The hotly contested debate over whether gasoline or diesel contributes more to smog continues with new findings from UC Berkeley showing that diesel is worse than gasoline.

In a paper published Oct. 22 in the journal Proceedings of the National Academy of Sciences, researchers examined how both fuels contributed to air pollution and found that diesel contributes more to a primary component of smog.

Drew Gentner, lead author of the paper and a graduate student in the campus department of civil and environmental engineering, said researchers analyzed 52 total gas and diesel samples collected across California, took field measurements of vehicular emissions at the Caldecott Tunnel and looked at results from a 2010 field study conducted by the Nexus of Air Quality and Climate Change that measured contributions to air pollution in places like Bakersfield and Pasadena.

The researchers found that both gasoline and diesel are important for air quality, but depending on a region’s fuel use, diesel can be responsible for 65 to 90 percent of secondary organic aerosol from vehicles.

Secondary organic aerosol is a type of atmospheric particulate matter — tiny pieces of solid or liquid matter suspended in the atmosphere — that contributes to smog. Sources of atmospheric particulate matter can be man-made or natural. Allen Goldstein, head researcher on the study and a professor in the departments of environmental science, policy and management and civil and environmental engineering, said the other important component of smog is ozone.

Robert Harley, a professor in the department of civil and environmental engineering who conducted the field studies in the Caldecott Tunnel, found that the reason diesel might contribute so much to SOA is that it contains a lot of hydrocarbons, which are a component of particulate matter.

Smog has been a major problem around the world not only for its unpleasant aesthetic but also because of its link to health problems. SOAs from diesel exhaust are small particles that can become lodged deep in the lungs upon inhalation, according to Harley. He said that SOA is known to be a human carcinogen.

Organizations like the Bay Area Air Quality Management District work to raise awareness of the harmful effects of atmospheric particulate matter through efforts like the Clean Air Plan of 2010.

According to Ralph Borrmann, a spokesperson for BAAQMD, the Clean Air Plan identified atmospheric particulate matter as being the most detrimental of air pollutants, causing serious health problems in children and seniors. Borrmann said that there are other contributors to atmospheric particulate matter, such as burning wood, and that the organization has taken several measures to reduce the presence of the pollutant in the air.

The results of this study contradict findings of a different study published in March that showed that gasoline emissions contributed more to SOA.

“This debate has been ongoing for several years now, and studies are always reaching different conclusions as to which fuel is more polluting,” Borrmann said. “But all these studies continue to point to the use of diesel as having very serious consequences.”

According to the researchers, California is at the forefront of aggressive measures against smog. Goldstein said that in recent years, the state has implemented new regulations for reducing emissions from diesel trucks.

“Air pollution in California has been a major success story,” Goldstein said. “But we still have pollution episodes where ozone and particulate matters exceed federal regulations. However, for the most part, we have dramatically improved our standards.”

Harley said that across the nation, new diesel trucks have been retrofitted with catalytic particulate filters, but California is the only state so far to have this regulation implemented in older diesel trucks too.

“California is at the forefront on this effort and has been really aggressive in its regulations,” Harley said. “The rest of the nation has been taking a more leisurely approach, but hopefully it will be more active in its cleanup roles now as well.”