If You Collect Environmental Data. fC"\
NEPA Dear Sir: We would like to concur with the letter submitted by Malcolm Baidwin of the Council on Environmental Quality in response to the NEPA article (€SAT, August 1976, p 757). In particular we would emphasize the need for a corrective article on NEPA to appear in your journal. Your readers deserve up-to-date information on such matters as EPA's "track record" on reviewing EIS's. Such information is readily available and this office is happy to supply it, but apparently the authors of the article made no effort to obtain current information. Such data would show a significant difference: for example, for the 12-month period ending on June 30, 1976, of the 1765 draft EIS's reviewed by EPA, 1411 were reviewed on time. This matter is complicated by the inability of Federal agencies to transmit copies of EIS's to EPA in a timely fashion. Rebecca W. Hanrner U.S. EPA
Washington, D.C. 20460
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CIRCLE 15 ON READER SERVICE CARD
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Environmental Science & Technology
Energy Dear Sir: That's a good Special Report on energy (€SAT, September 1976, p 854). But getting down to details, I think the common confusion between power and energy slipped in a couple of times. On page 858, column two, the Grandpa's Knob wind turbine cost is quoted as 3 miis/kW, etc. That's got to be kWh, not kW. A couple of paragraphs later, the Enertech system is said to produce 200-310 kWh in an 8-mph wind. This time it should be kW, not kWh, unless you're talking about storage capacity. I note that capital costs are quoted for some systems, operating costs for others, and neither for still others. The lesson may be that there's still a long way to go. You might have mentioned that the Passamaquoddy proposal has been around for at least forty years. Construction costs have quintupled since the 1930's, while the price of a kWh has on the average not risen at all. So one wonders how a project that wasn't feasible then could be economic now. Ian 0. Huebsch Euclid Research Group Berkeley, Calif. 94707 Opacity, a federal view Dear Sir: The feature article, "Factors Influencing Plume Opacity" (€SAT, June 1976, p 539) by Weir et al. discussed 24 variables that affect plume opacity. We at EPA do not agree that the analysis accurately predicts plume opacities or that the article accurately reflects the use of opacity standards; consequently, we consider the article's conclusions to be incorrect. The analysis procedure used by Weir et al. does not evaluate plume opacity, but rather evaluates plume appearance. The distinction between plume opacity and plume appearance is impor-
tant for proper evaluation of Weir et al.'s conclusions. Opacity is considered by EPA, and others, to be the degree to which emissions reduce the transmission of light and obscure the view of an object in the background. The amount of light attenuated by a plume is dependent on the aerosol extinction coefficient, the concentration of particulate matter in the plume, and the plume width. Thus, the opacity of a plume is an intrinsic physical property of the particulate matter being emitted. Plume appearance has been shown by Conner and Hodkinson (Ref. 7) to be dependent on the amount of light scattered by the plume from the sun and its surroundings toward the viewers' eyes relative to the luminance of the background and the opacity of the plume. Weir et al.'s analysis predicts plume appearance rather than opacity because the analysis was based on the equation developed by Halow and Zeek (Ref. 2).The equations and relationships developed by Halow and Zeek consider the amount of light scattered toward the observer and attempt to predict plume appearance by calculating the contrast between the scattered light and the plume background. Consequently, neither Halow and Zeek or Weir et al.'s results are applicable to opacity. The opacity of a plume may be measured instrumentally by transmissometer, lidar, etc., or assessed by trained observers. Owing to greater general applicability to all types of stationary sources, EPA has developed Reference Method 9 for determining opacity by trained observers. Under the procedures of the method, observers are trained with the use of a standard smoke generator to relate the obscuration of visibility by the plume to the in-stack opacity as measured by an in-stack transmissometer. An observer's perception of plume opacity depends on a number of variables, which include observer location relative to the sun and the plume and environmental lighting and background contrast. Consequently, the observer location criteria of Method 9 were established to minimize the potential for positive errors in observations. In addition, EPA has long been aware that environmental lighting and background contrast conditions are variables that may not be controllable in the field. These variables can affect the apparent opacity of the plume as viewed by the observer, and it is known that these variables can affect the ability of a trained observer to accurately assign opacity values to the observed plume. However, the results of field studies conducted by EPA (Ref. 3) show that the positive errors caused by environmental lighting and background contrast are much smaller than suggested by Weir et al. The field studies showed that qualified observers are able to consistently read opacities of black and white plumes with positive errors not exceeding 7 . 5 % opacity based on sets of 25 consecutive readings (Ref. 3). The positive errors of this magnitude
were infrequent and do not reflect a positive bias in Method 9. The effects of geographic location, time of day, and other illumination variables do not significantly affect the ability of trained observers to accurately evaluate plume opacities. The field studies were done by certified observers who were trained at different geographic locations, and on different smoke generators, and who had different degrees of experience reading opacity. They were positioned at different angles with respect to the sun (within the maximum angle allowed by Method 9), and the observations were taken at different times of the day and at different emission sources. Another point of concern to EPA is that Weir et al. apparently misunderstood the purpose and use of opacity standards. EPA establishes opacity standards to ensure that control systems required by mass or concentration standards are properly operated and maintained at all times. To accomplish this objective, opacity standards are set at levels that are not more restrictive than the corresponding mass-emission standard, considering all expected conditions of operation. Thus, as established by EPA, opacity standards are only an indirect measure of emissions. These opacity standards are separate regulatory requirements, and consequently it is not necessary to show that the mass, or concentration, emission standard is being exceeded in order to
support enforcement of the opacity standard. In conclusion, the article is only an application of Halow and Zeek's method for predicting plume appearance (Ringlemann numbers), not opacity. Consequently, the results are not applicable to opacity standards. Our studies indicate that the opacity concept is sound technically and that opacity standards provide the most practical and inexpensive means of ensuring that control equipment is adequately maintained and operated. References (1) Conner, W . D., and Hodkinson, J. R., Optical Properties and Visual Effects of SmokeStack Plumes. U S . Public Health Service. Cincinnati, Ohio. USPHS Publication No. 999-AP-30. NTlS Publication No. PB 174-705 (Springfield, Va.) 1967, 89 pp. (2) Halow, J. S., and Zeek, S. J., Predicting Ringelmann Numbers and Optical Characteristics of Plumes. J. Air Pollut. Control Assoc. 23, 676-684, August 1973. (3) Hamil, H. F., Thomas, R . E., and Swynnerton, N. F., Evaluation and Collaborative Study of Method for Visual Determination of Opacity of Emissions from Stationary Sources. Environmental Protection Agency. Research Triangle Park, N.C. EPA Report No. 650/475-009. January 1975. 70 pp.
Don R. Goodwin, Director Emission Standards and Engineering Div. EPA Research Triangle Park, N.C. 2771 1
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Volume 11, Number 1, January 1977
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