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TECHNICAL PAPER

Kean, Sawyer, Assoc. 50:1929-1939 ISSN 1047-3289 J. Air & Waste Manage. and Harley

Copyright 2000 Air & Waste Management Association

A Fuel-Based Assessment of Off-Road Diesel Engine Emissions

Andrew J. Kean and Robert F. Sawyer Department of Mechanical Engineering, University of California, Berkeley Robert A. Harley Department of Civil and Environmental Engineering, University of California, Berkeley

ABSTRACT The use of diesel engines in off-road applications is a significant source of nitrogen oxides (NOx) and particulate matter (PM10). Such off-road applications include railroad locomotives, marine vessels, and equipment used for agriculture, construction, logging, and mining. Emissions from these sources are only beginning to be controlled. Due to the large number of these engines and their wide range of applications, total activity and emissions from these sources are uncertain. A method for estimating the emissions from off-road diesel engines based on the quantity of diesel fuel consumed is presented. Emission factors are normalized by fuel consumption, and total activity is estimated by the total fuel consumed. Total exhaust emissions from off-road diesel equipment (excluding locomotives and marine vessels) in the United States during 1996 have been estimated to be 1.2 × 109 kg NOx and 1.2 × 108 kg PM10. Emissions estimates published by the U.S. Environmental Protection Agency are 2.3 times higher for both NOx and exhaust PM10 emissions than estimates based directly on fuel consumption. These emissions estimates disagree mainly due to differences in activity estimates, rather than to differences in the emission factors. All current emission inventories for off-road engines are uncertain because of the limited in-use emissions testing that has been performed on these engines. Regional- and state-level breakdowns in diesel fuel consumption by off-road mobile sources are also presented. Taken together with on-road measurements of diesel engine emissions, results of this study suggest that in 1996, off-road diesel equipment (including

agriculture, construction, logging, and mining equipment, but not locomotives or marine vessels) was responsible for 10% of mobile source NOx emissions nationally, whereas on-road diesel vehicles contributed 33%. INTRODUCTION While on-road vehicles such as cars and trucks have long been recognized as important mobile sources of air pollution, less attention has been paid to controlling off-road mobile source emissions. The importance of off-road sources is expected to grow as on-road vehicle emissions are controlled. Off-road engine applications include aircraft, locomotives, marine vessels, recreational vehicles, and equipment used for agriculture, construction, logging, mining, lawn and garden maintenance, and so on. The present study focuses on diesel engines, as these are the dominant off-road mobile source of nitrogen oxides (NOx) and exhaust particulate matter less than 10 µm in diameter (PM10). Emissions from aircraft (primarily powered by kerosene-type jet fuel and gasoline) and residential lawn and garden equipment (primarily powered by gasoline) are not considered here. All other off-road engine applications include at least some use of diesel fuel. Off-road mobile sources are defined by the U.S. Environmental Protection Agency (EPA) as engines used offroad that are moved at least once within a 12-month period.1 An engine's mobility is important because regulatory agencies generally separate the emissions of air pollutants into two main source categories: stationary sources and mobile sources. Making this distinction for many off-road sources has proved difficult because engines used in generators, pumps, compressors, and welders may be either mobile or stationary. This means that two of the same engines being used for similar purposes may be regulated differently. Total U.S. emissions during 1996 from off-road diesel engines including locomotives and diesel marine sources have been estimated by EPA to be 3.6 × 109 kg NOx and 3.2 × 108 kg exhaust PM10.2 Throughout this investigation,

Journal of the Air & Waste Management Association 1929

IMPLICATIONS The contribution of off-road diesel equipment to total emissions of NOx and PM may be lower than suggested by current emission inventories. As a consequence, control of these sources may not lead to air quality benefits that are as large as expected.

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NOx emissions are reported as NO2 equivalents (i.e., a molecular weight of 46 g/mol is assumed), even though most of the NOx is expected to be emitted as NO. Nationally, off-road diesel engines have been estimated by EPA to account for 34% of mobile source NOx emissions and 48% of mobile source exhaust PM10 emissions. Off-road diesel equipment used in agriculture, construction, industrial, commercial, mining, and logging sectors contribute more to these totals than locomotives and marine vessels do. With locomotive and marine vessel emissions excluded, off-road diesel equipment accounts for 25% of mobile source NOx emissions and 42% of mobile source exhaust PM10 emissions, according to EPA.2 Emissions estimates for these sources are based primarily on a methodology described in the Nonroad Engine and Vehicle Emissions Study,3 a major assessment of off-road engine activity and emissions at the national and local levels, including emissions from over 80 different types of equipment. Emissions, E, were estimated by EPA for most off-road engine applications as follows: E = N × P × LF × A × EF (1) consumed. This method is presently used to estimate emissions from marine vessels6 and locomotives,7 and has been demonstrated in estimating CO and VOC emissions from on-road motor vehicles,8 as well as NOx and black carbon particle emissions from heavy-duty diesel trucks.9 An important advantage of the fuel-based approach is that fuel consumption data can be obtained more readily than information on engine populations, load factors, and annual hours of use. The objectives of this study were to describe a fuelbased method for determining emissions from off-road diesel engines, to use this method to estimate emissions of NOx and PM10 from off-road diesel engines at national and regional levels, and to compare the fuel-based assessment of emissions with previous inventory estimates. METHOD A fuel-based emission inventory for off-road diesel engines was obtained by multiplying the diesel fuel consumed by off-road engines by emission factors that are normalized by fuel consumption (i.e., mass of pollutant emitted per unit of fuel consumed). Off-Road Diesel Engine Activity Previous fuel-based emission inventories for on-road vehicles8,9 have relied on highway fuel tax data to determine the quantity of fuel used. Diesel fuel used off-road is not subject to highway taxes, so in this study, diesel fuel use was determined from an annual survey of companies that sell distillate fuels to end users. The survey, the Annual Fuel Oil and Kerosene Sales Report,10 is conducted by the Energy Information Administration (EIA), part of the U.S. Department of Energy. The sample used for this survey was determined from prior EIA survey results in combination with a survey of ~30,000 companies that sell petroleum products. Companies in the EIA survey included all refiners and gas plant operators, those companies doing business in four or more states, and those companies that account for 5% or more of the distillate fuel sold in a particular end-use category in a state. Statistical procedures were used to determine the number of companies to be surveyed to achieve a 5% coefficient of variation (COV) for distillate fuel oil sales at the state level. Overall, ~4700 companies were included in the survey; the response rate was 93% for 1996. Missing data were imputed by EIA; these accounted for sales fractions ranging from 0.07% for oil company use to 15.8% for off-highway use of distillate fuel. The survey data were reviewed by EIA both manually and with an automated computer program to detect missing and outlying data. In addition, preliminary results were processed through a series of validation procedures to identify and fix potential misreporting of data. Where possible, survey results were compared and adjusted by EIA with

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where N is the engine population (number of engines in use); P is the average rated power of these engines; LF is the load factor, which indicates the typical operating load of the engines relative to their average rated power; A is activity (in average annual hours of use per engine); and EF is an emission factor expressed in units of work done by the engine (e.g., g/kW-hr or g/bhp-hr). EPA is currently developing a computer model known as NONROAD to estimate emissions from off-road vehicles, again relying on eq 1 as the framework.4 Emissions from off-road engines in Europe have been estimated using a similar approach. It was found that off-road diesel engine emissions accounted for 23% of mobile source NOx emissions and 40% of mobile source exhaust PM10 emissions in the European Union in 1990.5 The large number of off-road diesel engines and applications makes it difficult to quantify the factors appearing in eq 1 accurately. Unlike on-road vehicles, there is no registration database for most off-road engines. This makes it difficult to estimate engine populations. Load factors depend on specific engine applications, which are numerous and varied. Typically, engine power, population, activity, and load factors for each application are estimated from engine manufacturer sales surveys and surveys of users of these engines. It is also possible to estimate emissions from off-road diesel engines using a top-down approach that is based on fuel consumption. In this approach, emission factors are normalized by fuel consumption, and engine activity is estimated using the total quantity of diesel fuel

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alternate source data such as known regional distillate fuel oil production at refineries, accounting for imports, exports, and changes in reserves. The adjustment factors used by EIA for this purpose were 0.98 for East Coast, 0.98 for Central, 0.85 for South Central, 1.30 for Rocky Mountain, and 0.66 for West Coast regions (these regions are defined in Table 1). The same adjustment factor was applied to all offroad fuel uses except electric utilities, for all states within a given region. A list of distillate fuel end use categories with typical off-road equipment that falls into each end use is presented in Table 2. National distillate fuel oil sales are broken down into these end uses, as shown in Table 3. The fractions of these sales that are used by mobile off-road diesel engines also are included in Table 3. Distillate fuel oil is a general classification for one of the petroleum fractions produced by conventional refinery distillation operations. There are three types of distillate fuel oil: No. 1, No. 2, and No. 4, with No. 1 having the lowest distillation temperatures and No. 4 the highest. Most of the diesel fuel used in the United States is a form of No. 2 distillate fuel oil, although No. 1 and No. 4 diesel fuels are also used. Further description of the different types of distillate fuel oil is provided in Table 4. Current national regulations limit the sulfur content of diesel fuel used on-road to less than 0.05% by mass. Use of low-sulfur diesel fuel for off-road purposes is allowed, but use of high-sulfur diesel fuel is not allowed in on-road engines. Diesel fuel sold for on-road use is subject to a highway tax, whereas diesel fuel intended for off-road use is not taxed. In some states, onroad diesel fuel use by government vehicles, school buses, and transit vehicles is not taxed; the on-highway diesel sales provided by EIA10 include only taxable fuel sales. Sales of tax-exempt, low-sulfur diesel fuel for government vehicles, school buses, and so on are included by EIA in the commercial end-use category.11 For both commercial and industrial end-use categories, total No. 2 distillate fuel sales presented in Table 3 were broken down by EIA into low-sulfur diesel, high-sulfur diesel, and No. 2 fuel oil. Consequently, for this study, off-road diesel use in the commercial and industrial end-use categories was assumed to equal the high-sulfur diesel fuel sales. California is a special case where all on-road and off-road diesel fuel is required to have low sulfur content; in that case, all of the tax-exempt low-sulfur fuel was assumed to be used off-road, as the detail required to separate onroad and off-road diesel fuel use in the commercial sector was not available. For other end uses, the distillate sales survey data were not broken down into distillate type. In the case of farm and military distillate sales, diesel sales were still separated from the other types of distillate fuels. In those cases, all of the diesel fuel sold was assumed to be used by offroad diesel engines. For the remaining end uses, diesel fuel was not separated from other distillate fuels, and the following assumptions were made based on advice from EIA:11 no use of distillate fuels in the residential or electric utility sectors occurs in off-road diesel engines; 50% of oil company distillate fuel oil is diesel (all of which is assumed to be used off-road); and all railroad, vessel bunkering, construction, and other distillate is diesel fuel used

Table 1a. National and regional distillate fuel sales and off-road diesel fuel sales by end-use category, 1996. Region East Coast c Central d South Central e Rocky Mountains f West Coast Total U.S.

b

Distillate Sales (10 L) 74 59 31 9.0 22 196

a

9

Off-Road Diesel Fuel (10 L) 9.9 18 12 3.6 5.5 49

9

Table 1b. Distribution of off-road diesel fuel sales among end uses (%). Region East Coastb Centralc South Centrald Rocky Mountainse West Coastf Total U.S.

a

Commercial 7.7 3.1 1.7 1.5 7.4 4.0

Industrial 12.4 7.6 4.7 3.2 12.3 8.0

Farm 13.5 34.4 22.1 27.9 19.5 25.0

Oil Company 0.2 0.5 7.5 1.8 1.7 2.4

Railroad 21.7 30.2 17.2 47.4 19.2 25.3

Vessel Bunkering 19.6 10.2 29.4 <0.1 16.4 16.8

Military 1.6 0.4 4.9 0.3 7.1 2.5

Construction 19.3 11.5 9.2 14.5 11.7 12.7

Other 4.2 2.2 3.2 3.5 4.8 3.2

Includes all diesel fuel sold for on-road and off-road use, as well as fuel oils No. 1, No. 2, and No. 4; bPetroleum Administration for Defense (PAD) District I: CT, DE, DC, FL, GA, ME, MD, MA, NH, NJ, NY, NC, PA, RI, SC, VT, VA, and WV; cPAD District II: IL, IN, IA, KS, KY, MN, MI, MO, NE, ND, OH, OK, SD, TN, and WI; dPAD District III: AL, AR, LA, MS, NM, and TX; ePAD District IV: CO, ID, MT, UT, and WY; fPAD District V: AK, AZ, CA, HI, NV, OR, and WA.

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Table 2. Off-road diesel fuel end-use categories and associated engine applications. End-Use Category Commercial Industrial Farm Oil company Railroad Vessel bunkering Military Construction Other

a

Examples of Engine Applications Welders and pressure washers Forklifts, aerial lifts, and mining equipmenta Tractors and combines Bore/drill rigs and other oil field equipment Locomotives and railway maintenance equipment Recreational boats, tugboats, ferries, and ocean-going vesselsb Military trucks and other equipment Cranes, paving, and earth-moving equipment Logging equipment

deterioration in emission factors is not addressed here because the impact of increasing age on emissions from uncontrolled diesel engines is reported to be negligible for NOx emissions and small for PM10 emissions.3,7 Off-Road Diesel Equipment. Most of the emission factors for diesel engines used in the construction, agriculture, mining, and logging sectors that have been reported are for steady-state tests on new engines and have been provided by engine manufacturers.3 Correction factors typically are applied to these emission factors by EPA to adjust for non-steady-state operating conditions.3 Others report that most off-road equipment operates at quasi-steadystate conditions and argue that correction factors are not needed.13 Information that can be used to develop fleetaveraged emission factors for these engines is contained in EPA's NONROAD model and its associated documentation.14 The NONROAD model estimates fuel consumption and emissions of NOx and PM10 by off-road engines. In NONROAD, the range of emission factors for engines that contribute significantly to overall fuel consumption is 38­ 65 g NOx per kg fuel and 2.8­10.2 g PM10 per kg fuel. PM10 emissions from these engines are high compared with other off-road diesel engines and show significant variability. Table 5 presents fleet-averaged emission factors for off-road equipment, as well as the emission factors for major types of off-road equipment. Locomotives. The development of fleet-averaged emission factors for locomotives is easier than for other types of off-road engines because there are effectively only two basic engines that comprise the entire U.S. locomotive fleet.15 These engines are used for three primary applications: line-hauling, switching, and passenger transportation. The duty cycles, and consequently the emission factors, vary depending on the application. EPA has estimated that ~85% of the total locomotive fuel is consumed by national freight line-haul locomotives, 7% by national freight switching locomotives, 5% by local and regional freight locomotives, and 3% by passenger locomotives.16 Little is known about the duty cycles and emissions characteristics of local and regional freight locomotives,7 but it is assumed here that they are most similar to switch locomotive emissions. Local and regional freight locomotives, like switch locomotives, are typically older and have lower power duty-cycles compared with national freight line-haul locomotives.7 Consequently, fuel used by local and regional freight locomotives has been grouped with fuel used by switch locomotives. Average emission factors for each locomotive engine application have been presented by EPA.16 Typically, switch locomotives have higher fuel-based emission factors than line-haul locomotives do. Additional emission factors for

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EIA includes mining equipment in the industrial category, whereas EPA groups mining and construction equipment together; bMost recreational boats are gasoline powered; large ocean-going vessels primarily use residual fuel oil. Engine emissions from the use of gasoline and residual fuel oil are not included in the present study.

by off-road engines. All fuel sales to railroads were assumed to be used in locomotives, and all vessel-bunkering sales were assumed to be used by main propulsion engines in marine vessels. The fuel sales survey data used in this study are subject to sampling errors because fuel sales are determined using a sample of fuel vendors rather than an all-inclusive census. Estimates of these sampling errors, stated as coefficients of variation, have been calculated by EIA. Here, the COV is defined as the standard deviation of the estimated nonresidential retail distillate fuel sales volume divided by the total nonresidential retail distillate fuel volume. In 1996, four of five regions of the United States had COVs in fuel sales of 2.2% or less. The remaining region (South Central) had a COV of 13%, due to large sampling errors in Texas. The U.S. average COV was 3%. Therefore, uncertainties in fuel sales arising from statistical sampling of fuel vendors appear to be small at the national and regional levels compared with other uncertainties involved in this method. Emission Factors Unlike on-road vehicles, where emissions most often are reported in mass per distance traveled, emission factors for off-road engines typically are reported in mass per unit of brake work output from the engine. In most cases, emission factors are developed using an engine dynamometer test with a duty-cycle consisting of a set of steady-state modes that are time-weighted to represent in-use conditions.12 Due to engine size, locomotive and marine engine emissions are sometimes measured "in-use." By dividing the measured emission factor by brake-specific fuel consumption determined during the same test, the emissions can be normalized to fuel consumption. The question of

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Table 3. National 1996 distillate fuel salesa and estimated fractions that are diesel and used in off-road engines.

are not typically used for dedicated domestic shipping. Marine diesel enEnd Use Distillate Type Distillate Sales % of Sales Included gines are used on vessels ranging from (109 L) As Off-Road Diesel recreational boats to ocean-going tankers. Because of the wide range of Residential No. 1 distillate 0.7 0 engines used, marine diesel engines No. 2 distillate 24.7 0 are often subdivided based on engine Commercial No. 1 distillate 0.5 0 speed, with slower, larger engines genNo. 2 distillate 11.2 15 b erally being more powerful. No. 2 fuel oil 5.7 0 b Another difficulty in estimating Low-sulfur diesel 3.8 6 b emissions from marine engines is deHigh-sulfur diesel 1.7 100 termining what type of fuel they use. No. 4 distillate 1.3 0 Industrial No. 1 distillate 0.2 0 Very large marine diesel engines typiNo. 2 distillate 7.5 49 cally use less expensive residual fuel No. 2 fuel oilb 1.7 0 oil or a combination of diesel fuel and Low-sulfur dieselb 2.0 6 residual fuel oil to reduce their fuelHigh-sulfur dieselb 3.8 100 ing costs.19,21 Therefore, only emisNo. 4 distillate 0.2 0 sions from combustion of diesel fuel Farm Diesel 12.2 100 in main propulsion marine engines Other distillate 0.4 0 are addressed in this investigation. c Electric utility Unspecified 2.6 0 Residual fuel contains significantly Oil company Unspecified 2.4 50 more impurities and is more viscous Railroad Unspecified 12.4 100 than diesel fuel, and consequently the Vessel bunkering Unspecified 8.3 100 emissions characteristics are different. On-highway Diesel 102 0 Military Diesel 1.2 100 Both NOx and PM10 emissions are Other distillate 0.2 0 higher from the combustion of reConstruction Unspecified 6.2 100 sidual fuel versus that of diesel fuel.20,21 Otherd Unspecified 1.6 100 A final difficulty in determinTotal distillate sales 196 25 ing emission factors is that very few emissions tests have been performed a Source of the fuel sales data is EIA;10 bNo. 2 fuel oil, low-sulfur diesel, and high sulfur diesel are all subcategories of No. 2 on marine engines. Emissions data distillate; cType of distillate fuel was not specified on fuel sales survey form; dFuel sales for use in logging equipment is from primarily 18 marine diesel enincluded here. gines have been used by EPA to estimate baseline emissions from all marine diesel engines.20 Information regarding the brakeline-haul and passenger locomotives are available in the 7,15,17,18 literature. specific fuel consumption for these engines was not proIn total, data are available from emissions tests on ~25 locomotive engines. Emission factors for lovided, so to develop fuel-based emission factors, fuel consumption data from similar land-based engines were comotives all fall in the range of 60­100 g NOx per kg fuel used.13,20,21 This is appropriate because the emissions test consumed and 1.4­2.4 g PM10 per kg fuel. A fleet-averaged emission factor, presented in Table 5, was determined duty cycles of marine and land-based diesel engines have by weighting EPA emission factors16 for each locomotive been shown to be similar.20 Marine diesel emission facapplication by the distribution of fuel use mentioned tors ranged from 30 to 100 g NOx per kg fuel and 0.7 to above; the resulting emission factors are therefore heavily 2.1 g PM10 per kg fuel. Small, high-speed engines had lower influenced by values used to represent line-haul locomoNOx emission factors and large engines had higher NOx tive emissions. emission factors, but no such trend was present for PM10 emissions. Marine Vessels. It is more difficult to estimate fleet-average Lloyd's Register,22 a worldwide ship classification soemission factors for marine diesel engines than for locociety, has performed emissions tests on 50 vessels, with motives for several reasons. First, the types of engines used results similar to EPA baseline emissions.21 Lloyd's has esand number of applications are significantly larger in the timated fleet-average emissions factors to be 57 g NOx marine sector. Most locomotive engines have a rated power per kg fuel and 1.2 g PM10 per kg fuel for medium-speed between 1100 and 3000 kW, whereas marine diesel engines marine diesel engines and 87 g NOx per kg fuel and 7.6 g range from <100 to 30,000 kW, though the largest engines PM per kg fuel for low-speed marine diesel engines.21,22

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Table 4. Classification of distillate fuels.10 Distillate Type No. 1 Subcategory Diesel fuel Description Used in high-speed engines subject to wide variations in speed and load. Includes type C-B diesel fuel used in city buses. Light distillate intended for use in vaporizing pot-type burners. Used in high-speed engines generally operated under uniform speed and load conditions. Includes type R-R used by railroad locomotives and type T-T used by diesel trucks. Low-sulfur fuel contains <0.05% sulfur by weight. Used in atomizing-type burners for domestic space heating and moderate capacity commercial/industrial-type burners. Used in low- and medium-speed diesel engines such as marine engines.a Used in large burner installations not equipped with fuel preheating facilities, especially industrial plants. It is a blend of distillate and residual fuel oils.

in Table 5 were used for all marine engines burning diesel fuel.

RESULTS The fuel-based methodology discussed above was applied to the United States at the national and regional levels. Total diesel fuel used by Fuel oil off-road engines is shown in Table 1. The ratio of diesel fuel used off-road relative to the total No. 2 Diesel fuel volume of distillate fuels sold ranged from 13 to 40% at the regional level and was 25% at the national level in 1996. At the national level, Fuel oil 2.8 × 1010 L diesel fuel was sold for use in offroad equipment, 1.2 × 1010 L was sold for railNo. 4 Diesel fuel road use, and 8.2 × 109 L was sold for marine vessel use. For comparison, ~1.0 × 1011 L diesel Fuel oil fuel was consumed by highway vehicles.10 National emissions estimates for off-road diesel equipment, railroad locomotives, and marine vessels based on fuel sales are shown a Most ocean-going marine engines use residual fuel oil. Residual fuel oil is also used for electric power in Figure 1a for NOx and Figure 1b for PM10. generation, commercial space heating, and various industrial purposes. For comparison, EPA Emissions Trends Report estimates2 are also provided in Figure 1. EPA's nationwide estimate of emissions from off-road diesel Low-speed diesel engines typically are used by oceanequipment is 2.3 times the fuel-based estimate for both going vessels burning residual fuel oil.20 High-speed dieNOx and exhaust PM10 emissions. In contrast, the ratios sel engine emissions data are limited, leading, in a previous 21 of EPA emission estimates to the fuel-based estimates preinvestigation, to the use of medium-speed engine emissented here are 1.1 for NOx and 1.2 for PM10 from locosion factors to estimate emissions from high-speed engines. In the present study, the emission factors presented motives, and 0.34 for NOx and 1.8 for PM10 from diesel marine engines. Figure 2 presents emissions estimates from off-road Table 5. Diesel engine exhaust emission factors.a diesel equipment for five regions of the United States, using the emission factors described above. Emissions at Engine Application NO (g/kg) PM (g/kg) x 10 the state level can be estimated using fuel consumption by off-road engines presented in the appendix in combiOn-road diesel trucksb,c 42 ± 5 2.5 ± 0.2 nation with emission factors given in Table 5. Emissions d Off-road equipment (fleet average) 48 ± 6 5.1 ± 2.3 from locomotives and marine vessels were not modeled Commercial 45 5.9 at the regional level due to uncertainties in the regional Industrial 45 6.2 breakdown of fuel consumption by these engines. Farm 53 3.8

Construction and Mining Logging Locomotives7,15-18 (fleet average) Line-Haul Switch Marine vessels (fleet average) 13,20-22 High-speed Medium-speed Slow-speede

a

46 43 75 ± 9 74 81 55 ± 13 40 57 87

5.5 4.7 1.9 ± 0.3 1.8 2.1 1.3 ± 0.6 1.2 1.2 7.6

DISCUSSION As shown in Figure 1, significant differences exist between the emissions estimates for off-road diesel equipment and diesel marine engines, whereas emissions estimates for locomotives are in reasonable agreement. Off-Road Diesel Equipment EPA estimates2 of both NOx and PM10 emissions from offroad diesel equipment are more than twice the values presented here based on national diesel fuel consumption, even though emission factors used to derive both inventory estimates were similar. This means that differences in activity for these engines are the primary reason for differences

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A diesel fuel density of 0.85 kg/L is assumed for all cases; bOn-road measurements from 1997 in a San Francisco Bay area highway tunnel;23 cExhaust emission factor for particulate matter is PM for on-road diesel trucks; dOff-road equipment emission 2.5 factors were obtained from EPA's NONROAD model;14 eSlow-speed emission factors were measured on engines consuming residual fuel oil.

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Figure 1. National off-road diesel engine emissions of (a) NOx and (b) PM10 for 1996 from the present study with comparisons to EPA Emissions Trends Report estimates.2

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Figure 2. Regional breakdown of off-road diesel equipment emissions. The regions are defined in the footnotes to Table 1.

in the estimated emissions. EPA's NONROAD model, currently under development, estimates national diesel fuel consumption by off-road diesel equipment to be 6.3 × 1010 L in 1996. This is 2.2 times the value derived from EIA fuel sales surveys,10 suggesting that the total activity of off-road diesel equipment is substantially overestimated by EPA's use of eq 1 with the information currently available. Even if it was assumed that all tax-exempt diesel fuel, regardless of sulfur content, was used off-road, the ratio of fuel use by off-road diesel equipment as estimated by EPA's NONROAD model to the value derived from fuel sales surveys would still be 1.9. The differences between EPA and fuel-based diesel engine activity estimates may be larger than stated here, because some distillate fuel oil other than diesel fuel has been included in the present study's estimates, as has diesel fuel used by stationary off-road engines (e.g., stationary generators, pumps, compressors, and welders). Another factor complicating the assessment of engine activity is the potential for crossover in fuel sold for on-road versus off-road use. Entities operating the same engines both on- and off-road may choose to use taxable diesel fuel at all times to simplify fuel supply issues, and fuel that is exempt from highway taxes at times may be used illegally in on-road engines. Figure 2 shows the regional breakdown of off-road diesel engine equipment emissions. Emission factors may

1936 Journal of the Air & Waste Management Association

vary depending on the mix of equipment types and engines used in a given region, but for these large regions, the variation in the emission factors is likely to be small. EPA's NONROAD model also suggests that fleet-averaged emission factors do not vary significantly by region. Locomotives All of the emissions estimates for locomotive engines were in good agreement. This was expected, because EPA uses a fuel-based approach to estimate emissions from these engines, instead of relying on the methodology presented in eq 1. These EPA emission estimates are based on fuel consumption data obtained from the Association of American Railroads,7 which indicate that 1.36 × 1010 L diesel fuel was used by locomotives. This is 10% higher than the estimated 1.24 × 1010 L diesel fuel used by railroads during 1996 as given by EIA,10 supporting the assumption made earlier that virtually all fuel sold to railroads is used in locomotives. Given uncertainties resulting from the small number of engines tested and uncertainties in fuel sales, emissions estimates for locomotives presented here are effectively the same as EPA estimates. Marine Vessels For diesel marine vessels, EPA emission estimates2 are 0.34 times our fuel-based estimate for NOx and 1.8 times our

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fuel-based estimate for PM10. It is not clear why EPA estimates are lower for NOx but higher for PM10. Corbett and Fischbeck6,21 have presented detailed analyses of emissions from marine engines and reported similar findings when their estimates were compared with previously published EPA emission estimates. One source of uncertainty in these comparisons arises due to the international nature of shipping via ocean-going commercial vessels. In this case, fuel can be consumed far away from the point of sale. However, ocean-going vessels with large low-speed engines typically burn heavier residual fuels, not diesel fuel, in their propulsion engines.20 By focusing only on diesel fuel use in the marine sector, we emphasize emissions from river and lake, harbor, and near-shore vessels, which tend to be smaller and have medium- or high-speed diesel engines. Readers seeking a more detailed analysis of marine vessel emissions are referred to the work of Corbett and Fischbeck.6,21 Overall Mobile Source Inventory In contrast to the results for off-road engines, based on a similar fuel-based analysis, we found that on-road heavyduty diesel engine NOx emissions were approximately twice those of past EPA inventory estimates. On-road measurements of diesel engine emissions in tunnels and by remote sensing have been reviewed by Kirchstetter et al.,23 who found an emission factor of 42 ± 5 g NOx per kg fuel to be near the middle of the range of values reported. Using this emission factor in combination with the taxable on-road diesel fuel sales from EIA, we estimated national emissions from on-road heavy-duty diesel trucks in 1996 to be 3.6 × 109 kg NOx, whereas EPA estimated NOx emissions from these vehicles to be 1.8 × 109 kg.2 A breakdown of EPA's national mobile source NOx emission inventory2 is presented in Figure 3a. Note that aircraft emissions include only those associated with landing and takeoff operations, idling, taxiing, and climbing below 910 m (3000 feet) altitude. In Figure 3b, fuel-based emissions estimates derived in the present study for offroad diesel equipment, locomotives, marine vessels, and on-road heavy-duty vehicles have been used in place of EPA estimates. As seen by comparing Figures 3a and 3b, NOx emissions from off-road diesel equipment are less important than current inventories suggest, whereas emissions from on-road diesel engines appear to be underestimated. Also, a fuel-based analysis showed that locomotives were responsible for 34% of NOx emissions from off-road diesel sources, compared with 23% when EPA estimates are used. Despite large differences in emissions estimates for both on-road and off-road diesel engines, overall mobile source NOx emissions calculated in the present study agree with previous inventories. The breakdown between on-road and off-road contributions is still important because different emission standards, fuels, and timetables for controlling emissions apply in these sectors; this breakdown is important when determining the most effective control strategies to be pursued. CONCLUSIONS A fuel-based approach was used to estimate emissions of NOx and PM10 from U.S. off-road diesel engines. Fuel sales data can be obtained more readily than information on

Figure 3. Breakdown of mobile source NOx emissions by engine type and application, showing (a) current official inventory estimates for 19962 and (b) fuel-based emission estimates developed in the present study. Total NOx emissions = 1.1 × 1010 kg in both cases.

Volume 50 November 2000 Journal of the Air & Waste Management Association 1937

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populations, average load factors, and annual hours of use, which is required in current methods of estimating emissions. For 1996, U.S. emissions by all off-road diesel engines using this fuel-based method were estimated to be 2.3 × 109 kg of NOx and 1.5 × 108 kg of PM10. Emissions from off-road diesel equipment were 1.2 × 109 kg of NOx and 1.2 × 108 kg of PM10. Past estimates of emissions by these engines were higher by a factor of 2.3 for both NOx and exhaust PM10. These differences appear to be the result of overstated engine activity in the off-road diesel equipment sector; similar emission factors were used in both methods. Emissions from locomotives estimated in this study agree with EPA estimates, while unexplained differences in marine vessel emissions remain. EPA estimates suggest off-road diesel equipment was responsible for 25% of national mobile source NOx emissions in 1996. The estimate was reduced to 10% when fuelbased estimates of on- and off-road engine emissions were used instead. For all applications of off-road diesel engines, emissions estimates will improve as further engine activity and emissions test data are obtained. Based on currently available off-road engine emissions data, control of off-road diesel equipment emissions may not lead to air quality benefits as large as have been projected. The use of diesel fuel in off-road engines is nevertheless a significant source of NOx and PM10, and controlling these emissions will therefore be important to improving air quality. ACKNOWLEDGMENTS This material was based upon work supported by the National Science Foundation under Grant No. BES-9623385. We thank Alice Lippert of EIA, and Greg Janssen, Joseph Somers, and Alan Stout of EPA for their helpful comments and suggestions. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. Nonroad Engine Population Estimates; Report No. NR-006a; Office of Mobile Sources, U.S. Environmental Protection Agency: Ann Arbor, MI, 1998. National Air Pollutant Emission Trends Update, 1970-1997; Report No. EPA 454/E-98-007; Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency: Research Triangle Park, NC, 1998. Nonroad Engine and Vehicle Emission Study; Report No. EPA-21A-2001; Office of Mobile Sources, U.S. Environmental Protection Agency: Ann Arbor, MI, 1991. Average Life, Annual Activity, and Load Factor Values for Nonroad Engine Emissions Modeling; Report No. NR-005a; Office of Mobile Sources, U.S. Environmental Protection Agency: Ann Arbor, MI, 1998. Samaras, Z.; Zierock, K.H. Off-Road Vehicles: A Comparison of Emissions with Those from Road Transport; Sci. Total Environ. 1995, 169, 249-255. Corbett, J.J.; Fischbeck, P.S. Emissions from Ships; Science 1997, 278, 823-824. Locomotive Emission Standards, Regulatory Support Document; Office of Mobile Sources, U.S. Environmental Protection Agency: Ann Arbor, MI, 1997. Singer, B.C.; Harley, R.A. A Fuel-Based Inventory of Motor Vehicle Exhaust Emissions in the Los Angeles Area during Summer 1997; Atmos. Environ. 2000, 34, 1783-1795. 9. 10. 11. 12. Dreher, D.B.; Harley, R.A. A Fuel-Based Inventory for Heavy-Duty Diesel Truck Emissions; J. Air & Waste Manage. Assoc. 1998, 48, 352358. Fuel Oil and Kerosene Sales 1996; DOE/EIA-0535(96); Energy Information Administration, U.S. Department of Energy: Washington DC, 1997. http://www.eia.doe.gov, accessed June 1998. Lippert, A. Energy Information Administration, U.S. Department of Energy, Washington, DC. Personal communication, 1998. Pollack, A.K.; Lindhjem, C.E. NONROAD MOBILE Emissions Modeling; ENVIRON International Corporation, Novato, CA, and Office of Mobile Sources, U.S. Environmental Protection Agency: Ann Arbor, MI, 1997. Documentation of Input Factors for the New Off-Road Mobile Source Emissions Inventory Model; Report to the California Air Resources Board by Energy and Environmental Analysis, Inc.: Arlington, VA, 1997. Exhaust Emission Factors for Nonroad Engine Modeling--CompressionIgnition; Report No. NR-009a; Office of Mobile Sources, U.S. Environmental Protection Agency: Ann Arbor, MI, 1998. Fritz, S.G.; Cataldi, G.R. Gaseous and Particulate Emissions from Diesel Locomotive Engines; Transactions of the ASME 1991, 113, 370376. Emission Factors for Locomotives, Technical Highlights; Report No. EPA420-F-97-051; Office of Mobile Sources, U.S. Environmental Protection Agency: Ann Arbor, MI, 1997. Fritz, S.G. Exhaust Emissions from Two Intercity Passenger Locomotives; Transactions of the ASME 1994, 116, 774-783. Fritz, S.G.; Markworth, V.O.; Cataldi, G.R. Exhaust Emissions from Inuse Locomotives; 95-ICE-4, The American Society of Mechanical Engineers: New York, 1995. Woodward, J.B. Low Speed Marine Diesel; Robert Kreiger Publishing Company: Malabar, FL, 1988. Draft Regulatory Impact Analysis: Control of Emissions from Compression-Ignition Marine Engines; Office of Mobile Sources, U.S. Environmental Protection Agency: Ann Arbor, MI, 1998. Corbett, J.J.; Fischbeck, P.S. Commercial Marine Emissions Inventory for EPA Category 2 and 3 Compression Ignition Marine Engines in United States Continental and Inland Waterways; Prepared for U.S. Environmental Protection Agency, Order No. 8A-0516-NATX; Carnegie Mellon University: Pittsburgh, PA, 1998. Carlton, J.S.; Danton, S.D.; Gawen, R.W.; Lavender, K.A.; Mathieson, N.M.; Newell, A.G.; Reynolds, G.L.; Webster, A.D.; Wills, C.M.R.; Wright, A.A. Marine Exhaust Emissions Research Programme; Lloyd's Register Engineering Services: London, 1995. Kirchstetter, T.W.; Harley, R.A.; Kreisberg, N.M.; Stolzenburg, M.R.; Hering, S.V. On-Road Measurement of Fine Particles and Nitrogen Oxide Emissions from Light- and Heavy-Duty Motor Vehicles; Atmos. Environ. 1999, 33, 2955-2968.

13. 14. 15. 16. 17. 18. 19. 20. 21.

22.

23.

About the Authors Andrew Kean is a graduate student in the Department of Mechanical Engineering at the University of California at Berkeley. He holds an M.S. degree in Mechanical Engineering from UC Berkeley and a B.E. degree in Mechanical Engineering from Cooper Union. Robert Sawyer and Robert Harley are faculty members in the College of Engineering at UC Berkeley. Please direct correspondence to R. Harley, Department of Civil and Environmental Engineering, 631 Davis Hall, University of California, Berkeley, CA 947201710.

1938 Journal of the Air & Waste Management Association

Volume 50 November 2000

Kean, Sawyer, and Harley

APPENDIX

Table 1A. Diesel fuel by end use and total distillate fuel oil sales (109 L). Total Off-Road d Equipment 5.82 0.05 0.07 0.01 0.95 0.86 0.12 0.31 0.16 0.04 0.18 0.31 0.68 0.54 0.03 0.32 0.04 0.72 0.43 10.5 1.20 0.60 0.91 0.76 0.97 0.56 0.90 0.52 0.74 0.44 0.88 0.50 0.32 0.61 0.63 6.52 0.68 0.48 1.32 0.53 0.20 3.30 1.89 0.59 0.44 0.46 0.23 0.18 3.57 0.26 0.37 1.77 0.10 0.32 0.28 0.47 28.3 Total Distillate e Sales 74.1 3.58 0.61 0.38 6.25 6.63 2.42 3.49 5.53 1.27 5.66 11.5 5.31 9.88 0.97 2.47 0.93 5.73 1.48 59.2 6.06 5.69 3.21 2.69 4.49 4.74 3.92 4.42 2.71 1.35 7.19 3.25 1.06 4.38 4.07 31.23 3.68 2.64 6.29 2.31 1.58 14.7 8.97 2.36 1.51 1.56 1.58 1.98 22.3 1.40 2.59 10.7 0.77 1.51 2.24 3.02 196

Region East Coast Connecticut Delaware District of Columbia Florida Georgia Maine Maryland Massachusetts New Hampshire New Jersey New York North Carolina Pennsylvania Rhode Island South Carolina Vermont Virginia West Virginia Central Illinois Indiana Iowa Kansas Kentucky Michigan Minnesota Missouri Nebraska North Dakota Ohio Oklahoma South Dakota Tennessee Wisconsin South Central Alabama Arkansas Louisiana Mississippi New Mexico Texas Rocky Mountains Colorado Idaho Montana Utah Wyoming West Coast Alaska Arizona California Hawaii Nevada Oregon Washington United States

On-Road 31.7 0.79 0.22 0.08 3.64 5.09 0.51 1.35 1.19 0.23 2.02 3.50 3.01 4.43 0.17 1.73 0.36 2.87 0.50 34.5 3.35 3.92 1.74 1.34 2.22 2.92 1.83 2.90 1.27 0.51 4.83 2.05 0.54 2.86 2.26 17.1 2.45 1.88 1.92 1.51 1.19 8.17 4.14 1.03 0.71 0.53 0.97 0.90 14.6 0.13 2.06 8.01 0.15 0.93 1.46 1.81 102

a

Farm 1.34 0.01 0.02 0.0 0.28 0.38 0.01 0.09 0.01 0.00 0.02 0.08 0.11 0.10 0.00 0.07 0.01 0.13 0.01 6.08 0.76 0.34 0.74 0.61 0.12 0.22 0.62 0.22 0.65 0.39 0.33 0.28 0.28 0.22 0.31 2.70 0.12 0.35 0.20 0.30 0.06 1.67 1.00 0.33 0.23 0.32 0.07 0.05 1.08 0.00 0.07 0.72 0.02 0.02 0.10 0.16 12.2

b

Construction 1.91 0.03 0.03 0.001 0.42 0.19 0.05 0.09 0.09 0.02 0.10 0.18 0.21 0.20 0.01 0.08 0.01 0.18 0.03 2.03 0.24 0.09 0.12 0.10 0.14 0.16 0.17 0.14 0.06 0.02 0.25 0.11 0.03 0.20 0.19 1.12 0.22 0.05 0.35 0.06 0.06 0.39 0.52 0.20 0.06 0.11 0.08 0.07 0.65 0.04 0.09 0.32 0.02 0.04 0.04 0.11 6.23

c

a Diesel fuel that was taxed for use on-road; bDiesel fuel used by farm equipment; cDiesel fuel used by construction equipment; dDiesel fuel used by all off-road diesel equipment, including farm and construction e 10 equipment; Total distillate fuel oil sales.

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