LETTERS Aerosols & sulfate particles The article “What are aerosols made of?” (ES&T, April 1978, p 374) covered progress in studies of aerosol composition reported at the mid-winter meeting of the American Meteorological Society. Related to that article, a March 20-22, 1978 Conference on Carbonaceous Aerosols at Lawrence Berkeley Laboratory in Berkeley presented a great deal of progress in this special area of aerosol research. A major focal point of the latter conference was the possible importance of carbon soot particles in the conversion of SO2 to sulfates and sulfuric acid. Gas-phase reactions of OH, H 0 2 and similar photochemically produced radicals seem capable of accounting for the typical 1-2%/h
conversion rate on warm, sunny days. Yet another mechanism must surely be important during Fall and Winter episodes, which are often accompanied by low temperature, haze and little sunlight. The “dirty water” mechanism, involving SO2 oxidation by dissolved 0 2 in droplets, catalyzed by Fe, Mn or V ions, has long been suggested, but as yet not quantitatively established as being important. In 1973, Novakov et al. at Berkeley reported that SO2 could be catalytically oxidized in the laboratory on carbon soot particles. The importance of this mechanism in the field, however, was not established. Recently, at the 4th Conference on Environmental Sensing (New Orleans, November 1977) and at the Berkeley
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conference, Novakov et al. reported several observations that strengthen the case for the importance of the soot mechanism: (1) Raman lines of soot are found in most ambient samples. (2) Soot causes most of the blackness of filter deposits. (3) Blackening correlates strongly with total carbon and the slope of the curve suggests that about half of the carbon under most conditions (photochemical and otherwise) is on primary soot particles. (4) Soot suspended in sulfite solution catalyzes its oxidation. (5) Sulfate/SOz ratios in Berkeley in Winter are comparable with those in Los Angeles in Summer, suggesting that high photochemical activity in the
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latter case does not increase the oxidation rate. Despite these advances, many controversies and uncertainties remain regarding the importance of the mechanism. For example, Daniel Grosjean (UC, Riverside) finds that the major fraction of carbon in Los Angeles area samples is soluble in organic solvents. This suggests that most of the carbon is condensed organic material (Le., secondary) rather than primary soot, although even some primary diesel exhaust material is soluble. There are also questions about the relative humidity (RH) at which the soot mechanism becomes active. Bruce Appel (Calif. Dept. of Public Health, Berkeley) found little sulfate to be produced on soot formed in combustion of acetylene at R H 90%. Novakov's group finds enhanced yields with increasing R H , but some yield at low RH. It's probably necessary that water be condensed on the particle, but this wasn't controlled in early experiments. There are also many questions about the quantitative significance of the soot mechanism. How much soot is released by various sources? What is the capacity of soot, i.e., the maximum sulfate/carbon ratio? Is this enough to explain a substantial fraction of the sulfate formation in the field? Thus, although many questions remain to be answered about this and other mechanisms for sulfate conversion, the information presented at the meeting represents a big step forward in this field. The field has at least progressed to the point that one knows the right questions to attack by further work. Proceedings of the conference will be available this Fall through NTIS. Glen E. Gordon University of Maryland College Park, Md. 20742
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Asbestosis Dear Sir: Tens of thousands of people already have it; hundreds of thousands more may become afflicted. Lung diseases caused by asbestos dust are becoming one of the greatest health problems of all time. ABC News Closeup, Asbestos: The Time Bomb Explodes (Friday, July 14, 1O:OO- 1 1:00 p.m.), examines the history of the disease; the steps that were, or were not, taken to protect asbestos workers, and the current enforcement of standards in industry and the United States Navy.
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