Lubricating
Oil Dominates Primary Organic Aerosol
Emissions from Motor Vehicles
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Abstract
Motor
vehicles are major sources of primary organic aerosol (POA),
which is a mixture of a large number of organic compounds that have
not been comprehensively characterized. In this work, we apply a recently
developed gas chromatography mass spectrometry approach utilizing
“soft” vacuum ultraviolet photoionization to achieve
unprecedented chemical characterization of motor vehicle POA emissions
in a roadway tunnel with a mass closure of >60%. The observed POA
was characterized by number of carbon atoms (<i>N</i><sub>C</sub>), number of double bond equivalents (<i>N</i><sub>DBE</sub>) and degree of molecular branching. Vehicular POA was observed
to predominantly contain cycloalkanes with one or more rings and one
or more branched alkyl side chains (≥80%) with low abundances
of <i>n</i>-alkanes and aromatics (<5%), similar to “fresh”
lubricating oil. The gas chromatography retention time data indicates
that the cycloalkane ring structures are most likely dominated by
cyclohexane and cyclopentane rings and not larger cycloalkanes. High
molecular weight combustion byproducts, that is, alkenes, oxygenates,
and aromatics, were not present in significant amounts. The observed
carbon number and chemical composition of motor vehicle POA was consistent
with lubricating oil being the dominant source from both gasoline
and diesel-powered vehicles, with an additional smaller contribution
from unburned diesel fuel and a negligible contribution from unburned
gasoline