Products of Alkoxy Radical Reactions in the Atmospheric Aqueous Phase: Impact of Chemical Structure on Product Distributions

LEXY LEMAR, Victoria Barber, Yaowei Li, Frank Keutsch, Jesse Kroll, MIT

     Abstract Number: 312
     Working Group: Aerosol Chemistry

Abstract
The atmospheric oxidation of organic species, which leads to the formation of secondary organic aerosol (SOA) and other pollutants, can occur both in the gas phase and in the condensed phase. Oxidation typically occurs via key radical intermediates (R, RO, and RO₂), and the reactions of these are not well known in the atmospheric aqueous phase (i.e., deliquesced aerosol particles). Reactions that are too slow in the gas phase due to low concentrations could be important in the aqueous phase. In this study, we investigate how the reaction pathways of alkoxy (RO) radicals in bulk aqueous solution differ from those in the gas phase and how these differences depend on the carbon skeleton. Single isomer RO radicals are generated photolytically from alkyl-nitrite precursors in both an environmental chamber (gas phase) and in solution (aqueous phase). Single-isomer radical generation simplifies the subsequent chemistry and provides clear linkage between specific radicals and oxidation end products. For aqueous phase experiments, the reaction mixture is fed into an atomizer and injected directly into an ammonium chemical ionization mass spectrometer (CIMS). This provides the concentrations and molecular formulas of organic products, yielding insight into the mechanisms and branching ratios in the aqueous solution. The same instrument is used to determine products in the gas phase experiments for direct comparison. For pathways that became more dominant in the aqueous phase, an enhancement factor was calculated to compare across different species. Initial data demonstrate that the phase-dependent differences in downstream chemistry are highly dependent on the carbon skeleton of the organic radical. For example, the enhancement factor for the 1,5-H shift isomerization changes with degree of substitution. Results from this study provide insight into not only the aqueous phase chemistry of alkoxy radicals and insight to the competition between the various pathways as a function of phase.