Heterogeneous OH Oxidation
of Motor Oil Particles
Causes Selective Depletion of Branched and Less Cyclic Hydrocarbons
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Abstract
Motor oil serves as a useful model system for atmospheric
oxidation
of hydrocarbon mixtures typical of anthropogenic atmospheric particulate
matter, but its complexity often prevents comprehensive chemical speciation.
In this work we fully characterize this formerly “unresolved
complex mixture” at the molecular level using recently developed
soft ionization gas chromatography techniques. Nucleated motor oil
particles are oxidized in a flow tube reactor to investigate the relative
reaction rates of observed hydrocarbon classes: alkanes, cycloalkanes,
bicycloalkanes, tricycloalkanes, and steranes. Oxidation of hydrocarbons
in a complex aerosol is found to be efficient, with approximately
three-quarters (0.72 ± 0.06) of OH collisions yielding a reaction.
Reaction rates of individual hydrocarbons are structurally dependent:
compared to normal alkanes, reaction rates increased by 20–50%
with branching, while rates decreased ∼20% per nonaromatic
ring present. These differences in rates are expected to alter particle
composition as a function of oxidation, with depletion of branched
and enrichment of cyclic hydrocarbons. Due to this expected shift
toward ring-opening reactions heterogeneous oxidation of the unreacted
hydrocarbon mixture is less likely to proceed through fragmentation
pathways in more oxidized particles. Based on the observed oxidation-induced
changes in composition, isomer-resolved analysis has potential utility
for determining the photochemical age of atmospheric particulate matter
with respect to heterogeneous oxidation