Environ. Sci. Technol. 2001, 35, 4959-4960
Response to Comment on “Environmental Implications on the Oxygenation of Gasoline with Ethanol in the Metropolitan Area of Mexico City” SIR: Gaffney and Marley have pointed out some secondary pollutants implications of using ethanol in gasoline, derived from the increase in postcombustion acetaldehyde emissions. That is, the higher the acetaldehyde emitted, the higher the formation of peroxyacetyl nitrate (PAN). Indeed the scope of our study (1) did not contemplate implications on secondary pollutants (e.g., PAN, peroxypropionyl nitrate (PPN), and peroxybutyryl nitrate (PBN) among others) except ozone. Gaffney and co-workers have performed peroxyacyl nitrates (PANS) measurements in Mexico City in 1999 in a single location, and they simply extrapolated this one-point average result to the rest of the Metropolitan Area of Mexico City (MAMC). The average PAN concentration of a megacity with an area of approximately 5000 km2, with varying motor vehicle density, circulation patterns, local winds, etc., cannot be inferred from measurements made at one single location. We agree with Gaffney and Marley that increased levels of acetaldehyde may result in a negative impact for secondary pollutants, but on the other hand, lower levels of formaldehyde emissions from the use of ethanol-gasolines may offset this condition. In agreement with other comparative studies using MTBE- and ethanol-gasoline blends (2), our results showed a reduction of formaldehyde emissions in the case of ethanol-gasoline tested on vehicles equipped with catalytic converters (model year after 1992) in comparison with those of MTBE-gasoline (1). Table 6 in our work (1) shows an estimation in the emission inventory variation for the three ethanol fuels with respect to the emission inventory calculated for the MTBE blend of year 1999. For that reason, Table 6 correctly indicates, by a negative sign, a decrease in the inventory for benzene, 1,3butadiene, and formaldehyde and, with a positive sign, an increase for acetaldehyde emissions. In the year 1999 (Figure 1), the acetaldehyde inventory amounts to 192 ton/yr in the case of MTBE and 325 ton/yr for the 10% ethanol fuel, a net increase of 133 ton. In the case of formaldehyde, Table 6 correctly points out that in the year 2010 a net decrease of 137 ton/yr could be achieved. From those results, one can conclude that their increase in overall aldehydes emissions would not be significant. With the same view, Table 5 (and not Figure 3 as Gaffney and Marley pointed out) shows that NOx emissions would increase by 13.7 thousands ton/yr in the year 2010 if 10% ethanol blend is used. One must take into account that in the year 1999, 62.7% of the fleet was composed of vehicles with oxidative (highly deactivated) catalysts or without emissions control systems at all, while in the year 2010 this vintage represents only 25% of the entire formaldehyde emissions, derived from ethanol-gasoline, represents a health risk benefit as compared to use of MTBEgasoline. Motor vehicle population in the MAMC has a model-year distribution typical of developing countries: Old cars (unequipped with any emissions control technology) make up a great portion of the total population. The impact of using ethanol-containing gasoline is very important in terms of 10.1021/es0112036 CCC: $20.00 Published on Web 11/15/2001
2001 American Chemical Society
FIGURE 1. Total risk factor for MTBE- and ethanol-gasolines for the year 2010.
CO emissions reductions (26% lower in a fleet without catalyst than when using MTBE/gasoline (1)). As Gaffney and Marley suggest and as we have noted earlier (1), removal of old cars is of utmost importance to improve MAMC’s air quality. Recently, Mexico’s economic stability and lower inflation rates have prompted motor vehicle dealers to lower their down payments and interest rates, thus making it feasible for low income people (e.g., those that own an old car) to buy a new one. This recent condition may speed up motor vehicle turnover rates in the MAMC. As Gaffney and Marley noted above, further improvement of the MAMC’s air quality will depend not only on using ethanol in lieu of MTBE. At present, brand new motor vehicles, although equipped with three-way catalysts (TWC), still emit significant amounts of toxic unregulated compounds (e.g., aldehydes). We have reported recently that TWC performance in brand new vehicles deteriorates considerably after 60 000 km, either because of defective TWC materials or because of the high sulfur content of Mexican gasoline (8). It is clear that car-makers need to improve their catalytic converter performance to cope with ultralow toxic emissions demands.
Literature Cited (1) Schifter, I.; Vera, M.; Diaz, L.; Guzma´n, E.; Ramos, F.; Lo´pezSalinas, E. Environ. Sci. Technol. 2001, 36, 1893. (2) Motor Vehicle Pollution: Reduction Strategies beyond 2010; Organization for Economic Cooperation and Development: 1995; ISBN 92-64-14312-2; p 27. (3) Jones, A.; Wilson, M.; Norbeck, J.; Hurley, R.; Schuetzle, D. Environ. Sci. Technol. 2001, 35, 3418. (4) Air Toxic Hot Spots Program Risk Assessment Guidelines. Part II: Technical Support Document for Describing Available Cancer Potency Factors; Air Toxicology and Epidemiology Section, Office of Environmental Health Hazard Assessment, California Environmental Protection Agency: April 1999. VOL. 35, NO. 24, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(5) Potential Health Risks of Ethanol in Gasoline; Office of Environmental Health Hazard Assessment, California Environmental Protection Agency: October 29, 1999.
I. Schifter,* M. Vera, L. Dı´az, E. Guzma´ n, F. Ramos, and E. Lo´ pez-Salinas
(6) Schifter, I.; Diaz, L.; Lo´pez-Salinas, E.; Ramos, F.; Avalos, S.; Lo´pez-Vidal, G.; Castillo, M. Environ. Sci. Technol. 2000, 34, 3606.
Instituto Mexicano del Petroleo Competencia de Estudios Ambientales Eje Central Lazaro Ca´rdenas 152 Mexico D.F. 07730, Mexico
(7) Carter, W. P. L. SAE Pap. 1990, No. 900710. (8) Diaz, L.; Schifter, I.; Rodriguez, R.; Avalos, S.; Lo´pez, G.; Lo´pezSalinas, E. J. Air Waste Manage. Assoc. 2001, 51, 174.
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