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

The team used airborne and ground-based measurements of organic aerosol (OA) in the Los Angeles (LA) Basin, California during May and June 2010 to assess the amount of SOA formed from diesel emissions. Diesel emissions in the LA Basin vary between weekdays and weekends, with 54% lower diesel emissions on weekends. Despite this difference in source contributions, in air masses with similar degrees of photochemical processing, formation of OA is the same on weekends and weekdays, within the measurement uncertainties.

The surprising result we found was that it wasn’t diesel engines that were contributing the most to the organic aerosols in LA. This was contrary to what the scientific community expected.

—CIRES research scientist Roya Bahreini

Due to the harmful nature of SOA and the fact that they can also impact the climate and can reduce visibility, scientists want to understand how they form, Bahreini said. Researchers had already established that SOAs could be formed from gases released by gasoline engines, diesel engines, and natural sources—biogenic agents from plants and trees—but they had not determined which of these sources were the most important, Bahreini said.

Los Angeles proved to be an ideal location. Flanked by an ocean on one side and by mountains to the north and the east, it is, in terms of air circulation, relatively isolated. At this location, 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.

From their measurements, the scientists were able to confirm, as expected, that diesel trucks were used less during weekends, while the use of gasoline vehicles remained nearly constant throughout the week. The team then expected that the weekend levels of SOAs would take a dive from their weekday levels, Bahreini said. 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 exhaust, the study’s result pointed directly to gasoline as the key source.

According to the findings, diesel exhaust’s contribution to SOA is at max 20%; that leaves gasoline contributing the other 80% or more.

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

—Roya Bahrein

If the scientists were to apply their findings from the LA study to the rest of the world, a decrease in the emission of organic species from gasoline engines may significantly reduce SOA concentrations on a global scale as well. This suggests future research aimed at understanding ways to reduce gasoline emissions would be valuable.

Resources

• Bahreini, R., et al. (2012), Gasoline emissions dominate over diesel in formation of secondary organic aerosol mass, Geophys. Res. Lett., doi: 10.1029/2011GL050718, in press