Heterogeneous oxidation of unsaturated organic aerosols

A significant mass fraction of atmospheric aerosols is composed of a variety of oxidized organic compounds with varying functional groups that may affect the rate at which they chemically age. Here we study the heterogeneous reaction of OH radicals with different sub-micron, alkenoic acid particles: Oleic acid (OA), Linoleic acid (LA), and Linolenic acid (LNA), in the presence of H2O2 and O2. This work explores how OH addition reactions initiate chain reactions that rapidly transform the chemical composition of an organic particle. Particles are chemically aged in a photochemical flow tube reactor where they are exposed to OH radicals (~ 1E11 molecule cm-3 s) that are produced by the photolysis of H2O2 at 254 nm. The aerosols are then sized and their composition analyzed via Atmospheric Pressure Chemical Ionization mass spectrometry (APCI-MS). Kinetic measurements, done by tracking the decay of the alkenoic acid parent ion with increasing OH exposure, show that the reactive uptake coefficient, which is defined as the fraction of gas-phase collisions that result in a reaction, is larger than 1, indicating the presence of secondary chain chemistry occurring within the aerosol. The reactive uptake coefficient is found to scale linearly with the number of double bonds present in the molecule. In addition, the reactive uptake coefficient is found to depend sensitively upon the concentrations of O2 and H2O2 in the photochemical flow tube reactor, indicating that O2 and H2O2 play roles as propagators and terminators in the secondary chain reactions. Elemental analysis and mechanistic pathways will also be presented herein.