Single particle Raman spectroscopy for investigating atmospheric heterogeneous reactions of organic aerosols

Heterogeneous reactions of organic aerosols with atmospheric oxidants are important processes that affect the hygroscopicity and cloud condensation nuclei (CCN) activities of atmospheric aerosols. An electrodynamic balance (EDB) coupled with Raman spectroscopy is a particularly attractive platform for studying atmospheric reactions since it allows long-duration (days) particle levitation and reactions at atmospherically relevant low-oxidant concentrations can be investigated. In this study, we demonstrated the use of an EDB/Raman system to investigate the heterogeneous reactions of oleic acid particles with ozone (240–280 ppb) under ambient temperatures (22–24 °C) and dry conditions (relative humidity <5%) over a period of 20 h. The Raman signatures of the ozone-processed oleic acid particles indicate the formation of oxidation products predominately consisting of peroxidic compounds (OO groups of peroxides and/or ozonides), carbonyl (CO) and hydroxyl (OH) functional characteristics, which are consistent with the predictions of the Criegee mechanism as well as the results reported in the literature. We also confirmed that the Raman signatures of the reacted particles at atmospheric and much higher (>10 ppm) ozone concentrations are practically the same, which provides assurance to the use of elevated ozone concentrations in reaction studies of the oleic acid–ozone system in the literature. The ratio of the percentage of mass loss (due to evaporation of volatile organic products) to the percentage of oleic acid conversion was estimated to be 0.05. Furthermore, the oxidation products that remained in the particle phase were more hygroscopic than were their hydrophobic parent molecules.