Atmospheric Changes From Ethanol Certain, but Utlimately Unknown

In light of increased reliance on biofuels and ethanol-laced gasoline, a team of researchers has studied potential future atmospheric scenarios in North America.

The study uses an ensemble of aircraft measurements combined with the GEOS-Chem chemical transport model and was a collaboration by researchers from the University of Minnesota; University of Colorado, Boulder; National Center for Atmospheric Research; and NASA Ames Research Center.

In their analysis the researchers found that ethanol emissions are underestimated in Western North America by 50 percent, and overestimated in the East by a factor of two.

Considering only tailpipe emissions – and not other sources of ethanol such as from industrial sources – the team analyzed two scenarios.

One hypothetical model assumed a complete transition from gasoline to 85-percent ethanol fuel (E85) and looked at long-range changes to atmospheric chemistry.

The other hypothetical scenario analyzed was based on the assumption of meeting the 36 billion gallon per year biofuel requirement set by the 2007 U.S. Energy Independence and Security Act (EISA). The team assumed meeting this entirely with ethanol fuel and assumed gasoline is now blended with 10 percent ethanol. They then increased E85 into the hypothetical usage mix up to where total U.S. ethanol consumption reached 36 billion gallons.

This scenario forecast consumption of 126 billion gallons per year of E10 – which has 97 percent the energy content of gasoline – and 28 billion gallons per year of E85 – which has just 71 percent the energy content of gasoline.

While they are planning to do additional work to quantify full life-cycle emission changes for various biofuel production strategies, the study found a number of facts, including North American ethanol emissions increase by 2.6 times in the All-E85 scenario, and by 1.3 times in the EISA scenario. Respectively for the two scenarios, the continental U.S.’s emissions increase by 3.4 times and 1.4 times.

The study goes on to say, “in the atmosphere, ethanol is a precursor of acetaldehyde (CH3CHO) and peroxyacetyl nitrate (PAN), so that changing ethanol emissions have the potential to affect urban air pollution and associated long-range transport.

Acetaldehyde is a hazardous air pollutant (HAP) regulated under Title III of the Clean Air Act Amendments. PAN is an ingredient of photochemical smog created by ultraviolet component of sunlight reacting with hydrocarbons and NOx in the air.

Following is an excerpt from the original article’s abstract:

For both scenarios, increased ethanol emissions lead to higher atmospheric acetaldehyde concentrations (by up to 14% during winter for the All-E85 scenario and 2% for the EISA scenario) and an associated shift in reactive nitrogen partitioning reflected by an increase in the peroxyacetyl nitrate (PAN) to NOy ratio. The largest relative impacts occur during fall, winter, and spring because of large natural emissions of ethanol and other organic compounds during summer. Projected changes in atmospheric PAN reflect a balance between an increased supply of peroxyacetyl radicals from acetaldehyde oxidation, and the lower NOx emissions for E85 relative to gasoline vehicles. The net effect is a general PAN increase in fall through spring, and a weak decrease over the U.S. Southeast and the Atlantic Ocean during summer. Predicted NOx concentrations decrease in surface air over North America (by as much 5% in the All-E85 scenario). Downwind of North America this effect is counteracted by higher NOx export efficiency driven by increased PAN production and transport. 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.

However, the study concedes, “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.”

Using a supercomputer to model its two hypothetical scenarios, the study does offers insights into future changes in air quality. If you want to read a far more detailed article from Environmental Science & Technology, click here to download.


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