Site Name

IMPROVE: GRCA1 (Hopi Point #1), GRCA2 (Hance Camp at Grand Canyon NP)
INGA1 (Indian Gardens)

CASTNET: GRC474

Region

Colorado Plateau

Terrain

Two IMPROVE sites are located in the vicinity of the south rim of the Grand Canyon: GRCA2 and INGA1. There is also a CASTNET site, GRC474, with meteorological data collected since 1989. GRCA2 is at a regionally exposed location on the south rim, elevation 2,267 m (7,346 ft). Terrain in the vicinity of GRCA2 is forested rolling hills to the south, and drops into the Grand Canyon to the north. INGA1 is located near Bright Angel trail near Indian Gardens, about 3 km from the rim and at an elevation of 1,166 m (3,824 ft), some 430 m (1,410 ft) above Colorado River elevations at the bottom of the canyon. INGA1 measurements are thus taken at about ¼ of vertical distance from the river to the rim. Ground cover near INGA1 is sparse shrubs. The CASTNET site, where meteorological data are collected, is at an elevation of 2,073 m (6,799 ft) on the canyon rim. Terrain and ground cover are similar to the GRCA2 site.

Terrain in the area is depicted in the detailed 20 km terrain map, 2 km Hance Camp terrain map, 2 km Indian Gardens terrain map.

Representativeness

Aerosol data collected at the Hance Camp IMPROVE site (GRCA2) should be representative of visibility conditions at the Grand Canyon rim. It should represent conditions at lower elevations of the Grand Canyon when the atmosphere is uniformly mixed. Visibility measured at the Hance Camp site may not represent visibility at lower elevations within the Canyon when it is in a different stable layer, e.g., when lower canyon elevations are within a stable (inversion) layer with height below the Canyon rim. Visibility at inner canyon locations is better represented by measurements at the INGA1 (Indian Gardens) site, especially during very stable or inversion conditions.

Nearby Population/Industrial Centers

Population is sparse in the vicinity of the monitoring site. The nearest population center is Grand Canyon Village, the south rim tourist center located ~ 17 km (~10 mi) northwest from GRCA2. The nearest city is Flagstaff, population 53,000 (2002), 95 km (60 mi) south. The nearest major metropolitan area is Phoenix, population ~ 1.5 million, ~ 300 km (200 mi) south.

There are several coal fired power plants in the region, including the Navajo generating station near Page, Arizona, some ~ 100 km to the north near the Colorado River. SO2 emissions from the Navajo plant have in the past been a major concern with respect to haze and visibility in the Grand Canyon. Emissions from the Navajo plant have been drastically reduced since completion of the Navajo Scrubber Project in 1999. Other major point sources potentially affecting Grand Canyon NP include the Mojave Generating Station in southern Nevada, and the Four Corners and San Juan power plants in northwestern New Mexico.

Local sources of particulate matter that have been identified as possible contributors to visibility impairment in Grand Canyon NP include road dust and wildland fires (Grand Canyon Visibility Transport Commission). The Navajo power plant might also be considered a local source in view of its proximity to the Colorado River drainage a short distance upstream from Grand Canyon NP.

Nearby Data Stations

This map shows the location of the nearest air quality and meteorological monitoring sites, with respect to the Hance Camp IMPROVE monitoring site. Meteorology is not presently monitored at the site. The CASTNET Grand Canyon NP site, GRC474, is located on the Canyon rim near Grand Canyon Village, about 20 km northwest of the Hance Camp (GRCA2) site. It is near the GRCA1 site shown on the 20 km terrain map. Meteorological monitoring started at GRCA1 in 1989, and data from that site should represent surface conditions near the Hance Camp (GRCA2) site.

There are also a few RAWS network sites in the vicinity. Information on these sites, including period of record, is available from the U.S. Climate Archive.

The nearest upper air site is the Flagstaff RAOB site, which conducts atmospheric soundings twice daily. Sounding data from Flagstaff should be representative of regional upper air structure.

Wind Patterns

Canyon rim:

The following wind roses are based on surface meteorological data collected at the CASTNET GRC474 site for the period 1996 – 2001 and indicate annual and seasonal wind direction (direction from which the wind is blowing) frequencies. Since both GRC474 and GCRA2 are located on the south rim and are separated by about 20 km, the annual and seasonal wind roses should be representative of wind conditions at the Hance Camp site, GCRA2, and generally representative of well-exposed locations at the Canyon rim. Wind roses for GRC474 are also very similar to long term wind roses from the Flagstaff NWS site for January, April, July, and October, indicating a regional representation for exposed areas. The wind roses show a predominance of south to south-westerly wind directions, especially in the summer, with a significant frequency of more northerly wind directions during the fall and winter. Summertime southerly flow may in part result from the North American Monsoon, bringing with it frequent precipitation events.

Diurnal variability may be more site-specific, as GRC474 is located near a north flowing drainage, with drainage towards the canyon to the north. This could explain the nighttime southerly flow as drainage flow. However, drainage would be towards the canyon in any case.

Canyon Interior:

Wind directions and resulting transport flow at lower elevations within the Canyon should be significantly different from those for rim locations. Directions will be steered more in line with canyon orientation, and in side canyons such as the south to north drainage where the Indian Gardens (INGA1) site is located should have a well-defined diurnal drainage/upslope pattern. Interior locations should thus be more susceptible to aerosol transport via canyon winds. Average canyon flow should be predominantly downriver during the winter and upriver during the summer, in line with average synoptic scale pressure gradients. During nighttime drainage conditions, drainage from the canyon rim should bring regionally representative air to the INGA1 site and other side canyon locations. During daytime upslope conditions, aerosol measurements at INGA1 should better represent canyon conditions.

Inversions/Trapping

At lower elevations within the canyon there is a potential for trapping inversions. In such cases aerosol measurements from the INGA1 site 430 m (1410 ft) above the Colorado River at the canyon bottom would be most representative of conditions within the canyon, as long as the inversion height was above the INGA1 site elevation. Such cases would be indicated by significantly larger aerosol concentrations at INGA1 than at GRCA2. These situations are most likely to occur in the fall and winter, when surface cooling at the canyon bottom is greatest.

At canyon rim locations, inversions will not be trapping inversions of the sort that occur in deep valleys and canyons, but subsidence inversions associated with buildup and stagnation of synoptic high pressure ridges. These are most likely to occur during the summer (July – September), when pressure and temperature gradients in the region are weakest, and wind circulations therefore weaker.

Climatological Statistics

The following table shows long-term climatological statistics for Flagstaff. Data in this table should represent canyon rim locations. Climatological patterns at canyon interior locations are not well represented by data from Flagstaff and will be significantly different, with generally much higher temperatures and lower relative humidity, especially in the summer.