Technique measures ethanol isotopes from vehicle, plant emissions

By Kris Bevill | August 04, 2011

Researchers at the University of Miami Rosentiel School of Marine and Atmospheric Science recently developed a technique to measure ethanol isotopes and discovered that the ethanol found in urban air and vehicle exhaust has a distinct isotopic signature compared to ethanol emitted from green plants. The ethanol in urban air and exhaust emissions was found to contain enriched amounts of carbon-13 (13C), which stems from the corn used to produce it, according to Brian Giebel, recent graduate of the university’s doctoral program for marine and atmospheric chemistry and corresponding author for the group’s research paper recently published in the journal Environmental Science & Technology.

“Corn is a very unique plant. It’s considered to be a C4 photosynthesizing plant, meaning that as CO2 is drawn from the atmosphere into the plant it is converted into a four-carbon atom molecule,” he said. “Most other plants in the world are C3 plants; they convert CO2 to three-carbon compounds.” Giebel collected air samples from downtown Miami and Everglades National Park and, based on the concentration of 13C in the samples, concluded that 75 percent of the ethanol in Miami’s air was produced from vehicles, while most of the ethanol found in the Everglades came from plants.

Giebel said the ability to differentiate between sources of ethanol found in the air using his measurement technique could be useful to atmospheric chemists studying pollution. “They like to have ways to identify air,” he said. “If you envision a plume that leaves the middle of a city, it spreads out a little bit and will move along with the air in the atmosphere. One of the ways we like to study things is by knowing where that air is. We could use this type of isotopic fingerprint to understand maybe where that plume is. We could [also] use this isotopic signature to get a better understanding of which sources may be contributing pollution at any given time.”

Much of the ethanol in the atmosphere is quickly converted to acetaldehyde—a toxic volatile gas known to have a tendency to increase ozone production. As it stands currently, plants far outnumber vehicles in the world and therefore plants can be accountable for two to three times more ethanol emissions than manmade sources. But as the amount of ethanol used in vehicles increases, manmade ethanol emissions could eventually exceed the amount produced naturally, Giebel said. This causes some concern when considering the resulting concentrations of acetaldehyde added to the atmosphere, but more extensive research needs to be conducted before it should be considered alarming, he said. “I wouldn’t want to jump the gun and say I’m too concerned, but the possibility exists that we should be potentially concerned,” he said.

Giebel’s isotopic measurements of ethanol also demonstrated that not all of the ethanol blended with gasoline is combusted in vehicle engines. This causes one to question just how much ethanol in an E10 blend, for example, is actually burned by the vehicle’s engine, but Giebel points out that not all gasoline is converted by engines either.

In addition to more potential testing in the U.S., Giebel said he’d also like to see his isotopic measurement technique put to use in Brazil where sugarcane, which is also a C4 plant, is the primary source of ethanol. Previous tests conducted there by other researchers have suggested high amounts of ethanol and acetaldehyde in the air, he said.