Oligomers in the early stage of biogenic secondary organic aerosol formation and growth

The formation of secondary organic aerosol (SOA) by reaction of ozone with monoterpenes (beta-pinene, delta3-carene, limonene, and sabinene) was studied on a short time scale of 3-22 s with a flow tube reactor. Online chemical analysis was performed with the Photoionization Aerosol Mass Spectrometer (PIAMS) to obtain molecular composition and the Nanoaerosol Mass Spectrometer (NAMS) to obtain elemental composition. Molecular composition data showed that dimers and higher order oligomers are formed within seconds after the onset of reaction, indicating that there is no intrinsic kinetic barrier to oligomer formation. Because oligomer formation is fast, it is unlikely that a large number of steps are involved in their formation. Therefore, ion distributions in the PIAMS spectra were interpreted through reactions of intermediates postulated in previous studies with monomer end products or other intermediates. Based on ion signal intensities in the mass spectra, organic peroxides appear to comprise a greater fraction of the aerosol than secondary ozonides. This conclusion is supported by elemental composition data from NAMS that gave C:O ratios in the 2.2-2.7 range.