Abstract. Field observations over the past decade indicate that a significant fraction of organic aerosol in remote areas may contain highly oxidized molecules. Aerosol processing or further oxidation (aging) of organic aerosol has been suggested to be responsible for their formation through heterogeneous reaction with oxidants and multigenerational oxidation of vapours by OH radicals. In this study we investigated the influence of several aging processes on the molecular composition of secondary organic aerosols (SOA) using direct infusion and liquid chromatography high-resolution mass spectrometry. SOA was formed in simulation chamber experiments from ozonolysis of a mixture of four biogenic volatile organic compounds (BVOC): α-pinene, β-pinene, Δ3-carene and isoprene. The SOA was subsequently aged under three different sets of conditions: in the dark in the presence of residual ozone, with UV irradiation and OH radicals, and using UV light only. Among all studied conditions, only OH radical-initiated aging was found to influence the molecular composition of the aerosol and showed an increase in carbon oxidation state (OSC) and elemental O / C ratios of the SOA components. None of the aging processes produced an observable effect on the oligomers formed from ozonolysis of the BVOC mixture, which were found to be equally abundant in both "fresh" and "aged" SOA. Additional experiments using α-pinene as the sole precursor demonstrated that oligomers are an important group of compounds in SOA produced from both ozonolysis and OH radical-initiated oxidation processes; however, a completely different set of oligomers is formed under these two oxidation regimes. SOA from the OH-initiated oxidation of α-pinene had a significantly higher overall OSC and O / C compared to that from pure ozonolysis experiments confirming that the OH radical reaction is more likely to be responsible for the occurrence of highly oxidized species in ambient biogenic SOA.
Environmental context. Ambient fine aerosol from forested sites contains secondary organic aerosol from the oxidation of monoterpenes that are emitted by the vegetation, mainly by conifers. These biogenic aerosols can have varying lifetimes in the atmosphere because they contain first-generation oxidation products of a-pinene as well as aged products formed through further photooxidation, fragmentation, hydrolysis, and dimerisation reactions. We focus on the structural characterisation of secondary organic aerosol products that are simulated in a smog chamber experiment and can serve as potential tracers for aging processes in biogenic aerosols.Abstract. Secondary organic aerosol (SOA) from the oxidation of a-pinene is a very complex and dynamic mixture containing products with a different chemical nature and physicochemical properties that are dependent on chemical evolution or aging processes. In this study, we focus on the chemical characterisation of major products that are formed upon a-pinene ozonolysis SOA and subsequent aging through OH-initiated reactions in the absence of NO x , which include known as well as unknown tracers. The mass spectrometric data obtained for selected unknown compounds that show an increased relative abundance upon aging are interpreted in detail and tentative structures for them are proposed taking into account their formation through photooxidation of a-pinene. Known tracers for a-pinene SOA aging that were identified include norpinic acid, 10-hydroxypinonic acid, diaterpenylic acid acetate, and diesters formed by esterification of pinic acid with terpenylic acid or 10-hydroxypinonic acid. Novel tracers for a-pinene SOA aging that were tentatively identified include dinorpinic acid and 8-hydroxypinonic acid. In addition, reaction mechanisms are proposed to explain the formation of the observed a-pinene SOA tracers.
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