tree approached t‘hat of a solid object. This was not the case. Also, the random motion of the leaves flapping violently would cause considerably different flow conditions to prevail than the case of the single trailing leaf with little motion. Even though the fluid dynamics are different, the functional form and value of the results are the same. The demonstration of the validity and reproducibility of our results gives promise that a dose-response curve can be quantitatively determined for plants existing in an environment where the ambient aerosol cloud concentration is known. The simplicity of the results suggests that mathematical models for plant deposition via the diffusion process are possible. This would be a first step toward formulation of a quantitative model to assess the potential deleterious effects of aerosols on crop yields.
Literature Cited (1) Rolfe, G. L., J . Enu. Qual., 2, 153 (1973).
(2) Baumhardt, G. R., Welch, L. F., ibid., 1,92 (1972). (3) Lagenverff, J. V., Soil Science, 111, 129 (1971). (4) Buchauer, M. J., Enuiron. Sci. Technol., 7,131 (1973). (5) Lagerwerff, J. V., Specht, A . W., ibid.,7,583 (1970). (6) Wedding, 3. B., Stukel, J . J., Int. J . Multi-Phase Flou, in press, (1974). (7) Wedding, J. B., MS thesis, University of Illinois, Urbana, Illinois, 1972; Wedding, J. B., Stukel, J. J . , Enuiron. Sei. Technol., 8,456 (1974). (8) Green, H. L., Lane, W. R., “Particle Clouds, Dusts, Smokes and Mists,” E. and F. N. Spon, Ltd., London, 1957. (9) Baines, W. D., Peterson, E. A . , Trans. ASME, p 467 (1951). Received for reuieu; June 7, 1974. Accepted Oct 18, 1974. Work supported in part by Grant N S F GI 31605 from the R A N N program of the National Science Foundation.
Detection of Acrolein in Engine Exhaust with Microwave Cavity Spectrometer of Stark Voltage Sweep Type Mitsutoshi Tanimoto* and Hiromichi Uehara
Sagami Chemical Research Center, Nishi-Ohnurna 4-4-1, Sagarnihara, Kanagawa, 229, Japan
Acrolein in automobile engine exhaust is detected using a microwave cavity spectrometer of Stark voltage sweep type. The cavity operates in the TE1,0.20 mode at a fixed microwave frequency in the X-band region. The exhaust gas is collected through a glass tube packed with phosphorus pentoxide and trapped on a cold adsorbent in a dry ice-acetone cold bath. A sample of the engine exhaust is found tcl contain about 5 ppm of acrolein. Aldehydes are produced by incomplete combustion processes of various organic substances and appear to be important chemical species in photochemical air pollution. Formaldehyde i; the richest component of the aldehydes, and a specific analytical procedure has been developed ( I ) . Much attention has been paid also to acrolein, for it produces significant eye irritation. Acrolein has been determined colorimetrically using 4-hexylresorcinol as the color-producing reagent (1, 2). The colorimetric method suffers from interference from molecules having structural features in common with acrolein, such as crotonaldehyde and butadiene (2) and formaldehyde ( 3 ) .Microwave spectroscopy, however, is appropriate t o the detection of trace contaminants in complex mixtures because of its high sensitivity and high resolution. Recently a new cavity spectrometer has been constructed and applied to the detection of formaldehyde in automobile exhaust ( 4 , 5 ) . The present paper reports the detection of acrolein in engine exhaust by means of the cavity spectrometer with a help of a preconcentration technique.
Experimental As the details of the cavity spectrometer used in the present investigation was described elsewhere ( 4 ) , only a brief explanation is given here. A rectangular cavity (10 mm in height, 20 mm in width, and 500 mm in length) operates in the ‘I’El.o,zo mode at about 8.9 GHz. It is coupled to a microwave system through an iris diaphragm and is terminated by a movable short-circuiting plunger.
A copper plate is inserted as a Stark electrode a t the center of the cavity so as to cut the microwave electric field perpendicularly. The plate is mounted on narrow grooved strips made of Teflon. The Stark electrode is fed with a variable dc and 100 kHz sine-wave voltage. The microwave absorption is detected by a lock-in amplifier. The output of the amplifier is displayed on a recorder in the form of the derivative of the absorption spectrum. A number of samples were taken from the exhaust of an automobile engine connected with a dynamometer (GoPower DA-300). The samples were collected in a glass sample holder of about 300-ml capacity through a glass tube packed with phosphorus pentoxide. T o apply the low-temperature trapping technique (6), 1 gram of the adsorbent (diatomaceous earth firebrick) was placed in the sample holder, which was evacuated before the sampling. The exhaust gas was carefully introduced into the sample holder after the moisture which initially deposited in the sampling line upstream to the dehydrating reagent was revaporized by the heat of the exhaust gas. This procedure was necessary to avoid undesirable reactions of sample components with deposited water. The sample holder was then immersed in an acetone-dry ice cold bath with recurrent agitation. After half an hour, noncondensable gases such as Nz, 0 2 , and COZ were pumped out, and the adsorbent was heated up to about 200°C. The desorbed gas was introduced into the spectrometer cavity, and the pressure was reduced to 0.20 torr. The Stark dc voltage was varied, the microwave frequency being fixed a t 8910.5 MHz. The frequency was close to the zero-field rotational 000 8902.2 MHz, which was calculated transition 101 from the rotational constants derived by Cherniak and Costain ( 7 ) .
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Results and Discussion The spectrum of acrolein in automobile engine exhaust (4000 rpm; torque, 3.5 kg m) is shown in Figure 1.Two other signals are recognized in the spectrum. One is the strong signal on the side of the higher Stark voltage and is due to formaldehyde contained in the exhaust gas as much as 20 Volume 9, Number 2 , February 1975
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Figure 2. Spectrum of the standard 260 pprn acrolein in air.
Figure 1. Spectrum of acrolein contained in a sample of engine exhaust.
0 ppm ( 4 ) . The signal has its peak outside the region swept in the present investigation and shows only a steeply rising envelope. The signal of acrolein is observed on the tail of the signal of formaldehyde. The other weak signal (A, in Figure 1) around the lowest Stark voltage in the spectrum remains unidentified. Figure 2 shows, as an indication of the sensitivity, the spectrum of 260 ppm acrolein in air. The sample was directly introduced into the cavity, and the pressure was adjusted to 0.20 torr, the same pressure as that described above. From the signal-to-noise ratio of this spectrum, it is reasonable to expect that the detection of 10 ppm will be possible without preconcentration. However, direct detection of acrolein in the engine exhaust was not successful; the preconcentration was necessary. To determine the concentration of acrolein in the exhaust whose spectrum is shown in Figure 1, a calibration curve was drawn. A known amount of acrolein was introduced into a flask, which was then filled with dry air of 1 atm. Applying the same procedure as that for the exhaust gas, we observed the absorption signal and obtained the calibration curve shown in Figure 3. From the curve, the acrolein content in the exhaust gas (Figure 1) was determined to be about 5 ppm. This value is only as an indication of the concentration of acrolein in the exhaust gas, since the acrolein content depends upon the operating condition of the automobile engine. The concentration determined above is well compared with the value (5-10 ppm) determined by Cohen and Altshuller (2) by means of the 4-hexylresorcinol procedure with the subsequent spectrophotometric method. It was found that only one seventh of acrolein was recovered by the procedure described above, although it was the most efficient of the procedures we tested. Acrolein of ppm order was thus difficult to handle with the present technique. The recovering efficiency was especially low when liquid nitrogen trapping of condensables was used.
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Environmental Science & Technology
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4
6 P P ~
Acrolein Concentration Figure 3. Calibration curve for acrolein content
It remains unsettled what amount of acrolein is lost in each step of the treatment and how the efficiency of preconcentration can be improved. However, to our knowledge, the present study is the first example in which acrolein in engine exhaust has been detected by a physical method. Another difficulty in the simultaneous use of the present cavity and the preconcentration technique was the adsorption of some rich components on the cavity wall. This was the case of formaldehyde, contained so much that the direct observation would be possible. Once the preconcentrated sample was introduced into the cavity, trace amounts of formaldehyde persisted on the cavity wall even after pumping for several hours. For accurate measurement, the effects of gas adsorption to and desorption from the cavity wall should carefully be eliminated.
Acknowledgment The authors are grateful to Prof. Yonezo Morino for his interest and stimulation throughout this work. Literature Cited (1) Levaggi, D. A,, Feldstein, M., J . A i r Pollut. Contr. Ass., 20, 312-13 (1970). (2) Cohen, I. R., Altshuller, A. P., Anal. Chem., 33, 726-33 (1961). (3) MaHek, V., Gesundheits-lng., 92,245-46 (1971) (Ger.). (4) Uehara, H., Ijuuin, Y., Chem. Phys. Lett. in press. ( 5 ) Uehara, H., Tanimoto, M., Ijuuin, Y., ibid., 26, 578-81 (1974). (6) Uehara, H., Arimitsu, S., Anal. Chem., 45, 1897-900 (1973). (7) Cherniak, E. A , , Costain, C . C., J . Chem. Phys., 45, 104-110 (1966).
Received for review July 8, 1974. Accepted October 9, 1974.
