4 research outputs found
Long-Term Trends in Motor Vehicle Emissions in U.S. Urban Areas
A fuel-based
approach is used to estimate long-term trends (1990–2010)
in carbon monoxide (CO) emissions from motor vehicles. Non-methane
hydrocarbons (NMHC) are estimated using ambient NMHC/CO ratios after
controlling for nonvehicular sources. Despite increases in fuel use
of ∼10–40%, CO running exhaust emissions from on-road
vehicles decreased by ∼80–90% in Los Angeles, Houston,
and New York City, between 1990 and 2010. The ratio of NMHC/CO was
found to be 0.24 ± 0.04 mol C/mol CO over time in Los Angeles,
indicating that both pollutants decreased at a similar rate and were
improved by similar emission controls, whereas on-road data from other
cities suggest rates of reduction in NMHC versus CO emissions may
differ somewhat. Emission ratios of CO/NO<sub><i>x</i></sub> (nitrogen oxides = NO + NO<sub>2</sub>) and NMHC/NO<sub><i>x</i></sub> decreased by a factor of ∼4 between 1990
and 2007 due to changes in the relative emission rates of passenger
cars versus diesel trucks, and slight uptick thereafter, consistent
across all urban areas considered here. These pollutant ratios are
expected to increase in future years due to (1) slowing rates of decrease
in CO and NMHC emissions from gasoline vehicles and (2) significant
advances in control of diesel NO<sub><i>x</i></sub> emissions
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 69–96% of emissions and 79–97% 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
“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
Ethylene Glycol Emissions from On-road Vehicles
Ethylene glycol (HOCH<sub>2</sub>CH<sub>2</sub>OH), used as engine
coolant for most on-road vehicles, is an intermediate volatility organic
compound (IVOC) with a high Henry’s law coefficient. We present
measurements of ethylene glycol (EG) vapor in the Caldecott Tunnel
near San Francisco, using a proton transfer reaction mass spectrometer
(PTR-MS). Ethylene glycol was detected at mass-to-charge ratio 45,
usually interpreted as solely coming from acetaldehyde. EG concentrations
in bore 1 of the Caldecott Tunnel, which has a 4% uphill grade, were
characterized by infrequent (approximately once per day) events with
concentrations exceeding 10 times the average concentration, likely
from vehicles with malfunctioning engine coolant systems. Limited
measurements in tunnels near Houston and Boston are not conclusive
regarding the presence of EG in sampled air. Previous PTR-MS measurements
in urban areas may have overestimated acetaldehyde concentrations
at times due to this interference by ethylene glycol. Estimates of
EG emission rates from the Caldecott Tunnel data are unrealistically
high, suggesting that the Caldecott data are not representative of
emissions on a national or global scale. EG emissions are potentially
important because they can lead to the formation of secondary organic
aerosol following oxidation in the atmospheric aqueous phase