To their surprise, scientists have found that gasoline vehicles, rather than diesel, and contributing more to a major type of air pollution.
Secondary organic aerosols (SOAs) are tiny particles that make up 40 to 60 percent of the aerosol mass in urban environments, causing human health effects such as heart or respiratory problems.
"The surprising result we found was that it wasn't diesel engines that were contributing the most to the organic aerosols in LA," says Cooperative Institute for Research in Environmental Sciences (CIRES) research scientist Roya Bahreini. "This was contrary to what the scientific community expected."
Researchers had already established that SOAs could be formed from gases released by gasoline engines diesel engines, and natural sources - but hadn't determined which of these sources was the most significant.
Los Angeles proved the perfect location to separate out the three causes, as the surrounding ocean and mountains leave it relatively isolated. The team made three weekday and three weekend flights with the NOAA P3 research aircraft, equipped with instruments designed to measure different aspects of air pollution.
"Each instrument tells a story about one piece of the puzzle," says Bahreini. "Where do the particles come from? How are they different from weekday to weekend, and are the sources of vehicle emissions different from weekday to weekend?"
The measurements confirmed, as expected, that diesel trucks were used less during weekends, while the use of gasoline vehicles remained nearly constant throughout the week. The scientists expected to find that the weekend levels of SOAs would be lower than weekday levels as a result.
However, contrary to expecations, the levels of the SOA particles remained relatively unchanged from their weekday levels, pointing directly to gasoline as the key source.
"The contribution of diesel to SOA is almost negligible," says Bahreini. "Even being conservative, we could deduce from our results that the maximum upper limit of contribution to SOA would be 20 percent."
The findings imply that cutting the emission of organic pollutants from gasoline engines could significantly reduce SOA concentrations on a global scale.