Atmospheric aerosols are a complex mixture of inorganic and organic compounds from different sources. They exert a variety of effects on our macro- and micro-environments, including those to the global climate, regional visibility and human health. These effects are not very well understood because of the formation processes of atmospheric aerosols, especially organic aerosols, remain poorly constrained. For better prediction of organic aerosol formation using modeling approaches, studies of the formation processes of organic aerosols in mechanistic details are in great need....[ Read more ]

Atmospheric aerosols are a complex mixture of inorganic and organic compounds from different sources. They exert a variety of effects on our macro- and micro-environments, including those to the global climate, regional visibility and human health. These effects are not very well understood because of the formation processes of atmospheric aerosols, especially organic aerosols, remain poorly constrained. For better prediction of organic aerosol formation using modeling approaches, studies of the formation processes of organic aerosols in mechanistic details are in great need.

In this study, we investigated two chemical processes that can lead to formation of secondary organic aerosols (SOA), which are defined as organic aerosols involving chemical reactions after emissions as volatile organic compounds (VOCs). The first process is the gas-phase oxidation of biogenic VOCs. A sesquiterpene β-caryophyllene, which is one of the most abundant sesquiterpenes emitted by plants, was used as a model compound. The gas-phase oxidation was conducted in a continuous-flow chamber that allows steady-state equilibrium of the gas-phase oxidation and gas-to-particle partitioning, forming SOA. Particles were characterized by an ultra performance liquid chromatograph-time of flight mass spectrometer (UPLC-ToF-MS). Results show that highly oxygenated compounds are formed under ozone-rich conditions. Multi-functional products with 3 to 5 oxygen atoms, with one up to 7, were identified. In contrast to products from previous studies that focused on first-generation reactions, these second-generation products are less volatile and are predicted to be predominantly in the particle phase.

The second process is the particle-phase acid-catalyzed reactions of semi-volatile organic compounds (SVOCs). An aliphatic aldehyde (octanal) and biogenic alkenes (limonene and terpineol) were used as model compounds to study the sulfuric acid-catalyzed reactions of two types of functional groups (C=O and C=C). A flow cell reactor with acidic particles deposited on a substrate and gas-phase organics passing through the reactor was used to perform the reactions. Product identification and semi-quantification were carried out using gas chromatography-mass spectrometry (GC-MS). Multiple steps of aldol condensation reactions were observed for octanal under extremely acidic conditions, i.e., low relative humidity (RH), while no significant uptake was observed under high RH conditions. For biogenic alkenes, the reactive uptake was also strongly dependent on RH, with large amounts of dimers and trimers formed under low RH conditions and only hydroxylated products under high RH conditions.