May 6, 2015
Ethanol fuel refineries could be releasing much larger amounts of ozone-forming compounds into the atmosphere than current assessments suggest, according to a new study based on a field campaign that included a NASA sensor.
Airborne measurements made downwind from an ethanol fuel refinery in Decatur, Illinois, in 2013 found ethanol emissions 30 times higher than government estimates. The measurements also showed emissions of all volatile organic compounds (VOCs), which include ethanol, were five times higher than government numbers, which estimate emissions based on manufacturing information. VOCs and nitrogen oxides react with sunlight to form ground-level ozone, the main component of smog.
The measurements included those made by the In Situ Airborne Formaldehyde instrument, which was built at NASA's Goddard Space Flight Center, Greenbelt, Maryland. The instrument measures formaldehyde, which forms in the atmosphere when VOCs are present and is a strong indicator of VOC emissions, said Goddard's Thomas Hanisco, principal investigator of the ISAF instrument and a co-author of the new study.
If emissions at the more than 200 fuel other ethanol refineries in the U.S. are also being underestimated, these plants could be a higher source of VOC emissions than currently thought, according to the new findings accepted for publication in the Journal of Geophysical Research: Atmospheres, a publication of the American Geophysical Union.
Ethanol, a renewable transportation fuel made from corn, constitutes approximately 10 percent of the fuel used in gasoline vehicles in the U.S. The renewable fuel standard mandating use of ethanol and other renewable fuels aims to reduce greenhouse gas emissions and petroleum imports, while encouraging development and expansion of the U.S. renewable fuels sector, according to the U.S. Environmental Protection Agency.
The new study is one of the first and most detailed investigations of emissions from ethanol fuel refining, according to its lead author Joost de Gouw, a scientist at the Cooperative Institute for Research in Environmental Science at the University of Colorado – Boulder and NOAA's Earth System Research Laboratory in Boulder, Colorado. Information about the refining process is one piece of examining the entire cycle of ethanol fuel emissions, from growing the corn used to make the fuel to the effect of emissions on urban air quality, he said.
"Over the past decade, because of the renewable fuel mandate, we have added 10 percent of ethanol to all the gasoline that is sold in the U.S. and so the question is: what does that do to the environment," de Gouw said. "That is a very complicated question and it has many different aspects. One of the aspects is the air-quality implications and, to get at them, we have to know what are the emissions associated with producing ethanol and using ethanol. That is where this study fits in."
To make the measurements they report, de Gouw and his colleagues flew an airplane downwind of the Archer Daniels Midland ethanol refinery, the third largest producer of fuel ethanol in the U.S. and took air-quality readings at three different distances from the plant. The researchers used those to calculate emissions of various gases, including VOCs, nitrogen oxides and sulfur dioxide.
They then compared their findings with government emissions estimates from 2011. Emissions of sulfur dioxide and nitrogen oxides – compounds generated by the coal-burning plant – were in-line with government estimates, but emissions of VOCs, including ethanol, were higher than government estimates. De Gouw said the VOC emissions are likely generated by the refining process, not the coal-burning that powers it.
The measurements of formaldehyde were approximately 10 times higher than expected, according to the study. Formaldehyde has a lifetime of about two hours in the atmosphere, Hanisco said. So while some level of formaldehyde is typically present at background levels – it is also emitted by trees, for instance – large spikes indicate a separate source.
"When it's elevated above those background levels," Hanisco said, "you know an additional source is near."
The researchers also used government estimates and ethanol production numbers from the Renewable Fuels Association to analyze emissions from all fuel ethanol refineries in the U.S. and compare those to emissions from motor vehicles. While several studies in recent years have attempted to quantify the air quality impact of ethanol refining and use, it remains a difficult question to answer precisely.
Prevailing estimates had indicated that refining ethanol fuel and burning it in cars and trucks generate equivalent amount of VOCs, including ethanol. But, the new emissions measurements from the Decatur plant show that ethanol emissions from production of one kilogram of ethanol at the refinery are 30 times higher than what comes out of a vehicle burning the same amount of ethanol, de Gouw noted. If the Decatur refinery is like most other refineries in the U.S., he added, "the higher emissions of ethanol and VOCs that we calculated from our data would make the refining process a larger source than burning the fuel in your car."
"Obviously, this was just one refinery that we looked at, so we'd like to do more and see if these findings are more universal or if this plant was just exceptional," de Gouw added.
The new study points to the need for more measurements of emissions coming from ethanol fuel refineries, said Dylan Millet, an associate professor of atmospheric chemistry at the University of Minnesota in St. Paul. He was not involved with the new research. Additional observational data will help scientists better understand the emissions and their impact on air quality, he said.
"If we are going to accurately assess the air-quality implications of our fuel choices, then these are important emissions to know," Millet said.
Image: Air-quality readings over the Midwest were made from an aircraft in 2013 at three different distances downwind from an ethanol refining plant in Decatur, Illinois. The measurements were used to calculate emissions of various gases, including VOCs, nitrogen oxides and sulfur dioxide.