Science News - April 8, 2004

Study raises questions about diesel emissions

Results from a first-of-its-kind mobile laboratory reveal that the composition of particulate emissions from heavy-duty diesel trucks depends strongly on how the vehicle is operated. Experts speculate that the findings, published recently on ES&T’s Research ASAP website, may provoke changes in current models and human health risk assessments and even in how we plan our neighborhoods.

Traditionally, emissions from heavy-duty diesel vehicles are collected and measured in stationary labs. Other groups have tried using “chase vehicles” to quantify truck emissions under real-world conditions. Researchers at the University of California, Riverside, managed to cram into a 53-foot trailer all of the equipment required to comply with the California Air Resources Board’s (CARB) latest testing protocol for diesel particulate emissions. A double-insulated metal “snorkel” directs the exhaust of the diesel truck cabs—which are known as tractors—into the mobile lab trailer where automated equipment collects data, leaving little doubt about the origin of the emissions (Environ. Sci. Technol. 2004, 7, 2182–2189). David Cocker, the paper’s corresponding author, says that his group spent three years working out how the sensitive equipment dealt with literal bumps in the road. A professional driver operated 11 different tractors through four modes: idle, creep (congested traffic), transient (lower speeds), and cruise (highway travel).

Steve Walag Researchers at the University of California, Riverside, packaged equipment to collect emissions from a diesel tractor into a 53-foot trailer. The trailer is connected to a tractor being tested.

Using their specially designed trailer, the Riverside researchers made the first reported on-road measurements of elemental carbon (EC) and organic carbon (OC) fractions of particulate emissions from heavy-duty diesel truck engines. They found that trucks emit 8.1 times more OC and 1.9 times more EC per mile sitting in traffic than cruising at higher speeds on the highway.

Aniket Sawant Inside the trailer: Lead author Sandip Shah is working on the secondary dilution tunnel system, and the student in the background is working with the data acquisition system.

Although the OC fraction contains most of the mutagenic and carcinogenic chemicals in the air, scientists have traditionally used EC as a marker for human exposure to diesel exhaust. Modelers and risk assessors also assumed that the ratio of EC to OC remains constant. That assumption “may be flawed,” says Joseph Norbeck, director of the Center for Environmental Research and Technology at Riverside and a paper coauthor. We found that “the ratio changes dramatically, depending on the operation of the vehicle,” he says.

The Riverside researchers say that their new data’s implications for health policy could be dramatic. For example, the CARB’s assessment for cancer risk from diesel particulate matter (PM) assumes that the amounts of OC and EC are identical when a truck is driving on a highway and when it is idling. Therefore, “If one assumes that the organic fraction dominates the carcinogenic and toxic effects of the diesel PM, the human health risk increases almost an order of magnitude in highly congested traffic as compared with free-flowing traffic,” the authors say in the paper.

Voluntary and regulatory standards as part of the Diesel Risk Reduction Program are already in place to reduce emissions from mobile and stationary diesel engines within the next decade, but “if you could control the traffic flow of heavy-duty diesel vehicles to times that are free-flow or less congested, you could reduce the organic emissions rates by 80–90%,” says Cocker.

The University of Denver’s Donald Stedman, one of the on-road testing pioneers, commends the study. Although he is curious whether these results represent the whole fleet and all its emissions, he adds that these kinds of results “will help us to write intelligent regulations.”

Or help us create communities more intelligently, says Timo Ålander, a research scientist who investigates diesel emission studies at the University of Kuopio in Finland. “This information should be exploited in traffic planning, including how the roads and residential areas are situated.” For example, warehouses that supply trucks should be near highways and away from residential areas.

The Riverside researchers plan to evaluate train and jet engines next. —RACHEL PETKEWICH