composed of monoterpenes and sesquiterpenes
All sesquiterpenes combined accounted for 55 %–70 % of the total VOC mass concentration in the Scots pine experiments 1–3 and for 40 % in Scots pine experiment 4, based on proton-transfer-reaction time-offlight mass spectrometer (PTR-MS) measurements
Summary
The chemical transformation of primary VOC emissions to SOA particles, which have an important climate impact (Hallquist et al, 2009), is a complicated cascade of gas-phase oxidation and multiphase ageing reactions. The physical properties of SOA are dictated by the chemical complexity of the initial VOC emissions and the oxidative conditions they experience (Glasius and Goldstein, 2016). The formation and growth of SOA particles are often described by the absorptive partitioning of organic vapours (e.g. terpenoid oxidation products) between the gas and particle phase (Donahue et al, 2011; Pankow, 1994). The main property determining how readily organic molecules enter and stay in the particle phase is their volatility, usually expressed as saturation vapour pressure (Psat) or saturation mass concentration (C∗) in air. The uptake of oxidation products may involve or be facilitated by heterogeneous reactions
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