2 research outputs found
Daily Trends and Source Apportionment of Ultrafine Particulate Mass (PM<sub>0.1</sub>) over an Annual Cycle in a Typical California City
Toxicology studies indicate that
inhalation of ultrafine particles
(<i>D</i><sub>p</sub> < 0.1 μm) causes adverse
health effects, presumably due to their large surface area-to-volume
ratio that can drive heterogeneous reactions. Epidemiological associations
between ultrafine particles and health effects, however, have been
difficult to identify due to the lack of appropriate long-term monitoring
and exposure data. The majority of the existing ultrafine particle
epidemiology studies are based on exposure to particle number, although
an independent analysis suggests that ultrafine particle mass (PM<sub>0.1</sub>) correlates better with particle surface area. More information
is needed to characterize PM<sub>0.1</sub> exposure to fully evaluate
the health effects of ultrafine particles using epidemiology. The
present study summarizes 1 year of daily PM<sub>0.1</sub> chemistry
and source apportionment at Sacramento, CA, USA. Positive matrix factorization
(PMF) was used to resolve PM<sub>0.1</sub> source contributions from
old-technology diesel engines, residential wood burning, rail, regional
traffic, and brake wear/road dust. Diesel PM<sub>0.1</sub> and total
PM<sub>0.1</sub> concentrations were reduced by 97 and 26%, respectively,
as a result of the adoption of cleaner diesel technology. The strong
linear correlation between PM<sub>0.1</sub> and particle surface area
in central California suggests that the adoption of clean diesel engines
reduced particle surface area by similar amounts. PM<sub>0.1</sub> sulfate reduction occurred as a result of reduced primary particle
surface area available for sulfate condensation. The current study
demonstrates the capability of measuring PM<sub>0.1</sub> source contributions
over a 12 month period and identifies the extended benefits of emissions
reduction efforts for diesel engines on ambient concentrations of
primary and secondary PM<sub>0.1</sub>
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 “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