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Eligibility of Freight Projects and Diesel Engine Retrofit Programs

Clean Air and Transportation Diesel Engine Retrofit January 2002

Issue: Emissions generated by diesel vehicles are a considerable challenge for both transportation and air quality planners around the country. How can EPA's diesel engine retrofit program contribute to mobile source emissions reductions?

Background: Recent estimates indicate that diesel exhaust from heavy-duty vehicles contributes 63 percent of the Nation's on-road particulate matter (PM) inventory. These same diesels generate about 42 percent of on-road nitrogen oxides (NOx), or 13 percent of total NOx. Even more critical, heavy-duty vehicles off the highways, like construction or farm equipment, are estimated to generate 22 percent of the Nation's NOx inventory-a total that also includes point and area sources. EPA reports that diesel engines are among the largest sources of PM, other than natural sources such as wildfires.

Truck emissions are a necessary element of the transportation demand forecasting and emissions analysis conducted by the Nation's major metropolitan areas. Controlling and reducing the emissions of these vehicles in metropolitan air quality nonattainment areas has been one of the more formidable clean air challenges since implementation of the 1990 amendments of the Clean Air Act. The transportation conformity provisions of the Act require areas to meet the mobile source emissions budgets included in State Implementation Plans (SIPs).

As the Nation's passenger vehicle fleet has become much cleaner, heavy-duty diesel vehicles have not kept pace. From 1970-1999, NOx emissions from passenger vehicles fell more than 30 percent. Over that same period, NOx emissions from heavy-duty diesel trucks rose more than 115 percent. Total on-road emissions of this ozone precursor increased over 15 percent during the 30-year period.

The emissions performance of off-highway mobile sources has been even more troubling. For volatile organic compounds (VOC), on-road emissions were down 59 percent from 1970-1999, but non-road engines-many of which are diesel-registered an increase of 72 percent over the period. While on-road emissions of NOx increased a modest 16 percent during this same period, the off-road inventory rose 186 percent. These increases pose significant challenges for areas to reach their emissions reduction targets. In addition, much of this fleet is at least indirectly linked to transportation programs, e.g. highway construction vehicles, port handling equipment, and other off-road mobile sources.

To address the continuing problem with diesel truck emissions, EPA has embarked on a number of initiatives in recent years. Aside from revised heavy-duty vehicle standards targeted at future fleets, the agency also has developed a program aimed at emissions reductions for in-use vehicles, both on- and off-highway. The diesel retrofit program was unveiled in March 2000, and has been taking hold in a number of States. There are more than 20 active diesel retrofit programs around the country with several more advancing through the proposal stage.


The growth in interstate commerce has spawned a corresponding growth in diesel truck traffic. Much of this increase in diesel vehicle miles traveled has been concentrated around large metropolitan areas that function as port cities or other transportation hubs. Many of these cities and regions are designated nonattainment for ozone and are bound by the provisions of the transportation conformity rule. As new measures and strategies are implemented to help meet the motor vehicle emissions budgets in SIPs, expanded application of the diesel engine retrofit program could be an opportunity in many areas.

New tailpipe standards are looming for heavy-duty diesel vehicles in addition to tighter restrictions on fuel sulfur. Both these developments on the regulatory front stand to drive emissions down, especially NOx and fine PM. The positive outlook notwithstanding, relatively little is available for States and local governments to counter diesel emissions today. While initiatives such as truck inspection and maintenance (I/M) programs and anti-idling technologies address existing emission inventories, these and other similar programs are conceptual or in the early stages of implementation and have had to contend with technological, financial, and political barriers of their own.

The new standards are expected to generate considerable long-term progress toward attainment. The NOx from diesel trucks are projected to fall 88 percent between 2007 and 2030, while PM is expected to drop 64 percent over the same period. However, the long service lives of commercial trucks and the durability of engines between rebuilds places most of the value of the new diesel standards in the outyears. The emissions reduction benefits of the new tailpipe standards will not be realized until a sizable portion of the truck fleet has turned over-well beyond the 2007 implementation date, as some estimates point to full turnover in approximately 25 years.

Diesel retrofit technology is ready and available. It is supported by EPA and carries established, calculable emissions benefits for use in SIP and conformity analyses. While emissions reduction credit varies with each local program, some estimates point to substantial clean air gains. In support of Boston's Big Dig project, a diesel retrofit program is expected to net declines in fine PM of 200 tons over 5 years and reduce toxic pollutants by about 70 percent.

Several types of retrofit emission control technologies are available with varying levels of demonstrated effectiveness at reducing PM, hydrocarbons (HC), carbon monoxide (CO), and air toxics. The commercially available technology that offers the potential capability to reduce particulates to near zero is the diesel particulate filter. These devices have demonstrated diesel PM reductions of over 85 percent depending on the operating cycle. In addition to the filters, advanced oxidation catalysts also show great promise in reducing emissions of PM, HC, and CO. This retrofit basically consists of a specially designed replacement muffler.

Closely related to the more targeted retrofit programs are diesel engine replacement initiatives; the two are sometimes cast as a complementing package. Cost-effectiveness of the engine replacement programs has been rated by California's Carl Moyer program at just under $1,200 per ton of NOx reduced for locomotives and just over $10,000 per ton of NOx reduced for diesel school buses. As with EPA's retrofit efforts, the value of such programs and technologies lies in the near real-time application of benefits.

Off-road diesel vehicles contribute a great deal to the emissions burden of most nonattainment areas. While the transportation conformity process may have only an indirect role in controlling non-highway emissions, new avenues of support for further application of retrofit technology to the off-road fleet may be a valuable tool in the drive to cleaner air. Such contributions notwithstanding, procedural difficulties in crediting these emissions reductions in the conformity process will prompt decision-makers to carefully consider their transportation investments.

Support for diesel retrofit in construction vehicles has been demonstrated by a number of State departments of transportation. The gains cited in the Boston example above are derived largely from off-road construction vehicles. While the emissions of off-road construction vehicles are not part of the inventory accounted for under the conformity process, retrofits of this equipment can reduce emissions effectively.

The technology applied in the Big Dig retrofit project included PM traps and oxidization catalyst devices. Program managers in the Boston area estimate a cost of between $1,000 and $3,000 for the diesel modifications installed on each retrofitted construction vehicle. The Massachusetts Department of Environmental Protection estimates that cost-effectiveness for the Big Dig retrofit as low as $1,000 for each ton of emissions removed.

Taking credit in the conformity process for such off-road investments requires revision to SIPs, possibly under EPA's Economic Incentive Program. The FHWA is not aware of any cases where this mechanism has been implemented. Discussions with EPA of a more streamlined process have begun.

The benefits of diesel retrofit technology in reducing PM, ozone precursors, and other pollutants are well documented. New engine technology, advances in fuel formulas, and other enhancements historically have been successful in reducing these more familiar emissions. A number of other pollutants, however, are advancing to the forefront. New technologies and approaches to clean air attainment will need to confront the growing emphasis on fine PM, mobile source air toxics, and other pollutants.

The EPA is obligated to regulate 188 hazardous air pollutants as outlined in the Clean Air Act. Of this total, 21 have been identified as mobile source air toxics or MSATs. A number of highway projects around the country have been targeted by environmental and health advocacy groups over concerns with air toxics mitigation. Although the FHWA is opening a dialogue with EPA and other concerned groups, the tools and strategies for dealing with MSATs are as yet undeveloped.

Along with NOx and PM reductions, diesel retrofit programs have been known to reduce VOC and associated toxic pollutants. As diesel emissions have been targeted as a principal producer of toxic gases and particles, the retrofit program has been credited with lowering air toxics, including benzene, formaldehyde, and 1-3 butadiene, by as much as 70 percent from conventional engine technology.

New national ambient air quality standards (NAAQS) for ozone and PM are about to be implemented around the Nation. These new NAAQS will prompt the expansion of existing nonattainment areas or the increase in numbers of nonattainment counties in many regions. Areas that face clean air challenges for the first time will need a wide variety of strategies at their disposal, since the unique nature of regional pollutants usually demands unique solutions.

Efficient, effective emission reduction measures will be difficult to identify for many of the new areas subject to the more stringent NAAQS. Early projections indicate that some of these new areas could be designated nonattainment for both the 8-hour ozone and the fine PM standard. With considerable emphasis placed on early attainment, especially for areas that fall on the margin, measures that rely on behavioral change and sweeping changes in regional travel patterns and priority may not be appropriate. Similar to fuel reformulation and I/M, near immediate benefits will be realized through the retrofit program.

Updated: 7/6/2011
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