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Haze in the Grand Canyon: An Evaluation of the Winter Haze Intensive Tracer Experiment l BACKGROUND The Grand Canyon is one of the most spectacular natural sights on earth. The diversity and color of the geological formations are extraordinary, and the geographical scales are immense. In some places the canyon is 1.6 km deep and 30 km wide, and panoramic views typically extend to over 100 km. The aesthetic effect of the canyon depends on many aspects of visibility in addition to distance: the luminous quality of the air; the interplay of color, light, and shadow; the colossal scale; and the clarity of the view. These magnificent and unique qualities attract approximately 4 million visitors to Grand Canyon National Park (GCNP) each year. The atmosphere in the Grand Canyon is generally very clear, and under ideal "blue-sky conditions (particle-free, also known as Rayleigh, conditions), visibility approaches the ultimate value of 400 km. However, even small increases in aerosol) concentrations can change the appearance of views- dulling the colors, shifting the color spectrum, reducing the contrasts, and decreasing the visual range (see Appendix 1~. When visibility is impaired in the Grand Canyon, suspended fine particles In the air are usually the major cause. Fine particles, which typically are 0.1- to 1.0-pm diem., scatter and absorb light, and the viewer perceives haze. In the Grand Canyon region, sulfates (SO4=) account for approximately one- third to one-half of the fine-particle-mass concentration in the air. SO4= plays a major role in visibility degradation in summer and winter (NPS, 1988; Trijonis et al., 1989~; SO4= in this area is predominantly the result of atmo- 1An aerosol is strictly defined as suspension of particles in a gas. In this report, "aerosol" is used synonymously with "particle," in conformity with common usage in the atmospheric chemistry literature (e.g., see Buat-Menard et al., 1989, p. 252~. 9
10 · HAZE IN THE GRAND CANYON spheric transformation of sulfur dioxide (SO2) emissions from anthropogenic sources. Wintertime hazes are believed to result from the accumulation of emissions from local sources during conditions of air stagnation; such condi- tions occur more frequently in winter than in summer. The National Park Service (NPS) the managing agency for the GCNP believes that the Navajo Generating Station (NGS) is an important source of SO4= aerosols that cause wintertime haze in GCNP. NGS is a coal-fired power plant placed in operation in stages between 1974 and 1976. It is owned jointly by the U.S. Department of the Interior's Bureau of Reclamation, the Salt River Project (SRP),2 and several electric utilities. NGS is located ap- prox~mately 25 km from the GCNP border at its closest point and about 110 km northeast of the Grand Canyon Village tourist area near Hopi Point (Fig. 1~. NGS has a generating capacity of 2,400 MW gross (2,250 MW net), which makes it one of the largest power plants in the United States west of the 100th meridian. In January and February 1987, a large-scale experiment was carried out by NPS to investigate the causes of wintertime haze in the region between Grand Canyon and Canyonlands National Park. The Winter Haze Intensive Tracer Experiment (WHITEX) was a research project sponsored by a consortium of utilities and governmental agencies called SCENES (Subregional Cooperative Electric Utility, Department of Defense, National Park Service, and Environ- mental Protection Agency Study). The original objectives of WHITEX were (1) to evaluate an empirical approach for assessing the relative contribution of an isolated source to aerosols at specified locations and (23 to determine the relative contributions of individual aerosol constituents to haze at these locations (SCENES, 1987~. The initial plan was for Ha scoping study to investi- gate the feasibility of more extensive source attribution studies in future years (SCENES, 1987~. The original experimental design focused on the area be- tween NGS and Canyonlands National Park, because this region was believed to be most susceptible to effects from NGS emissions due to the presumed prevailing wind flow toward the east. NPS added additional sampling sites in northeastern Arizona and southeastern Utah, including one at Hopi Point on the south canyon rim near Grand Canyon Village to the southwest of NGS. Except for the Hopi Point site, all of the NPS sampling sites were north and east of GCNP (Figs. 1 and 2~. Measurements included atmospheric opti- cal properties, particle concentrations and composition, SO2 concentrations, 2The SRP, a political subdivision of Arizona (akin to a special district), supplies consumers with water and electrical power. The project is the operating agent of the NGS.
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EVALUATION OF VI/HITEX · 13 meteorological variables, and trace elements. Source tracers included particu- late selenium (Se), assumed to be a generic tracer for coal-fired power plants; particulate arsenic (As), assumed to be a generic tracer for copper smelters; and deuterated methane (CD4), which was injected into the NGS stacks to serve as a unique tracer for NGS emissions. UPS analyzed the data from WHITEX to evaluate the effects of NGS emissions on visibility In GCNP. NPS recently issued a final report on the WHITEX study (NPS, 1989), subse- quently referred to as the NPS-WHITEX report. WHITEX focused on NGS because it has large sulfur emissions. NGS is one of the largest single sources of SO2 in the United States west of the 100th meridian. It emits more SO2 than is emitted in the Los Angeles basin but less than is emitter! in the area of California south of Point Conception.3 NGS emits approximately the same amount of SO2 as the combined output of two power plants at Four Corners and San Juan, New Mexico (Appendix 2, p. 80~4 (EL\, 1987~. However, NGS emits less than half the total SO2 of the group of copper smelters in southeast Arizona and Mexico. During WHITEX, NGS emitted less SO2 than two or three individual copper smelters whose emissions have since been reduced.s NGS has no technological controls on its SO2 emissions, although the plant was designed so that flue-gas desulfurization could be incorporated later (SRP, 1971~.6 NGS limits its SO2 emissions by burning low-sulfur coal, typ;- cally 0.45~-O.55%o- sulfur, which allows NGS to meet Arizona's emission limit of 1.0 lb SO2/million btu for the plant. Nevertheless, at full operation, NGS emits more than 200 tons SO2/day through three 236-m tall stacks.7 These emissions qualify NGS as a major stationary source, defined by Section 3About 120 tons SO2/day were emitted in 1987 in the Los Angeles Basin and about 300 tons SO2/day were emitted for all of southern California in 1987 (California Air Resources Board, 1990~. 4Appendix 2 consists of selected pages from the NPS-WH1TEX report. SAccording to the Arizona Department of Environmental Quality (1989), the three Arizona copper smelters currently emit a total of 170 tons SO2/day, and the two Mexican smelters a total of 300 tons S02/day. During WH1TEX, the estimates by ADEQ were 450 tons S02/day total from the Arizona smelters and 620 tons SO2/day total from the Mexican smelters. 6NGS is equipped with electrostatic precipitators that limit primary particle emis- sions to approximately 6.4 tons/day under typical conditions (Appendix 2, p. 82). During WHITEX, the average emission rate was reported to be 163 tons S02/ day (Appendix 2, p. 78~. This emission rate is reasonably consistent with coal- consumption records for January and February 1987: 648,000 and 488,000 short tons, respectively, with sulfur contents of 0.47% and 0.46% (EIA, 1987~.
14 · HAZE IN THE GRAND CANYON 169A(g)~7) of the Clean Air Acts as a source that emits over 250 tons of a regulated pollutant in a year (i.e., 0.68 ton/day). In an innovative step forward in the field of source attribution, WHITEX injected CD4 as a tracer into the NGS stacks (Appendix 3~. Significant con- centrations of CD4 later were detected at Hopi Point during some haze epi- sodes (periods during which visibility was particularly poor). WHITEX inves- tigators focused their analyses on these episodes. Receptor modeling and statistical techniques were used to estimate quantitatively the fraction of par- ticulate SO4= at Hopi Point that could be attributed to NGS. As is often the case when new techniques and approaches are being used, however, unexpect- ed problems arose that made quantitative evaluations of the experimental data difficult. These are discussed in detail in this report. The MPS-WHITEX report concluded that NGS causes wintertime haze in GCNP (Appendix 2, p. 74~. The report claimed that during the days CD4 measurements were made at Hopi Point, NGS was responsible for about 70% of the mean particulate SO4= and about 40% of the mean aerosol-related light extinction. During some wintertime haze episodes,9 the report claimed that NGS contributed as much as 60% of the aerosol-related light extinction. On the basis of the WHITEX results, the U.S. Environmental Protection Agency (EPA) initiated regulatory action under Section 169A of the Clean Air Act (U.S. EPA, 1989~. This section requires the installation of the best avail- able retrofit technology (BART) on any "major stationary source" placed in operation after August 7, 1962, that "emits any air pollutant which may reason- ably be Anticipated to cause or contribute to any impairment of visibility,' in a Class I areas for which EPA has determined that visibility is an important value (Section 169A(b)~2~(A)~. EPA has chosen to take a phased approach to the implementation of Section 169A. Consequently the agency regulations References are to the Clean Air Act as amended, 42 USC § 7401-7626. CD4 was released for 43 days, from January 7 to Febma~y 18, 1987, and was sampled continuously throughout this period. Based on examination of meteorological and other available data, a minority of the CD4 samples was selected for analysis. At Hopi Point, the selected samples covered 36 half-day periods. The selection process and criteria are not documented in the NPS-WH1TEX report, but had the effect of emphasizing periods of higher than average SO4= concentrations. 10Class I areas are those areas subject to the most restrictive limits on growth in air-pollution concentrations under the Clean Air Act's Prevention of Significant Deteri- oration program. Section 162(a) of the act classifies 158 national parks, international parks, and wilderness areas as Class I areas whose designation may not be altered. Section 164(a) allows states and Indian tribes to designate additional areas as Class I; to date, this authority has been little used.
