(2) We have observed variation of inorganic species with depth profiling; however, it may be difficult to separate actual variation of oxidation state with depth from changes induced by the ion-bombardment process itself. (3) Two samples showed evidence of toxic elements on their surfaces. Arsenic was detected on the NBS 1648 sample at a concentration greater than the average volume level, and lead was detected on the surface of the Anacostia sample. There was no significant variation of the P b and As signals with depth over a range of 200-400 A. We did not, however, detect Pb on the surface of the NBS 1648 sample despite its identification in the bulk. Acknowledgments
We thank J. Small and D. Newbury for providing some of the particulate samples and bulk analyses used here. Registry No. Al, 7429-90-5; Ca, 7440-70-2; Cl,, 7782-50-5; Fe, 7439-89-6; K, 7440-09-7; Mo, 7439-98-7; Nz, 7727-37-9; Na, 7440-23-5;Oz, 7782-44-7;S, 7704-34-9; Si, 7440-21-3;P , 7723-14-0; Pb, 7439-92-1; Br2, 7726-95-6; Zn, 7440-66-6; A1203, 1344-28-1; Sod2-,14808-79-8; SiOz, 7631-86-9; C, 7440-44-0; sulfide, 1849625-8.
Literature Cited (1) Novakov, T. “Analysis of Airborne Particles by Physical
Methods”; Malissa, H., Ed,; CRC Press: West Palm Beach, F1 1978; Chapter 10. (2) Barbaray, B.; Contour, J. P.; Mouvier, G. Atmos. Environ. 1977, 11, 351. (3) Barbaray, B.; Contour, J. P.; Mouvier, G. Environ. Sei. Technol. 1978,12, 1294. (4) Campbell, J. A.; Smith, R. D.; Davis, L. E. Appl. Spectrosc. 1978, 32, 316. ( 5 ) Keyser, T. R.; Natusch, D. F. S.; Evans, C. A,,; Linton, R. W. Environ. Sei. Technol. 1978, 11, 768. (6) Certificate of Analysis, Standard Reference Material 1649, National Bureau of Standards, Washington, DC. (7) Certificate of Analysis, Standard Reference Material 1648, National Bureau of Standards, Washington, DC. (8) Carter, G.; Colligon, J. ”Ion Bombardment of Solids”; Elsevier: New York, 1968. (9) Small, J. A. Ph. D. Dissertation, University of Maryland, College Park, MD, 1976. (10) Lichtman, D.; Craig, J. H.; Sailer, V.; Drinkwine, M. Appl. Surf. Sci. 1981, 7, 325. (11) “Handbook of X-Ray Photoelectron Spectroscopy”; Muilenberg, G. E., Ed.; Perkin-Elmer Corp.: Eden Prairie, MN, 1979.
Received for review October 19, 1982. Revised manuscript received March 9,1983. Accepted August 9,1983. This work was supported by the Office of Health and Environmental Research of the U.S. Department of Energy.
CORRESPONDENCE
Comment on “Acid Fog” S I R The March issue carried a feature article on “acid fog” ( I ) . Concern is based on pH measurements made on samples obtained with a centrifugal collector. The low pH levels sound very damaging, but no damage to materials or health has been established. This lack of effect might be expected since damage depends on amount of acidmaking contact as well as the degree of acidity as measured by pH. This follows because acid-damage actions consume acid and because buffers that neutralize acid are invariably present in some degree. Whether the buffer is sufficient to neutralize the acid depends on relative amount of buffer to acid. The amount of acid in acid fog is small relative to the buffering capacity of most biological systems. In the case of humans, the pulmonary system buffering capacity, partially ammonia, appears to exceed the acid content of inhaled air. The acidity per volume of air is the important factor to consider. Maximum acid content in Los Angeles basin air is about 30 pg/m3 as nitric acid, and the amount of air inhaled per hour is about 0.4 m3. The ammonia content of expired air, expressed as nitric acid equivalent, was found to vary from 107 to 1927 pg/m3 (oral) and from 48 to 170 pg/m3 (nasal) (2). I conclude that pH data on fog is meaningless in predicting harmful acidic effects without information on acid concentration based on air volume as well as the degree of buffering. In the extreme case, nearly pure nitric acid
could theoretically be obtained by centrifugal collection methods. Literature Cited (1) Hileman, B. Environ. Sci. Technol. 1983, 17, 117.4. ( 2 ) Larson, T. V.; Covert D.; Frank, R.; Charlson, R. J. Science (Washington, D.C.) 1977, 197, 161-163.
W. B.
Innes
724 Kilbourne Drive Upland, California 91786
SIR: Mr. W. B. Innes has stated that “low pH levels sound very damaging, but no damage to materials or health has been established. This lack of effect might be expected...”. I strongly disagree with Mr. Innes’ suggestion that fogs with pH of 2 and lower will have no impact on plants, materials, or humans. With respect to potential effects, the aqueous concentrations of fog droplets, the acidity per volume of air, and the acid deposition rate are all important factors to consider. The pH of urban fogs does not tell the whole story; however, it is sufficient information to prompt us to examine the severity of the situation and review past findings on the subject. Envlron. Scl. Technol., Vol. 18, No. 1, 1984 %
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