Consideration of Black Carbon and Primary Organic Carbon

Sep 26, 2014 - Envisioning a transition to advanced vehicle technologies and alternative fuels in the United States, we aim to systematically evaluate...
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Consideration of Black Carbon and Primary Organic Carbon Emissions in Life-Cycle Analysis of Greenhouse Gas Emissions of Vehicle Systems and Fuels Hao Cai* and Michael Q. Wang Systems Assessment Group, Energy Systems Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: The climate impact assessment of vehicle/fuel systems may be incomplete without considering short-lived climate forcers of black carbon (BC) and primary organic carbon (POC). We quantified life-cycle BC and POC emissions of a large variety of vehicle/fuel systems with an expanded Greenhouse gases, Regulated Emissions, and Energy use in Transportation model developed at Argonne National Laboratory. Life-cycle BC and POC emissions have small impacts on life-cycle greenhouse gas (GHG) emissions of gasoline, diesel, and other fuel vehicles, but would add 34, 16, and 16 g CO2 equivalent (CO2e)/mile, or 125, 56, and 56 g CO2e/ mile with the 100 or 20 year Global Warming Potentials of BC and POC emissions, respectively, for vehicles fueled with corn stover-, willow tree-, and Brazilian sugarcane-derived ethanol, mostly due to BC- and POC-intensive biomass-fired boilers in cellulosic and sugarcane ethanol plants for steam and electricity production, biomass open burning in sugarcane fields, and diesel-powered agricultural equipment for biomass feedstock production/harvest. As a result, life-cycle GHG emission reduction potentials of these ethanol types, though still significant, are reduced from those without considering BC and POC emissions. These findings, together with a newly expanded GREET version, help quantify the previously unknown impacts of BC and POC emissions on life-cycle GHG emissions of U.S. vehicle/fuel systems.

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requirement of 36 billion gallons of biofuels by 2022.6 The California Air Resources Board implemented California’s Low Carbon Fuel Standard, which requires a reduction in the average carbon intensity of California’s transportation fuel mix by 10% by 2020.7 The U.S. Environmental Protection Agency has set the Tier 3 vehicle standard to reduce both tailpipe and evaporative emissions from passenger cars, light-duty trucks, medium-duty passenger vehicles, and heavy-duty vehicles, beginning with MY 2017 vehicles.8 A transition to energy-efficient, low-carbon, and clean vehicle/fuel systems, as promoted by the aforementioned regulations and standards, requires a holistic evaluation of the energy use and GHG and air pollutant emissions of vehicle/fuel systems. Life-cycle analysis (LCA) has been applied to accomplish this goal. Traditionally, LCA studies have focused on GHG emissions of long-lived climate forcers (LLCFs), namely CO2, methane (CH4), and nitrous oxide (N2O), whereas short-lived climate forcers (SLCFs), for example, black carbon (BC), which has short atmospheric lifetime ranging from days to about 2 weeks,9,10 and organic carbon (OC),

INTRODUCTION In 2013, the U.S. transportation sector accounted for 72% of total consumption of petroleum energy,1 contributed to 28% of the total greenhouse gas (GHG) emissions,2 and was responsible for 59%, 54%, 23%, and 6% of nitrogen oxides, carbon monoxide, volatile organic compounds, and particulate matter with a diameter ≤2.5 μm (PM 2.5 ) emissions, respectively.3 To reduce petroleum consumption, GHG emissions, and air pollutant emissions from the transportation sector, policies and regulations are in place to facilitate development of advanced vehicle technologies with improved energy efficiencies, deployment of advanced vehicle emission reduction technologies, and a transition from petroleum-based transportation fuels to a portfolio of low-carbon alternative fuels. For example, the Corporate Average Fuel Economy standard increases the fuel economy of light-duty vehicles to 54.5 miles per gallon by model year (MY) 2025 and may stimulate the introduction and adoption of battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and fuel cell electric vehicles (FCEVs).4 Also, fuel consumption standards and carbon dioxide (CO2) emissions standards were established for the first time for medium- and heavy-duty vehicles.5 The Energy Independence and Security Act of 2007 mandates and expands the Renewable Fuel Standard with a total volumetric © XXXX American Chemical Society

Received: August 7, 2014 Revised: September 13, 2014 Accepted: September 26, 2014

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Figure 1. System boundaries of WTW analysis of evaluated vehicle/fuel systems.

which has atmospheric lifetime of about 1 week,11 are generally not considered. Despite much shorter atmospheric lifetimes of days to weeks in the lower troposphere, SLCFs can cause both warming and cooling of climate in both the near- and longterm.11 For example, BC is recognized as the second most important human emission after CO2 in terms of its climate forcing in the present-day atmosphere.12,13 On the other hand, OC causes an overall negative forcing but with uncertainties.11 When SLCFs are considered together with LLCFs, mitigation choices to contain climate change can be different from considering only LLCFs.14−17 The transportation sector was the dominant source of BC emissions in the United States in 2005, accounting for about 52.3%, of which 93% came from on-road and off-road diesel vehicles.9 Conventional and alternative vehicle/fuel systems generate both LLCF and SLCF emissions primarily from combustion of fossil and/or biofuels in their life-cycle stages that include feedstock production and transportation, feedstock conversion to fuels, fuel transportation and distribution, and vehicle end-use of fuels. Studies on the impacts of SLCFs on GHG emissions of vehicle/fuel systems on an LCA basis are very limited. Peters et al.18 found that both the inclusion of SLCFs and the choice of emission metric can alter LCA results of European diesel passenger cars, long-distance trains, conventional diesel buses, and passenger aircrafts, and thereby change mitigation priorities. However, such a systematic

evaluation of the climate impacts of SLCFs in the United States has yet to be conducted. Envisioning a transition to advanced vehicle technologies and alternative fuels in the United States, we aim to systematically evaluate the impacts of BC and primary OC (POC), two key SLCFs that are coemitted in most combustion sources, on the life-cycle GHG emissions of a large variety of alternative vehicle/fuels systems that include vehicles powered by ethanol, biodiesel (BD), renewable gasoline (RG), renewable diesel (RD), Fischer−Tropsch diesel (FTD), compressed natural gas (CNG), liquefied petroleum gas (LPG), electricity, and hydrogen. We expanded the Greenhouse gases, Regulated Emissions, Energy use in Transportation (GREET) model developed at Argonne National Laboratory19 with an emission database of BC and POC from stationary, mobile, and open burning emission sources that are involved in various life-cycle stages of both conventional and alternative vehicle/fuel systems. This enables that the life-cycle BC and POC emissions of vehicle/fuel systems are evaluated with a consistent LCA platform. The time frame of this analysis is 2015.

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DATA AND METHODS LCA System Boundary. The LCA system boundary is from wells to wheels (WTW) of vehicle/fuel systems, which separates well-to-pump (WTP) and pump-to-wheels (PTW) stages. The WTP stage models the life-cycle of fuel production, B

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Table 1. Vehicle/Fuel Systems for Evaluation vehicle technologies SI ICEVsa

CIDI ICEVsb

fuel and feedstock options 90% petroleum gasoline with 10% corn ethanol by volume renewable gasoline: corn stover via pyrolysis renewable gasoline: forest residue via pyrolysis compressed natural gas: conventional and shale natural gas liquefied petroleum gas: natural gas and petroleum petroleum LSDc BD20d: SB renewable diesel: corn stover via pyrolysis renewable diesel: forest residue via pyrolysis Fischer−Tropsch diesel: natural gas Fischer−Tropsch diesel: coal Fischer−Tropsch diesel: forest residue

vehicle technologies SI FFVse

fuel and feedstock options E85:85% corn ethanol and 15% petroleum gasoline by volume E85:85% CS ethanol and 15% petroleum gasoline by volume E85:85% willow ethanol and 15% petroleum gasoline by volume E85:85% SC ethanol and 15% petroleum gasoline by volume

SI ICE HEVsf SI ICE PHEV40sg BEV100sh FCEVsi

E85:85% SS ethanol and 15% petroleum gasoline by volume 90% petroleum gasoline with 10% corn ethanol by volume 90% petroleum gasoline with 10% corn ethanol by volume, and U.S. mix electricity electricity: U.S. mix electricity: CA mix hydrogen: NG hydrogen: coal hydrogen: poplar hydrogen: WE with U.S. average grid

a Spark-ignition internal combustion engine vehicles. bCompression-ignition direct injection internal combustion engine vehicles. cLow-sulfur diesel with a sulfur content of 15 ppm. d20% biodiesel blended with 80% petroleum diesel by volume. eFlexible fuel vehicles. fSpark-ignition internal combustion engine hybrid electric vehicles. gSpark-ignition internal combustion engine plug-in hybrid electric vehicles with an all-electric range of 40 miles. hBattery electric vehicles with an all-electric range of 100 miles. iFuel-cell electric vehicles.

from the U.S. average electric grid and California (CA) electric grid with low-carbon intensity. WTW BC and POC Emission Modeling. The production of fuels and their end-use by vehicles involves combustion processes that produce BC and POC emissions. We considered BC and POC emissions from combustion sources, because both BC and POC are normally formed from incomplete fuel combustion. We did not consider BC and POC emissions from noncombustion sources such as coal mining dusts, except for vehicle brake and tire wear (BTW) emissions. Secondary OC (SOC), which are formed via complex atmospheric chemical and physical processes that are subject to both primary precursor emissions and the concentrations and kinetics of atmospheric oxidants,22 are beyond our consideration. The process activities associated with fuel production pathways consume a diversified mix of process fuels by multiple combustion technologies with varying energy efficiencies and emission performances. For a given vehicle/ fuel system, we applied a combustion technology-based approach to estimating the BC and POC emissions of each life-cycle stage and then aggregated these emissions to produce WTW results on per mile of vehicle driven basis with the GREET model (Supporting Information, Section 2). We summarized the key GREET life-cycle parametric assumptions for calculating WTW GHGw/oBC+POC emissions of the examined vehicle/fuel systems in the Supporting Information, Section 3. Data for BC and POC Emissions. Unlike CO2 emissions from fuel combustion, which can be estimated on a carbon mass balance basis, BC and POC are not predictable from overall stoichiometry, because their formation and destruction are limited by kinetics, not equilibrium states.23 As a result, BC and POC emission factors have to be determined via direct measurements of combustion processes. Thermal-optical and filter-based optical techniques have been adopted to measure BC and POC emissions,9 and the results are often expressed as mass fractions of the primary PM2.5 (hereinafter, PM2.5 refers to primary PM2.5) emissions.9,24 We estimated the BC and POC emissions using source profiles that report the BC and POC mass fractions of the PM2.5 emissions and the corresponding

including feedstock recovery/production, processing, transportation, and storage; and fuel production, transportation, distribution, and storage, while the PTW stage models the vehicle operations. Figure 1 illustrates the system boundaries of the vehicle/fuel systems evaluated in this study. We defined GHGBC, GHGPOC, GHGBC+POC, GHGw/BC+POC, and GHGw/oBC+POC as the CO2 equivalent (CO2e) emissions of BC, POC, BC and POC, and CO2, N2O, and CH4 emissions with and without GHGBC+POC included, respectively. We used the Global Warming Potentials (GWPs) of N2O, CH4, BC, and POC as recommended in the Fifth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC)11 to calculate GHGw/BC+POC and GHGw/oBC+POC. Both GWP100 and GWP20 on a time horizon of 100 and 20 years, respectively, (Supporting Information, Table S1) were used to evaluate the long-term and short-term climate impacts of GHGBC+POC on WTW GHG emissions of vehicle/fuel systems. Vehicle/Fuel Systems. Table 1 lists the vehicle/fuel systems evaluated in this study. Vehicles running on petroleum gasoline blended with 10% corn ethanol by volume that is widely used in the United States (gasoline vehicles [GVs]) and petroleum low-sulfur diesel vehicles (LSDVs) serve as the baseline vehicle/fuel systems. About 15 million flexible fuel vehicles (FFVs) capable of running on gasoline blended with up to 85% ethanol by volume (E85) are on U.S. roads.20 For E85, we included ethanol from corn, cellulosic corn stover (CS), cellulosic willow trees as a typical short-rotation woody crop, Brazilian sugarcane (SC), and U.S. sweet sorghum (SS). We included BD20, which has 20% of BD by volume from soybeans (SB), currently the largest BD feedstock in the U.S.21 In addition, we included RG and RD produced via pyrolysis of CS and forest residue (FR). Moreover, we evaluated FTD from NG, coal, and biomass. Furthermore, we evaluated FCEVs running on hydrogen produced from steam methane reforming of NG, gasification of coal and biomass, and water electrolysis (WE). We evaluated HEVs, PHEVs with an all-electric range (AER) of 40 miles (PHEV40), and BEVs with an AER of 100 miles (BEV100). In particular, we included BEV100s charged C

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Figure 2. WTW GHGBC and GHGPOC of fossil- and electricity-powered vehicle systems on the (a) 100 year and (b) 20 year time horizon, respectively. Error bars depict the standard deviation of WTW GHGBC and GHGPOC.

parts would be altered when BC and POC emissions are considered. The model takes into account the parametric uncertainties that affect WTW GHG w / B C + P O C and GHGw/oBC+POC of vehicle/fuel systems and diagnoses the chances for specific alternative vehicle/fuel systems to achieve certain levels of WTW GHG emission reductions from the baseline counterparts with and without considering BC and POC emissions.

PM2.5 emissions of a specific emission source (see the Supporting Information, Table S10). We express emissions in emission factors in grams per million Btu (mmBtu) of fuel burned. BC emissions factors range from 0.004 to 40.8 g/ mmBtu and those for POC range from 0.09 to 65.1 g/mmBtu, depending on the source (see the Supporting Information, Table S12). See the Supporting Information, Section 4 for details of the methodology, data, and results of BC and POC mass fractions and BC and POC emission factors for stationary, mobile, and open combustion emission sources. Vehicle Fuel Economy. For this analysis, the vehicle fuel economy was assumed to be 27.4 miles per gasoline gallon equivalent (MPGGE) for gasoline cars,25 and 32.9 MPGGE for diesel cars, assuming diesel cars are 20% more energy efficient than the gasoline counterparts.26 We assumed no relative change in MPGGE between gasoline vehicles and E85 FFVs according to Yanowitz et al.27,28 Furthermore, we assumed a relative fuel economy of 95% for compressed natural gas vehicles (CNGVs), of 100% for liquefied petroleum gas vehicles (LPGVs), of 100% for RG-fueled vehicles, of 120% for LSDVs, of 120% for BD20 vehicles, of 120% for FTD vehicles, of 120% for RD-fueled vehicles, of 140% for HEVs, of 341% and 111% for the charge-depletion and charge-sustaining mode of PHEV40s, of 400% for BEV100s, and of 210% for FCEVs, all relative to gasoline cars.26 Probabilistic Diagnosis Model. We developed a probabilistic diagnosis model (Supporting Information, Section 6) to analyze how the statistically true chances for alternative vehicle/ fuel systems to gain GHG emission reduction benefits under uncertainties compared with their gasoline or diesel counter-

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RESULTS WTW BC and POC Emissions of Fossil- and ElectricityPowered Vehicle Systems. The WTW GHGBC+POC add a small amount of GHG emissions to the WTW GHGw/oBC+POC of fossil- and electricity-powered vehicle/fuel systems, accounting for less than 2% or 5%, on average, with 100 year GWPs and 20 year GWPs, respectively, of the WTW GHGw/oBC+POC of the same vehicle systems (Supporting Information, Figure S1). Figure 2 shows the WTP and PTW GHGBC and GHGPOC and WTW GHGBC+POC of fossil- and electricity-powered vehicle systems. The WTW BC to POC emission ratio ranges from 0.36 to 0.68 among these vehicle/fuel systems. However, the WTW GHGBC+POC were dominated by positive WTW GHGBC, in contrast with much less negative WTW GHGPOC for GVs, LSDVs, and their alternative vehicle/fuel counterparts. This is attributed to the much stronger climate warming potential of BC emissions than the climate cooling potential of POC emissions, as indicated by the GWPs of BC and POC emissions. As a result, the WTW GHGBC+POC range from 0.7 and 2.7 g CO2e/mile, on average, for hydrogen FCEVs via WE D

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Figure 3. WTW GHGBC+POC from combustion-technology-specific direct fuel combustion and upstream emissions of biofuel-powered vehicle systems with 20 year GWPs. Other boilers include coal, distillate fuel oil, residual fuel oil, liquefied petroleum gas, coke, and crude oil boilers; other engines include natural gas and gasoline engines; turbines include diesel and natural gas combustion turbines.

associated combustion technologies such as NG-based or oilbased combustion processes and from minimum use of GHGBC+POC-intensive combustion technologies such as biomass-fired boilers and diesel-fired agricultural equipment. WTW BC and POC Emissions of Biofuel-Powered Vehicle Systems. Biofuel-powered vehicle systems differ from the fossil- and electricity-powered vehicle systems in that various types of biofuels can be produced via a variety of conversion technologies from various biomass feedstocks with different farming practices, which could result in very different BC and POC emissions. WTW GHGBC+POC vary significantly among biofuel-powered vehicle systems, ranging from 2.5 or 9.0 g CO2e/mile, on average, with 100 year and 20 year GWPs, respectively, for poplar-based hydrogen FCEVs, to 34.2 or 124.6 g CO2e/mile, on average, for SC-derived E85 FFVs. The WTW GHGBC+POC of E85 FFVs fueled with SC-, CS-, and willow-derived ethanol are significantly higher than those fueled with corn ethanol, CS- and FR-derived RG and RD, SB-based BD20, FR-derived FTD, and poplar-derived hydrogen (Supporting Information, Figure S4). Again, the WTW GHGBC+POC of these biofuel-powered vehicles are dominated by GHGBC, despite a WTW BC to POC emission ratio of 0.42 to 0.57 among the vehicle/fuel systems, except for a ratio of 1.05 for poplar-based hydrogen FCEVs. Domination of BC effects is primarily caused by the much stronger climate warming potential of BC emissions than the climate cooling potential of POC emissions. This domination of GHGBC to GHGBC+POC indicates the importance of BC emission mitigation for reducing the WTW GHG emissions of these vehicle/fuel systems. WTW GHGBC+POC add a significant amount of the GHG emissions for SC-, willow- and CS-derived E85 FFVs, which account for 34.2, 15.7, and 15.5 g CO2e/mile, on average, that represent an increase of WTW GHGw/o,BC+POC by about 18%, 15%, and 9% with the 100-year GWPs and for 124.6, 56.2, and

to 2.7 and 9.8 g CO2e/mile, on average, for GVs on the 100 year and 20 year time horizon, respectively, representing small contributions by the WTW GHGBC+OC for FCEV and GVs (see the Supporting Information, Figure S1). The large error bars show that WTW GHGBC+POC of these vehicle/fuel systems have significant uncertainties, which are primarily due to the uncertainties of the IPCC’s current understanding of the climate forcing effects of BC emissions, as indicated by the wide variations in their GWP100 and GWP20, and to the parametric uncertainties we quantified for the PM2.5, BC, and POC emissions of various emission sources. A breakdown of WTW GHGBC+POC of these vehicle/fuel pathways reveals that tailpipe and BTW emissions from vehicle operations are the major contributors to the WTW GHGBC+POC of GVs, CNGVs, LPGVs, LSDVs, HEVs, NG- and coal-based FTD vehicles, and WE- and coal-based hydrogen FCEVs (Supporting Information, Figure S2). Transportation of feedstocks and fuels contributed to 16−20% for GVs, HEVs, and LSDVs, whereas diesel engines that are primarily used during crude recovery and refining, NG recovery and processing, and coal mining contributed to 9−22% for GVs, CNGVs, LPGVs, LSDVs, HEVs, and coal-sourced hydrogen FCEVs. For a BEV100 powered by the U.S. electricity generation mix, coal-fired boilers dominated the WTW GHGBC+POC, whereas biomass-fired boilers were an important driver of the BEV100 charged by the CA generation mix, despite its very small share (0.7%) of the CA generation mix. The total BC and POC mass fractions, the BC to POC emission ratio (Supporting Information, Figure S3), and the PM2.5 emission factor of each emission source (Supporting Information, Table S10) are important parameters that determined the GHGBC+POC of a combustion-technologyspecific emission source. The low WTW GHGBC+POC of fossiland electricity-powered vehicle systems result primarily from the use of low GHGBC+POC-intensive process fuels and E

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Figure 4. Change of cumulative probability of WTW GHG emission reduction with 100 and 200 year GWPs, respectively, by (a) CS-, (b) willow-, (c) SC-, and (d) SS-sourced E85 FFVs relative to GVs due to inclusion of BC and POC emissions. In panel a, the y-axle values of point “A” and point “B” represent the probabilities for CS-derived E85 FFV to achieve a GHG emission reduction represented by the x-axle value of the points, which is 200 g/mile in this case, without and with BC and POC emissions with their 20-year GWPs considered, respectively. The drop from point “A” to point “B” as a result of considering BC and POC emissions shows a reduced probability for CS-derived E85 FFV to achieve a GHG emission reduction by 200 g/mile.

GHGBC+POC. The much higher amount of coproduced electricity from SS ethanol32 than that of SC ethanol33 provides much higher coproduct credits and results in much lower WTW GHGBC+POC from biomass combustion for SS ethanol. Besides, diesel agricultural equipment operations for biomass feedstock production and harvest are another major contributor to WTW GHGBC+POC of biofuel-powered vehicle systems (Figure 3). Tailpipe and BTW GHGBC+POC constitute a minor portion of WTW GHGBC+OC of willow-, CS-, SC-, and SSderived E85 FFVs, but become significant for other biofuelpowered vehicle systems with low WTW GHGBC+POC. With a PM2.5 emission factor 9 times higher than that for NG-fired boilers, biomass-fired boilers are about 11 times more GHGBC+POC-intensive than NG-fired boilers (Supporting Information, Table S10), despite similar BC to POC emission ratios of the two fuel combustion technologies and a lower total mass fractions of BC and POC emissions for biomass-fired boilers (Supporting Information, Figure S3). Another GHGBC+POC-intensive emission source is biomass open field burning, which has a BC-to-POC emission ratio of about 1:2, but has a very high PM2.5 emission factor that is about 8 times higher than that of biomass-fired boilers.34 This explains why CS-, willow-, SC-, and SS-derived E85 FFVs, of which the ethanol production involves a fair amount of biomass combustion in boilers and in the open field in the case of SC ethanol, have much higher GHGBC+POC than other biofuelpowered vehicles whose fuel production does not employ biomass combustion. Therefore, emission controls of these GHGBC+POC intensive biomass combustion sources are important for mitigating the impacts of BC and POC emissions

55.5 g CO2e/mile, on average, that represent an increase of WTW GHGw/o,BC+POC by about 57%, 51%, and 30% with the 20 year GWPs (Supporting Information, Figure S5). The choice between GWP100 and GWP20 has significant effects on both WTW GHG B C + P O C and the resulting WTW GHGw/o,BC+POC changes. Figure 3 shows a breakdown of emission sources of WTW GHGBC+POC of the biofuel-powered vehicle systems. For SC-, CS-, and willow-derived E85 FFVs, biomass-fired boilers, which are employed to combust part of the biomass feedstock to generate steam and electricity to meet the demand for process energy in ethanol plants and to export electricity, are the dominant source of WTW GHGBC+POC of these vehicle/fuel systems, accounting for about 60%, 65%, and 74%, on average, of their WTW GHGBC+POC, respectively. We estimated that an SC or CS ethanol plant with an ethanol production capacity of 50 million gallons per year would need a biomass-fired boiler that has a heat input of about 320 mmBtu/ hour from combusting about 30% of the biomass feedstock to provide sufficient process energy for ethanol production. For biomass-fired boilers of this size, we estimated a PM2.5 emission factor of 32.8 g/mmBtu (Supporting Information, Table S10), even with PM emission controls.29 For SC-derived E85 FFVs, preharvest open field burning of SC straws and residues associated with manual harvest of Brazilian SC,30 which we assumed currently accounted for 15% of Brazilian SC fields,31 was another major contributor to the WTW GHGBC+POC of Brazilian SC ethanol, accounting for 18% of the emissions. For SS ethanol, the much lower WTW GHGBC+POC, compared with that of its Brazilian sugar-based ethanol counterpart, is mainly from boiler combustion of SS bagasse for steam and electricity generation in ethanol plants that account for 44% of WTW F

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multicyclones.29 If 100% ESPs could be applied to improve the PM2.5 emission control efficiencies, the PM2.5 emission factor would decrease from 32.8 to 24.4 g/mmBtu, representing a reduction by 26%. Consequently, the GHGBC+POC of CS- and willow-derived E85 FFVs would be reduced by 17% and 19%, respectively, showing that better emission controls are important to reduce the impacts of GHGBC+POC on the WTW GHG emissions of cellulosic ethanol fueled FFVs. Preharvest burning of biomass in SC fields to help manual harvest of SC in Brazil has been subject to regulations and agreements that aim to eventually eliminate SC field burning due to air pollution concerns.36 Given that preharvest open field burning of Brazilian SC would be phased out in the near future, the GHGBC+POC of SC-derived E85 FFVs would be reduced by about 5 or 19 g CO2e/mile with 100 year and 20 year GWPs, respectively, which represents a WTW GHGBC+POC reduction by 16%. Consequently, SC bagasse combustion in boilers and an increased use of diesel harvesters due to increased machinery harvesting would become major contributors to WTW GHGBC+POC of SC-derived E85 FFVs. Historically, both on-road and off-road diesel vehicles are known to be major PM2.5 emission sources.37 The U.S. has phased in stringent Tier 2 tailpipe emission standards for onroad vehicles since 2000.38 Yet, tailpipe BC emissions are still a major contributor to WTW GHGBC+POC of fossil-fueled vehicles in the United States. With the U.S. Tier 3 vehicle emission standards targeting new on-road vehicles starting in 2017, the tailpipe PM2.5 emissions of light- and heavy-duty diesel vehicles would be reduced by about 60%.39 The tailpipe GHGBC+POC, however, could be significantly higher in countries like India and China where the vehicular PM2.5 emissions are severe due to a shortage of low-sulfur fuels that prevents nationwide adoption of stricter vehicle emission standards, particularly those equivalent to Euro 4 and 5 vehicle emission standards.40,41 The U.S. Tier 4 emission standards for new offroad diesel engines, which are phased-in over the period of 2008 to 2015, require off-road diesel with a sulfur content of 15 ppm and would reduce the PM2.5 emissions of diesel agricultural equipment employed for biomass feedstock production by more than 90%, owing primarily to application of particulate filters as an advanced aftertreatment technology.42 These stringent emission standards are important to mitigate BC emissions of biofuel production, as the GHGBC+POC of corn-, CS-, and willow-derived E85 FFVs would be 8 or 29, 27 or 96, and 12 or 42 g CO2e/mile higher with 100 year or 20 year GWPs, respectively, if MY 2005 diesel heavy-duty vehicles were employed for feedstock and fuel transportation and MY 2005 diesel agricultural equipment was used for biomass feedstock production. Considering the climate impacts of SOC on life-cycle GHG emissions requires in-depth knowledge of the formation, constituents, properties, and transformation of SOC. Sophisticated laboratory experiments, field measurements, and modeling studies of primary precursor emissions associated with specific life-cycle activities and the abundance, formation, reactivity, and kinetics of atmospheric oxidants are needed to gain adequate understanding of the climate impacts of SOC so that it can be eventually included in LCA of vehicle/fuel systems. With the small negative GWP values for OC, exclusion of SOC may have minimum effects on vehicle/fuel systems evaluated in this study. The 100 year GWP metric has been widely applied to compare the effect of different GHGs with a wide range of

on the WTW GHG emissions of cellulosic and SC ethanol driven vehicles. WTW GHGw/oBC+POC, GHGw/BC+POC, and GHGBC+POC per mega-joules of fuels used for fossil-, electricity-, and biofuelpowered vehicle systems are given in Supporting Information for information purposes. Impacts of WTW GHGBC+POC Uncertainties on WTW GHG Emission Reductions of Alternative Vehicle/Fuel Systems. We applied the probability diagnosis model to develop two sets of cumulative probability distribution curves that depict the reduction of WTW GHGw/oBC+POC and GHGw/BC+POC that could be achieved by alternative vehicle/ fuel systems from those of baseline vehicles. Figure 4 illustrates that WTW GHGBC+POC significantly reduce the maximum WTW GHG emission reductions by CS-, willow-, SC-, and SSderived E85 FFVs. For example, Figure 4a shows that including WTW GHGBC+POC would decrease the maximum WTW GHG emission reduction from 233 to 220 g CO2e/mile or from 240 to 191 g CO2e/mile that could be achieved by CS-derived E85 FFVs with at least a 60% probability with 100 year or 20 year GWPs, respectively. For willow-derived E85 FFVs, the impacts of including WTW GHGBC+POC on the maximum achievable WTW GHG emission reduction for a given probability are even bigger than those for CS-derived E85 FFVs, as shown by the bigger horizontal gap between the pairs of curves in red or black in Figure 4b. For example, including WTW GHGBC+POC decreases the maximum WTW GHG emission reduction of willow-derived E85 FFV from 315 to 301 g CO2e/mile or from 334 to 283 g CO2e/mile with 100-year or 20-year GWPs, respectively, with at least a 60% probability of achieving emission reduction. Between the two sugar-based ethanolfueled E85 FFVs, as shown in Figure 4c,d, the maximum WTW GHG emission reduction for SC- and SS-derived E85 FFVs to achieve with at least a 60% probability is reduced from 225 to 192 and from 267 to 263 g CO2e/mile, respectively, or is reduced from 230 to 98 and from 284 to 263 g CO2e/mile, respectively, when WTW GHGBC+POC are considered with their 100 year or 20 year GWPs, respectively. On the other hand, the cumulative probability distribution curves with and without WTW GHGBC+POC considered for CNGVs, LPGVs, BEV100s charged by the U.S. average and CA electricity mixes, and hydrogen FCEVs are barely discernible, indicating that consideration of BC and POC emissions for such alternative vehicle/fuel systems would barely make a difference in their statistical probabilities of achieving certain levels of WTW GHG emission reductions relative to GVs. This is also true for diesel alternative fuel/vehicles including vehicles fueled with NG- and FR-derived FTD, SB-based BD20, and CS-derived RD (Supporting Information, Figure S6). This demonstrates that the diversified levels and impacts of WTW GHGBC+POC on a variety of vehicle/fuel systems have to be examined individually.

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DISCUSSION As a key driver of GHGBC+POC of cellulosic ethanol production pathways, the high PM2.5 emissions from biomass combustion in boilers are an emission control priority to reduce the impacts of GHGBC+POC on the WTW GHG emissions of these biofuel pathways. PM2.5 emissions from biomass boilers depend largely on the deployment of PM2.5 control devices,35 and such PM2.5 controls are effective at removing BC and POC.9 We assumed that the boiler PM2.5 emissions are controlled with 57% electrostatic precipitators (ESPs), 39% wet scrubbers, and 4% G

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and BC emissions from diesel and FTD fueled vehicles. We thank the anonymous reviewers of this paper for their helpful comments.

atmospheric lifetimes. For BC and POC with atmospheric lifetimes much less than 100 years, GWP100 has the potential drawback of not capturing the near-term climate impacts of these SLCFs. The 20-year GWP metric may better reflect the potency during their short lifetime. However, there is no agreement as for which of the two metrics should be used to assess climate impacts of GHGs.11 Inclusion of both GWP metrics in this analysis provides an opportunity to evaluate the impact of the metric choice on the effects of considering BC and POC emissions on life-cycle GHG emissions of GVs and LSDVs and their alternative counterparts. The GWPs of BC and POC are both very uncertain, as their radiative forcings depend on the location, timing, and source of the emission. Therefore, the impacts of BC and POC can be location-dependent. We adopted the global-scale GWPs for BC and POC from IPCC11 because BC and POC can be transported over long distances and beyond the geographic boundary of the U.S. where their climate effects could occur. Spatial considerations of the climate effects of BC and POC on WTW GHG emissions of vehicle/fuel systems are warranted in further studies. Compared with the wide variation in the GWPs of BC (Supporting Information, Table S1), the GWPs of POC without any recommendations of its uncertainties by the IPCC11 probably indicate that they remain too uncertain to be quantified. Due to the counter climate effects of BC and POC, the uncertainties associated with the net climate impact of BC and POC emissions would probably be better understood when the uncertainties of POC emissions could be quantified.

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ABBREVIATIONS AER all-electric range BC black carbon BD biodiesel BEV battery electric vehicle CA California CH4 methane CNG compressed natural gas CNGV compressed natural gas vehicle CO2 carbon dioxide CO2e carbon dioxide equivalent CS corn stover E85 85% ethanol blending by volume ESP electrostatic precipitator FCEV fuel cell electric vehicle FFV flexible fuel vehicle FR forest residue FTD Fischer−Tropsch diesel g gram(s) GHG greenhouse gas GREET Greenhouse gases, Regulated Emissions, and Energy use in Transportation GV gasoline vehicle GWP global warming potential HEV hybrid electric vehicle IPCC Intergovernmental Panel on Climate Change kg kilogram(s) LCA life-cycle analysis LLCF long-lived climate forcer LPG liquefied petroleum gas LSDV low-sulfur diesel vehicle MPGGE miles per gasoline gallon equivalent MY model year N2O nitrous oxide NG natural gas OC organic carbon PHEV plug-in electric vehicle PM2.5 particulate matter with an aerodynamic diameter of 2.5 μm or less PTW pump-to-wheels RD renewable diesel RFO residual fuel oil RG renewable gasoline SB soybean SC sugarcane SLCF short-lived climate forcer SS sweet sorghum BTW brake and tire wear WE water electrolysis WTP well-to-pump WTW well-to-wheels

ASSOCIATED CONTENT

S Supporting Information *

GWPs of BC, POC, N2O, and CH4 emissions, description of GREET methodology, key parametric assumptions of life-cycle analysis of vehicle/fuel systems with the GREET model, methodology for and results of developing the BC and POC emission factor database, illustration of BC and POC emission characteristics of stationary, mobile, and open combustion emission sources, description of the probabilistic diagnosis model, additional results of WTW GHGBC+POC per MJ and their impacts on fossil-, electricity-, and biomass-powered vehicle/fuel systems. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*H. Cai. E-mail: [email protected]. Tel.: 630-252-2892. Fax: 630252-3443. Notes

The authors are solely responsible for the contents and results of the paper. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Bioenergy Technologies Office and the Vehicle Technologies Office of the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy under Contract DE-AC02-06CH11357. We are grateful to Drs. David Streets, Zifeng Lu, and Fang Yan of Argonne National Laboratory for their inputs on BC and OC emissions of various combustion processes. We thank Drs. Grant Forman, Paul Schaberg, and Andre Swarts from Sasol Corporation for sharing data and insights on tailpipe PM

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