measured, as given by Sinclair (9)where the aerosol “owl” is described. This red band is part of the higher order Tyndall spectra. Operation of the owl utilizes the human eye as the scattered light detector, thwaffording rapid measurement of a mean droplet size. Angular measurements made (46’) indicate a droplet size of 0.70 km a t very dense aerosol conditions. The droplet size measurement was made a t the location in the mixing tube where the aerosol first formed (-1 cm from HC1 injection nozzle). This far upstream location was chosen to minimize any change in the size distribution from coagulation. In fact, even a t this location, the red band was faint due to the lack of uniformity in droplet size. Further downstream in the mixing tube, the red band was not observable. Assuming mass conservation and utilizing the measured number concentration of condensation nuclei and droplet size, the liquid aerosol mass concentration was 640 mg/m3. At the aerosol formation threshold, however, the aerosol mass concentration was considerably less. Depending slightly on the conditions within the mixing tube, the aerosol formed approximately 1 cm downstream from the HCl injection nozzle. The resultant time lapse for aerosol formation was, a t most, 2 msec. Since the flow in the mixing tube was laminar, the aerosol was confined to an annulus of diameter equal to the outside diameter of the injection nozzle. During the experiments, the tube Reynolds
CORRESPONDENCE
SIR: Jack G. Calvert in his paper, “Hydrocarbon Involvement in Photochemical Smog Formation in Los Angeles Atmosphere” [ES&T, 10 (3), 256 (1976)],presents an analysis of LARPP data to test current theories of hydrocarbon involvement in photochemical smog formation. This is a very important proposition, and Professor Calvert is to be congratulated on his skillful handling of a complex and difficult analysis. In his analysis of the LARPP hydrocarbon data, Calvert has apparently overlooked the fact that the data which are presented in Figure 6 for n-pentane and isopentane indicate that these hydrocarbons do not react to a significant extent in the Los Angeles atmosphere. A statistical analysis of the seven data points presented for isopentane and the seven data points presented for n-pentane shows that the slopes of the pentane/acetylene vs. time curves are both not significantly different from zero (cu = 0.05). In the Calvert analysis, it is necessary that a negative slope be demonstrated for the hydrocarbon/acetylene curves in order to conclude that a hydrocarbon exhibits a significant reaction rate relative to acetylene used to normalize the data. Since it is not possible to show a negative slope significantly different from zero with these data, it cannot be shown that the paraffins react. Further, an examination of the data presented in Figure 6 will also reveal that the first data point for each pentane, taken a t about 0800, is crucial for estimating any negative slope whatsoever. The data taken a t 0815 agree with that taken later in the day and cast considerable doubt on the validity of the 0800 data. Without the 0800 data the isopentane/acetylene and n -pentane/acetylene slopes determined by regression analysis using the remaining six points for each pentane become zero and slightly positive, respectively. Thus, the regression analyses of Figure 6 data, both with and without the 0800 data, fail to establish significant evidence that either n-pentane or isopentane reacts a t a measurable rate in the Los Angeles atmosphere. Therefore, Calvert’s subsequent calculations become speculative, and the resulting conclusion that 1162
Environmental Science & Technology
number varied between 720 and 590, thus maintaining laminar flow conditions where the mist formation was observed. Acknowledgment The guidance of Len DeVries, J. Briscoe Stephens, John W. Kaufman, and Earl 0. Knutson is acknowledged. Literature Cited (1) Knutson, E. O., Fenton, D. L., Walanski, K., Stockham, J. D.,
“Washout Coefficients for Scavenging of Rocket Exhaust HCl by Rain”, Sixth Conf. on Aerospace and Aeronautical Meteorology, Amer. Met. SOC., El Paso, Tex., Nov. 12-13,1974. (2) Kerker, M., Hampl, V., J. Atmos. Sei., 31,1368-78 (1974). (3) Rhein, R. A,, “Some Environmental Considerations Relative to the Interaction of the Solid Rocket Motor Exhaust with the Atmosphere”, NASA Tech. Memo. 33-659, December 1973. (4) Twomey, S., Geofis. Pura Appl., 43,227-42 (1959). (5) Gillespie, G. R., Johnstone, H. F., Chem. Eng. Prog., 74F-80F (1955). (6) “International Critical Tables, Vol. 111”,p 301, McGraw-Hill, New York, N.Y., 1928. (7) Whitbv. K. T.. Clark. W. E.. Tellus. 18.573-86 (1966). (8) Zebel, G., Z. Aerosol korsck. Ther.,’B, 1 (1956). (9) Sinclair, D., “Handbook on Aerosols”, pp 106-10, AEC, Washington, D.C., 1950. ~I
Receiued for reuieu J u n e 16, 1975. Accepted J u n e 10, 1976. Work supported by NASAIGeorge C. Marshall Space Flight Center under Contract No. NAS8-29668.
alkanes are important reactants in the smog mixture is not justified. Furthermore, there is no direct experimental evidence given in the paper that most of the other hydrocarbons listed in Table IV are reacting. The purpose of the LARPP experiment is to provide direct evidence from real atmospheric measurements as to which hydrocarbon species participate in smog formation. The only hydrocarbon species shown to be reacting to form oxidant from the data provided in the paper are the olefins, propylene (Figure 3), 1-butene (Figure 4),and isobutene t-2-butene (Figure 5). This lack of observable paraffin reaction, within experimental error limits, shown by the LARPP study, agrees with two other EPA studies (1, 2 ) which were designed to detect oxidant formation from light saturated hydrocarbons in the real atmosphere by downwind aircraft flight sampling of plumes containing these hydrocarbons. In the first study ( 1 ) the authors concluded, “On each of these flights we feel sampling occurred in the area where plume associated ozone could have developed. However, no significant increase in ozone levels was observed.” In the other study ( 2 )the authors concluded, “Local contributions to ground level ambient ozone, a t least out to 30 km, appear to be definitely a minority (
