Molecular identification of organic acid molecules from α-pinene ozonolysis.

Oxidized organic molecules from ozonolysis of α-pinene are believed to participate in atmospheric new particle formation in forest regions. However, the detailed process of nucleation originated from α-pinene ozonolysis remains unclear. In this study, an atmospheric pressure interface time-of-flight mass spectrometer coupled with a planar differential mobility analyzer (DMA-MS) was applied to investigate the gas-phase reaction of α-pinene and ozone. Ion mobility and mass to charge ratio of the oxidized molecules were simultaneously measured. The ion mobility distribution of these oxidation products highlighted three mobility diameter ranges: 0.89–0.99 nm, 1.16–1.24 nm and 1.31–1.38 nm, where the dominating molecules were CH 2 O 2 , C 7-10 H 10-20 O 3-7 and C 19-20 H 30-32 O 6-9 , respectively. Combining the DMA-MS measurements, quantum chemistry calculations, and ion mobility modeling, the dominant monomer (C 10 H 16 O 3) was confirmed as pinonic acid while the primary dimer (C 20 H 32 O 6) was considered as pinonic acid cluster ((C 10 H 16 O 3) 2). Considering the yield of pinonic acid produced from α-pinene ozonolysis (2–4%), (C 10 H 16 O 3) 2 potentially accounts for part of the nocturnal C 20 dimers and ungrowable sub-2-nm particles in forests. Our work provides a new method to facilitate the understanding of atmospheric nucleation and initial growth. • A novel method to characterize molecular structures is introduced. • The dominating products of α-pinene ozonolysis are confirmed to be organic acid molecules and clusters. • Pinonic acid cluster potentially accounts for part of the nocturnal C 20 dimers and ungrowable sub-2-nm particles in forests. [ABSTRACT FROM AUTHOR]