Effects of Condensed-Phase Oxidants on Secondary Organic Aerosol Formation

In this study we investigate the hypothesis that oxidants present within atmospheric particles can promote the formation of highly oxidized organic aerosol (OA) via oxidation reactions in the condensed phase. Secondary organic aerosol (SOA) was generated from the ozonolysis of α-pinene and isoprene in an environmental chamber, with seed particles systematically varied in order to assess the effects of condensed-phase oxidant levels on SOA loading and composition. The effects of particle phase (aqueous vs dry), condensed-phase oxidant source (none vs H₂O₂ vs Fenton chemistry), and irradiation (none vs UV) were all examined. For experiments conducted with aqueous particles but without any added oxidants, UV irradiation resulted in a small but measurable enhancement in the oxygen-to-carbon ratio (O/C). OA formed in the presence of aqueous oxidants was substantially more oxidized, with the highest oxidant concentrations leading to OA with an O/C as high as 1.4 for α-pinene and 2.0 for isoprene, strongly suggesting the formation of oxalate. High aqueous oxidant levels also resulted in increased loss of carbon from the condensed phase. This OA was more oxidized than in any other ozonolysis experiment reported to date, indicating that, when present, aqueous oxidants can have a dramatic effect on SOA formation. However, oxidant concentrations within atmospheric aqueous particles remain poorly constrained, making it difficult to assess the impacts of aqueous-phase oxidation on the loadings and oxidation state of atmospheric OA

Formation of Secondary Organic Aerosol from the Direct Photolytic Generation of Organic Radicals

The immense complexity inherent in the formation of secondary organic aerosol (SOA)due primarily to the large number of oxidation steps and reaction pathways involvedhas limited the detailed understanding of its underlying chemistry. As a means of simplifying such complexity, here we demonstrate the formation of SOA through the photolysis of gas-phase alkyl iodides, which generates organic peroxy radicals of known structure. In contrast to standard OH-initiated oxidation experiments, photolytically initiated oxidation forms a limited number of products via a single reactive step. As is typical for SOA, the yields of aerosol generated from the photolysis of alkyl iodides depend on aerosol loading, indicating the semivolatile nature of the particulate species. However, the aerosol was observed to be higher in volatility and less oxidized than in previous multigenerational studies of alkane oxidation, suggesting that additional oxidative steps are necessary to produce oxidized semivolatile material in the atmosphere. Despite the relative simplicity of this chemical system, the SOA mass spectra are still quite complex, underscoring the wide range of products present in SOA

Secondary Organic Aerosol Formation and Chemistry from the OH-Initiated Oxidation of Monofunctional C₁₀ Species

The formation of secondary organic aerosol (SOA), even from a simple hydrocarbon, is a complex, heterogeneous, multigenerational process involving hundreds of radical intermediate isomers and reaction pathways. Here, we compared the SOA generated from the reaction of the OH radical with five precursor species that differed in the identity of their primary functional group: n-decane, cyclodecane, 2-decanol, 2-decylnitrate, and 2-decanone. We compared results from smog chamber experiments and an explicit oxidation/gas-particle partitioning model of first-generation oxidation chemistry (Framework for 0-Dimensional Atmospheric Modeling–Washington Aerosol Module, F0AM-WAM) under two NOx regimes: lower NOx where RO2 + HO2 dominates and higher NOx where RO2 + NO dominates. Our results show that while functional group identity impacted the vapor pressures of the precursor species, this alone was unable to explain trends in experimental yields. Functional groups also directed the site of initiation with the OH radical and the propagation and termination reactions that follow, with the most significant differences noted for 2-decanol. SOA production was greater in the lower NOx experiments for n-decane, 2-decanol, 2-decylnitrate, and 2-decanone due to production of the low volatility hydroperoxides and oxidized hydroxycarbonyls. Cyclodecane, however, produced more aerosol in higher NOx experiments, potentially due to the enhanced formation of low volatility acetals or dimers in the presence of greater concentrations of nitric acid. Finally, we predicted that as much as 67% of the first-generation products may undergo subsequent oxidation to later-generation species. While model results from first-generation chemistry alone are unable to predict experimentally observed yields and chemistry, this work provides a foundation for the incorporation of additional (e.g., later-generation or heterogeneous oxidation chemistry, condensed-phase reactions, etc.) processes