Tracing Bioethanol In The Atmosphere

The popularity of ethanol biofuels has grown, as policymakers look for ways to reduce greenhouse gas emissions from gasoline. But the fuels aren’t completely environmentally friendly. Most unburned ethanol degrades to toxic acetaldehyde in the atmosphere. Now, scientists report a method to track atmospheric ethanol using a unique chemical signature that distinguishes between ethanol from car exhaust and natural emissions from plants (Environ. Sci. Technol., DOI: 10.1021/es200982t). Measuring man-made ethanol emissions could improve monitoring of regional air quality, the researchers say.

About 80% of U.S. transportation fuel contains ethanol, according to the Renewable Fuels Association. Most of that fuel is a gasoline-ethanol blend called E10, which contains 10% ethanol by volume. The amount of ethanol fuel in the U.S. will likely grow: The Energy Independence and Security Act called for increased production of renewable fuels from 9 billion gal in 2008 to 36 billion by 2022.

Currently, according to global emission estimates, plants release three times as much ethanol as manmade sources emit. But scientists lack a direct method to pinpoint the source of ethanol plumes.

Fortunately, ethanol’s carbon atoms contain a clue to its origins, says Brian Giebel, a graduate student at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science. The abundance of stable carbon isotopes in a plant’s sugars depends on its photosynthetic process. Corn and sugarcane, the two main crops used to make biofuels, incorporate more 13C into their sugars than most plants do. As a result, the ethanol in biofuels has a higher 13C-to-12C ratio than natural emissions do.

Giebel and colleagues used these stable isotopic signatures to identify the ethanol emission sources in air samples collected in downtown Miami and the Everglades National Park in Florida. They concentrated each sample, separated its constituents using gas chromatography, and completely burned each component. The resulting carbon dioxide then flew through a mass spectrometer and the researchers measured the abundance of each carbon isotope.

Using the 13C-to-12C isotopic ratio, the researchers found that about 75% of ethanol in the urban air came from biofuels. Meanwhile, ethanol from plants dominated the air from the Everglades—even though city pollution and exhaust from a nearby road wafts into the park.

Giebel says isotopic ratios also could help scientists track ethanol plumes as they drift away from urban areas. Ethanol with 13C atoms degrades more slowly than 12C ethanol does. By knowing the isotopic ratio of a city’s ethanol emissions, scientists could estimate how the ratio would change over time and use those calculations to identify the urban plume in the atmosphere.

Andrew Rice, at Portland State University, praises the study, saying it involved “really challenging analytical work” that required high-precision measurements on samples with low concentrations. He adds that the isotopic signature method could serve as an effective tool to help scientists track emission sources, especially as gasoline blends change their makeup.