10 research outputs found
Long-Term Trends in California Mobile Source Emissions and Ambient Concentrations of Black Carbon and Organic Aerosol
A fuel-based
approach is used to assess long-term trends (1970–2010)
in mobile source emissions of black carbon (BC) and organic aerosol
(OA, including both primary emissions and secondary formation). The
main focus of this analysis is the Los Angeles Basin, where a long
record of measurements is available to infer trends in ambient concentrations
of BC and organic carbon (OC), with OC used here as a proxy for OA.
Mobile source emissions and ambient concentrations have decreased
similarly, reflecting the importance of on- and off-road engines as
sources of BC and OA in urban areas. In 1970, the on-road sector accounted
for ∼90% of total mobile source emissions of BC and OA (primary
+ secondary). Over time, as on-road engine emissions have been controlled,
the relative importance of off-road sources has grown. By 2010, off-road
engines were estimated to account for 37 ± 20% and 45 ±
16% of total mobile source contributions to BC and OA, respectively,
in the Los Angeles area. This study highlights both the success of
efforts to control on-road emission sources, and the importance of
considering off-road engine and other VOC source contributions when
assessing long-term emission and ambient air quality trends
Effects of Switching to Lower Sulfur Marine Fuel Oil on Air Quality in the San Francisco Bay Area
Ocean-going vessels burning high-sulfur
heavy fuel oil are an important
source of air pollutants, such as sulfur dioxide and particulate matter.
Beginning in July 2009, an emission control area was put into effect
at ports and along the California coastline, requiring use of lower
sulfur fuels in place of heavy fuel oil in main engines of ships.
To assess impacts of the fuel changes on air quality at the Port of
Oakland and in the surrounding San Francisco Bay area, we analyzed
speciated fine particle concentration data from four urban sites and
two more remote sites. Measured changes in concentrations of vanadium,
a specific marker for heavy fuel oil combustion, are related to overall
changes in aerosol emissions from ships. We found a substantial reduction
in vanadium concentrations after the fuel change and a 28–72%
decrease in SO<sub>2</sub> concentrations, with the SO<sub>2</sub> decrease varying depending on proximity to shipping lanes. We estimate
that the changes in ship fuel reduced ambient PM<sub>2.5</sub> mass
concentrations at urban sites in the Bay area by about 3.1 ±
0.6% or 0.28 ± 0.05 μg/m<sup>3</sup>. The largest contributing
factor to lower PM mass concentrations was reductions in particulate
sulfate. Absolute sulfate reductions were fairly consistent across
sites, whereas trace metal reductions were largest at a monitoring
site in West Oakland near the port
High-Resolution Mapping of Sources Contributing to Urban Air Pollution Using Adjoint Sensitivity Analysis: Benzene and Diesel Black Carbon
The adjoint of the Community Multiscale
Air Quality (CMAQ) model
at 1 km horizontal resolution is used to map emissions that contribute
to ambient concentrations of benzene and diesel black carbon (BC)
in the San Francisco Bay area. Model responses of interest include
population-weighted average concentrations for three highly polluted
receptor areas and the entire air basin. We consider both summer (July)
and winter (December) conditions. We introduce a novel approach to
evaluate adjoint sensitivity calculations that complements existing
methods. Adjoint sensitivities to emissions are found to be accurate
to within a few percent, except at some locations associated with
large sensitivities to emissions. Sensitivity of model responses to
emissions is larger in winter, reflecting weaker atmospheric transport
and mixing. The contribution of sources located within each receptor
area to the same receptor’s air pollution burden increases
from 38–74% in summer to 56–85% in winter. The contribution
of local sources is higher for diesel BC (62–85%) than for
benzene (38–71%), reflecting the difference in these pollutants’
atmospheric lifetimes. Morning (6–9am) and afternoon (4–7
pm) commuting-related emissions dominate region-wide benzene levels
in winter (14 and 25% of the total response, respectively). In contrast,
afternoon rush hour emissions do not contribute significantly in summer.
Similar morning and afternoon peaks in sensitivity to emissions are
observed for the BC response; these peaks are shifted toward midday
because most diesel truck traffic occurs during off-peak hours
Quantifying On-Road Emissions from Gasoline-Powered Motor Vehicles: Accounting for the Presence of Medium- and Heavy-Duty Diesel Trucks
Vehicle
emissions of nitrogen oxides (NO<sub><i>x</i></sub>), carbon
monoxide (CO), fine particulate matter (PM<sub>2.5</sub>), organic
aerosol (OA), and black carbon (BC) were measured at the
Caldecott tunnel in the San Francisco Bay Area. Measurements were
made in bore 2 of the tunnel, where light-duty (LD) vehicles accounted
for >99% of total traffic and heavy-duty trucks were not allowed.
Prior emission studies conducted in North America have often assumed
that route- or weekend-specific prohibitions on heavy-duty truck traffic
imply that diesel contributions to pollutant concentrations measured
in on-road settings can be neglected. However, as light-duty vehicle
emissions have declined, this assumption can lead to biased results,
especially for pollutants such as NO<sub><i>x</i></sub>,
OA, and BC, for which diesel-engine emission rates are high compared
to corresponding values for gasoline engines. In this study, diesel
vehicles (mostly medium-duty delivery trucks with two axles and six
tires) accounted for <1% of all vehicles observed in the tunnel
but were nevertheless responsible for (18 ± 3)%, (22 ± 6)%,
and (45 ± 8)% of measured NO<sub><i>x</i></sub>, OA,
and BC concentrations. Fleet-average OA and BC emission factors for
light-duty vehicles are, respectively, 10 and 50 times lower than
for heavy-duty diesel trucks. Using measured emission factors from
this study and publicly available data on taxable fuel sales, as of
2010, LD gasoline vehicles were estimated to be responsible for 85%,
18%, 18%, and 6% of emissions of CO, NO<sub><i>x</i></sub>, OA, and BC, respectively, from on-road motor vehicles in the United
States
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
Effects of Particle Filters and Selective Catalytic Reduction on Heavy-Duty Diesel Drayage Truck Emissions at the Port of Oakland
Effects
of fleet modernization and use of diesel particle filters
(DPF) and selective catalytic reduction (SCR) on heavy-duty diesel
truck emissions were studied at the Port of Oakland in California.
Nitrogen oxides (NO<sub><i>x</i></sub>), black carbon (BC),
particle number (PN), and size distributions were measured in the
exhaust plumes of ∼1400 drayage trucks. Average NO<sub><i>x</i></sub>, BC, and PN emission factors for newer engines (2010–2013
model years) equipped with both DPF and SCR were 69 ± 15%, 92
± 32%, and 66 ± 35% lower, respectively, than 2004–2006
engines without these technologies. Intentional oxidation of NO to
NO<sub>2</sub> for DPF regeneration increased tailpipe NO<sub>2</sub> emissions, especially from older (1994–2006) engines with
retrofit DPFs. Increased deployment of advanced controls has further
skewed emission factor distributions; a small number of trucks emit
a disproportionately large fraction of total BC and NO<sub><i>x</i></sub>. The fraction of DPF-equipped drayage trucks increased
from 2 to 99% and the median engine age decreased from 11 to 6 years
between 2009 and 2013. Over this period, fleet-average BC and NO<sub><i>x</i></sub> emission factors decreased by 76 ±
22% and 53 ± 8%, respectively. Emission changes occurred rapidly
compared to what would have been observed due to natural (i.e., unforced)
turnover of the Port truck fleet. These results provide a preview
of more widespread emission changes expected statewide and nationally
in the coming years
Effects of Particle Filters and Selective Catalytic Reduction on Heavy-Duty Diesel Drayage Truck Emissions at the Port of Oakland
Effects
of fleet modernization and use of diesel particle filters
(DPF) and selective catalytic reduction (SCR) on heavy-duty diesel
truck emissions were studied at the Port of Oakland in California.
Nitrogen oxides (NO<sub><i>x</i></sub>), black carbon (BC),
particle number (PN), and size distributions were measured in the
exhaust plumes of ∼1400 drayage trucks. Average NO<sub><i>x</i></sub>, BC, and PN emission factors for newer engines (2010–2013
model years) equipped with both DPF and SCR were 69 ± 15%, 92
± 32%, and 66 ± 35% lower, respectively, than 2004–2006
engines without these technologies. Intentional oxidation of NO to
NO<sub>2</sub> for DPF regeneration increased tailpipe NO<sub>2</sub> emissions, especially from older (1994–2006) engines with
retrofit DPFs. Increased deployment of advanced controls has further
skewed emission factor distributions; a small number of trucks emit
a disproportionately large fraction of total BC and NO<sub><i>x</i></sub>. The fraction of DPF-equipped drayage trucks increased
from 2 to 99% and the median engine age decreased from 11 to 6 years
between 2009 and 2013. Over this period, fleet-average BC and NO<sub><i>x</i></sub> emission factors decreased by 76 ±
22% and 53 ± 8%, respectively. Emission changes occurred rapidly
compared to what would have been observed due to natural (i.e., unforced)
turnover of the Port truck fleet. These results provide a preview
of more widespread emission changes expected statewide and nationally
in the coming years
On-Road Measurement of Gas and Particle Phase Pollutant Emission Factors for Individual Heavy-Duty Diesel Trucks
Pollutant concentrations in the exhaust plumes of individual
diesel
trucks were measured at high time resolution in a highway tunnel in
Oakland, CA, during July 2010. Emission factors for individual trucks
were calculated using a carbon balance method, in which pollutants
measured in each exhaust plume were normalized to measured concentrations
of carbon dioxide. Pollutants considered here include nitric oxide,
nitrogen dioxide (NO<sub>2</sub>), carbon monoxide, formaldehyde,
ethene, and black carbon (BC), as well as optical properties of emitted
particles. Fleet-average emission factors for oxides of nitrogen (NO<sub><i>x</i></sub>) and BC respectively decreased 30 ±
6 and 37 ± 10% relative to levels measured at the same location
in 2006, whereas a 34 ± 18% increase in the average NO<sub>2</sub> emission factor was observed. Emissions distributions for all species
were skewed with a small fraction of trucks contributing disproportionately
to total emissions. For example, the dirtiest 10% of trucks emitted
half of total NO<sub>2</sub> and BC emissions. Emission rates for
NO<sub>2</sub> were found to be anticorrelated with all other species
considered here, likely due to the use of catalyzed diesel particle
filters to help control exhaust emissions. Absorption and scattering
cross-section emission factors were used to calculate the aerosol
single scattering albedo (SSA, at 532 nm) for individual truck exhaust
plumes, which averaged 0.14 ± 0.03
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