plumes emitted into dry, clean air. It is sometimes not true for plumes emitted into a background with high moisture or ozone levels. It also may not apply to plumes emitted from smelters or urban areas. Since visibility is a sensitive political issue and since VISTTA is sponsored jointly by government and industry, it is very important to us that the results not be generalized to conditions for which they might not apply. Aside from these minor problems, you have performed a great service by clearly summarizing the status of the visibility issue. Donald L. Blumenthal
VISTTA Program Coordinator L. W. Richards
VISTTA Project Manager Meteorology Research, Inc. Santa Rosa, Calif. 95401 PAHs Dear Sir: I am writing this in regard to your article “Polynuclear aromatic hydrocarbons,” which appeared in a recent issue of ES& T (January 198 1, p. 20). This article describing important new developments in the analysis of hydrocarbons and their metabolites is well-written and of general interest.
1 was rather pleased to see that our recent accomplishments in the separation and analyses of benzo[a] pyrene metabolites from living organisms were reviewed in some detail on p. 22. Regrettably, however, one sentence-in the third paragraph under the subheading “Metabolite analysis”-describing our technique is incorrect. The paragraph should read as follows: “In order to separate BaP metabolites more clearly-in this case from liver lipids-Varanasi developed two-dimensional thin-layer chromatography (TLC). This involved successive developments of a TLC plate containing the sample in two different solvent systems.” Usha Varanasi
Northwest & Alaska Fisheries Center Seattle, Wash. 98 1 12
ES&T features Dear Sir: I am certain you get more letters of criticism or “helpful suggestions” than compliments concerning Environmental Science and Technology, since most of us are moved to action only when we are displeased. However, I have found the improvement in the “Features” section
of ES& T so dramatic in recent months that I feel a letter of thanks is very much in order. I’m sure there are many people who, like me, wish to keep abreast of the state of the art in diverse aspects of environmental engineering and science. However, it is difficult to find the time to read all the relevant research papers, and often the level of detail in such papers is beyond our needs or interest. Recent feature articles, which provide an extensive overview of a topic of broad interest combined with up-to-date experimental results and an appraisal of “where we go from here,” have been excellent at addressing these needs. The authors consistently have been recognized leaders in their fields, and the articles have been uniformly well-written. You can be assured that, though you may not receive many tangible expressions of thanks, your efforts at improving ES&T are neither unnoticed nor unappreciated. Mark M. Benjamin University of Washington Seattle, Wash. 98195
Correction February 1981, p. 124. The cover credits should have included David E. Patterson of Washington University.
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