In situ analysis of single secondary organic aerosol from the ozonolysis of α-pinene using laser trapping and Raman spectroscopy

To form secondary organic aerosols (SOAs) from volatile α-pinene via ozone oxidation reactions, the generation of low-vapor pressure compounds capable of acting as condensation nuclei is essential. Both laboratory and field studies have identified low-volatile dimers as key components in SOA formation. Although mechanisms involving stabilized Criegee intermediates and peroxy radicals have been proposed for dimer formation in the gas phase, the detailed processes of SOA formation remain widely debated due to the high diversity of reaction products and the inherent complexity of their formation mechanisms. To clarify certain steps of the SOA formation mechanism from ozone-initiated oxidation of α-pinene, a laser trapping technique coupled with Raman spectroscopy was employed for in situ analysis of individual SOA particles. In this study, single micrometer-sized α-pinene SOA particles were successfully trapped in air. To our knowledge, this represents the first successful application of laser trapping of α-pinene SOA particles directly in air. An intense Raman peak observed at about 852 cm-1 was attributed to the O-O stretching vibration of peroxides. This finding provides direct evidence for the presence of peroxy functional groups in the dimers, likely formed via stabilized Criegee intermediates. These results demonstrate the capability of laser trapping combined with Raman spectroscopy to detect specific molecular features, such as peroxide bonds, within α-pinene-derived SOAs in air.