LOS ANGELES In a surprising finding, a new study concludes that the exhaust fumes from gasoline vehicles are a bigger polluter than diesel exhaust when it comes to a specific type of pollution.

Gasoline-powered vehicles contribute more to the production of secondary organic aerosols, or SOAs, say scientists from the Cooperative Institute for Research in Environmental Sciences (CIRES), NOAA's Earth System Research Laboratory and other researchers.

SOAs are tiny particles that are formed in air and make up typically 40-60 percent of the aerosol mass in urban environments. Fine-particle pollution can cause human health effects, such as heart or respiratory problems.

Due to the harmful nature of these particles and the fact they also can impact the climate and reduce visibility, scientists want to understand how they form, explains CIRES research scientist Roya Bahreini, who led the study and also works at NOAA's ESRL.

Researchers had already established that SOAs could be formed from gases released by gasoline engines, diesel engines, and natural sources, but they had not determined which of these sources were the most important, she said.

In Los Angeles, the scientists made three weekday and three weekend flights using the NOAA P3 research aircraft, which were loaded with an arsenal of instruments designed to measure different aspects of air pollution.

Because diesel-powered vehicles, especially trucks, are used less during weekends, while the use of gasoline vehicles remained nearly constant throughout the week, the team expected the weekend levels of SOAs would take a dive from their weekday levels. But that was not what they found.

Instead the levels of the SOA particles remained relatively unchanged from their weekday levels. Because the scientists knew that the only two sources for SOA production in this location were gasoline and diesel fumes, the study's result pointed directly to gasoline as the key source.

"The contribution of diesel to SOAs is almost negligible," Bahreini said. "Even being conservative, we could deduce from our results that the maximum upper limit of contribution to SOAs would be 20 percent."

That leaves gasoline contributing the other 80 percent or more of the SOAs, Bahreini said. The finding was published online in Geophysical Research Letters.

"While diesel engines emit other pollutants, such as soot and nitrogen oxides, for organic aerosol pollution they are not the primary culprit," Bahreini said.

And, of course, today's diesel engines put out far smaller amounts of soot (particulate matter) and NOx, because of increasingly stricter EPA regulations.