Diesels are Beijing’s biggest polluters

CORNELL (US) — The 62-mile, nine-day traffic jam in Beijing’s August heat made international headlines—and an epic amount of air pollution.

The biggest culprit? Diesel-fueled large trucks, says Max Zhang, assistant professor of mechanical and aerospace engineering at Cornell University.

His latest study of the city’s air quality, published online Sept. 21 in the journal Atmospheric Environment, particularly singles out a small number of the most severely polluting vehicles—”heavy emitters”—which contribute to a large share of total emissions.

Since 2007, Zhang has worked on quantifying air quality, using Beijing as his laboratory. His goal is to give governments and other organizations the data and tools necessary to combat the worsening air quality in major cities by accurately measuring it.

For six days in 2009, using a set of fast-response instruments that hang out of a minivan’s window, Zhang followed vehicles while measuring the emissions in their wake, quantified into categories called emission factors. He also tracked routes and license plates using a video camera.

Data on 230 trucks and 57 buses was collected. Particularly, the researchers looked at levels of carbon monoxide, black carbon (soot), and particulate matter with diameters of less than half a micron.

“We provided a very cost effective way to quantify emissions from large on-roads vehicles, complimentary to conventional laboratory emission testing,” Zhang says.

Just 5 percent of all trucks surveyed—those that were overloaded and not well-maintained—were responsible for about half the total soot emissions among trucks and buses sampled.

Surprisingly, the heaviest emitters they observed were rurally based three-wheeled diesel trucks that are largely unregulated by environmental standards.

“There are tens of millions of them, registered and unregistered, in China,” Zhang says. Less surprisingly, vehicles registered in Beijing were less polluting, while vehicles from less-regulated areas tended to spew dirtier air.

In the coming year, Zhang will study more closely another class of key pollutants, nitrogen oxides, working with his Chinese collaborators to try and start changing policies around heavy-emitting trucks and buses by regulating or curbing them from the roads.

“We want to work with local air regulators to find ways to effectively reduce emission factors from the transportation sector,” he says.

“It’s only the first step to identify the heavy emitters, but the real action is how to remove them.”

The research was funded primarily by the U.S. Environmental Protection Agency.

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