Investigation of heterogeneous reactions of organic aerosols using single particle levitation and Raman spectroscopy.

摘要

Atmospheric aerosols have been found to exert pronounced detrimental effects on human health, visibility, and global climate. In particular, organic aerosols are abundant in the atmosphere and hence are expected to have significant impacts on both physical and chemical properties of atmospheric aerosols. Organic aerosols can interact with various gas-phase oxidants, and reactive organic vapor can interact with inorganic aerosols throughout their atmospheric lifetime. These aging processes insert great uncertainty into the prediction of global climate change.;An electrodynamic balance (EDB) coupled with Raman spectroscopy is an attractive platform for studying atmospheric heterogeneous reactions since it affords long duration (>days) particle levitation and hence gas-phase reactants concentrations relevant to atmospheric conditions can be investigated. In addition to being as a levitation tool, the EDB is a pico-balance that has been shown to be very useful in examining particle hygroscopicity because of its high sensitivity to any changes in particle mass in response to changes in the surrounding air. This feature makes the EDB an ideal tool to directly measure the mass yield (or loss) of levitated particles when they interact with gas-phase reactants. Single particle Raman spectroscopy allows the in-situ composition monitoring of the levitated particle due to chemical reactions, and therefore the potential reaction mechanisms can be proposed. In this study, the EDB-single particle Raman spectroscopic system was used to investigate three heterogeneous reactions. They are (1) ozonolysis of oleic acid particles, (2) heterogeneous oxidation of poly-unsaturated fatty acids particles under ozone exposure, and (3) uptake of octanal vapor by sulfuric acid droplets.;Overall, the single particle studies clearly reveal that the gas-phase reactant concentrations play an important role in affecting the heterogeneous aging processes such as reaction mechanisms, kinetic behaviors, organic mass yield and particle morphology. The results evidently show that the heterogeneous oxidative processes are potentially important to modify the organic mass yields and enhance the hygroscopicity of atmospheric organic aerosols. Furthermore, acid-catalyzed reactions of organic vapor may be a vital atmospheric process to incorporate hydrophobic organics into hydrophilic aerosols and hence influencing the gas-particle partitioning of volatile/semi-volatile hydrophobic organics in the atmosphere.