As a fuel additive, ethanol changes the emissions profile of gasoline, creating a cleaner, safer motor fuel. Real-world evidence demonstrates that ethanol blending reduces municipal smog levels and cuts down on atmospheric concentrations of harmful toxins. As new technology reduces overall emissions from the US car fleet, the benefits of ethanol will become even clearer.
Ethanol is an oxygenate - a fuel additive that raises the octane level of gasoline, producing a motor fuel that burns more cleanly and completely and cutting down on emissions of carbon monoxide and other air pollutants. CO, in particular, is a major contributor to ground-level ozone (smog) formation,1 and in 1990, the federal Clean Air Act was amended to mandate that certain areas not meeting air quality standards blend oxygenates such as ethanol with gasoline in order to reduce wintertime levels of atmospheric CO. Ethanol has a proven success record on this front - according to the California Air Resources Board, blending ethanol with gasoline at a rate of 5.7% by volume reduces CO emissions by about 7.8g/vehicle/day.2 And unlike MTBE, an additive used as an oxygenate until the late 1990s when evidence emerged linking it to surface and groundwater pollution, ethanol decomposes rapidly in water and soil.3
As an oxygenate, ethanol also displaces high-octane aromatics in conventional gasoline, resulting in a reduction in soot and particulate emissions. The Renewable Fuels Association reports that ethanol can reduce tailpipe soot and particulate emissions by as much as 50% overall, with the greatest reductions being achieved in the highest-emitting vehicles.4 Given that the American Lung Association links these emissions to cancer, asthma, and heart attacks, ethanol blending can play an important role in improving public health.5
Similarly, ethanol replaces many of the toxic components of gasoline. Ethanol use decreases emissions of benzene, a hydrocarbon classified by the EPA as a known human carcinogen. Benzene accounts for about 70% of the total toxic emissions from vehicles running on conventional gasoline.6 According to the EPA's hazard summary, exposure to benzene can lead to blood disorders, including anemia, and higher instances of leukemia, as well as short-term impacts such as headache and respiratory irritation.7 Studies have indicated that blending ethanol with gasoline at a 10% rate can reduce benzene emissions by as much as 25%.8 Replacing gasoline with ethanol also reduces emissions of butadiene, a probable human carcinogen,9 and formaldehyde, a toxic air contaminant.10
Replacing conventional gasoline with ethanol blends also reduces greenhouse gas emissions by up to 40% with current production technology, and by about 86% once production of cellulosic ethanol becomes viable.11 Given the proven link between rising temperatures in increased smog levels, reducing greenhouse gasses is an important step to improving air quality.12
Because blending ethanol with gasoline may result in increased emissions of nitrous oxides (NOx) and some hydrocarbons and volatile organic compounds (VOCs), some critics have proposed a link between ethanol-blended fuel and increased levels of ground-level ozone (smog). Higher NOx emissions are attributable to the higher oxygen content of ethanol blends relative to conventional gasoline, while VOC emissions are generally a result of the higher rate at which ethanol-blended gasoline permeates the soft components of a vehicle's fuel system.13 Both NOx and VOCs may contribute to ground-level ozone. However, evidence linking NOx and VOC emissions directly to smog formation is inconclusive.14 Smog formation depends heavily on local weather conditions and atmospheric composition, making it difficult to establish a clear connection between the emissions profile of ethanol and deterioration of air quality.15
Additionally, modern vehicles are equipped with technology to reduce NOx emissions by automatically adjusting the oxygen content of fuels as they are burned.16 And the hydrocarbons emitted in ethanol combustion, such as acetaldehyde, are far less toxic than substances such as benzene.17 In fact, the EPA terms studies linking acetaldehyde to human health risks as "inadequate."18 The estimated increase in these VOC emissions from burning ethanol, about 1.1g/vehicle/day, is offset by a reduction in CO emissions that is seven times greater in magnitude,19 and by substantial reductions in benzene, butadiene, and formaldehyde emissions.
Studies that depict ethanol as having a negative impact on air quality are based on computer modeling of hypothetical scenarios, and are consistently out of sync with on-the-ground results of ethanol blending mandates in states and cities across the US. Recently, a study by Stanford University' Mark Jacobson sparked debate about ethanol's air quality impact. Jacobson claims that "Due to [ethanol's] ozone effects, future E85 may be a greater overall public health risk than gasoline."20 However, Jacobson's study has been criticized by the Renewable Fuels Association and the Natural Resources Defense Council for making unrealistic assumptions and omitting crucial factors that affect real-world outcomes. For example, Jacobson assumes that all vehicles will run on ethanol in 2020, failing to account for the fact that the oldest, highest-emitting vehicles won't be capable of accepting an higher-concentration ethanol blend.21 Additionally, he focuses on the few potential emissions increases from ethanol and does not acknowledge the substantial air quality benefits, including reductions in emissions of CO, particulate matter, and many toxins.
Most importantly, Jacobson's concerns simply aren't reflected in the real-world data. For example, ozone exceedance days dropped 16% in Wisconsin after adoption of a 10% ethanol blend. California's South Coast Air Management District, one of the most polluted areas in the country, saw a 22% reduction in ozone levels after statewide introduction of E6 (a 6% ethanol blend) in 2004. Impacts have been even more dramatic in New York and Connecticut, which reduced their ozone exceedance days 68% and 48% respectively by switching from conventional gasoline to an E10 blend.22
While both critics and proponents have made much of the air quality impact of ethanol, the outcome of switching from conventional gasoline to alternative fuels may in fact be relatively minimal, considering the range of other forces that affect vehicular emissions. Chief among those forces are improvements in the overall emissions performance of the US car fleet. According to the Environmental Protection Agency, emissions of carbon monoxide, hydrocarbons, and particulate matter from on-road vehicles have all fallen by at least one-half between 1970, when emissions standards were first enacted, and are projected to be at least 20 times less than 1970 levels by 2020.23 As vehicular emissions contribute a smaller and smaller share to overall air pollution, the question of whether ethanol reduces or increases emissions becomes more and more irrelevant.
Weather and climate conditions can also alter the equation when it comes to air quality and automotive fuel. For example, evaporative emissions of any fuel increase in warmer temperatures, compounding the effect of a slight increase in base evaporative emissions with ethanol fuel. However, evaporation of ethanol-based fuel releases fewer ozone-forming hydrocarbons - an important consideration on hot days when ozone forms more rapidly.24 In cold temperatures, when vehicles tend to emit more CO, and concentrations of CO near ground level increase, the pollution-reduction effect of ethanol blending is all the more important.25 Fluctuations in weather and temperature render the impact of any one fuel source on air quality relatively ambiguous, and this effect illustrates the importance of continuing to develop emissions-control technology that will reduce pollution under any conditions.
Vehicles operating on ethanol-based fuel have to meet the same air quality standards as conventional gasoline vehicles, so ethanol-fueled vehicles will have to keep up with the steady overall improvements in the automotive technology. However, evidence suggests that ethanol blends will in fact remain a step ahead of gasoline when it comes to air quality. Newer vehicles, equipped with modern pollution-control technologies, will minimize evaporative emissions and maximize the air quality benefits of ethanol.26
1 Gary Whitten, "Air Quality and Ethanol in Gasoline," Smog Reyes, December 2004.
2 California Air Resources Board, "The Ozone Impact of Permeation VOC Relative to Carbon Monoxide," March 2006.
4 Renewable Fuels Association, "Ethanol Facts."
5 Brett Husley and Brook Coleman, "Clearing the Air with Ethanol," Better Environmental Solutions and Renewable Energy Action Project, March 2006.
6 Gary Whitten, "Air Quality and Ethanol in Gasoline," December 2004.
7 Environmental Protection Agency, "Benzene Hazard Summary," revised January 2000.
8 Whitten, "Air Quality and Ethanol in Gasoline."
9 Environmental Protection Agency, "Benzene Hazard Summary," revised January 2000.
10 Renewable Fuels Association, "Ethanol Facts - Environment."
11 Renewable Fuels Association, "Talking Ethanol."
12 Natural Resources Defense Council, "Statement on New Study."
13 Natural Resource Defense Council, "Unlocking the Promise of Ethanol," February 2006.
14 R. Brooke Coleman, "A Northeast Regional Biofuels Action Plan," Renewable Energy Action Project, March 2007.
15 Brett Husley and Brook Coleman, "Clearing the Air with Ethanol," Better Environmental Solutions and Renewable Energy Action Project, March 2006.
16 Natural Resources Defense Council, "Unlocking the Promise of Ethanol."
17 Renewable Fuels Association, "Talking Ethanol," April 2007.
18 Environmental Protection Agency, "Acetaldehyde Hazard Summary," revised January 2000.
19 California Air Resources Board, "The Ozone Impact of Permeation VOC Relative to Carbon Monoxide," March 2006.
20 Mark Jacobson, "Effects of Ethanol (E85) versus Gasoline Vehicles on Cancer and Mortality in the United States," Environmental Science and Technology, April 18, 2007.
21 Natural Resources Defense Council, "Statement on New Study of Ethanol (E85) Impact on Air Quality," April 26, 2007.
22 Husley and Coleman, "Clearing the Air with Ethanol"
26 Natural Resources Defense Council, "Unlocking the Promise of Ethanol," February 2006.