since removal of PAH from potable waters is desirable. However, it must be borne in mind that little is known of the reaction products from the chlorination of PAH, and the concentrations of these compounds in drinking water as well as their toxicological properties should be investigated. Literature
Cited
(1) Graf, W., Nothhafft, G., Arch. Hyg. Bakteriol., 147, 135-46
(1963). (2) Trakhtman, N. N., Manita, M. D., Hyg. Sanit., 31, 316-9 (1966). (3) Scassellati Sforzolini, G., Savino, A., Merletti, L., Boll. SOC.Ztal. Biol. Sper., 46,903-6 (1970).
(4) Scassellati Sforzolini, G., Savino, A., Monarca, S., Lollini, M. N., Zg Mod., 66,309-35 (1973). (5)-Scassellati Sforzolini, G., Savino A., Monarca S., ibid., pp 595619. (6) Reichert, J. K., Arch. Hyg. Bakteriol., 152,37-44 (1968). ( 7 ) Harrison, R. M., Perry, R., Wellings, R. A., Enuiron. Sci. Technol., 10,1151 (1976). (8) Acheson, M. A., Harrison, R. M., Perry, R., Wellings, R. A., Water Res., 10,207-12 (1976). (9) Hine, J . , “Physical Organic Chemistry”, 2nd ed., pp 358-63, McGraw-Hill, New York, N.Y., 1962.
Received for review February 24,1976. Accepted June 9, 1976.
Aerosol Formation Threshold for HCI-Water Vapor System Donald L. Fenton* and Madhav B. Ranade IIT Research Institute, 10 West 35th Street, Chicago, Ill. 60616
The conditions under which liquid droplet growth (aerosol formation) occurs in the presence of the hygroscopic vapor, hydrogen chloride (HCl),are experimentally investigated. The HC1 vapor has no impact on the nucleation process itself. The HC1-water equilibrium tables accurately predict the conditions where droplet growth occurs for HC1 concentrations ranging from 10 to 40 mg/m3. In the presence of HC1, a relative humidity of 81%promotes droplet growth. With the advent of NASA’s Space Shuttle Program, considerable quantities of hydrogen chloride (HC1) will be released in the earth’s atmosphere. The estimated mass of HC1 generated is approximately 18 000 kg/launch for the ammonium perchlorate propellant planned to be used. An environmental assessment is therefore necessary concerning this airborne HCl. Under the support of NASA, tropospheric washout of HC1 due to rainout was investigated by Knutson et al. ( 1 ) . Important in the study was the partitioning of HCl between the vapor and liquid phases under appropriate conditions of temperature and relative humidity. If the HC1 was mostly in the liquid phase (droplets), a considerable reduction in the HC1 washout occurred relative to gaseous HCl absorption by the rain. This prediction was based on the recent work of Kerker and Hampl ( 2 ) ,which gives an empirical expression for the capture of small airborne droplets by falling raindrops. Since HC1 is a hygroscopic vapor, mixing with a moist air stream can form an aerosol (liquid droplets) under the appropriate conditions. [Formation of an aerosol is taken to mean the significant growth of droplet size (to -0.3 pm) from the initial size of the nucleating particles.] Formation of the aerosol is normally due t o the nucleation of a supersaturated mixture of a condensable vapor in air. The nuclei may have any origin whatsoever-minute crystals of a foreign material or molecular clusters of the condensable vapor, which have an excess of surface energy sufficient to produce the aggregate as a liquid phase. In the literature thus far, experimental work relating to the aerosol formation threshold of the HC1-water vapor system is limited. Rhein ( 3 ) ,however, did perform a chemical equilibrium analysis where the conditions were determined under which HC1 and water vapor in the Space Shuttle’s exhaust cloud should form an aerosol. The temperatures and relative humidities a t which the aerosol exists were predicted for 1160
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various weight ratios of air-to-exhaust. Twomey ( 4 ) performed experiments to carefully measure the conditions under which HC1 aerosol droplets grow in size. Gillespie and Johnstone ( 5 )made measurements of particle-size distributions with various hygroscopic aerosols, including HCl. Relative humidity of the air, addition of foreign nuclei, concentration of the aerosol particles, and time were studied in reference to the particle-size distribution. The experimental system used is somewhat similar to the one constructed in this work. Aerosol formation was noted to result for relative humidities greater than 78% with the presence of both sodium chloride and ammonium chloride (soluble in HC1) nuclei material. Measurement of the aerosol’s size distribution, though difficult, indicated a mean diameter (mass basis) of 5.5 pm for a loading of 1 g/m3. Since none of the above studies was concerned with the conditions of actual HCl aerosol formation (significant droplet growth), an experimental program was initiated. The relative humidity, HCl mass concentration, and addition of foreign nuclei were varied in the experiment performed here. The criterion for aerosol formation was visual observation of the particles by means of light scattering. Formation of an HC1 aerosol has importance on precipitation scavenging because larger droplets are scavenged less efficiently by falling raindrops. Experimental Apparatus
A mixing tube was constructed to generate the HC1 aerosol. A glass tube with an inside diameter of 25.4 mm was used. A porous brass plate was located near the point where the humid air was introduced to reduce nonuniformities in the flow field a t the point of the HCl gas injection. The length of the mixing tube was approximately 30 cm and of sufficient length for liquid aerosol formation. A schematic diagram of the apparatus is shown in Figure 1.All the experiments were conducted a t 25 “C-ambient temperature. Gaseous HCl was generated by bubbling clean, dry air through 19% hydrochloric acid. The Zeisberg (6) HC1-HpO vapor-liquid equilibrium tables give the amounts of water vapor and HCl gas added to the air flow. To check the average HC1 concentration, a sample from the mixing tube was passed through a bubbler containing distilled water. Analysis of the water indicated agreement with the calculated HC1 concentration to within 5% over the full range of flow rates. The flow rate varied from 0.1 to 2.3 lpm through the HCl acid bubbler. The HCl bubbler was situated adjacent to the mixing tube to
G l a s s WOO1 Filter Plug I
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Filter Plug
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HCI Acid Bubbler Flewmeter
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Figure
I. Schematic of apparatus
reduce any condensation that might occur. The injection nozzle was glass and had an inside diameter of 3.5 mm. Moist air was provided by bubbling dry filtered air through distilled water maintained a t a constant temperature. The relative humidity of the secondary air flow was regulated by the quantities of dry air mixed with the air passed through the water bubbler where the relative humidity was very nearly 100%.The wet and dry bulb temperature of the secondary air flow was measured prior to the experiments to verify the final relative humidity. Agreement was within 4% over the test flow conditions of the experiment. The total flow rate of the secondary air was monitored and maintained a t approximately 10 Ipm. The condensation nuclei material was potassium chloride (soluble in HCl). The nuclei were generated in a flask by increasing the temperature of a KC1 plug by means of a nichrome heating wire to a faint incandescence. Air flow to the flask was first dried and then filtered, and upon exit, transported the nuclei to the mixing chamber. The arithmetic mean diameter of the nuclei was approximately 0.02 Fm, measured with a particle mobility analyzer similar to Whitby and Clark (7). The air flow through the potassium chloride generator was added with the humid secondary air before introduction t o the mixing tube. Discussion Generation of the theoretical curve in Figure 2 utilizes the Zeisberg equilibrium values to determine the partition between the liquid and gas phases of HC1. The onset of water condensation is required before absorption of HC1 and droplet growth can occur. If no condensation is initiated, the nuclei will not grow; therefore, no mist will form. Because the existence of HC1 in the liquid phase corresponds t o droplet growth, the predicted condition where liquid-phase HCl exists is taken to be the region of aerosol formation. Conversely, where no liquid-phase HC1 exists, accelerated droplet growth cannot occur. The curve in Figure 2 separates these two regions and therefore defines the threshold of aerosol formation. The experimentally determined threshold for HCl aerosol formation is also given in Figure 2. Two data points are indicated for each HC1 mass concentration. The upper point indicates the existence of droplets, and the point below indicates no droplet growth. The experimental aerosol formation data agree with the equilibrium curve for HC1 mass concentrations less than 40 mg/m'. For concentrations greater than this, the deviation increases to approximately 10%relative to the humidity. From these results, regardless of the HC1 concentration, a liquid aerosol forms for relative humidities greater than 81%.Although the range of HCI concentration is not given in
Figure 2. Hydrogen chloride mist formation and relative humidity
Gillespie and Johnstone ( 5 ) ,their value of 78% is supported. This is not surprising because the nucleating material used by them was NaCl which is similar to the KC1 used in our experiments. Zebel ( 8 ) calculated that 0.02 gm NaCl particles will nucleate water vapor a t about 80% relative humidity. He included both the increase in vapor pressure over a curved surface due to high surface tension (Kelvin effect) and the lowering of vapor pressure due to presence of a solute. An error analysis performed on the experimental variables-HC1 mass concentration and relative humidity-resulted in 5 and 4%, respectively. With this degree of uncertainty, the result is not significantly different from Gillespie and Johnstone ( 5 )or Zebel(8). In conjunction with observations concerning the aerosol formation, the minimum droplet size detectable was approximately 0.3 wm. The vertical distance between the experimental test points on Figure 2 is a measure of the sensitivity in observing droplet growth (3% relative humidity variation). Attempts were made to verify the droplet size with the particle mobility analyzer; however, due to difficulties in matching the sample conditions, the droplets underwent large size changes. Another source of error results from the incomplete mixing a t the location where the growing droplets first became visible. The uncertainty in the relative humidity a t the position where the droplets were observed is most important. Fluid velocities, HCl concentration, and number concentration of the nuclei are not significant in regulating the droplet growth. Actually, the disparity in relative humidity between the secondary and primary (HC1) flows is small-typically less than 10% depending on the conditions. Therefore, this uncertainty is approximately the same magnitude as the other uncertainties. For the experimental conditions, the number concentration of the nuclei was measured to be 4 X 1O6/cm"with a condensation nuclei counter manufactured by the Gardner Instrument Co. Increasing the number of the condensation nuclei by approximately an order of magnitude did not in any way change the visual appearance of the aerosol a t each of the experimental conditions investigated. However, as with Gillespie and Johnstone ( 5 ) ,entirely eliminating the nuclei from the mixing tube resulted in no visual aerosol irregardless of the HCI mass concentration or relative humidity. Also, eliminating the HC1 resulted in no visible aerosol, again irregardless of the nuclei concentration or relative humidity. This proves that the HC1 significantly promoted droplet growth to the size where light scattering is effective. T o be sure, the KC1 at 81%or higher relative humidity went into solution, but droplet growth did not occur because of the HC1 absence. An indication of the droplet size was made by utilizing light scattering where the angular position of the first red band was Volume IO. Number 12, November 1976
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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
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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, El Paso, Tex., Nov. 12-13,1974. Amer. Met. SOC., (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 (
