Correspondence Comment on “Environmental Implications on the Oxygenation of Gasoline with Ethanol in the Metropolitan Area of Mexico City” SIR: Schifter et al. recently described the measurement of emissions from motor vehicles using gasoline blended with methyl tert-butyl ether (MTBE) and ethanol as well as the policy implications of the results for air quality control in Mexico City (1). The authors’ conclusion is as follows: “On the basis of the emissions results, an estimation of the change in the motor vehicle emissions of the metropolitan area of Mexico City was calculated for the year 2010 if ethanol were to be used instead of MTBE, and the outcome was a considerable decrease in all regulated and toxic emissions, despite the growing motor vehicle population.” The authors seem to have come to this conclusion based upon comparison of data from Tables 5 and 6 in their paper from ethanol blends in 2010 to the 1999 MTBE fuel (1), i.e., that the use of ethanol will lead to a decrease in all emissions. Figure 7 clearly shows that acetaldehyde emissions in 2010 with ethanol usage will be increased as compared to the 1999 MTBE fuel by at least 50% in the case of a 10% ethanol fuel usage. If Figure 7 is correct, and it is likely the case based upon emission measurements using this fuel (2), then the data in Table 6 should have a negative sign in front of them not a positive sign. As well, the data shown in Figure 3 indicates that NOx emission increases with 10% ethanol use. Thus, this analysis did not take into serious account some key considerations about the atmospheric chemistry of secondary pollutants, specifically the projected ethanolenhanced emissions of acetaldehyde and NOx and the connection of these pollutants to the formation of peroxyacetyl nitrate (PAN). As these researchers indicated, the most detrimental effect of ethanol addition to gasoline is an increase in acetaldehyde emissions (80-104%), regardless of the emission control technology tested. Although the researchers noted the connection of PAN to acetaldehyde emissions (3), their policy analysis did not include the potential effects of PAN formation on air quality in Mexico City and on regional and global scales. We would like to point out previous field measurements of PAN and analogues in Mexico City that are relevant to the policy analysis of oxygenated fuel usage. Measurements of PAN, peroxypropionyl nitrate (PPN), and peroxybutyryl nitrate (PBN) have been measured in Mexico City during the use of MTBE-gasoline blends and leaded fuels (4). The data, taken at the Instituto Mexicano de Petroleo in the winter of 1997, showed very high levels of all three peroxyacyl nitrates. Levels of PAN (the predominant peroxyacyl nitrate) were found to exceed 30 ppb during a number of episodes, while PPN concentrations reached 6 ppb, and PBN was readily detected at ppb levels. These PAN concentrations were the highest reported worldwide since levels of 50 ppb were measured in Los Angeles during the late 1970s (4). Increased acetaldehyde emissions due to use of ethanol-gasoline blends (with accompanying NOx increases) would probably exacerbate this problem. As Schifter et al. (1) noted, the use of MTBE is expected to increase levels of formaldehyde as compared to ethanol use. MTBE use is also expected to produce isobutene as a companion emission to formaldehyde (4) because MTBE will crack to yield those two products. Measurements before and 10.1021/es0110832 CCC: $20.00 Published on Web 11/15/2001
2001 American Chemical Society
during a holiday (when motor vehicle traffic was reduced) clearly showed a reduction in isobutene emissions consistent with MTBE use (4). Formaldehyde levels of 100 ppb reported during the early morning rush hours in Mexico City (4, 5) have been linked in previous work to MTBE use (6). We are in agreement that the use of MTBE to reduce CO emissions has a number of chemically deleterious effects besides the water contamination issue noted by the authors. However, increases in acetaldehyde and NOx emissions due to use of ethanol-gasoline blends are likely to be significant (1, 7), especially in the absence of postcombustion catalysts. Briefly, the production of PAN from the reaction of acetaldehyde with OH in the presence of NO2 proceeds as follows:
CH3CHO + OH f CH3CO + H2O
(1)
CH3CO + O2 f CH3CO-O-O
(2)
CH3CO-O-O + NO2 T CH3CO-O-O-NO2 (PAN) (3) At ambient temperatures in Mexico City, PAN is reasonable stable. Its slow thermal decomposition yields the peroxyacetyl radical (CH3CO-O-O) and NO2, with which it is in equilibrium (reaction 3). Nitrogen dioxide photolyzes to produce ozone, while the peroxyacetyl radical reacts with NO as follows:
CH3CO-O-O + NO f CH3CO-O + NO2
(4)
CH3CO-O f CH3 + CO2
(5)
CH3 + O2 f CH3O2
(6)
CH3O2 + NO f CH3O + NO2
(7)
CH3O + O2 f CH2O + HO2
(8)
CH2O + hν f CO + H2
(9a)
f HCO + H f (O2) f 2 HO2 + CO
(9b)
HO2 + NO f OH + NO2
(10)
One can see from reactions 4-10 that the decomposition of PAN is expected to produce formaldehyde and CO as well as to convert a number of NO molecules to NO2, which will produce ozone. If significant PAN is formed (and its analogues), significant amounts of NO2 can be temporarily sequestered, leading to a decrease in the ozone-forming process (i.e., NO2 photolysis). Thus, some caution is required in assessing the impact of using alternative oxygenated fuels to decrease ozone. PAN is also an important oxidant that has clearly been implicated as a potentially harmful substance. Although it is not currently a “regulated” pollutant (i.e., a criteria pollutant), PAN was identified in the early 1950s as a potent lachrymator (eye irritant) and phytotoxin (7, 8). We also note that PAN can transport NOx long distances, adding to regional ozone and oxidant problems and secondary aerosol formation (2-4, 7, 8) and potentially having significant radiative balance effects as well as health and ecological effects. Schifter et al. (1) indicated that three-way catalyst (TWC) systems lead to essentially the same emissions from blends of oxygenated organics with gasoline, with the possible VOL. 35, NO. 24, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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exception of increased NOx emissions with 10% ethanol as noted in Figure 3. Because TWC systems will lead to these same emissions with gasoline alone (2), we would caution against limiting policy assessment to the use of ethanol instead of MTBE in Mexico City as a means of improving air quality across the board. The use of TWC systems with simultaneous removal of older cars in this and other megacities will clearly improve air quality without the potential for regional-scale impacts due to secondary oxidants (PAN and its analogues). Furthermore, control strategies that focus on hydrocarbon control without NOx control are likely to lead to increases in ozone on urban and regional scales. Thus, both hydrocarbon (and aldehyde) emissions and NOx need to be reduced. Air quality control strategies must account for the complete impacts of emissions on all scales and with regard to total air quality, not simply currently regulated pollutants.
Acknowledgments Work supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, under Contract W-31-109-Eng-38.
Literature Cited (1) Schifter, I.; Vera, M.; Diaz, L.; Guzman, E.; Ramos, F.; LopezSalinas, E. Environ. Sci. Technol. 2001, 35, 1893-1901.
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(2) Gaffney, J. S.; Marley, N. A. Alternative Fuels. In Air Pollution Reviews: Volume 1. The Urban Air Atmosphere and Its Effects; Brimblecombe, P., Maynard, R., Eds.; Imperial College Press: London, U.K., 2000; Chapter 6, pp 195-246. (3) Kirchner, F.; Thuneer, L. P.; Becker, K. H.; Donner, B.; Zabel, F. Environ. Sci. Technol. 1997, 31, 1801-1804. (4) Gaffney, J. S.; Marley, N. A.; Cunningham, M. M.; Doskey, P. V. Atmos. Environ. 1999, 33, 5003-5012. (5) Andraca, G. L. A.; Ruiz-Suarez, L. G.; Montero, G. Gas-Phase Determination of Aldehydes, Hydrogen Peroxide, and Acids at Three Sites in Mexico City. Paper 2308; Presented at the Fifth Chemical Congress of North America, Cancun, Mexico, 1997. (6) Bravo, H. A.; Rosaura Camacho, C.; Guadelupe Roy-Octola, R.; Rodolfo Sosa, E.; Ricardo Torres, J. Atmos. Environ. 1991, 25B, 285-288. (7) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Chemistry of the Upper and Lower Atmosphere: Theory, Experiments, Applications; Academic Press: San Diego, 2000. (8) Gaffney, J. S.; Marley, N. A.; Prestbo, E. W. Peroxyacyl nitrates (PANs): Their Physical and Chemical Properties. In Handbook of Environmental Chemistry, Volume 4/Part B (Air Pollution); Hutzinger, O., Ed.; Springer-Verlag: Berlin, 1989; pp 1-38.
Jeffrey S. Gaffney* and Nancy A. Marley Environmental Research Division Argonne National Laboratory Argonne, Illinois 60439-4843 ES0110832
