Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Natural and Anthropogenic Ethanol Sources in North America and Potential Atmospheric Impacts of Ethanol Fuel Use” Jason Hill† Julian D. Marshall† Hanwant B. Singh∥ Christopher W. Tessum†
n their comment on our article “Natural and Anthropogenic Ethanol Sources in North America and Potential Atmospheric Impacts of Ethanol Fuel Use”,1 Wallington et al. point out that volatile organic compound (VOC) emissions from vehicles in the U.S. have been decreasing in recent years. They further note that the trend is likely to continue in the future. That projection is plausible, but it does not alter any of the conclusions or findings in our paper. Wallington et al. also state that our paper attempts to project emissions for a future vehicle fleet, which is incorrect. Our analysis applied an ensemble of aircraft measurements and a chemical transport model to derive new constraints on ethanol emissions from North America during the 2004−2006 time frame. Our best estimate for North American emissions during that time was 670 GgC/y (440 GgC/y in the continental U.S.). Based on this new knowledge, we then carried out a sensitivity analysis (for the same time frame) to examine how atmospheric acetaldehyde, peroxyacetyl nitrate (PAN), and reactive nitrogen partitioning changed with varying amounts of ethanol fuel use in the model. The sensitivity analyses we employed included (i) ethanol fuel use similar to the biofuel mandate of the Energy Independence and Security Act (EISA) (36 billion gallons/ year) and (ii) a complete switch to E85 fuel. These two scenarios led to North American ethanol emissions of 845 GgC/y and 1710 GgC/y respectively for year-2005 (i.e., increases of 30% and 2.6× compared to baseline emissions for the same year). Wallington et al. incorrectly imply that we were attempting to predict emissions for some specific year in the future. This is not the case. Rather, we sought to investigate the sensitivity of some key atmospheric gases to varying amounts of ethanol fuel use relative to a similar fleet running mainly on gasoline. Actual future emissions will depend on fleet evolution, regulatory emission controls, U.S. economic growth or recession, and other related changes that occur between now and then. Because of the uncertainty in projecting such changes and their effects on VOC and NOx emissions, we employed 2005 as a test year for our sensitivity study. Nevertheless, a finding of our paper is that the modeled atmospheric impacts driven by an ethanol fuel increase similar to that required under EISA are quite small, even using the 2005 vehicle fleet as a diagnostic. This is consistent with other recent work.2 For instance, atmospheric acetaldehyde concentrations increased by only a maximum of 2% for this scenario, while PAN increased by a maximum of 1%. This is largely because of the significant sources of atmospheric acetaldehyde that already exist,3 including biogenic and anthropogenic emissions of acetaldehyde itself, as well as of ethanol and other precursor VOCs.
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† University of Minnesota, Minneapolis-St. Paul, Minnesota, United States ‡ National Center for Atmospheric Research, Boulder, Colorado, United States § University of Colorado, Boulder, Colorado, United States ∥ NASA Ames Resarch Center, Moffett Field, California, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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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 (15), 8484−8492. (2) de Gouw, J. A.; Gilman, J. B.; Borbon, A.; Warneke, C.; Kuster, W. C.; Goldan, P. D.; Holloway, J. S.; Peischl, J.; Ryerson, T. B.; Parrish, D. D.; Gentner, D. R.; Goldstein, A. H.; Harley, R. A. Increasing atmospheric burden of ethanol in the United States. Geophys. Res. Lett. 2012, 39, L15803 DOI: 10.1029/2012GL052109. (3) Millet, D. B.; Guenther, A.; Siegel, D. A.; Nelson, N. B.; Singh, H. B.; de Gouw, J. A.; Warneke, C.; Williams, J.; Eerdekens, G.; Sinha, V.; Karl, T.; Flocke, F.; Apel, E.; Riemer, D. D.; Palmer, P. I.; Barkley, M. Global atmospheric budget of acetaldehyde: 3D model analysis and constraints from in-situ and satellite observations. Atmos. Chem. Phys. 2010, 10, 3405−3425.
Dylan B. Millet*,† Eric Apel‡ Daven K. Henze§
Published: December 17, 2012 © 2012 American Chemical Society
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dx.doi.org/10.1021/es305112s | Environ. Sci. Technol. 2013, 47, 2141−2141
