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

Secondary organic aerosol (SOA) from the oxidation of alpha-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 alpha-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 alpha-pinene. Known tracers for alpha-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 alpha 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 alpha pinene SOA tracers.