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Executive Summary . . . BACKGROUND The Grand Canyon is one of the most spectacular natural sights on earth. Approximately 4 million visitors travel to Grand Canyon National Park (GCNP) each year to enjoy its majestic geological formations and intensely colored views. However, visibility in GCNP can be impaired by small increas- es In concentrations of fine suspended particles that scatter and absorb light; the resulting visibility degradation is perceived as haze. Sulfate (SO4=) parti- cles- largely the result of the atmospheric transformation of sulfur dioxide (SO2) emissions from anthropogenic sources are a major factor in visibility impairment at Grand Canyon in summer and winter. Many wintertime hazes at GCNP are believed to result from the accumula- tion of emissions from local sources during conditions of air stagnation, which occur more frequently In winter than In summer. In January and February 1987, the National Park Senice (NPS)the managing agency for the GCNP- carried out a large-scale experiment known as the Winter Haze Intensive Tracer Experiment (WHITEX) to investigate the causes of wintertime haze the region of GCNP and Canyonlands National Park. The overall objective of WHITEX was to assess the feasibility of attributing visibility impairment In specific geographic regions to emissions from a single point source. The experiment called for the injection of a tracer, deuterated methane (CD4), into one of the stacks of the Navajo Generating Station (NGS), a major coal- fired power plant located 25 km from the GCNP boundary and 110 km north- east of Grand Canyon Village. A network of field stations was established in the vicinit~mostly to the northeast of GCNP and NGSto measure CD4 concentrations, atmospheric aerosol and optical properties, and other chemical and physical attributes. During some haze episodes, significant concentrations of CD4 were detect- ed at the Hopi Point sampling station on the south rim of the Grand Canyon

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2 HAZE IN THE GRAND CANYON near Grand Canyon Village to the southwest of NGS. The NPS analyzed data from WHITEX and recently issued a final report concluding that NGS con- tributed to wintertime visibility impairment in the GCNP during the study period. The report asserted that NGS was responsible for approximately 70% of the mean particulate SO4= and approximately 40% of the mean aerosol- related light extinction for selected wintertime periods of haze at the Hopi Point sampling station. In response to the MPS-WHITEX report, the U.S. Environmental Protection Agency (EPA) published a proposal to attribute visibility impairment in the GCNP to NGS and took steps to propose that the level of allowable SO2 emissions from NGS be reduced by 90%. THE NATIONAL RESEARCH COUNCIL STUDY This evaluation of WHITEX was prepared by the Committee on Haze in National Parks and Wilderness Areas, which was convened in February 1990 by the National Research Council's Board on Environmental Studies and Toxicology in collaboration with the Board on Atmospheric Sciences and Climate of the Commission on Geosciences, Environment, and Resources. The committee comprises members appointed for their expertise in meteorol- ogy, atmospheric chemistry, atmospheric aerosols, air pollution monitoring and modeling, statistics, environmental engineering, control technology, and envi- ronmental law and public policy. The committee's overall charge is (1) to develop working principles for assessing the relative importance of anthropogen~c emission sources that contribute to haze in Class I areas (which mclude many national parks and wilderness areas) and for assessing alternative source control measures and (2) to recommend strategies for improving scientific understanding and techni- cal information on relative source contributions to haze formation, regional and seasonal factors affecting haze, relevant air quality models, and various emission control measures. The committee's work is sponsored by the U.S. Department of the Interior (National Park Service, Bureau of Reclamation, and Office of Environmental Quality), the U.S. Department of Energy, the U.S. Environmental Protection Agency, and the Arizona Salt River Project (SRP). The committee's final report, which will address this charge, will be issued in 1991. In addition to the final report, the committee was also asked to provide this special report evaluating WHITEX, the recent site-specific study conducted by UPS. This report evaluates the scientific evidence relevant to EPA's recent- ly proposed finding that NGS contributes to "impairment of visibility`' in GCNP. Specifically, the committee was asked to review NPS data and analyses upon

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EXECUTIVE SUMMARY 3 which the EPA determination was based and other data and analyses related to source attribution for Grand Canyon haze. It was also asked to evaluate the contribution of the WHITEX study toward the science of source appor- tionment. The committee reviewed the December 1989 NPS-WHilEX re- port, other relevant published materials, and some unpublished information. In March 1990, the committee conducted site visits to the GCNP aunt NGS near Page, Arizona, and heard technical presentations from NPS, SRP, and their scientific consultants. This information was used as part of the basis for the committee's evaluation. The committee based its evaluation solely on the scientific aspects of WHITEX. It did not consider whether or how EPA should regulate NGS. QUALITATIVE ASSESSMENT On the basis of the data presented in the NPS-WHITEX report, the com- mittee concludes that, at some times during the study period, NGS contributed significantly to haze in GAP. The committee bases this assessment on evalu- ations of meteorological, photographic, chemical, and other physical evidence. The committee's conclusion is supported by the following qualitative evidence: (1) the haze episodes (periods when visibility was particularly poor) observed during WHITEX occurred under conditions of air stagnation, when sulfur from local sources would be expected to accumulate; (23 SO4= aerosol was a significant contributor to haze at Hopi Point during periods when visibility was particularly poor and when CD4 was detected; (3) NGS is a large source of SO2 emissions near GCNP and it could potentially account for the total sulfur (SO2 and SO4=) observed at Hopi Point during some haze episodes; (4) meteorological analyses and photography indicate air movement from NGS to GCNP during some haze episodes; (5) significant quantities of CD4 released from NGS were observed at Hopi Point; and (6) the presence of clouds and fog in the vicinity of GCNP and NGS during haze episodes would accelerate the conversion of NGS SO2 to form SO4= aerosol. The detection of CD4 at Hopi Point is an unambiguous indicator that air parcels containing NGS emissions did impinge on the GCNP on several occa- sions. The use of CD4 in WHITEX was an innovative and important step forward in the field of source attribution, and NPS and the WHITEX team are to be commended for including this tracer in WHITEX.

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4 HAZE IN THE GRAND CANYON QUANTITATIVE ASSESSMENT .Y The NPS-WHITEX report based its quantification of the NGS contribution to SO4= aerosol at Hopi Point on two empirical modelsTracer Mass Balance Regression (TMBR) and Differential Mass Balance (DMB) - oth of which are based on multiple linear regression techniques. Although multiple linear regression has been used before to apportion primary (directly emitted as particles) source contributions to ambient aerosol mass, there has been little verification of its applicability in the case of a predominantly secondary (parti- cles formed in the atmosphere) species, such as SO4=. Moreover, the com- mittee identifies problems in the implementation and interpretation of multi- ple linear regression in the WHITEX analysis: (1) satisfactory tracers were not available for all major sources; (2) the interpretation did not adequately account for the possible covariance between NGS contributions and those from other coal-fired power plants in the region; and (3) both models employ inadequate treatment of sulfur conversion, which is an important controlling factor in the formation of haze at GCNP. Therefore, the committee concludes that WHITEX did not quantitatively determine the fraction of SO4= aerosol and resultant haze in CGNP that is attributable to NGS emissions. The report did not adequately quantify the sensitivity of the analyses to departures from model assumptions, nor did it establish an objective and quantitative rationale for selecting among various statistical models. The conceptual framework for DMB involved physically unrealistic simplifications, the effect of which on quantitative assessments was not addressed. The data base contained weaknesses; especially damaging were the absence of measurements below the rim of the Grand Canyon and the paucity of background measurements. In addition, the background mea- surements that were made were not adequately incorporated into the data analyses. ESTIMATES OF THE RANGE OF POSSIBLE IMPACTS FROM NGS The committee used the WHITEX data obtained during a haze episode in GCNP to estimate the ranges of possible impacts from NGS. These estimates consist of a series of mass-balance calculations made on the basis of simple, but reasonable, assumptions. These calculations were made for illustrative purposes; they cannot, in themselves, be used to quantify the impacts of NGS on visibility in GCNP, in part because measurements needed to confirm some of the assumptions were not made during WHITEX. The committee's analysis

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EXECUTIVE SUMMARY 5 indicates that NGS sulfur emissions are sufficiently large to account for even more than the total sulfur concentration measured in GCNP. However, the actual impact of NGS emissions on haze in GCNP depends on the rate of conversion of SO2 to SO4=. The extent of conversion is extremely sensitive to meteorological conditions and the availability of oxidants; the extent could range from a very small percentage to nearly 100%. Conversion rates would tend to be high during cloudy and fogy conditions, such as those observed during WHITEX haze episodes. Under these conditions, NGS emissions could produce SO4= concentrations similar to those measured In GCNP. However, the WHITEX data also show that there are important sulfur sources in addition to NGS that appear to contribute to regional background SO4= aerosol levels. These additional sources evidently account for a signifi- cant fraction of the haze observed in GCNP. Thus, if NGS emissions were controlled, then wintertime haze at GCNP would most likely be reduced, but not eliminated. THE COMMI1lEE'S CONCLUSIONS IN PERSPECTIVE The committee cautions that its conclusions do not resolve whether EPA should require NGS to install the best available retrofit technology (BART). First, the committee's conclusions are not binding on EPA or any other gov- ernment agencies. Second, even if accepted by EPA, the committee's conclu- sions would no't''dictate a particular result to EPA's rule making. Section 169A of the Clean Air Act generally requires the installation of BART on any Major stationary source" built after 1962 if that source "emits any air pollutant which may reasonably be anticipated to cause or contribute to any impairment of visibility,' in a listed Class I area, such as GCNP. The phrase "may reasonably be anticipated" suggests that Congress did not intend to require EPA to show a precise relationship between a source's emissions and all or a specific frac- tion of the visibility impairment within a Class I area. Rather, EPA is to assess the risk in light of policy considerations regarding the respective risks of overprotection and underprotection. These considerations transcend scien- tific issues and are, therefore, outside this committee's purview.

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Haze in the Grand Canyon An Evaluation of the Winter Haze Intensive Tracer Experiment

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