Probing Reaction Pathways of RO2 Radicals in the Atmospheric Aqueous Phase
LEXY LEMAR, Seamus Frey, Yaowei Li, Frank Keutsch, Jesse Kroll, MIT
Abstract Number: 489
Working Group: Aerosol Chemistry
Abstract
Organic peroxy radicals (RO2) are key intermediates in most oxidation processes, and their branching has a governing influence on product distributions, including secondary organic aerosol (SOA) formation. RO2 chemistry has received considerable study in the gas phase, but significantly less in aqueous environments (e.g., deliquesced aerosols, cloud droplets), and represents a major uncertainty in our understanding of atmospheric aqueous-phase oxidation, including the formation of aqueous SOA. In this study, we probe how the reaction pathways of RO2 radicals differ between the gas and aqueous phases. The reactions of these intermediates are not well constrained in the atmospheric aqueous phase and likely differ significantly from that in the gas phase due to locally high concentrations, solvent effects, and competition with reactions unique to the aqueous phase, such as oligomerization or hydrolysis. We assess the impact of these differences by direct comparison of the product distributions through gas-phase experiments, carried out in an environmental chamber, and aqueous experiments, carried out both in the bulk phase (by oxidation within an aqueous reactor) and in the aqueous particulate phase (by oxidation within deliquesced particles in the environmental chamber). In both types of aqueous experiments, we modulate the chemical environment by varying the concentrations of water-soluble organic species and OH precursors (H2O2, HONO, and NO2-) and initiate oxidation chemistry using UV lights. A number of analytical instruments, including an aerosol mass spectrometer, a chemical ionization mass spectrometer, and a scanning mobility particle sizer, are used to provide real-time concentrations of both gas- and aerosol-phase products. Direct comparison of the evolution of various products across phases (gas phase vs. bulk aqueous phase vs. aqueous particulate phase), provides new insight into the mechanisms and branching ratios governing oxidation product formation in the atmospheric aqueous phase, improving our understanding of aqueous-phase RO2 chemistry and hence the aqueous generation of SOA and gas-phase products.