Environ. Sci. Technol. 1902, 26, 1670-1671
additional factor may be heterogeneous reactions such as the formation of methyl nitrite discussed above. In summary, while the loss of methanol and formaldehyde in exhaust samples from methanol-fueledvehicles may not be significant during a few hours of storage in large Tedlar bags, caution must be exercised in extending these results to the identification and measurement of species such as methyl nitrite, which under certain conditions can be formed in significant amounts as artifacts during storage. Thus, based upon results to date, methyl nitrite does not appear to be a significant component of the exhaust from methanol-powered vehicles. Registry No. MN, 624-91-9; MeOH, 67-56-1.
Literature Cited Andino, J. M.; Butler, J. W. Enuiron. Sci. Technol. 1991, 25, 1644.
Lipari, F. J. Chromatogr. 1990,503, 51. Jonsson, A.; Berg, S.; Bertilsson, B. M. Chemosphere 1979, 11/12, 835. Okada, J.; Koda, S.; Akita, K. Fuel 1985, 64, 553. Jonsson, A.; Bertilsson, B. M. Enuiron. Sci. Technol. 1982, 16, 106. Hanst, P. L.; Stephens, E. R. Spectroscopy 1989, 4, 33. He, Y.; Sansers, W. A.; Lin, M. C. J . Phys. Chem. 1988,92, 5474. Hanst, P. L.; Hanst, S., personal communication. Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry: Fundamentals and Experimental Techniques;Wiley: New York, 1986. Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Int. J . Chem. Kinet. 1982, 14, 1199. Takagi, H.; Hatakeyama, S.; Akimoto, H.; K d a , S. Enuiron. Sci. Technol. 1986, 20, 387. Akimoto, H.; Takagi, H. Enuiron. Sci. Technol. 1986,20, 393.
B. J. Flnlayson-Pltts," J. N. Pltts Jr. Department of Chemistry & Blochemistry California State University, Fullerton Fullerton, California 92634
A. C. Lloyd Technology Advancement Office South Coast Air Management District 21865 East Copley Drive Diamond Bar, California 91765
We are grateful to the Technology Advancement Office o f the South Coast Air Management District for support of this work.
SIR: It has recently been suggested (1) that methyl nitrite (MN), which can be formed by reaction methanol with nitrogen dioxide, is present in significant quantities in the exhaust of methanol-fueled vehicles. This suggestion is at odds with previous reports (2). This is an issue of considerable importance because MN is extremely reactive in the atmosphere due to its photolysis (3),and its presence in the exhaust of methanol-fueled vehicles would significantly increase the atmospheric reactivity of the exhaust from such vehicles. In turn, this could make it more difficult for vehicles operating on methanol-based fuels to meet California's reactivity-based emission standards. This correspondence and the accompanying one by Finlayson-Pitts, Pitts, and Lloyd ( 4 ) deal with a number of aspects of the MN problem and together demonstrate that MN levels in the exhaust of methanol-fueled vehicles 1670
Environ. Sci. Technol., Vol. 26, No. 8, 1992
Table I T (K) K , (atm-*)
dil tube tailpipe
298 376
1370 7.68
[NO21
[CH,OHI
(pprn)
(pprn)
dil ratio
2.9
30 510
17 1
49
will be much lower than suggested by Hanst and Stephens (1).
The chemistry of the formation of methyl nitrite is adequately described by (1) CH30H + 2N02 = CH30N0 + HN03 where an equilibrium, described by an equilibrium constant K,, is established between the products and the reactants ( 5 , 6 ) . It is unlikely that MN could result from the combustion process because of its thermal instability (7). Representative values for the levels of NOz and methanol in the exhausts of typical methanol-fueled vehicles are available, and such values are presented in the table, for the tailpipe and for a typical dilution tube analysis, for emissions collected during the transient stabilized-phase data (bag 1)of a UDDS test for a catalyzed vehicle running on M85 (8). While dilution tube concentrations were reported, the tailpipe concentrations could be calculated from the dilution ratio (17:1, calculated from COzconcentrations). The temperature in the dilution tube can reasonably be assumed to have been 25 "C,while the temperature at the tailpipe (undiluted exhaust) was assumed to be 103 "C (it is not unusual to see water condensing at the tailpipe). The conditions are listed in the table. With the use of these data and literature values for the temperature dependence of the equilibrium constant (5, 6), one can predict the levels of MN (relative to methanol) expected in the vehicle exhaust. Assuming that nitric acid is formed only from reaction 1,and that its concentration in the exhaust is equal to that of MN, then at 25 "C, K = 1370 atm-l = [Mn][HN03]/ [CH30H][N0z]2and [ d N ] = 0.587 ppm; and [MN]/ [CH30H] = 0.0196. At 103 "C, Kp = 7.68 atm-l and [MN] = 3.07 ppm; and [MN]/[CH,OH] = 0.00601. These values should be considered upper limits to the MN concentration for the following reasons: (1)it is assumed that there are no other sources of nitric acid in the exhaust, which is not the case; (2) these results do not reflect the true kinetics of the formation, which may be too slow to allow for the maximum methyl nitrite formation before dilution in the atmosphere eliminates formation completely; e.g., another 10-fold dilution of the exhaust at 25 "C would lower the and (3) when a conversion, [MN]/[CH,OH], to 6 X dilution tube is used, NOz levels may overpredict tailpipe values because the termolecular conversion of NO to NOz may occur in the dilution tube or in the bag. As is noted in detail in the companion discussion ( 4 ) , the formation of MN is also heterogeneously catalyzed and may occur on the walls of the dilution tube or the sampling bag. However, the catalysis process changes only the rate at which equilibrium is attained and not the position of the equilibrium. The calculated values presented in the table accurately reflect real exhaust conditions. Thus, the maximum level of MN in the exhaust should be -0.01 of that of methanol (on a volume basis). This is consistent with the upper limit value of 0.002 reported by Jonsson and Bertilsson (2) for undiluted exhaust from a 1978 Volvo running on M85 (85% methanol, 15% gasoline) with a three-way catalyst, exhaust ratios for uncatalyzed trucks in the range from 0.001 to 0.04 (2).
0013-936X/92/0926-1670$03.0010
0 1992 American Chemical Society
1.2
1.0
1
1.2
1
I c
2 U
I
I
I
I
I
750
800
850
900
950
a 2
t
I
I
1000 1050
0.8
0.6
I
1100 1150
750
800
850
Wavenumber
900
950
1000 1050 1100 1150
Wavenumber
Figure 1. FTIR spectrum of a standard sample of methyl nitrite (30 ppm in air; 21.75-m path length).
Figure 2. FTIR spectrum of diluted exhaust from a vehicle fueled with M85 (21.75-m path length).
The results are interesting because they clearly point out that the conversion of methanol to MN is favored at the low-temperature, intermediate-dilution conditions present in a dilution tube or in a bag sample. Higher temperatures and higher concentrations reflective of tailpipe conditions and ambient-temperature, high-dilution conditions reflective of on-road vehicle operation lead to lower yields of MN. This clearly points out that care must be taken in the measurement of MN levels in vehicle exhaust so that the results are not artifacts of the sampling conditions. To further investigate the importance of methyl nitrite in the exhaust of vehicles operating on methanol-based fuels, exhaust from vehicles operating on M85 was introduced into a dilution tube, and diluted exhaust gas samples were collected in 60-L Tedlar bags. Details of the vehicle operation and sampling techniques have been published (8). Samples were taken from a catalyzed, 3.0-L FFV Taurus and from two different catalyzed, 5.0-L FFV LTD/Crown Victorias during the transient phase of the cold-start cycle of the Federal Test Procedure. The bag samples were analyzed by FTIR immediately after collection. The methanol and NOz levels were similar to the dilution tube levels indicated in the table. Comparison of a 30 ppm methyl nitrite standard spectrum (in a 21.75-m path-length cell-Figure 1)to spectra of vehicle exhaust (see Figure 2, as an example), resulted in no detectable methyl nitrite in the vehicle exhausts. Based on comparison of the standard spectrum and the exhaust spectra, an effective upper limit of 1.2 ppm in the bag samples can be placed on the methyl nitrite concentration. Analysis of the diluted exhaust from a number of vehicles operating on M85 showed no methyl nitrite above the system detection limit of 1.2 ppm. Under these conditions, the methanol concentration in the diluted exhaust was on the order of 30 f 5 ppm and, therefore, [MN]/
[CH30H] I 0.04. This is consistent with the value of [MN]/[CH30H] I0.02 calculated above for diluted exhaust and with the highest values measured previously by Jonsson and Bertillsson (2) for undiluted exhaust. On the basis of this analysis and that in the accompanying discussion (5), it seems very likely that the high exhaust levels of MN reported by Hanst and Stephens (1) were artifactual, probably being exacerbated by heterogeneous chemistry taking place while the exhaust was allowed to cool while it was still highly concentrated. Registry No. CH,ONO, 624-91-9;CH,OH, 67-56-1. *(
Literature Cited (1) Hanst, P.L.; Stephens, E. R. Spectroscopy 1989,4, 33. (2) Jonsson, A,; Bertilsson, B. M. Enuiron. Sci. Technol. 1982, 16, 106. (3) Atkinson, R.;Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. J.-Air Pollut. Control Assoc. 1981,31, 1090. (4) Finlayson-Pitts, B. J.; Pitts, J. N., Jr.; Lloyd, A. C. Enuiron. Sci. Technol., preceding letter in this issue. (5) Fairlie, A. M.,Jr.; Carberry, J. J.; Treacy, J. C. J. Am. Chem. SOC.1953, 75,3786. (6) Silverwood, R.;Thomas, J. H. Trans. Faraday SOC.1967, 63,2476. (7) He, Y.;Sansers, W. A.; Lin, M. C. J. Phys. Chem. 1988,92, 5474. (8) Andino, J. M.; Butler, J. W. Enuiron. Sci. Technol. 1991, 25,1644. James W. Butler," Jean M. Andlno, Steven M. Japar Chemistry Department Research Staff Ford Motor Company Dearborn, Michigan 48121
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