Characterisation of tracers for aging of α-pinene secondary organic aerosol using liquid chromatography/negative ion electrospray ionisation mass spectrometry

Environmental contextAmbient 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 α-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. AbstractSecondary organic aerosol (SOA) from the oxidation of α-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 α-pinene ozonolysis SOA and subsequent aging through OH-initiated reactions in the absence of NOx, 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 α-pinene. Known tracers for α-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 α-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 α-pinene SOA tracers.