E10 Emissions
E10 is a low-level fuel blend composed of 10% ethanol, 90% gasoline. The fuel is sold in all 50 states and accounts for about one-third of all U.S. gasoline. It is used to meet air quality standards and increase octane in vehicles.
Tailpipe Emissions
Two literature reviews provide summaries of the numerous (and often conflicting) studies of tailpipe emissions from light-duty vehicles operating on E10. One is Ethanol in Gasoline: Environmental Impacts and Sustainability (Renewable and Sustainable Energy Reviews, Vol. 9, Issue 6, December 2005, pp. 535-555), and the other is Ethanol Fueled Motor Vehicle Emissions: A Literature Review (PDF 231 KB). Download Adobe Reader.
Both reviews concluded that, when compared to gasoline, E10 reduces carbon monoxide (CO), hydrocarbons (HC), particulate matter (PM), and air toxics (except acetaldehyde, which forms directly from the combustion of ethanol). The first paper (Ethanol in Gasoline) explains that E10 has mixed results on formaldehyde emissions and discusses a study that showed formaldehyde emissions to be dependent on ambient temperature.
Of the 12 studies Ethanol in Gasoline analyzed for oxides of nitrogen (NOx) findings, six showed that E10 increases NOx emissions, three show mixed results, and three show similar or lower emissions. However, NOx emissions exhibit a strong dependence on the fuel/air ratio, implying that engine optimization for E10 could decrease emissions. This agrees with a study that finds NOx emissions depend on the engine operating condition more than the ethanol content of gasoline/ethanol blends. It also agrees with a study that found E10 to increase NOx emissions in vehicles manufactured after 1986 but decrease them in older vehicles.
Evaporative Emissions
The Ethanol in Gasoline report also reviewed four studies that found E10 to increase evaporative emissions by 20% to 80%. This is largely because at low-blend levels, ethanol increases the Reid vapor pressure (RVP) of gasoline (making it evaporate more easily) and makes the vapor more permeable through fuel lines. It should be noted that the RVP for E10 can be, and frequently is, adjusted to adhere to the same volatility standards as gasoline. Evaporative and permeative emissions are also highly dependent on temperature, vehicle activity, and vehicle system materials. The majority of these emissions occur when the car is sitting or refueling, so they are not dispersed as widely as tailpipe emissions.
Combined Tailpipe and Evaporative Emissions
When both tailpipe and evaporative emissions are considered, Niven found that E10 showed increases in total hydrocarbon, non-methane organic species, and air toxic emissions, while substantially increasing the ozone forming potential relative to gasoline.
| Pollutant | E10 Tailpipe | E10 Tailpipe Plus Evaporative* |
|---|---|---|
| CO | Reduce | Reduce |
| HC | Reduce | Increase |
| PM | Reduce | NA |
| NOx | Increase (mixed) | Increase |
| 1,3-Butadiene | Reduce | NA |
| Benzene | Reduce | NA |
| Formaldehyde | Mixed** | Increase |
| Acetaldehyde | Increase | Increase |
| Ozone forming potential | NA | Increase |
| NMHC | Reduce | Increase |
*When assessing evaporative emissions, there are caveats that make it less ubiquitous and consistent than tailpipe emissions. The results shown in this table do not reflect these caveats.
**In a follow-up study, formaldehyde emissions were found to change according to temperature.
Lifecycle GHG Emissions and Petroleum Use
In the 2005 report, Updated Energy and Greenhouse Gas Emission Results of Fuel Ethanol (PDF 1.7 MB), Argonne National Laboratory analyzed the greenhouse gas (GHG) emissions reduction from E10 on a per-mile basis and found that corn-based E10 reduces GHG emissions 2% below the emissions of regular gasoline. Download Adobe Reader. E10 also reduces petroleum use by 6.3%. In addition to lifecycle GHG emissions and petroleum use, numerous studies have looked into the lifecycle energy balance of ethanol. These studies are addressed on the Ethanol Lifecycle page.