montmorillonite has previously been discussed-in particular, the effect of pH on the adsorption reaction. The influence of the metals provides additional evidence regarding the significance of specific chemical adsorption for phosphate interactions with clay minerals.
Literature Cited (1) Stumm, W., Leckie, J. O., “Phosphate Exchange with Sediments’’, in Proc., 5th Intern. Conf. Water Pollution, Pergamon, 1971. (2) Stumm, W., Zobrist, J., “Phosphate Adsorption on Goethite”, Summary Progress Report to the Federal Water Quality Administration, Harvard University, 1971. (3) Chen, Y. S., Butler, J. N., Stumm, W., Enuiron. Sci. Technol., 7,327-32 (1973). (4) Chen, Y. S., Butler, J. N., Stumm, W., J . Colloid Interface Sci., 43,421-36 (1973). (5) Kerr, P. F., et al., “Analytical Data on Reference Clay Minerals”, American Petroleum Institute Research Project 49, Columbia University, New York, 1950. (6) Olsen, S. R., Dean, L. A,, “Methods of Soil Analysis, Part 2”, C. A. Black, Ed., pp 1035-49, Amer. SOC.Agron., Madison, Wis., 1965. (7) American Public Health Association, “Standard Methods for the Examination of Water and Wastewater”, APHA, WPCF, AWWA, pp 527-30, 13th ed., 1971. (8) Muljadi, D., Posner, ‘A. M., Quirk, J. P., J. Soil Sci., 17, 212-29 (1966).
(9) Kuo, S., Lotse, E. G., Soil Sci. SOC.A m . Proc., 36, 725-9 (1972). (10) Carritt, D. E., Goodgal, S., Deep Sea Res., 1,224-43 (1954). (11) Black, C. A., Soil Sci. SOC.A m . Proc., 7,123-32 (1942). (12) Hall, J. K., Baker, D. E., ibid., 35,876-81 (1971). (13) Parks, G., “Aqueous Surface Chemistry of Oxides and Complex Oxide Minerals”, in “Equilibrium Concepts in Natural Water Systems”, R. F. Gould, Ed., Am. Chem. SOC., Washington, 1967. (14) Michaels, A. S., Bolger, J. C., Ind. Eng. Chem. Fundam., 3, 14 (1964). (15) Stumm. W.. Morean, J . J.. “Aauatic Chemistrv”, Wilev-Interscience, New Yorc, N.Y., 1970. (16) James. R. O., Healy, T. W., J . Collozd Interface Sci., 40, 65-81 (1972). (17) Huang, C., “Adsorption of Phosphate at the Hydrous y-Al203 Electrolyte Interface”, ibid., 53, 178-86 (1975). (18) Mortimer, C. H., Limnol. Oceunogr., 16,387-404 (1971). (19) Serruya, C., ibid., pp 510-21. (20) Williams, J. D. H., Syers, J. K., Shulka, S. S., Harris, R. F., Armstrong, D. E., Enuiron Sci. Technol., 5, 1113-20 (1971). (21) Upchurch, J. B., Edzwald, J. K., O’Melia, C. R., ibid., 8, 56-8 (1974). ~
Receiued for reuiew March 17, 1975. Accepted December 29, 1975. Presented i n part ut the 37th Annual Meeting of the American Society of Limnology and Oceanography, Seattle, Wash. This research was supported in part by the National Science Foundation under Grant No. GK-37438 and by Clarkson College of Technology. D. C. T . received support from the Enuironmental Protection Agency under Training Grunt T-900125-04.
NOTES
Direct Evidence of Heterogeneous Aerosol Formation in Los Angeles Smog Rudolf B. Husar Air Pollution Research Laboratory, Department of Mechanical Engineering, Washington University, St. Louis, Mo. 63 130
Warren H. White and Donald L. Blumenthal Meteorology Research, Inc., 464 W. Woodbury Road, Altadena, Calif. 91001
Electron micrographs of smog aerosol samples reveal that particles were liquid droplets, containing an electronopaque nucleus. This observation provides direct evidence that supports previous hypotheses that .the Los Angeles smog aerosol formation occurs by heterogeneous nucleation-Le., by gas deposition onto existing nuclei.
It is well established that the major fraction of the Los Angeles smog aerosol is of secondary origin-i.e., it is formed in the atmosphere as a consequence of chemical reactions of aerosol precursor gases (1-3). Unfortunately, the chemical and physical mechanisms that control or influence the smog aerosol formation are still not well understood. A related question pertains to the mode of gas-particle conversion. Is it governed by homogeneous or heterogeneous condensation? From the point of view of aerosol growth kinetics, a major question of interest is: What is the rate-controlling step for gas-particle conversion-a reaction in the gas phase, reaction in the liquid phase, or a t the particle surface? In recent years, attempts have been made to identify and to establish the role that physical processes play in smog aerosol formation ( 2 , 4 ) .Based on observations of size 490
Environmental Science 8, Technology
spectrum kinetics and conversion rate of smog aerosols, Husar and Whitby ( 4 ) suggested that the rate-limiting step is a gas phase reaction. From their data, they also inferred that the gas-particle conversion proceeds by heterogeneous nucleation and they argued that there were always adequate numbers of foreign nuclei present in the Los Angeles basin to accommodate the condensable vapors. They also proposed an empirical criterion for the occurrence of homogeneous and heterogeneous nucleation. The purpose of this note is to present direct electronmicrographic evidence that the smog aerosol formation occurs primarily by gas deposition on existing nuclei-Le. by heterogeneous nucleation rather than homogeneous or selfnucleation.
Experimental Smog aerosol samples were collected on September 29, 1972, using the instrumented research aircraft of Meteorology Research Inc. ( 5 ) .The sampling location was at 390-m elevation over ground level at Pasadena, Calif. Pressed against the San Gabriel mountains, and capped by an inversion, was a pocket of haze with horizontal extent of about 10 km. Probing the vertical structure of the haze layer revealed that the light-scattering coefficient was higher (3.4 X m-]) at the top of the haze layer than a t lower altitudes. For purposes of electronmicroscopy, smog
Results A sample of aerosol deposits from impactor stage 4 is shown in Figure 1. From their shadows, solid particles can be distinguished from flat residues of liquid droplets. Nearly all particles appearing in Figure 1 are circular droplet residues. Without exception, each of the droplet residues contains a nucleus of high electron opacity. These nuclei, about 0.02 pm in diameter in appearance resemble poppy seeds. Inspection of the electron micrographs a t high magnification reveals that the dry nuclei have compact spherical or cubical shape with no evidence of agglomeration during the evaDoration process. Two likely mechanisms which could acmmint. for the in-... -. -..-. . . clusion of these nuclei in the lianid dror)lets are diffusional collection of the nuclei by existing drol)lets and growth of droplets upon the nuclei by heteroger ieous condensation from the eas nhase. Diffusional collecticm would result in a w r l n ; in oaoh A mn. Poisson distribution for the number of ir l.Yr let. If diffusional collection produced nuclei in just 95% of the droplets, i t would produce multiple nuclei in 80% of the droplets, and three or more nuclei in 58% of the droplets. These high incidences of multiple nuclei are not observed in the micrographs, and it is thus probable that each of the droplets formed around an initial nucleus The above phorographir evidenre supports previous hy. vuthesrs that t h e L(,s Angeles . ’ smog aerosol. formation proceeds by heterogenei )us nucleation-i.e. by growth on existing particles rather t han by self-nucleation.
____
I
-
Figure 1. Electron micrograph of smog aerosol sampled in elevated layer over Pasadena, Calif. Note the opaque nuclei in each of the flat droplet residues aerosol sampling in elevated haze layers has the advantage that it is not “contaminated” by local sources such as fresh automobile exhaust. The isolation of elevated pollutant layers makes these layers “outdoor smog chambers” in which secondary pollutant formation and aging can be studied. The aerosol was collected by a five-stage round jet inertial impactor, with nominal cutoff diameters of 025 pm (stage 51, 0.5 pm (stage 4), 1.0 pm (stage 3), 2.0 pm (stage 2), and 4.0 pm (stage 1).The impaction plates had indentations to hold 3-mm diameter electron microscope (EM) grids directly under each of the five jets. For the collodion film on the EM grids to withstand the jet pressure and rhnnr. t,he m i d a
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Lit,erature Cited
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(1) U.an.n.9mi+ ..y61..~ A J.,Ind. Eng. Chern., 44,1342 (1952). (2) Husar, R. Whitby, K. T., Liu, B. Y. H., J. Colloid Interface Sci., 39,211 (1972). (3) Gartrell, G.Jr., Friedlander, S. K., Atmos. Enuiron., 9,279-99
e.,
119761 II,
R. B., WIhitby, K. T., Enuiron. Sei. Teehnol., 7, 241-6
.. . .
( 5 ) Blumenmai, u..L., Smith, T. B., White, W. H., Marsh, S. L., Ensor, D. S., Husar, R. B., McMurry, P. S., Heisler, S. L.,
Owens,P., Report #MRI 74 FR-1261, prepared for the Califor-
Volume 10, Number 5, May 1976 491
