1Johannes Gutenberg-Universität, Institute for inorganic Chemistry and analytical Chemistry, Duesbergweg 10–14, 55128 Mainz, Germany
2Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research (IMK-AAF), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
3Forschungszentrum Jülich GmbH, ICG-2: Troposphäre, 52425 Jülich, Germany
4Carnegie Mellon University, Department of Chemical Engineering, 5000 Forbes Avenue, Pittsburgh, PA 15213-3890 USA
Abstract. This paper presents the results of mass spectrometric investigations of the OH-initiated oxidative aging of α-pinene SOA under simulated tropospheric conditions at the large aerosol chamber facility AIDA, Karlsruhe Institute of Technology. In particular, the OH-initiated oxidation of pure pinic and pinonic acid, two well-known oxidation products of α-pinene, was investigated. Two complementary analytical techniques were used, on-line atmospheric pressure chemical ionization/mass spectrometry (APCI/MS) and filter sampling followed by liquid chromatography/mass spectrometry (LC/ESI-MS). The results show that 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), a known and very low volatile α-pinene SOA product, is formed from the oxidation of pinonic acid and that this oxidation takes place in the gas phase. This finding is confirmed by temperature-dependent aging experiments on whole SOA formed from α-pinene, in which the yield of MBTCA scales with the pinonic acid fraction in the gas phase. Based on the results, several feasible gas-phase radical mechanisms are discussed to explain the formation of MBTCA from OH-initiated pinonic acid oxidation.