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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. W e 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 M R I 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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