Contrary to what the scientific community expected, the exhaust fumes from gasoline vehicles are a bigger polluter than diesel exhaust when it comes to a specific type of pollution, according to a new study.

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

SOAs are tiny particles that are formed in air and make up typically 40-60% 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 that they can also impact the climate and can 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 with the NOAA P3 research aircraft, which hosted an arsenal of instruments designed to measure different aspects of air pollution.

Because diesel trucks are used less during weekends, while the use of gasoline vehicles remained nearly constant throughout the week, the team expected that 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 SOA is almost negligible," Bahreini said. "Even being conservative, we could deduce from our results that the maximum upper limit of contribution to SOA would be 20%."

That leaves gasoline contributing the other 80% or more of the SOA, Bahreini said. The finding was published online March 1 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, thanks to strict EPA regulations.

The study was funded by the National Oceanic and Atmospheric Administration's Climate Change and Air Quality Programs, the California Air Resources Board and The National Science Foundation.