Emissions from Waterborne Commerce Vessels in United States

Jun 21, 2000 - Nearly all emissions in U.S. waters occur in shipping channels outside of port regions, either on rivers or within 200 miles of shore. ...
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Environ. Sci. Technol. 2000, 34, 3254-3260

Emissions from Waterborne Commerce Vessels in United States Continental and Inland Waterways JAMES J. CORBETT* AND PAUL S. FISCHBECK Department of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213

We present an inventory of emissions from marine vessels engaged in waterborne commerce (i.e., cargo transport) on the U.S. navigable waters. Emissions are estimated for 1997 for various U.S regions and types of traffic, including oceangoing (international), coastwise (domestic), inland-river system, and Great Lakes. Nearly all emissions in U.S. waters occur in shipping channels outside of port regions, either on rivers or within 200 miles of shore. NOx emissions from commercial marine engines considered in this study account for about half of the U.S. EPA baseline inventory of ∼1000 tons per year for all marine vessels (1). This equals 4% of all U.S. transportation emissions, more than double previous nationwide inventories of vessel emissions (2). Waterborne commerce emissions are not negligible when compared to other sources. In fact, in many regions NOx emissions from waterborne commerce rank higher than other source categories, which are regulated at the state and federal levels. NOx emissions from river commerce in the top 20 states with waterborne trade account for 65% of total waterborne commerce emissions in those states. In contrast, 72% of SOx emissions in these states occurs along coastal (Ocean or Great Lake) areas, where high-sulfur fuels are more commonly used.

Introduction Nationwide emissions inventories from shipping have traditionally ignored shipping channels outside of port regions. In the past, inventories have been based on in-port emissions from a few ports where direct inventories were performed, extrapolated to the remaining ports, and summed to estimate nationwide emissions from ships (3-5). While this approach may be valid when considering local air-quality impacts, regional effects cannot be properly estimated without consideration of vessel emissions occurring outside of port areas. Recent research on the regional character of many air pollutants (including NOx) illustrates the importance of considering the impacts of emissions that may originate outside of air-quality nonattainment areas (6). Ship emissions along the coasts, Great Lakes, and inland-river system are considered in this paper. This paper provides estimates of oxides of nitrogen (NOx), particulate matter (PM), hydrocarbons (HC), carbon monoxide (CO), and sulfur dioxide (SO2) from 1997 cargo movements (domestic and foreign) on U.S. navigable * Corresponding author current address: College of Marine Studies, University of Delaware, Newark, Delaware; phone: (302)8310768; fax: (302)831-6838; e-mail: [email protected]. 3254

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waterways (i.e., on inland waterways and the Great Lakes within approximately 200 miles of U.S. coastlines).

Methodology and Calculations We estimate emissions from foreign and domestic cargo transport in U.S. waters by estimating the average emissions per ton-mile of cargo moved and deriving the pollution emitted from cargo transported on trade routes in U.S. waters. This method provides an estimate of emissions from foreign and domestic shipping on U.S. waterways distributed regionally. This methodology relies on a database of registered cargo-carrying ships containing such information as propulsion type, brake horsepower, and engine data (7). For inlandwaterway estimates, registered tug- and tow-boat data are used. For each method, the analysis used marine diesel emission factors reported by Lloyd’s Register Engineering Services in the Marine Exhaust Emissions Research Programme (8). These emission factors are generally similar to EPA emission factors in AP-42 (9) for similar CI engines. Daily fuel consumption was calculated from ship brake horsepower (BHP) and engine brake-specific fuel consumption (BSFC). BHP for nearly all of these engines is reported in the Lloyd’s registry data. The rated BHP values used in the fuel-consumption estimates were adjusted downward, in consideration of the fact that ship engines generally operate at maximum loads no higher than 80% of rated load. The adjusted BHP for each engine was multiplied by a typical average brake-specific-fuel-consumption (BSFC) factor for the horsepower range indicated. Generalized fuel-consumption characteristics for direct-drive, geared, and diesel-electric CI marine engines were used to establish the relationship between average BSFC and engine load for these propulsionplant configurations (10). This aggregated study did not apply specific engine performance curves but made the average BSFC factor linearly proportional to load for all enginess generally similar to fuel-power curves for marine engines. These fuel-consumption characteristics were confirmed by comparison with other sources (11-13). Units were converted to tons-fuel per day (tpd). Fuel-based emission factors were applied to get emissions per day, making adjustments for usage using marine duty cycles from the International Organization for Standardization (ISO) (14), estimated time in service, and time underway per year. To make the fuel-consumption estimates more realistic, the E3 duty cycle for heavy-duty marine engines was used (14, 15). This duty cycle was developed to represent typical overall engine loads for exhaust emission measurement for commercial vessels. Application of a common duty cycle allows for a direct estimate of emissions for all types of vessels engaged in waterborne commerce. These estimates appear reasonable when compared with fuel consumption reported by other sources (16, 17). To further validate this methodology, we estimated annual fuelusage for the international commercial cargo fleet using these assumptions. This approach overestimates international marine bunker sales by 20% of the 140 million metric tonnes reported by the Engergy Information Administration (18). We consider this to be good agreement, given the general nature of the assumptions applied. Emission factors for NOx and PM are different for mediumspeed and slow-speed engines (8), so engines were divided into two subcategories according to cylinder displacement, based on U.S. EPA categories (see supplemental information) (1, 19). Engines with cylinder displacements less than 20 L (EPA categories 1 and 2) were considered to be mediumspeed engines. For engines in EPA category 3 with cylinder 10.1021/es9911768 CCC: $19.00

 2000 American Chemical Society Published on Web 06/21/2000

TABLE 1. Estimated Fuel-Consumption for CI Engines in the U.S. and Foreign Fleets (tpd)a engine categoryb 1

2

3A

3B

weighted av

vessel service

U.S.

foreign

U.S.

foreign

U.S.

foreign

U.S.

foreign

U.S.

foreign

Container RoRo Transport Tug

NA NA 4 3

NA 7 6 6

5 6 6 5

9 15 8 8

NA 48 31 24

19 22 16 12

56 40 29 31

46 24 23 17

55 28 22 5

46 19 19 8

a Foreign fleet waterborne commerce estimates include only Container, RoRo, and General Transport vessels. b EPA engine categories are used: category 1 engines have cylinder displacements less than 5 L; category 2 engines have cylinder displacements 5 L or greater to 20 L; category 3A engines have cylinder displacements 20 L or greater to 60 L; and category 3B engines have cylinder displacements 60 L or greater.

displacements greater than 20 L but less than 60 L (subcategory 3A), we assumed that half of these engines were medium-speed engines, and half were slow-speed engines. EPA category 3 engines greater than 60 L were assumed to be slow-speed engines (subcategory 3B). Weighted-average fuel-consumption estimates for cargo-carrying vessels are presented separately for each engine size range in Table 1. (These are weighted-average estimates; the number of ships in each category is provided in the Supporting Information.) These fuel-based emission factors were then multiplied by the estimated daily fuel-consumption to obtain an estimate for the daily emissions for each pollutant in kg per day. To obtain an estimate of annual emissions, the daily emissions per ship were multiplied by the number of ships and by the number of days per year to estimate annual emissions, assuming a conservative in-service factor of 80%. An informal telephone survey of several commercial tug and barge operators (Crowley Maritime Corporation, Foss Maritime Company, American Commercial Barge Lines Company, National Marine Inc., and the Port of Pittsburgh) confirmed that this assumption was also reasonable for tugs and utility vessels.

Foreign and Domestic Cargo Transport Fleet Inventory We used cargo movements and waterway data to calculate the total tons and ton-miles moved by ships annually in and around the U.S. Then “average” cargo ships needed to carry this cargo were characterized, and emissions per ton-mile for these hypothetical cargo ships are calculated. Annual total emissions from cargo transportation are the product of these two values. Navigable U.S. waterways are inventoried in the U.S. Army Corps of Engineers (USACE) Waterway Link Network. Shipping lanes and open-water passage lanes are represented by over 5000 line segments or “links” . The National Waterway Network (NWN) geographic database includes physical and location information about each of these links (20). For this paper, 1997 commodity movements (in tons) across each NWN link (21) were used to create a comprehensive picture of cargo movement in and around the U.S. The cargo movements in ton-miles for each region were then calculated by summing the product of the number of tons shipped along each link in a region by the length of that link, as shown below 3

C)

ni

∑ ∑L

ij

* Tij

(1)

i)1 j)1

where C ) cargo movement in ton-miles, i ) geographic region (ocean, inland, Great Lakes), L ) length (in miles) of link i, T ) tonnage (total tons shipped in 1993) of link i, j ) each link in the NWN and waterborne commerce statistics center (WCSC) data for a given geographic region, and n ) number of links in geographic region.

TABLE 2. Summary of Cargo Movements in 1993 by Major Waterway region

distance (miles)a

106 tons movedb

109 ton-miles

ocean great lakes inland rivers unassigned total

116 000 8380 43 600 120 168 000

15 970 7660 33 060 90 56 780

765 85 494 2 1346

a From NWN data (20). b From WCSC data (20). Note that tonnage moved is not the same as tons received or sent by a given port; the same tons may move across several links.

A summary of these calculations is shown in Table 2. Ton-miles of cargo moved over inland rivers equal 64% of the ton-miles of cargo transported in U.S. coastal waters. However, since the navigable miles on the inland rivers equal only about one-third of the navigable ocean miles, emissions from these cargo movements occur in a much smaller area and may be regionally more significant. To estimate emissions per ton-mile from commercial marine vessels, the estimated daily emissions per ship were used, along with vessel average deadweight tonnage (DWT) and speed data from Lloyd’s registry. Deadweight tonnage is a measure of the total contents of a ship including cargo, fuel, crew, passengers, food, and water aside from boiler water. Because DWT describes more than the cargo carrying capacity of a ship, the DWT reported in Lloyd’s was multiplied by 80% to obtain an estimate of the maximum cargo tons that could be carried; this is consistent with typical voyage estimating factors (22). Average DWT for cargo vessels was computed for all vessels in Lloyd’s registry (including U.S. flag) and separately for U.S. flag vessels. The average DWT for cargo ships in the world fleet is 34 400 DWT; the average DWT for cargo ships in the U.S. flag fleet is 15 500 DWT. However, ships do not typically operate fully loaded with cargo. Many ships, particularly tankers, may carry cargo in one direction and return empty (or with ballast only). Other ships carry cargo both directions but rarely carry their full capacity. Most ships carry cargo loads that average 50-65% capacity; when cargo capacities exceed 70%, it can be an indication that too few ships are available for the route (23). This analysis applied a cargo capacity factor of 50% to vessels operating on ocean routes and a 60% cargo capacity factor to inland rivers and Great Lakes vessels. This is consistent with reported statistics (24). Moreover, barges can be added or removed from a group of as many as 35 barges (25), and therefore barge towboats/pushboats may also transport higher average capacities. The higher capacity factor for inland river cargo transport follows from the understanding that inland vessels are smaller, with shallower drafts and smaller total capacities per vessel. This fact, combined with the large tonnages moved on inland rivers, implies that these vessels are loaded to higher capacities than oceangoing and coastwise transport. VOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Average Speed (in Knots) of U.S. and Foreign Fleet Cargo Ships (Adjusted for Duty Cycle)a engine categoryb 1

2

3A

3B

weighted av

vessel service

U.S.

foreign

U.S.

foreign

U.S.

foreign

U.S.

foreign

U.S.

foreign

Container RoRo Transport weighted av

NA 6 5 5

NA 6 5 5

5 8 7 7

6 8 6 6

NA 10 8 9

13 14 12 12

10 10 8 9

16 14 12 12

10 9 7 9

16 12 11 11

a The average speed of tugs moving barges on inland waters equals 79% of the speed of U.S. cargo vessels; this is taken as the average speed on the inland rivers. b EPA engine categories are used: category 1 engines have cylinder displacements less than 5 L; category 2 engines have cylinder displacements 5 L or greater to 20 L; category 3A engines have cylinder displacements 20 L or greater to 60 L; and category 3B engines have cylinder displacements 60 L or greater.

Speed data reported in Lloyd’s represents the rated design speed of the vessel, similar to the BHP data. Therefore, the E-3 marine duty-cycle load factors were applied to speed, in the same way as for BHP above (14, 15). To be consistent with the 80% adjustment made to the maximum BHP, the maximum speed was adjusted by employing the relationship between horsepower and the cube of the speed (26). This resulted in a maximum speed that is 93% of the rated design speed of the vessel. Average speeds for cargo ships in the U.S. flag fleet and the world fleet are shown in Table 3. From this information, the emissions per ton-mile and emissions per year can be calculated, according to the following equations

ETM ) Ed ÷ (DWT * CCF * V * 24)

(2)

where ETM ) emissions per ton-mile, Ed ) emissions per day, DWT ) average DWT per ship, CCF ) cargo capacity factor (0.5 for oceans, 0.6 for inland rivers and Great Lakes), V ) average speed of vessel across duty cycle (adjusted for max BHP), and 24 ) hours per day to convert ship speed to ship miles per day and

EY ) ETM * TMY

(3)

where EY ) emissions per year and TMY ) ton-miles per year. To distinguish between domestic and foreign trade, port data indicating the amount of domestic and foreign cargo was taken from 1993 and 1997 WCSC data (20, 21). Here, the cargo tons delivered or shipped from the 179 ports reported in the data was used, because waterway link detail does not distinguish between domestic and foreign cargoes. In 1993, 49% of the cargo delivered to ports in U.S. ocean regions (Atlantic, Pacific, and Gulf Coasts) was domestic. In Great Lakes, 80% of the trade was domestic; 70% of trade shipped on the inland rivers was domestic. These ratios can change over time. In 1997, domestic cargo delivered to ports in U.S. ocean regions, Great Lakes, and inland rivers accounted for 45%, 69%, and 74%, respectively. To estimate the number of U.S. ships engaged in foreign trade, this analysis used U.S. Census Bureau data indicating the number of port visits for foreign commerce (i.e., that involved U.S. Customs) in 1993 and 1997 (20, 21). Each year, some 90 000 ship visits involving foreign cargo were reported nationwide. In 1993, approximately 15% of these vessels were registered as U.S. flag ships; in 1997, this had decreased to 12%. For this study, a 15% factor was applied to separate foreign trade between U.S. ships and foreign ships. (This 15% assumption amounted to 68% of the cargo ships in U.S. registered fleet as discussed above.) Our emission estimate assumes CI marine engines propel all vessels. This assumption is generally valid for non-U.S.flag cargo vessels (which are nearly all diesel-propelled), but it is not true for U.S. flag cargo ships. While ships with steam3256

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TABLE 4. Emissions from Maritime Transport of Cargo in U.S. Waterwaysa

U.S. region

NOx PM HC CO (metric (metric (metric (metric tons tons tons tons NOx/year) PM/year) HC/year) CO/year)

Oceangoing (Foreign Cargo) U.S. ship engines 9800 800 300 all U.S. CI engines 8700 400 250 U.S. steam/other 1100 400 50 foreign ship engines 103 800 8400 3000 oceangoing total 113 600 9200 3300 Coastwise (Domestic Cargo) all U.S. CI engines 56 500 2700 1600 U.S. steam/other 7400 2500 300 coastwise total 63 900 5200 1900 Inland Rivers U.S. ship engines 94 500 4800 3400 all U.S. CI engines 83 500 2500 2800 U.S. steam/other 11 000 2300 550 foreign ship engines 32 000 1600 1100 inland rivers total 126 500 6400 4510 Great Lakes U.S. ship engines 11 600 900 330 all U.S. CI engines 10 200 500 280 U.S. steam/other 1300 400 50 foreign ship engines 900 100 30 Great Lakes total 12 500 1000 360 Total U.S. Emissionsb U.S. ship engines 179 800 11 700 5800 all U.S. CI engines 158 900 6000 4900 U.S. steam/other 20 900 5700 900 foreign ship engines 136 700 10 100 4200 grand total 316 600 21 800 10 000 (all ships)

900 800 100 9200 10 100 5000 700 5700 10 400 9200 1200 3500 13 900 1000 900 100 100 1100 18 000 15 900 2100 12 800 30 800

a There may be some differences in totals due to rounding of numbers. b Emissions estimated to a distance of 200 miles from shore; this is approximately the same distance from shore as most of the major shipping lanes.

turbine engines only account for 6% of the U.S. flag fleet, they equal 52% of the U.S. flag cargo-carrying vessels. Marine steam-turbine engines emit significantly less NOx, HC, and CO and about the same PM as CI marine engines (4). This analysis corrected the raw calculations to account for U.S. steam ships by taking 52% of the raw estimate and multiplying it by the ratio of steam-engine-emission factor to dieselengine-emission factor for each pollutant. Last, these emissions were characterized by engine size. World-fleet characteristics for cargo vessels were assumed to apply to foreign-flag vessels transporting cargo in U.S. waters. Emissions estimated for foreign ships were distributed according to the percent of foreign-cargo vessels with engines in each size range. Estimates for emissions from cargo transport are shown by region separated by domestic and foreign vessels and steam and diesel categories, in Table 4.

FIGURE 1. Top 20 states in terms of NOx from Waterborne Commerce. There are four primary sources of uncertainty in the estimates presented here: (1) the use of average emission factors; (2) simplified duty cycle assumptions for main propulsion and exclusion of auxiliary engines; (3) assumptions about the distance that cargo moves on each waterway link; and (4) the use of an “average vessel” (particularly to inland rivers). Uncertainty in our emission factors is due to the small sample of vessels in published data sets. This uncertainty may be significant, and its implications are discussed in previous work (27, 28). To a lesser degree the general application of a single duty cycle in the fuel consumption estimates (discussed above) also contributes to uncertainty in our emissions calculation. We believe that both of these estimates are unbiased and that they can be significantly improved through additional monitoring emissions testing of inland and oceangoing vessels. By not explicitly considering auxiliary engines aboard these vessels, our estimates are likely biased low. Ship auxiliary load varies from vessel to vessel but can increase the total vessel fuelconsumption by as much as 10-20% in addition to main propulsion. Uncertainty in our emissions calculations also arises because cargo movements may only transit partial link distances. For example, push boats (tugs) often deliver materials by barge from one industrial site to another within many river ports. Our analysis of the USACE data shows that 90% of the cargo moves on waterway links that are less than 36.4 miles long. Since most links are defined so that they terminate at ports along the navigable waterway network, we assumed that all waterborne commerce would transit the full length of each segment. This assumption is less likely to produce significant errors on the shortest links and the longest open-ocean links (>1000 miles) but may lead to an overestimate on the fewer but important links in the middle lengths. Any significant errors in these calculations would be caused by major shipping terminals not being nodes on the USACE network; this could point to the need for a redesign of data collection or navigable link definitions.

Last, uncertainty arises in our “average vessel” assumptions applied to entire inland waterway network. Our “average” inland water vessel is larger in both cargo capacity and brake horsepower than the typical vessel in the upper inland-river system but smaller than the typical vessel on the Lower Mississippi. The method applied here probably attributes too few vessels to, and underestimates total emissions in, the upper inland-river system. Bray et al. (29, 30) show that the energy efficiency, in ton-miles/gallon, can vary signficantly across the inland-river system. Their bargecost model efficiency estimates show that barges on the Lower Mississippi have the greatest fuel efficiency, with 917 tonmiles/gallon of fuel, while the lower fuel efficiency occurs on the upper inland rivers. A comparison of energy efficiencies implied by our work with those from Bray et al. (29, 30) show that our method accurately estimates fuel-efficiency on the Lower Mississippi (908 ton-miles/gallon). However, we underestimate the fuel efficiency on the Upper Mississippi, Missouri, and Ohio rivers by 30%, 300%, and 64%, respectively. The differences in emissions resulting from this error may not be as large, because decreased fuel efficiency in the upper river system may be attributed to differences in duty cycles or to up-river and down-river differences in fuel demand. In other words, a manuevering or down-bound towboat in the upper river system may produce less NOx due to lower average loads (and reduced peak temperatures in the cylinder) but may produce more CO or PM as it manuevers. This variability should be investigated and confirmed.

Results and Discussion While this paper presents a 1997 waterborne commerce emissions inventory for NOx, PM, HC, CO, and SO2, the discussion of results focuses on NOx emissions. NOx emissions from marine engines are more important relative to HC and CO emissions, and PM measurements from these engines require additional monitoring to discuss their importance according to size distribution and secondary particle formaVOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Top 20 states in terms of SO2 from Waterborne Commerce

TABLE 5. Waterborne Commerce NOx Emissions in Lower Mississippi River State

TABLE 7. Waterborne Commerce NOx Emissions in Upper Mississippi/Missouri River States waterborne commercea

waterborne commerce state

total NOx emissions (tpy)

river NOx (%)

all sources state totals (tpy)

Louisiana Arkansas Texas Alabama Tennessee Mississippi Oklahoma grand total

41 700 17 800 7210 4130 2340 980 160 74 320

91 100 10 80 99 45 100 84

915 800 286 500 1 692 000 642 400 838 500 338 600 466 400 5 180 200

TABLE 6. Waterborne Commerce NOx Emissions in Ohio River States waterborne

commercea

state

total NOx emissions (tpy)

river NOx (%)

all sources state totals (tpy)

Kentucky Indiana Ohio Pennsylvania West Virginia grand total

10 200 9400 8200 2200 1300 31 300

100 97 66 81 100 89

739 500 917 100 1 236 600 988 300 511 000 4 392 500

a In Indiana and Ohio, nonriver NO emissions are from waterborne x commerce on the Great Lakes; in Pennsylvania nonriver NOx emissions are from both the Great Lakes and Atlantic coast regions.

tion. Waterborne commerce emissions for NOx in the top 20 states are shown in Figure 1, separated by inland river, ocean, and Great Lake emissions. It is clear that waterborne commerce on inland rivers accounts for a significant fraction of statewide marine emissions sources, dominating in 12 out of the top 20 states. Table 4 shows that NOx emissions 3258

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state

total NOx emissions (tpy)

river NOx (%)

all sources state totals (tpy)

Illinoisb Missouri Minnesota Iowa Wisconsin grand total

19 143 3881 2807 1103 93 27 027

95 100 32 100 0 89

1 144 000 519 400 466 500 341 200 479 400 2 950 000

a In these states, nonriver NO emissions are from waterborne x commerce on the Great Lakes. b All of Illinois’ river emissions are assigned to the Upper Mississippi/Missouri Rivers.

from waterborne commerce on the inland rivers (126.5 thousand metric tons NOx) equal 71% of the combined emissions from oceangoing (foreign) and coastwise (domestic) shipping (177.6 thousand metric tons NOx). These comparisons vary by pollutant, according to the differing emissions profiles for different propulsion systems. Additional regional detail for the inland river states is provided in Tables 5-7. Overall statewide emissions are greater in states along the lower Mississippi River, which can accommodate larger cargo volumes. NOx emissions from river-based waterborne commerce in the Lower Mississippi River system (Table 5) are more than twice the NOx emissions in the Ohio River (Table 6) and nearly three times the NOx emissions occurring in the Upper Mississippi and Missouri Rivers (Table 7). The engine-focued, fuel-based methodology developed here applies well to those emissions that are strongly related to combustion conditions (NOx, CO, HC, PM). However, to estimate sulfur dioxide (SO2) emissions from waterborne commerce on a geographically resolved national basis, this methodology is necessary but not sufficient to perform a

FIGURE 3. Relative intensity of NOx emissions in annual tons NOx per mile of waterway (numbers in parentheses indicate the number of waterway links in each range). detailed calculation for SO2. This is because the fuel-sulfur content of marine residual fuels can vary significantly from coast to coast (and from refinery to refinery). For this paper, we have not collected the detailed market data needed to determine fuel-sulfur levels by point-of-sale and then estimated which waterborn commerce movements may have been associated by these fuels. However, we did perform a first-order estimate of SO2, with the specific assumption that inland river vessels use distillate fuel (diesel with 0.5% sulfur) and coastal vessels use high-sulfur residual fuel (3% sulfur). Under these additional assumptions, we estimate the 1997 annual SO2 emissions from waterborne commerce to be about 148 000 tons SO2. Comparing Figures 1 and 2, an important difference between NOx and SO2 from waterborne commerce in the United States is that while river regions account for most of the NOx emissions (65%), coastal regions account for most of the SO2 emissions (72%). However, 18 of the states in the top 20 for NOx are also in the top 20 for SO2sonly the ranking changes. This result has significant implications for regional air pollution models that include emissions from shipping channels in coastal, Great Lakes, and inland river states. A major policy and scientific insight of this work is that emissions from shipping channels between ports cannot be ignored in regional and statewide inventories. This can be demonstrated most clearly by comparing NOx emissions from waterborne commerce on a state-by-state basis with emissions from other source categories (31, 32). If statewide NOx emissions from waterborne commerce were ranked with other source categories, this industry would be the seventh largest source category on either a nationwide basis or

considering only the inland river states. To put this ranking in context, waterborne commerce as a source category would rank higher than regulated source categories such as metals processing, petroleum industries, and chemical manufacturing, but the absolute emissions are much lower than the top two categories (onroad vehicles and electric utility fossil fuel combustion). (See Supporting Information.) Moreover, while NOx emissions from waterborne commerce are generally 1-3% of total statewide NOx inventories in the top 20 states, the concentration of these emissions along waterways is important. (States in which waterborne commerce exceeds 3% include Alaska, Arkansas, and Louisiana, where the contribution is 21%, 6%, and 5%, respectively.) Figure 3 presents a graphical view of 1997 nationwide annual tons NOx emissions per mile of waterway. These units provide an indication of the annual emissions intensity along the nations waterways. These results are based on annual data for waterborne commerce, and seasonal resolution would provide additional insights. In particular, NOx emissions from ships during the summer season may be higher than these annual average estimates would imply. In the upper inland river system, Great Lakes, Northern Pacific Coast, and much of the Gulf Coast, 14-28 tons NOx per mile each year is common. One can compare these to the annual NOx emissions per mile of interstate highway by using an average automobile emission rate of 1 g per mile, reflecting a rough fleet-average factor for vehicles built since the 1980s (9). (This emission rate ignores NOx emissions from light- and heavy-duty truck traffic.) The number of automobiles that would produce an equivalent amount of NOx per mile of highway would range between 30 800 and 76 000 VOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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per day. In the Pittsburgh region, this would be roughly equivalent to annual emissions from automobile traffic coming through the Fort Pitt Tunnel on Interstate Highway 279 (33). This implies thatsat least on an annual basiss waterborne transport can produce as much NOx as a region’s freeways, even in large riverside cities (e.g., St. Louis, Nashville, and New Orleans) where significant automobile commuting occurs.

Acknowledgments The work was supported by NSF grant SBR9521914, EPA Contract 8A-0516-NATX, and by academic funds of the Department of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, PA.

Supporting Information Available Tables and figures of background information and additional comparisons of emissions from waterborne commerce emissions with other sources. In Tables 8-10, we describe the EPA engine size categories for CI marine engines, U.S. and foreign fleet registry data broken down by EPA engine category, and a comparison of emissions factors between Lloyd’s data for ships and AP-42 data for similar engines. In Table 11, we rank waterborne commerce NOx emissions with nationwide and several statewide inventories for other sources, and in Figures 4 and 5, we illustrate their relative magnitudes on a national basis. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) EPA. 40 CFR Parts 89, 92, and 94: Control of Emissions of Air Pollution from New CI Marine Engines at or above 37 kW, Final Rule; Regulation RIN 2060-AI17; Environmental Protection Agency, U.S. Government Printing Office: Washington, DC, November 23, 1999. (2) Davis, S. C. Transportation Energy Data Book, 18th ed.; ORNL6941; U.S. Department of Energy: Oak Ridge, TN, 1998. (3) EPA. Nonroad Engine and Vehicle Emission Study - Report; 21A-2001; United States Environmental Protection Agency, U.S. Government Printing Office: Washington, DC, November 1991. (4) Booz, Allen & Hamilton, Inc. Commercial Marine Vessel Contributions to Emission Inventories: Draft Final Report; EPA Report A-91-24; Booz, Allen & Hamilton, Inc.: Los Angeles, CA, September 12, 1991. (5) Heiken, J. G. Draft Detailed Design for the Marine Module; Environ: Novato, CA, Memorandum, July 8, 1997. (6) National Research Council. Rethinking the Ozone Problem in Urban and Regional Air Pollution: National Academy Press: Washington, DC, 1991. (7) Lloyd’s Maritime Information Services (LMIS). Dataset of Ships 100 GRT or Greater; Lloyd’s Maritime Information Services: Stamford, CT, 1996. (8) 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. (9) EPA. AP-42: Compilation of Air Pollutant Emission Factors; U.S. Environmental Protection Agency: Research Triangle Park, NC, 1997; Vol. I. (10) Harrington, R. L. Marine Engineering; Society of Naval Architects and Marine Engineers: Jersey City, NJ, 1992; p 953.

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Received for review October 13, 1999. Revised manuscript received April 20, 2000. Accepted May 2, 2000. ES9911768