Researchers report on potential long-range atmospheric emissions impacts of increased ethanol fuel use in North America
A study by a team of researchers from the University of Minnesota; University of Colorado, Boulder; National Center for Atmospheric Research; and NASA Ames Research Center have used an ensemble of aircraft measurements combined with the GEOS-Chem chemical transport model to to gauge potential long-range emissions impacts of increased ethanol fuel use in North America. Their paper is published in the ACS journal Environmental Science & Technology.
Ethanol is emitted to the atmosphere by both natural and anthropogenic processes; examples of the latter include industrial processes, biomass combustion and use as a biofuel mixed with gasoline. (Terrestrial plants are thought to be the dominant global source of atmospheric ethanol.) In the atmosphere, ethanol is a precursor of acetaldehyde (CH3CHO) and peroxyacetyl nitrate (PAN).
Acetaldehyde is classified as a hazardous air pollutants (HAP) regulated under Title III of the Clean Air Act Amendments. PAN, which is created by the action of the ultraviolet component of sunlight on hydrocarbons and NOx in the air, is an ingredient of photochemical smog.
...changing ethanol emissions have the potential to affect urban air pollution and associated long-range transport. However, the significance of this effect will depend on the size of the emission change compared to that of the existing source fluxes, which are poorly known. Here we use a global 3D chemical transport model (GEOS-Chem CTM) applied to an ensemble of airborne observations to derive new constraints on natural and anthropogenic ethanol sources in North America. We then employ this revised source estimate as a baseline for assessing some potential large-scale impacts of changing ethanol fuel use in the US.—Millet et al.
The team found that current ethanol emissions are underestimated by 50% in Western North America, and overestimated by a factor of 2 in the East. They estimated year-2005 North American ethanol emissions of 670 GgC/y, with 440 GgC/y from the continental US.
The researchers then applied these estimates to analyzing two scenarios for increased ethanol fuel use in the US:
A complete transition from gasoline to E85 (85% ethanol) fuel. They used this scenario as a diagnostic to evaluate the potential for E85 fuel use to affect long-range atmospheric chemistry relative to a similar vehicle fleet running mainly on gasoline.
Meeting the full requirements of the 36 billion gallons per year of biofuel required by the US Energy Independence and Security Act (EISA) of 2007 entirely with ethanol fuel. To meet the EISA mandate in the model scenario, they assumed all gasoline is blended with 10% ethanol, and then increased E85 use to the point where total US ethanol consumption reached 36 billion gallons.
This resulted in consumption of 126 billion gallons per year of E10 (at 97% the energy content of gasoline) and 28 billion gallons/year of E85 (at 71% the energy content of gasoline).
They considered only changes to tailpipe emissions in both scenarios; future work is planned to quantify full life-cycle emission changes for various biofuel production strategies. Among their findings:
North American ethanol emissions increase by 1040 GgC/y (2.6×) in the All-E85 scenario, and by 175 GgC/y (1.3×) in the EISA scenario. Emissions from the continental US increase by 3.4× and 1.4× for the two scenarios.
The relative perturbation is weakest in summer because of large natural emissions of ethanol (and other VOCs). In the All-E85 scenario, wintertime North American ethanol emissions increase by 600%, with ethanol alone then making up 6.7% of the total continental VOC source at this time of year. For the same scenario during summer, ethanol emissions increase by 70%, and account for only 1.7% of the total seasonal VOC source.
For both scenarios, increased ethanol emissions lead to higher atmospheric acetaldehyde concentrations—up to 14% during winter and by up to 5% during summer for the All-E85 scenario (but by only 2% and 1% for the EISA scenario). This acetaldehyde increase leads to a larger supply of peroxy acetyl (CH3C(O)OO) radicals and in turn a shift in reactive nitrogen partitioning, with PAN making up a larger fraction of total NOy.
PAN increases up to 6% in the All-E85 scenario (1% in the EISA scenario) during fall, winter, and spring. During summer there is an ample existing supply of CH3C(O)OO and other peroxyacyl radicals from oxidation of biogenic VOCs, so that the additional ethanol source represents a smaller perturbation, and the NOx effect becomes more important. The net effect is a general PAN increase in fall through spring, and a weak decrease over the US Southeast and the Atlantic Ocean during summer.
The All-E85 scenario shows a widespread decrease in surface NOx (as much as 5%) over North America due to the lower NO emissions for E85 relative to gasoline vehicles. This is accompanied by a modest regional decrease in surface ozone (of order 1%).
Downwind of the continent, the lower NOx emissions are offset by increased NOx export efficiency associated with enhanced PAN formation. Over the Atlantic Ocean, instead of a NOx decrease, they found a minor increase of up to 2% outside of the summer season. Only during summer, when biogenic VOC oxidation provides a strong continental source of peroxyacyl radicals, is there an overall (weak) NOx decrease downwind over the Atlantic associated with E85 use.
From the point of view of NOx export from North America, the increased PAN formation associated with E85 fuel use thus acts to offset the associated lower NOx emissions.
Dylan B. Millet, Eric Apel, Daven K. Henze, Jason Hill, Julian D. Marshall, Hanwant B. Singh, and Christopher W. Tessum (2012) Natural and Anthropogenic Ethanol Sources in North America and Potential Atmospheric Impacts of Ethanol Fuel Use. Environmental Science & Technology doi: 10.1021/es300162u