Terpenylic acid and related compounds: precursors for dimers in secondary organic aerosol from the ozonolysis of α- and β-pinene

In the present study, we have characterized the structure of a higher-molecular weight (MW) 358 alpha- and beta-pinene dimeric secondary organic aerosol (SOA) product that received ample attention in previous molecular characterization studies and has been elusive. Based on mass spectrometric evidence for deprotonated molecules formed by electrospray ionization in the negative ion mode and chemical considerations, it is suggested that diaterpenylic acid is a key monomeric intermediate for dimers of the ester type. It is proposed that cis-pinic acid is esterified with the hydroxyl-containing diaterpenylic acid, which can be explained through acid-catalyzed hydrolysis of the recently elucidated lactone-containing terpenylic acid and/or diaterpenylic acid acetate, both first-generation oxidation products. To a minor extent, higher-MW 358 and 344 diester products are formed containing other terpenoic acids as monomeric units, i.e., diaterpenylic acid instead of cis-pinic acid, and diaterebic acid instead of diaterpenylic acid. It is shown that the MW 358 diester and related MW 344 compounds, which can be regarded as processed SOA products, also occur in ambient fine (PM(2.5)) rural aerosol collected at night during the warm period of the 2006 summer field campaign conducted at K-puszta, Hungary, a rural site with coniferous vegetation. This indicates that, under ambient conditions, the higher-MW diesters are formed in the particle phase over a longer time-scale than that required for gas-to-particle partitioning of their monomeric precursors in laboratory alpha-/beta-pinene ozonolysis experiments.