A new study says diesel exhaust contributes 15 times more of a smog-causing chemical than gasoline engines, per liter of fuel burned.

The University of California-Berkely study looked at secondary organic aerosol (SOA), a major component of smog.

In the study, published this week in the Proceedings of the National Academy of Sciences, 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 gasoline is used compared with diesel.

The study was conducted in the summer of 2010 and was based on gas-chromatography and mass spectrometry analysis of two bores of the Caldecott tunnel in Oakland, California, one of which does not allow large trucks, according to published reports.

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, say researchers.

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.

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, 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.