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Cold temperature and biodiesel fuel effects on speciated emissions of volatile organic compounds from diesel trucks Ingrid George, Michael D. Hays, Richard Snow, James Faircloth, Barbara Jane George, Thomas Long, and Richard Baldauf Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es502949a • Publication Date (Web): 13 Nov 2014 Downloaded from http://pubs.acs.org on November 17, 2014
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Environmental Science & Technology
Cold temperature and biodiesel fuel effects on speciated emissions of volatile organic compounds from diesel trucks Ingrid J. George,1 Michael D. Hays,1,* Richard Snow,1 James Faircloth,1 Barbara J. George,2 Thomas Long1 and Richard W. Baldauf1 1
Office of Research and Development, National Risk Management Research Laboratory, United States Environmental Protection Agency, Research Triangle Park, NC 27711. 2
Office of Research and Development, National Health and Environmental Effects Research Laboratory, United States Environmental Protection Agency, Research Triangle Park, NC 27711.
*Corresponding Author: E-mail:
[email protected]; phone: +1 919-541-3984; fax: +1 919685-3346) Keywords: dynamometer, diesel exhaust, volatile organic compounds, carbonyls, mobile source air toxics, biodiesel
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ABSTRACT
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Speciated volatile organic compounds (VOCs) were measured in diesel exhaust from three
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heavy-duty trucks equipped with modern aftertreatment technologies. Emissions testing was
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conducted on a chassis dynamometer at two ambient temperatures (-7 °C and 22 °C) operating
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on two fuels (ultra-low sulfur diesel and 20 % soy biodiesel blend) over three driving cycles:
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cold start, warm start and heavy-duty urban dynamometer driving cycle. VOCs were measured
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separately for each drive cycle. Carbonyls such as formaldehyde and acetaldehyde dominated
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VOC emissions, making up ~72 % of the sum of the speciated VOC emissions (ΣVOCs) overall.
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Biodiesel use led to minor reductions in aromatics and variable changes in carbonyls. Cold
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temperature and cold start conditions caused dramatic enhancements in VOC emissions, mostly
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carbonyls, compared to the warmer temperature and other drive cycles, respectively. Different
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2007+ aftertreatment technologies involving catalyst regeneration led to significant
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modifications of VOC emissions that were compound-specific and highly dependent on test
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conditions. A comparison of this work with emission rates from different diesel engines under
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various test conditions showed that these newer technologies resulted in lower emission rates of
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aromatic compounds. However, emissions of other toxic partial combustion products such as
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carbonyls were not reduced in the modern diesel vehicles tested.
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INTRODUCTION
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Vehicle exhaust emissions have substantial negative impacts on air quality, human health and
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climate. Primary pollutants that are directly emitted from mobile sources include NOx, volatile
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organic compounds (VOCs), CO, CO2 and particulate matter (PM). Atmospheric chemistry of
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VOCs and NOx from vehicle emissions in the presence of sunlight creates secondary pollutants
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leading to enhanced ozone and secondary organic aerosol (SOA) formation, which contribute to
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photochemical smog.1 A subset of VOCs from vehicle exhaust emissions, termed mobile source
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air toxics (MSATs) that include some aromatics and carbonyls, are of particular concern because
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they are carcinogenic, mutagenic or are otherwise suspected to cause serious health effects.2 An
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accurate account of detailed mobile source emission profiles is vital information for local- and
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regional-scale air quality models to predict the environmental and health impacts of vehicle
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emissions.
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Diesel combustion, especially from heavy-duty vehicles, has become subject to progressively
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more stringent regulatory oversight in recent years due to the fact that diesel exhaust was
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emitting disproportionately more NOx and PM than gasoline combustion.3 Furthermore, diesel
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exhaust and specific components within that exhaust have been associated with acute and
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chronic adverse health effects.4,5 The 2007 Heavy-Duty Highway Rule was enacted by the U.S.
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EPA to reduce diesel exhaust emissions for NOx and PM over 90 % below previous standard
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levels, with full compliance required for model year 2010.6 The fuel standard for sulfur content
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in highway diesel fuel was reduced to 15 ppm, referred to as ultra-low sulfur diesel (ULSD), to
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allow for the employment of more sophisticated diesel exhaust aftertreatment technologies that
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are required to reach the new emission standards. Therefore, the implementation of these diesel
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emission standards resulted in a need to fully characterize the impact of the new aftertreatment
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technologies on diesel exhaust emissions.7-10
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There is a growing interest in the use of renewable biofuels, such as biodiesel, to replace
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petroleum-based transportation fuels to reduce dependence on limited fossil fuel resources and to
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address climate change. Biodiesel consists of fatty acid alkyl esters that are produced from
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transesterification of vegetable oils or animal fats with an alcohol. Biodiesel can be used directly
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in diesel engines as blends with petroleum diesel up to 20% by volume biodiesel without engine
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modification. After corn ethanol, biodiesel is the second most widely used biofuel in the U.S and
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its consumption has increased by approximately a factor of 10 since 2005.11 The U.S. Energy
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Independence and Security Act (EISA) was enacted with the ultimate goals of increasing
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domestic energy independence while reducing negative climate impacts of the transportation
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sector through greenhouse gas reductions.12 The legislation requires the implementation of
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renewable fuel standards with an increase in the future usage of renewable fuels as transportation
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fuels from 9 billion gallons in 2008 to 36 billion gallons per year by 2022. The European
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Parliament set forth analogous legislation recently requiring 10 % (by energy) of transportation
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fuel as renewable fuels by 2020.13
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As a result of the growing interest in biofuel use, there has been a dramatic escalation of
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scientific research in the past few years to quantify the environmental impacts of diesel engine
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exhaust using biodiesel.14 However, the majority of the research has focused on characterizing
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emissions of criteria pollutants, i.e., NOx, CO, total hydrocarbons (THC) and PM, from biodiesel
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combustion in diesel engines.14-17 In general, the addition of biodiesel to diesel fuel as blends
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(e.g., B20 for 20 % biodiesel blend) or as a replacement (i.e., B100 for 100 % biodiesel) has
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consistently been shown to reduce PM, CO and THC emissions significantly and lead to modest
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increase in NOx emissions in diesel engine exhaust as compared to conventional diesel fuel
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combustion. However, there is considerable variability in the pollutant emission rates due to
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various experimental conditions, including quality and characteristics of biodiesel feedstock,
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driving cycle, aftertreatment technologies, model year, vehicle test weight (VTW), engine type
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and vehicle age.
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Few studies have assessed the impact of biodiesel application for unregulated emissions,
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such as chemically speciated VOCs, and in particular MSATs. Although the effects of biodiesel
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on MSATs have been reviewed,14-16 a number of more recent studies have also reported biodiesel
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effects on carbonyl emissions18-25 and other MSAT emissions26-32 in diesel exhaust. While the
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majority of these studies reported decreases in aromatics and increases in carbonyls from the use
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of biodiesel, there is still no clear consensus on the impact of biodiesel due to conflicting trends
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observed in literature. As noted above, many factors can impact the diesel exhaust emissions,
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which may perhaps explain some of the inconsistencies in the literature on the effect of biodiesel
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on MSATs. Although ambient temperatures below freezing may have significant adverse effects
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on unregulated vehicle emissions,33 the effect of cold temperature on MSATs in diesel exhaust
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with biodiesel use is unknown.
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In this study, the MSAT emissions from soy B20 biodiesel blended with ULSD was
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compared to emissions from ULSD in three heavy-duty trucks equipped with aftertreatment
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technologies designed to meet EPA 2010 model year diesel emissions standards. Emission rates
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were measured using two chassis dynamometers, where one of which has been uniquely set up to
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test the vehicles under cold ambient temperature conditions. To our knowledge, this is the first
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study to characterize the unregulated emissions from biodiesel blend under cold temperature
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conditions. A complete set of emission rates for all conditions for MSATs is reported in the
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supporting information to facilitate their utilization in air quality modeling efforts and
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improvement of emissions inventories. However, the major focus of this work is to examine the
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influence of various experimental conditions on the VOC emissions of modern heavy-duty diesel
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vehicles. To that end, a systematic approach was taken to investigate the effects of operating
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conditions, including ambient temperature, driving cycle, fuel type and VTW.
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EXPERIMENTAL METHODS
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Dynamometer testing
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Emissions testing was conducted on three heavy-duty diesel trucks. Most of the testing
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occurred on a chassis dynamometer (#1) enclosed within a climate-controlled chamber that
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permitted low ambient temperature testing. Additional testing was performed using a second
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chassis dynamometer (#2) without low temperature capabilities, in cases where the anticipated
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simulated VTW exceeded the capacity of dynamometer #1 (i.e. ~5500 kg). Dynamometer #1 was a
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48-inch roll MIM 4800 dynamometer (Burke E. Porter Machinery Co., Grand Rapids, MI, U.S.),
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and dynamometer #2 was a 72-inch roll Emission Truck Chassis Dynamometer (Renk AG,
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Augsburg, Germany). The three test vehicles named Vehicle 1, 2 and 3 (V1, V2, V3) were all
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model year 2011 trucks in U.S. EPA heavy-duty (HD) truck classes HDV2B, HDV5 and HDV6,
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respectively. Odometer readings (converted to km) at the beginning of the study were 35,498 km
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for V1, 4,333 km for V2 and 5,850 km for V3. Further characteristics of the test vehicles are
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summarized in Table S2 in the supporting information. The stock exhaust aftertreatment systems
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for all three trucks were designed to meet the EPA 2010 model year PM and NOx emission
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standards for HD diesel trucks. All three trucks utilized a Diesel Particulate Filter (DPF) to control
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particulate emissions by trapping exhaust particles during vehicle operation. PM collected on the 7 ACS Paragon Plus Environment
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DPF was periodically removed (i.e., oxidized) typically by injection of fuel into the exhaust
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stream, resulting in an increase in exhaust and DPF temperatures. This process is referred to as
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active DPF regeneration. For NOx control, V1 used a NOx Adsorber Catalyst (NAC), which
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chemically stores NOx emissions. Once saturated, the NAC catalyst undergoes regeneration,
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releasing NOx under fuel rich conditions and reducing it to N2. Both V2 and V3 trucks utilized
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Selective Catalytic Reduction (SCR) for controlling NOx emissions, which combine NOx with urea
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over a catalyst to produce N2 and CO2. All three trucks included a Diesel Oxidation Catalyst
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(DOC) for the removal of hydrocarbons and CO.
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The test matrix shown in Table S3 in the supporting information summarizes the conditions for
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each test that included VOC sampling and observed fuel consumption. For each test condition, one
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vehicle preconditioning test with no sampling was conducted, followed by three replicates with
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sampling. Three driving cycles were performed for each test day. The speed vs. time trace for all
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three cycles is shown in Figure S1 in the supporting information. Two cycles were identical to the
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lower-speed transient mode of the CARB Medium Heavy Duty Truck three-mode test
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(MHDTLO).34 This cycle was performed twice for each test to simulate cold start and warm start
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driving conditions. The Federal Heavy-Duty Urban Dynamometer Driving Schedule (HD-UDDS)
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(Code of Federal Regulations (CFR), Title 40, Part 86.1216-85) was performed between the cold
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start and warm start driving cycles. A twenty-minute engine off period occurred between driving
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cycles. Note that both DPF and NAC regenerations occurred only during HD-UDDS cycles, but
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not simultaneously. During the testing, it was observed that the NAC of V1 underwent
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regenerations during every HD-UDDS cycle at 22 °C unless an active DPF regeneration occurred.
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NAC regenerations were considered as part of normal operation, but active DPF regeneration
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events were not. When a DPF regeneration occurred on a particular day, the test was repeated the 8 ACS Paragon Plus Environment
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following day to obtain triplicate measurements under normal operation mode (i.e. in absence of
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DPF regenerations).
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Two test fuels used in this study were Ultra Low Sulfur Diesel (ULSD) and 20 % soybean oil-
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based biodiesel (B20) that was splash-blended in the same ULSD used in the pure ULSD tests,
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which met ASTM D7467 specifications. Both fuels were obtained from Gage Products Co.
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(Ferndale, MI, U.S.). A fuel change procedure, as detailed in the supporting information, was
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performed with each change in fuel type in order to minimize residual effects from the previous
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fuel on the fuel and aftertreatment systems. The measured chemical and physical properties of the
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test fuels are listed in Table S4 in the supporting information. Testing was conducted for unladen
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(i.e., vehicle curb weight plus 68 kg; UNL) and laden (i.e., at 90 % of gross vehicle weight rating;
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LAD) VTW conditions for all vehicles. VOC sampling was undertaken for all conditions except
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under laden VTW conditions for V2 because the VOC sampling setup was not available for
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dynamometer #2 during the V2 testing period.
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Dynamometer #1 was housed inside a custom made temperature-controlled chamber (Luwa-
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Environmental Specialties, Raleigh, NC, U.S.) that enabled chassis dynamometer emissions testing
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to be conducted at ambient temperatures in the range of -30 to 45 °C. The chamber was designed to
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maintain temperatures within ±2 °C of setpoint temperature with up to a maximum applied heat
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load of 80,000 BTU. In this work, exhaust emissions were sampled under two ambient temperature
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conditions T = -7 °C and 22 °C for V1 (UNL and LAD) and V2 (UNL only) with dynamometer
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#1. Chamber temperature was monitored approximately 30 cm from the test vehicle’s air intake,
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where temperatures within ±2 °C of setpoint for cold and warm start cycles and within ±3 °C for
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HD-UDDS were measured. Prior to testing, the vehicles were conditioned for 12 hours at the test
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temperature. Because V3 exceeded weight limits of dynamometer #1, testing for V3 was
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conducted only at ~22 °C on dynamometer #2.
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Sampling methods
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A schematic of the setup for the constant volume sampler (CVS) dilution tunnel is included in
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the supporting information (Figure S2). In this study, both time-integrated and real-time
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measurements were made for the gaseous regulated pollutants (i.e., CO2, CO, NOx, CH4 and THC).
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Particulate mass was determined gravimetrically, and particles were further characterized by
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particle size, number and chemical composition. This paper focuses on measurements of speciated
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VOC and carbonyls emissions while the other gaseous and particulate measurements in this study
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will be covered elsewhere. The dilution flow in the CVS dilution tunnel consisted of ambient
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laboratory air that was passed through a charcoal bed to reduce volatile organics, then a HEPA
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filter to remove particles. Typical THC background values were in the range of 2.2-3.4 ppmC, of
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which ~80 % was methane. Dilution flow was pulled through the tunnel with a turbine (Spencer
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Turbine Company, Model 2025-H-SPEC, Windsor, CT, U.S.). The dilution air temperature was
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equilibrated with the laboratory room air temperature (~21 °C). The total flow through the dilution
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tunnel was maintained within ±2 % of the set flow rate of 29.7 standard m3 min-1 that was
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monitored and controlled by a critical flow venturi (CFV, Horiba Instruments Inc., CVS-48M, Ann
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Arbor, MI, U.S.).
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VOCs were sampled in SUMMA canisters and analyzed by gas chromatography/mass
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spectrometry (GC/MS) in accordance with U.S. EPA TO-15 Method. Carbonyls were sampled
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with dinitrophenylhydrazine (DNPH)-coated cartridges (Sigma-Aldrich Corp., St. Louis, MO,
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U.S., LpDNPH H30), and the hydrazones in acetonitrile extracts were analyzed by high 10 ACS Paragon Plus Environment
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performance liquid chromatography (HPLC) according to U.S. EPA Method TO-11A. Detailed
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descriptions of the TO-15 and TO-11A sampling and analytical procedures along with THC and
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methane measurements are provided in the supporting information. Before the test driving cycles
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commenced, one background sample of the dilution air was taken each day. Time-integrated
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samples of each medium were taken over each driving cycle: cold start, HD-UDDS and warm
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start. Background samples were analyzed to determine background-corrected VOC concentrations,
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which were used to estimate emission rates as described in the supporting information. One blank
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sample was also taken for each test condition to confirm that media were contamination-free.
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Blank samples were handled identically to other samples but with no flow passing into the
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sampling media (canister or cartridge). For TO-15 analysis of VOCs, method detection limits
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(MDLs) ranged from 15 to 186 ppt but were mostly
