Iridium Tracer Study Sheds
Further Light on Self Pollution

OEM's findings contradict those of University of California researchers

Part 2 in a series

By Ryan Gray | Senior Editor

LOS ANGELES - Researchers nationwide disagree over the scope of just how much diesel engine emissions filter back onto a school bus in comparison to other ambient on-road pollutants being spewed forth by other motorists.

Depending on who is asked, the issue of school bus diesel self pollution, the name given to the leaking of harmful emissions directly inside the cabin, is a legitimate threat to the health of students nationwide or is simply hype originating from faulty data.

Following a California Air Resources Board study conducted by a team of University of California scientists that found children on school buses breathe far more concentrated levels of harmful emissions than does the community-at-large from the same emissions at the tailpipe, a school bus OEM and diesel engine manufacturer says data does not prove those pollutants come from the school bus engine.

" There're good ways to do things and not good ways to do things," said Dr. Tom Hesterberg, director of product stewardship at International, such as "flowing" a specific pollution tracer directly into fuel that passes through combustion chamber and assigning a name to the pollutant. "If we find iridium on bus we can very precisely say what exhaust is getting on the bus and what part is PM 2.5."

Comparing the Tests

The 2002 UCLA and UC Riverside experiment used a mass flow controller metered delivery of the tracer gas sulfur hexafluoride (SF6) from a high-concentration cylinder into the school bus exhaust system. Onboard SF6 concentrations were measured at two locations (front and rear) with an electron capture detection analyzer. The scientists found concentrations of black carbon and particulate matter (PM 2.5) levels on six Los Angeles school bus runs were higher in the cabins than outside on the road. The scientists tested two high-emitting Crown Supercoaches with mid-bus mounted engines and four later-model conventional school buses, including one fitted with a particulate matter trap and one CNG bus.

It found that children inhale 34 percent more diesel particulate emissions during the morning commute and 70 percent more in the afternoon than do other motorists.

That study was most recently published in the April 15, 2005 issue of "Environmental Science and Technology." It differed from a study performed a year earlier from a non-peer reviewed study conducted by the Natural Resource Defense Council (NRDC) and the Coalition for Clean Air, which warned that school bus self-pollution could cause cancer in children, according to EPA and federal guidelines.

International Truck & Engine says it has data that shows self-pollution levels are between one and three times lower than what the UC study asserts.

In 2003 the company commissioned a study performed by a team of private researchers and scientists from the University of Maryland and University of West Virginia, with the goal of more accurately defining from where the diesel particulate matter originated, either from the diesel engine or from other contributing factors like emissions from other vehicles or burnt rubber from tires. Like the UC study, International's research stemmed from a school bus route run from South Central Los Angeles to the Westside suburb of Brentwood, a couple of miles west of the UCLA campus. Instead of six buses, International used one bus, an International 1995 conventional International running a DT466 A190 diesel engine with 90K miles on it, in 12 different roadway tests.

"Estimation of Diesel Particulate Matter Concentrations in a School Bus Using a Fuel-Based Tracer: Sensitive and Specific Method for Quantifying Vehicle Contributions," was published last year in the Transportation Research Board's journal "Energy and Environmental Concerns 2004." Linda Mason, a spokesperson for TRB, said that, while neither the TRB nor the National Academy of Sciences endorses the research, the data was subject to a scientific review by committee members and other outside experts.

The UC and International studies both traveled on freeways and arterial roads, made scheduled stops with the engine idling and stopped at railroad crossings, all to approximate actual route operations. International's researchers said that, unlike the UC study, they gauged the amount of on-road pollution ahead of the school bus so they could compare it with onboard readouts.

"We actually considered using SF6 but rejected it because we wanted a tracer that was consistently proportional to the bus' exhaust," said Ireson. "The e mission rate varies dramatically from second to second and minute to minute depending on the speed. You can't adjust the SF6 rate so the ratio between exhaust and fuel is unknown."

Instead of SF6, International's teams used iridium, a naturally occurring corrosion-resistant metal similar to platinum, which binds to diesel particulate matter (DPM). By spiking diesel fuel with iridium and then tracing the element in emissions readouts both inside the bus and at the tailpipe, the researchers concluded that the concentrations of DPM onboard the school bus were about thirty times less per mile than those found in the UC study and that 99.5 percent of particulate matter collected in the bus came from other sources. In addition, they said the iridium tracer was found to be 1,000 times more sensitive than SF6.

"ARB never measured emissions on their vehicles so they didn't know the levels of SF6 in emissions at a constant rate; they really didn't have a relationship," Hesterberg added. "We had a lead vehicle that was gas powered that we were measuring PM, five minutes ahead so we had a good understanding of background results."

He added that PM2.5 measured on the roadway was comparable to that recorded on the school bus.

"Actually, (on road) was a bit higher," Hesterberg said. "Just another piece that suggests that pollution on the bus is from the roadway and not from the bus."

Diverging Viewpoints

Arthur Winer, professor of Environmental Health Sciences in the UCLA School of Public Health who oversaw the 2002 CARB study, invalidated International's research by saying that iridium is known to act as a DPM suppressing agent.

"They're accusing our study of having serious design flaws and misinterpretations. (But) it's well known that iridium suppresses particulate formulation. It's a bogus experiment," he said. "They knew of course if there was no iridium they'd see the same results we did."

He said Dr. David Kittleson, a professor in the University of Minnesota's Department of Mechanical Engineering, questioned lead International researcher Rob Ireson at a scientific workshop in San Diego regarding why the Green Diesel study did not measure interior school bus particulates with and without the iridium tracer.

"Ireson had no answer to this obvious flaw in his methodology," Winer recounted.

But Kittleson said he now feels the International study is more representative of actual DPM readouts on the school bus.

"I don't think the comment I made changed the validity of the iridium tracer but that iridium could reduce soot," said Kittleson, who heads the Center for Diesel Research at the University of Minnesota. "The bottom line for me is, if you have the bus running under representative conditions on a chassis dyno and you measure the output and you drive around and measure the emissions content on the bus, the only uncertainty in that is matching the driving cycle. And I guess you could have a small uncertainty of how iridium hangs up (on the engine surface). I just said it should be considered, even measuring how lube oil suppress soot. The bottom line is it doesn't matter that there's a bit of soot suppression as long as you have the same soot suppression in both cases."

Kittleson added that he had yet to read the UC study.

The initial findings showed that between 75 and 80 percent of the iridium introduced in the diesel fuel disappeared when reading the onboard emissions. Based on those levels, Ireson said, onboard DPM could be reduced by another 400 percent.

Since no current studies exist to show the effect iridium tracers in diesel fuel has on particle formation, emission rates, size distribution, or particle shape, Ireson developed four hypotheses to shed further light on the subject. He culled data to determine the following: if the iridium tracer was removed by reactions with the fuel tank walls or possible contaminants within the fuel supply tank; if the tracer may have significantly altered the formation of particles during combustion, resulting in unrepresentative DPM emission rates for the bus; if iridium atoms embedded in the emitted DPM alter the density of individual particles, resulting in increased settling velocity and; if the iridium atoms embedded in the emitted DPM altered the physical and chemical properties of the primary particles causing them to exhibit different behavior in coalescing to form the larger accumulation mode particles.

"Although the iridium may have altered the size distribution of particles and the relative fractions by number and mass that are in the nucleation mode and accumulation mode, such changes would have occurred within the engine," Ireson wrote in his conclusion, adding that there is no basis for expecting that the DPM to iridium ratio in dynamometer samples would be different from of diesel exhaust particles sampled to ambient air. "We conclude that any effect of the iridium on particle composition, size distribution, and morphology will have no effect on the validity or concentration calculations of (DPM) based on in-bus (iridium) measurements."

Source: School Transportation News, August 2005. All rights reserved.