6 research outputs found
Heterogeneous OH Oxidation of Motor Oil Particles Causes Selective Depletion of Branched and Less Cyclic Hydrocarbons
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 âunresolved
complex mixtureâ 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 20â50%
with branching, while rates decreased âŒ20% 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
OH-Initiated Heterogeneous Oxidation of Cholestane: A Model System for Understanding the Photochemical Aging of Cyclic Alkane Aerosols
Aerosols
containing aliphatic hydrocarbons play a substantial role
in the urban atmosphere. Cyclic alkanes constitute a large fraction
of aliphatic hydrocarbon emissions originating from incomplete combustion
of diesel fuel and motor oil. In the present study, cholestane (C<sub>27</sub>H<sub>48</sub>) is used as a model system to examine the
OH-initiated heterogeneous oxidation pathways of cyclic alkanes in
a photochemical flow tube reactor. Oxidation products are collected
on filters and analyzed by a novel soft ionization two-dimensional
gas chromatography/mass spectrometry technique. The analysis reveals
that the first-generation functionalization products (cholestanones,
cholestanals, and cholestanols) are the dominant reaction products
that account for up to 70% by mass of the total speciated compounds.
The ratio of first-generation carbonyls to alcohols is near unity
at every oxidation level. Among the cholestanones/cholestanals, 55%
are found to have the carbonyl group on the rings of the androstane
skeleton, while 74% of cholestanols have the hydroxyl group on the
rings. Particle-phase oxidation products with carbon numbers less
than 27 (i.e., âfragmentation productsâ) and higher-generation
functionalization products are much less abundant. Carbon bond cleavage
was found to occur only on the side chain. Tertiary-carbon alkoxy
radicals are suggested to play an important role in governing both
the distribution of functionalization products (via alkoxy radical
isomerization and reaction with oxygen) and the fragmentation products
(via alkoxy radical decomposition). These results provide new insights
into the oxidation mechanism of cyclic alkanes
The Influence of Molecular Structure and Aerosol Phase on the Heterogeneous Oxidation of Normal and Branched Alkanes by OH
Insights
into the influence of molecular structure and thermodynamic
phase on the chemical mechanisms of hydroxyl radical-initiated heterogeneous
oxidation are obtained by identifying reaction products of submicrometer
particles composed of either <i>n</i>-octacosane (C<sub>28</sub>H<sub>58</sub>, a linear alkane) or squalane (C<sub>30</sub>H<sub>62</sub>, a highly branched alkane) and OH. A common pattern
is observed in the positional isomers of octacosanone and octacosanol,
with functionalization enhanced toward the end of the molecule. This
suggests that relatively large linear alkanes are structured in submicrometer
particles such that their ends are oriented toward the surface. For
squalane, positional isomers of first-generation ketones and alcohols
also form in distinct patterns. Ketones are favored on carbons adjacent
to tertiary carbons, while hydroxyl groups are primarily found on
tertiary carbons but also tend to form toward the end of the molecule.
Some first-generation products, viz., hydroxycarbonyls and diols,
contain two oxygen atoms. These results suggest that alkoxy radicals
are important intermediates and undergo both intramolecular (isomerization)
and intermolecular (chain propagation) hydrogen abstraction reactions.
Oxidation products with carbon number less than the parent alkaneâs
are observed to a much greater extent for squalane than for <i>n</i>-octacosane oxidation and can be explained by the preferential
cleavage of bonds involving tertiary carbons
Chemical Composition of Gas-Phase Organic Carbon Emissions from Motor Vehicles and Implications for Ozone Production
Motor vehicles are
major sources of gas-phase organic carbon, which includes volatile
organic compounds (VOCs) and other compounds with lower vapor pressures.
These emissions react in the atmosphere, leading to the formation
of ozone and secondary organic aerosol (SOA). With more chemical detail
than previous studies, we report emission factors for over 230 compounds
from gasoline and diesel vehicles via two methods. First we use speciated
measurements of exhaust emissions from on-road vehicles in summer
2010. Second, we use a fuel composition-based approach to quantify
uncombusted fuel components in exhaust using the emission factor for
total uncombusted fuel in exhaust together with detailed chemical
characterization of liquid fuel samples. There is good agreement between
the two methods except for products of incomplete combustion, which
are not present in uncombusted fuels and comprise 32 ± 2% of
gasoline exhaust and 26 ± 1% of diesel exhaust by mass. We calculate
and compare ozone production potentials of diesel exhaust, gasoline
exhaust, and nontailpipe gasoline emissions. Per mass emitted, the
gas-phase organic compounds in gasoline exhaust have the largest potential
impact on ozone production with over half of the ozone formation due
to products of incomplete combustion (e.g., alkenes and oxygenated
VOCs). When combined with data on gasoline and diesel fuel sales in
the U.S., these results indicate that gasoline sources are responsible
for 69â96% of emissions and 79â97% of the ozone formation
potential from gas-phase organic carbon emitted by motor vehicles
Lubricating Oil Dominates Primary Organic Aerosol Emissions from Motor Vehicles
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
âsoftâ 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 (â„80%) with low abundances
of <i>n</i>-alkanes and aromatics (<5%), similar to âfreshâ
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
Improved Resolution of Hydrocarbon Structures and Constitutional Isomers in Complex Mixtures Using Gas Chromatography-Vacuum Ultraviolet-Mass Spectrometry
Understanding the composition of complex hydrocarbon
mixtures is
important for environmental studies in a variety of fields, but many
prevalent compounds cannot be confidently identified using traditional
gas chromatography/mass spectrometry (GC/MS) techniques. This work
uses vacuum-ultraviolet (VUV) ionization to elucidate the structures
of a traditionally âunresolved complex mixtureâ by separating
components by GC retention time, <i>t</i><sub>R</sub>, and
mass-to-charge ratio, <i>m</i>/<i>z</i>, which
are used to determine carbon number, <i>N</i><sub>C</sub>, and the number of rings and double bonds, <i>N</i><sub>DBE</sub>. Constitutional isomers are resolved on the basis of <i>t</i><sub>R</sub>, enabling the most complete quantitative analysis
to date of structural isomers in an environmentally relevant hydrocarbon
mixture. Unknown compounds are classified in this work by carbon number,
degree of saturation, presence of rings, and degree of branching,
providing structural constraints. The capabilities of this analysis
are explored using diesel fuel, in which constitutional isomer distribution
patterns are shown to be reproducible between carbon numbers and follow
predictable rules. Nearly half of the aliphatic hydrocarbon mass is
shown to be branched, suggesting branching is more important in diesel
fuel than previously shown. The classification of unknown hydrocarbons
and the resolution of constitutional isomers significantly improves
resolution capabilities for any complex hydrocarbon mixture