Exchange of Comments: Measurement of Sulfuric Acid Aerosol, Sulfur Trioxide, and the Total Sulfate Content of the Ambient Air Sir: The article by Thomas et al. ( I ) is seriously flawed in several critical aspects. This is especially true with respect to: 1) An appreciation of the factors affecting the collection efficiencies of commonly used air sampling filters, and 2) The applicability of test results with pure compounds under carefully controlled laboratory conditions to the more variable field conditions and complex mixtures of co-contaminants. The discussion on filter efficiency fails to cite the pertinent literature, especially the factors affecting filter performance. Fluoropore membranes and Type A glass fiber filters have both been shown to collect essentially all particles of all sizes at all face velocities. Gelman Type A filters were tested with 0.3 pm DOP for face velocities between 7.2 and 283 cm/s, and the lowest efficiency was 99.979% a t the intermediate face velocity of 35 cm/s ( 2 ) .Both 0.5 pm and 1.0 pm Fluoropore have been shown to have collection efficiencies greater than 99.9%over a similar range of face velocties for particles ranging in diameter from 0.03 to 1pm ( 3 ) .Thus, it is not possible that “impregnated glass fibers were approximately 25% more efficient than the non-impregnated”, or that the impregnated glass fiber filters had increased efficiencies of 73 to 110%relative to Fluoropore as indicated in this paper. The apparent collection efficiency increases must have been due to misinterpretations of the experimental measurements. Several possible causes are apparent. One source of error is the assumption that the same volumes of air were pulled through each filter. Since the pressure drops through the different filters vary considerably, the rotameters were measuring flowrates at different inlet pressures. Under such conditions, the same rotameter readings do not indicate the same mass or standard volumetric flowrates. At a face velocity of 35 cm/s, the pressure drop through Type A filters is 23 mm Hg ( 2 ) ,while through 0.5 pm Fluoropore it is 100 mm Hg (3). Thus, the apparent efficiency increases may have been due, at least in part, to smaller sample volumes rather than to actual collection efficiency changes. Efficiencies also can be affected by the chemical reactions involved. Initial collection, reaction with the reagent, regeneration of S02, secondary collection in the reagent and measurement are all factors in the overall efficiencies given, but none are evaluated individually. The title of the paper is very misleading. The inclusion of sulfur trioxide is strange, since no data are presented or discussed relative to the collection efficiency of the filters for SO3 gas. Whatever is collected would cause an increase in the reported H2SO4 and there would be no way of separating the contributions of SO3 and HzSo4 to the total. The serious and acknowledged interference of is a major limitation of the method. Presumably, NH4HS04 would also interfere. These three major components of the accumulation mode ambient aerosols are not equivalent in their health significance. The statement: “However, because (NH4)2S04 is readily hydrolyzed to HzSO4 in moist air or in the lung, it may be realistic to include it with SO3 and H2S04 as an environmental hazard” is unsupportable on the basis of either chemical mechanism or toxicological effects. HzS04 is clearly much more toxic then (NH4)2S04, which is why a chemical method for its specific determination in the presence of its salts is very much needed. Collection of interfering substances such as metal oxides subsequent to the collection and reaction of the acid aerosol does not demonstrate elimination of an interference. In ambient sampling these aerosols may be mixed or separate in
space and in time, depending on the particular relationships and locations of sources, reactions in the air mass, and sampling positions. In a real world, collection of a metal oxide aerosol might well precede collection of an acid mist and reverse the results reported. Unfortunately, despite the claims made in this paper, this method does not yield sulfuric acid aerosol concentration, and does not solve the sampling problem for which it is claimed to have been developed.
LITERATURE CITED (1) R . L. Thomas, V. Dharmarajan, G. L. Lundquist, and Philip W. West, Anal. Chem.. 48, 639 (1976). (2) L. 6 . Lockhart, Jr., R. L. Patterson, and W. L. Anderson, “Characteristics of Air Filter Media Used for Monitoring Airborne Radioactivity,” NRL Report No. 6054, U.S. Naval Research Laboratory, Washington, D.C. March 20, 1964. (3) B. Y. H. Liu and K. W. Lee, fnviron. Sci. Techno/., 10, 345 (1976).
Morton Lippmann* Aerosol Research Laboratory Institute of Environmental Medicine New York University Medical Center New York, N.Y. 10016
Theo. J. Kneip Laboratory for Environmental Studies Institute of Environmental Medicine New York University Medical Center New York, N.Y. 10016 RECEIVEDfor review June 23, 1976. Accepted August 18, 1976.
Sir: Regarding the comments of Lippmann and Kneip on our article ( I ) , we regret that some questions have arisen; however, we appreciate the obvious concern and scientific responsibility of these authors. The following comments and additional data should clear the matter: (1)In response to the “efficiency comments” of Lippmann and Kneip, we’re sorry we neglected to state the obvious in our Experimental section. A complete description of our apparatus was referenced (Ref. lo), however, and states: “Because the pressure drop across different filters varies, a rotameter was incorporated before the filter, and the flow rates were adjusted to the same value using a needle valve. Once the flow rate was adjusted, the rotameter was removed and the sample collected for the time desired.” (2) The higher collection efficiency of PDA-Br impregnated filters over Fluoropore, and nonimpregnated filters is primarily a result of the stabilization of HzSO4 by the topochemical reaction with PDA-Br. Thus neutralization and reactions with copollutants are inhibited. This is the heart of our method which recognizes the chemical nature of the problem as well as its physical aspects. (3) The correlation of glass fiber and Fluoropore efficiencies with DOP tests and comparing them to H2SO4 collection efficiencies may not be valid since HzSO4 aerosol is a reactive species and presents far more than a physical problem.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
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