Formation mechanism of organic clusters originating from -pinene ozonolysis

Ozonolysis of -pinene has significant contribution on atmospheric particle nucleation in forestry regions. Homogeneous nucleation was simulated with -pinene+O3 flow tube experiments. The mobility, chemical composition and formation mechanism of the produced clusters were studied with APi-TOF-MS coupling with planar DMA (DMA P5) and secondary electrospray ionization (SESI) source with NO3- reagent ion. Clusters with chemical composition being C20H32O6(NO3)- and electric mobility being ~ 1.16 cm2 (V s)-1 were observed as the major dimer clusters, while C10H16O3(NO3)- was the main first-generation product. Quantum chemistry calculation and ion mobility modeling were performed to verify the structures of observed molecules and clusters. Initial structures of C10H16O3(NO3)- were generated randomly by Molclus and optimized semi-empirically at the PM6-DH+ level using the MOPAC. The results were re-optimized using the DFT method at B3LYP // 6-31G (d) level by Gaussian09. The lowest-energy structures were selected to calculate their ion mobility with Ion mobility software (IMoS, version 1.10) with Trajectory Method Lennard Jones (TMLJ). The good match (difference less than 3%) between simulated and observed mobility indicated that the detected C10H16O3(NO3)- was ionized thorough SESI instead of fragmented from larger clusters. The structures of C20H32O6(NO3)- were determined with optimized C10H16O3 as model input and the mobility difference between simulated structure and observed result was within 5%. We proposed series of C20H32O6(NO3)- structures and corresponding mobilitiesindicating the potential formation mechanism of pinonic acid clusters. Such nucleation process can be very important in real atmosphere, regarding large production yield of pinonic acid in -pinene ozonolysis cascade.