title: The effects of scrubber installation at the Navajo Generating Station on particulate sulfur and visibility levels in the Grand Canyon.
abstract: Grand Canyon National Park (GCNP) is a mandatory Class I federal area that is afforded visibility protection under the Federal Clean Air Act. In this paper, we have examined the effects on visibility and particulate sulfur (Sp) at GCNP as a result of reducing sulfur dioxide (SO2) emissions by 90% from the Navajo Generating Station (NGS). Scrubbers were retrofitted to each of the three units at NGS during 1997, 1998, and 1999. The Inter-agency Monitoring of Protected Visual Environments aerosol network database affords us an opportunity to examine trends in Sp and extinction both prescrubber and postscrubber. The NGS impacts GCNP primarily during the winter (December to February). During winter, at times, there are fogs, stratus, and high-relative humidity in the Grand Canyon. When the NGS plume interacts with these fogs and stratus, rapid conversion of SO2 to Sp can occur. A variety of analytical techniques were used, including cumulative frequency plots of Sp and extinction, and chemical mass balance and tracer source apportionment analysis. We also deployed P value statistical analysis of "extreme" Sp values. Before scrubbers were installed, values of Sp approaching 2 microg/m3 were occasionally observed. Because scrubbers have been installed, high levels of Sp have been markedly reduced. Statistical P value analysis suggests that these reductions were significant. Furthermore, we have also observed that Sp has been reduced throughout the cumulative frequency curve during winter by approximately 33% since scrubbers were installed. By contrast, during summer when the NGS impact on the Canyon is minimal, there has been only a relatively small decrease in Sp.
authors.affiliation: Division of Atmospheric Sciences, Desert Research Institute, Las Vegas, NV 89119, USA. green@dri.edu
authors.forename: Mark
authors.initials: M
authors.lastname: Green
authors.forename: Rob
authors.initials: R
authors.lastname: Farber
authors.forename: Nghi
authors.initials: N
authors.lastname: Lien
authors.forename: Kristi
authors.initials: K
authors.lastname: Gebhart
authors.forename: John
authors.initials: J
authors.lastname: Molenar
authors.forename: Hari
authors.initials: H
authors.lastname: Iyer
authors.forename: Delbert
authors.initials: D
authors.lastname: Eatough
mesh_terms: D000395:Air Pollutants, Occupational / Q000032:analysis*; D000397:Air Pollution / Q000517:prevention & control*; D001130:Arizona; D016208:Databases, Factual; D011210:Power Plants; D012621:Seasons; D013455:Sulfur / Q000032:analysis*; D014887:Weather
chemical_list: D000395:Air Pollutants, Occupational; D013455:Sulfur