M
LETTERS Visibility clarified
Dear Sir: I have read your article “Visibility clarified” (February 198 1, p. 135). It provided a good summary of the technical presentations made at the Grand Canyon symposium, including the state of our understanding of visibility, sources of visibility impairment, and the limitations of and difficulty with the tools (modeling and monitoring, etc.) to assess the part man’s activities play in this impairment. You have also pointed out a need to investigate this subject to fill up the gaps in our understanding, e.g., sources involved in visibility impairment, and linking human perception to physical parameters predicted by model calculation or measured by instruments. I have some comments which relate to corrections in this article.
1. Sponsorship of the symposium: The Salt River Project (SRP) was a cosponsor of this symposium along with EPA, NPS, and EPRI. 2. VISTTA is a joint EPA- and SRP-sponsored project. 3. The Rocky Mountains don’t block transport of pollutants from southern California; rather, it is the Sierras and other ranges. Prem S. Bhardwaja
Environmental Services Department
Salt River Project Phoenix, Ark. 85001
Dear Sir: We would like to thank you for your excellent article on visibility in the February issue of ES& T and for your prominent discussion of the VISTTA program. We do, however, have two comments on the discussion of the VISTTA results. First, since the VISTTA program was de-
signed and managed by Meteorology Research, Inc., and much of the research was performed by MRI scientists, it would have been nice to have been given credit for our results described in your article. Second, it is possible to interpret your discussion of the VISTTA results as applying to more situations than those for which our measurements were made. The VISTTA results mentioned may not apply to plumes with more particles than are found in the Navajo Generating Station (NGS) plume, or to plumes emitted into moister or more polluted background air. These conditions are undergoing continuing investigation in the VISTTA program. To be specific, your article mentioned that “the optical effects of the plume are due to NO;! and not particles.” The blue light extinction and brown coloration of the N G S plume was usually dominated by N02. However, even for the N G S plume, the extinction of green light and, thus, much of the degradation of contrast of distant objects due to the plume can be caused in large part by particles in the plume. In addition, you mentioned that “sulfate formation can be ignored in models.” So far, this has been shown only for power plant
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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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