Correspondence/Rebuttal pubs.acs.org/est
Comment on “Natural and Anthropogenic Ethanol Sources in North America and Potential Atmospheric Impacts of Ethanol Fuel Use” assessments of emissions from the future vehicle fleet. Vehicle manufacturers have a legal obligation to meet existing and future tailpipe emission standards, including VOC standards in the federal Tier 2 program and the new California LEV III program. Vijayaraghaven et al.3 have used the EPA MOVES model to estimate VOC emissions from the gasoline light-duty vehicle fleet in the continental U.S. in 2022 assuming either the current federal Tier 2 or the more stringent California LEV III emissions standards. Vijayaraghaven et al.3 find that annual average VOC emissions decline by a factor of approximately 3 from 6712 Mg day−1 in 2008 to either 2258 or 2116 Mg day−1 in 2022 for the Tier 2 or LEV III standards, respectively. The USEPA National Emission Inventory indicates there was a decline by approximately a factor of 1.3 in VOC emissions (4076 to 3036 thousand tons) from the on-road fleet from 2005 to 2008. Hence, combining the data from the National Emission Inventory for 2005 to 2008 and the data from Vijayaraghaven et al.3 for 2008 to 2022, we estimate a decrease in VOC emissions from the fleet by approximately a factor of 4 over the period 2005 to 2022. Unburned fuel is the largest single component in vehicle VOC emissions,4 and to a first order approximation, we can equate a reduction in total VOC emissions with a proportional reduction in emissions of the major fuel components. By not accounting for the decrease in vehicle VOC emissions between 2005 and 2022, reflecting improved emission control technology and fleet turnover, we argue that Millet et al.1 overestimate ethanol emissions associated with the EISA scenario by approximately a factor of 4. We note that the most significant change in the emissions from the on-road fleet over coming decade will result from old vehicles being replaced with new vehicles meeting increasingly
M
illet et al.1 reported an assessment of natural and anthropogenic ethanol sources in North America using aircraft measurements in 2004−2006. Their estimate for ethanol emissions from the continental U.S. in 2005 is 440 GgC yr−1. Millet et al.1 used this estimate to place in perspective the impact of potential future ethanol emissions associated with the biofuel requirements of the U.S. Energy Independence and Security Act (EISA) assuming the requirements of EISA would be met entirely with ethanol. The EISA requires the use of 36 billion (ethanol equivalent) gallons of renewable biofuels by 2022. Millet et al.1 considered vehicle tailpipe emissions and other anthropogenic ethanol emissions given in the U.S. Environmental Protection Agency (EPA) National Emissions Inventory for 20052 and used these data to estimate emissions from a future vehicle fleet. We point out that emissions from the on-road vehicle fleet in the U.S are decreasing and that this emission trend needs to be considered in assessments of future emissions. Millet et al.1 did not consider this emission trend and consequently overestimate the likely future emissions of ethanol by approximately a factor of 4. As shown in Figure 1, on a per vehicle per mile basis, the tailpipe emissions of volatile organic compounds (VOCs) have decreased by almost 2 orders of magnitude for new U.S. vehicles over the past 40 years as a result of improved engine design and emissions controls driven by federal and state legislation. The total emissions from U.S. on-road gasoline powered vehicles fell by a factor of approximately 4 from 1970 to 2010. The decrease in total fleet emissions is less than the decrease in per vehicle per mile emissions for new vehicles because of two main factors: an increase by a factor of approximately 3 in the total vehicle miles traveled by the fleet and an approximately 10−15 year lag for the vehicle fleet to turn over. The substantial and ongoing trend of decreased emissions from the light duty vehicle fleet needs to be incorporated in
Figure 1. Emissions of volatile organic compounds from gasoline powered highway light-duty vehicles in the U.S. (left) Representative per vehicle per mile emissions versus model year. Data for 1963−1967 (9.1 g/mile) and 1968−1971 MY vehicles (4.7 g/mile) are from Fegraus et al.5 The 1975/1976, 1991, 1994, 2004, and 2010 data (1.5, 0.41, 0.41, 0.125, and 0.09 g/mile) are U.S. federal standards. The data point at 2022 is the California combined standard for HC + NOx of 0.03 g/mile and is an upper limit for hydrocarbon emissions. (right) Absolute emissions from the on-road fleet in units of millions of tons per year taken from Davis et al.6 and the USEPA National Emissions Inventory.2 Published: December 17, 2012 © 2012 American Chemical Society
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Environmental Science & Technology
Correspondence/Rebuttal
stringent emissions standards, not from changes in fuel composition.
T. J. Wallington J. E. Anderson S. L. Winkler
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Systems Analytics and Environmental Sciences, Ford Motor Company, Mail Drop RIC-2122, Dearborn, Michigan 48121-2053, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Bob McCabe and Carolyn Hubbard for helpful discussions. While this article is believed to contain correct information, Ford Motor Company (Ford) does not expressly or impliedly warrant, nor assume any responsibility, for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, nor represent that its use would not infringe the rights of third parties. Reference to any commercial product or process does not constitute its endorsement. This article does not provide financial, safety, medical, consumer product, or public policy advice or recommendation. Readers should independently replicate all experiments, calculations, and results. The views and opinions expressed are of the authors and do not necessarily reflect those of Ford. This disclaimer may not be removed, altered, superseded, or modified without prior Ford permission.
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REFERENCES
(1) Millet, D. B.; Apel, E.; Henze, D. K.; Hill, J.; Marshall, J. D.; Singh, H. B.; Tessum, C. W. Natural and Anthropogenic Ethanol Sources in North America and potential atmospheric impacts of ethanol fuel use. Environ. Sci. Technol. 2012, 46, 8484−8492. (2) USEPA. National Emissions Inventory; http://www.epa.gov/ ttnchie1/trends/ (accessed Sept. 25, 2012). (3) Vijayaraghavan, K.; Lindhjem, C.; DenBleyker, A.; Nopmongcol, U.; Grant, J.; Tai, E.; Yarwood, G. Effects of light duty gasoline vehicle emission standards in the United States on ozone and particulate matter. Atmos. Environ. 2012, 60, 109−120. (4) Wallington, T. J.; Kaiser, E. W.; Farrell, J. T. Automotive fuels and internal combustion engines: A chemical perspective. Chem. Soc. Rev. 2006, 35, 335−347. (5) Fegraus, C. E.; Domke, C. J.; Marzen, J. Contribution of the vehicle population to atmospheric pollution. SAE Technical Paper 730530, 1973, doi: 10.4371/730530. (6) Davis, S. C.; Diegel, S. W.; Boundy, R. G. Transportation Energy Data Book, 31st ed., Publication ORNL-6987; U.S. Department of Energy: Washington, DC, 2012.
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