College of Natural Resources, UC Berkeley

Diesel Exhaust Findings Heat Up Air-Pollution Policy Debate

October 18, 2012

By Sarah Yang, UC Berkeley Public Affairs

Are gasoline-fueled cars or large diesel trucks the bigger source of secondary organic aerosol (SOA), a major component of smog? UC Berkeley researchers have stepped into this debate with a new study that says diesel exhaust contributes 15 times more than gas emissions per liter of fuel burned.

Diesel exhaust contributes more to a component of smog than gasoline-fueled cars, according to a new UC Berkeley study.

The study, published today in the Proceedings of the National Academy of Sciences, elucidates the contributions to air pollution from the two types of fuel emissions. The authors estimate that diesel exhaust is responsible for 65-90 percent of a region’s vehicular-derived SOA, depending upon the relative amounts of gasoline and diesel used in the area.

For example, the researchers noted that in the San Francisco Bay Area, about 10 times more gas is used compared with diesel. Diesel_truck_185.jpg

The findings hit on the debate — at times heated and controversial — over which vehicle emissions, diesel vs. gas, contribute more to secondary organic aerosols, a major component of smog and a contributor to human health hazards and poor air quality. Another study earlier this year came to an opposite conclusion, finding that gas-fueled vehicles were the bigger source of secondary organic aerosol.

Knowing which vehicles are contributing more to air pollution is important when establishing and continuing policies to reduce smog in the state. The California Air Resources Board has policies in place to reduce emissions from both gas and diesel vehicles.

"We can now say that while both motor vehicle sources are important for these 'secondary' particles, diesel is responsible for a larger portion, especially in regions such as the San Joaquin Valley with a lot of diesel use," said study principal investigator Allen Goldstein, professor with joint appointments in UC Berkeley's Department of Environmental Science, Policy and Management and the Department of Civil and Environmental Engineering. SOA contributes to respiratory problems and poor air quality, so pinpointing the major sources of the pollutant is important in evaluating current and future policies to reduce smog in the state.

The new findings contradict previous research that put the blame on gasoline-fueled vehicles as the predominant source of precursors that form secondary organic aerosol.

“We can now say that, while both motor vehicle sources are important for these ‘secondary’ particles, diesel is responsible for a larger portion, especially in regions such as the San Joaquin Valley with a lot of diesel use,” said study principal investigator and professor Allen Goldstein, who has joint appointments in the Department of Environmental Science, Policy and Management and the Department of Civil and Environmental Engineering.

For this study, Goldstein teamed up with Robert Harley, professor of civil and environmental engineering, and an expert on vehicle emissions and air quality.

The findings stand out because the researchers were able to tease out the chemical composition of fuel emissions.

“The data from our study contains the most comprehensive chemical detail to date on diesel and gasoline emissions,” said study lead author Drew Gentner, a recent UC Berkeley Ph.D. graduate in civil and environmental engineering. “This presents many opportunities to assess the chemistry of these compounds in the atmosphere and the impacts of these sources. We expect that these findings will help policymakers improve air pollution control measures in the state, and also other parts of the world.”

The California Air Resources Board and the U.S. Environmental Protection Agency helped support this research.

-Sarah Yang, UC Berkeley Public Affairs

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