Earth's atmosphere is made of 78% nitrogen (N2) and 21% oxygen (O2). The remaining 1% of the air consists of water vapor, carbon dioxide, sulfur dioxide, nitrogen oxides, ozone, various organic compounds (such as methane), and many other gases. Suspended in this gaseous mixture are droplets of liquid water, visible particles of dust and soot, and microscopic particles of water, ammonium sulfate, ammonium nitrate, nitric and sulfuric acid, carbon compounds, marine salt, lead, zinc, selenium, and hundreds of other elements and compounds.
What we call air quality depends on the exact composition of that 1% of atmospheric gases other than nitrogen and oxygen, and on the chemical nature of the particles suspended in the gas. Very small differences in the overall composition of the atmosphere can cause enormous effects on the environment and human health.
It is important to monitor these small variations in the atmosphere. Some aspects of air quality that should be monitored are visibility, gaseous pollutants, aerosol pollutants, and acid deposition. Great Basin National Park monitors all of these facets of air quality.
Visibility refers to the relative clarity of the air, which determines the distance, details, and colors that we can see in a landscape. Visibility is related to the absorption and scattering of light in the air. As more light is absorbed and scattered, whether by clouds, dust, or smog, visibility decreases. Even pure air absorbs and scatters light. Maximum visibility is about 390 km (240 miles) at sea level.
Weather and other natural conditions affect visibility, of course; but so does air pollution, to a surprising degree. Since the 1950's increasing air pollution has caused a marked deterioration in average visibility across the U.S., in both urban and rural areas (Malm 1989).
Nationwide studies indicate that the intermountain West enjoys the best visibility in the coterminous U.S., from the southern Cascades, eastward across the Great Basin and Snake River Plain, to the northern Colorado Plateau and central Rocky Mountains.
Great Basin NP, which is located in middle of this region and has been monitoring visibility since 1982, typically records some of the highest average visibility readings in the nation. The latest and most accurate data, from March 1993 through February 1994, indicates that the median annual non-weather-related standard visual range in the park is approximately 15O km (93 miles), and that values rarely fell below 106 km (66 miles) and rarely exceeded 241 km (149 miles). This places Great Basin National Park well within the top few sites in the nation, along with Denali NP, Alaska; Jarbridge Wilderness Area, Nevada; and Bridger National Forest, Wyoming (Copeland and others 1995).
Combined transmissometer and aerosol monitoring data shows that visibility at Great Basin NP was affected principally by organic carbon, soot, sulfates, and coarse soil aerosols. Nitrates, which have a great affect on visibility at urban sites, were very minor at Great Basin (Copeland and others 1995). Major sources of these aerosol types are discussed under aerosols below.
Like a clean white page, the relatively clear air of the Great Basin can be marred easily. Studies of the effect of visibility on park visitors show that slight increases in air pollution are much more distinct and objectionable when and where the air is cleanest (O'Leary 1988). At Great Basin NP, visibility declines after periods of sustained northeasterly winds, when a brown-yellow haze appears in Snake Valley, obscuring the mountains east of the park. Presumably the pollution comes from the Salt Lake City area and the Intermountain Power Plant near Delta, Utah. Fortunately, winds are seldom northeasterly for long periods. If similar pollution sources were built to the west, the park's visibility would be affected more frequently.
Gases of primary concern include ozone and sulfur dioxide.
Ozone molecules consist of three oxygen atoms (O3), in contrast with the more stable, biatomic form of oxygen (O2) which makes up most of the oxygen in the lower atmosphere. Ozone is a much more effective oxidizing agent than O2 because it tends to release oxygen atoms in chemical reactions. Consequently, ozone chemically damages plants and animals when it occurs in unnaturally high concentrations in the lower atmosphere. (Paradoxically, ozone in the upper atmosphere protects life forms by blocking much of the ultraviolet radiation striking the earth.)
Ozone forms when sunlight strikes certain pollutants, especially nitrogen oxides (NOX) and volatile organic compounds (VOC), causing them to release oxygen atoms, which combine with naturally occuring O2. The largest sources of VOC and NOX are automobile exhaust, refineries, and manufacturing processes using industrial solvents (Mangis and others 1991).
Average ozone concentration at Great Basin NP was 0.041 ppm from September 1993 through December 1995. The average monthly maximum reading during that period was 0.061 ppm. Over more than two years of continuous monitoring, hourly ozone levels reached or exceeded 0.070 ppm only 16 times. The highest recorded hourly concentration was 0.079 ppm.
No significant upward or downward trend is apparent in the Great Basin NP data; but this is due to the short period of monitoring at the site, not necessarily to long-term stability in ozone levels. Seasonal cyclicity is apparent--from winter lows to summer highs. This seasonal pattern is typical throughout the network and might reflects greater solar radiation during the summer, rather than increased emissions of ozone precursers.
Ozone concentrations at Great Basin NP are well within the current EPA health standard (0.120 ppm per hour), in contrast to ozone levels near many urban areas. However, knowledge of the biological effects of ozone pollution is incomplete at best, and environmental damage might be occurring at lower ozone concentrations. Several studies indicate that some plant species are harmed at ozone levels well below 0.120 ppm, including ponderosa pine, aspen, squawbush (Rhus trilobita), and other species found in Great Basin NP (Mangis and others 1991).
The EPA currently is revising its primary NAAQS for ozone and developing a secondary NAAQS, relating to vegetation (Musselman 1996). Proposals for the new primary standard range from 0.07 to 0.09 ppm ozone over an eight hour period. Proposals for the secondary NAAQS recognize the damaging accumulative effects of moderate ozone levels during the growing season, and agree that damage to plants occurs when ozone levels exceed 0.06 ppm for sustained periods.
Sulfur dioxide and nitrogen dioxide also problem gases because they can be absorbed into dew and precipitation to make sulfuric and nitric acids. They also join with ammonium to form visibility impairing sulfates and nitrates. Great Basin NP started monitoring SO2 and NO2 in 1995.
The term aerosol refers to any suspension of sub-microscopic solid or liquid particles (called particulates) in the air. Once airborne, particulates remain suspended for long periods and disperse over thousands of square miles. Fine particulates (by convention, those smaller than 2.5 microns) disperse farther, reduce visibility more, and affect overall air quality more than do larger particulates. Urban and industrial emissions account for the bulk of fine particulates in the U.S. (Cahill and others 1987).
Particulates generally measured in microns (one- millionth of a meter). An average human hair is 70 microns in diameter. Average pollen grains are 30 microns across. Particulates small enough to be inhaled deeply into the lungs are smaller than 10 microns. The particulates that reduce visibility the most are smaller than 2.5 microns.
Smaller particulates remain suspended in the atmosphere longer than coarser ones, and they disperse farther. This is unfortunate because, in general, smaller particulates are more harmful to human health and the environment, and have a greater affect on visibility. Natural particulates, such as windblown dust, volcanic ash, and soot from wildfires, tend to be coarser. Man-made particulates, such as sulfate, nitrate, many organic compounds, and trace metals, tend to be smaller.
Sulfate makes up about 25 to 50 percent of fine particulate mass at most sites in the U.S. Sulfur typically enters the atmosphere as sulfur dioxide gas, chiefly from fossil-fuel combustion. The gas converts to sulfuric and sulfurous acids, and then to ammonium sulfate ([NH4]2SO4). Other sulfates can form under certain conditions but ammonium sulfate accounts for nearly all elemental sulfur collected in aerosol samples (Cahill and others 1987). Ammonium sulfate is a chief cause of reduced visibility because it is so common and because it scatters light more effectively than nearly all other particulate types (Mangis and others 1994).
Organic carbon is another large component of aerosol. The category includes a variety of carbon-based organic molecules, which derive from smoke (from wildfires, planned burns, wood stoves, etc.) and from industrial hydrocarbon gas emissions and biological sources. In remote areas with comparatively low sulfate concentrations, such as Great Basin NP, organic carbons often comprise the majority of the total aerosol (Copeland 1995).
Nitrates typically form a minor portion of the aerosol, except near California urban areas, where nitrates sometimes exceed sulfates. Nitrate aerosol, which derives from automobile and oil refinery emissions, is a chief cause of ozone pollution and a major contributor to acid rain (Mangis and others 1991).
Soil particles also occur in the aerosol, often accounting for 20% of the total at Western sites. Concentrations vary widely with the season and with short-term weather conditions. Soil has less effect on visibility and health than other particulate types because the particles tend to be coarser and less reactive chemically.
Finally, various trace elements can occur in the aerosol, such as sodium and chlorine ions derived from ocean salts, and lead and zinc particles from smelters. Although they never comprise a large percentage of the total aerosol budget, heavy metals such as lead and selenium can cause significant environmental damage.
The largest sources of organic carbon and soot aerosols are diesel exhaust, smoke, and industrial solvents. Most sulfate emissions come from coal- and oil-fired power plants, refineries, and smelters. Most coarse soil aerosols come from wind-blown dust from eroding soil. Coarse soil aerosols are a minor factor in visibility impairment at most sites, in contrast to Great Basin NP, simply because they are overwhelmed by greater concentrations of human-made pollutants.
From 1959 through 1978, Lehman Caves NM participated in the National Air Surveillance Network. During the 20-year monitoring period, mean annual aerosol concentrations at Lehman Caves ranged from 6 to 17 μg/m3, and averaged 11 μg/m3. Maximum readings exceeded 100 μg/m3 only four times. Lehman Caves typically had the lowest aerosol levels of Nevada's 51 sites participating in the study(NDEP, 1978).
From September 1982 through March 1986 Lehman Caves NM participated in the NPS Particulate Monitoring Network. Compared with other network sites surrounding the Great Basin (Bryce Canyon, Grand Canyon, Joshua Tree, Death Valley, Yosemite, Crater Lake, and Craters of the Moon) from September 1982 through February 1985, Lehman Caves recorded the lowest average fine particulate concentration and the second lowest fine sulfur concentration of these eight sites (Cahill and others 1987).
The IMPROVE system measures concentrations of sulfates, nitrates, organic and elemental carbon, and a large suite of elements (H, S, Si, K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, As, Se, Br, Pb, Na, and Cl), plus light absorption and the concentrations of PM10 and PM2.5 (that is, particulates smaller than 2.5 microns).
For the two-year period from March 1993 through February 1995, the median annual PM10 concentration at Great Basin NP was 6.5 μg/m3, and the median annual PM2.5 concentration was 2.9 μg/m3. During that same period, the average composition of the fine particulate mass (PM2.5) was: 36% organic carbon, 22% ammonium sulfate, 18% soil, 6% elemental carbon, and 5% ammonium nitrate. The remaining 13% includes various trace elements, unmeasured nitrates, and residual water.
Aerosol monitoring over the last few decades indicates that the cleanest air in the continguos U.S. extends from the southern Cascades, across the Great Basin and the Snake River Plain to the central Rockies and the northern Colorado Plateau. Great Basin NP, located near the middle of this region, typically records aerosol concentrations that are among the lowest in the nation. For example, from March 1994 through February 1995 (the most recent period for which published data is available), only Lassen Volcanic NP, Crater Lake NP, Bryce Canyon NP, and Jarbridge Wilderness Area recorded lower readings for major aerosol types. In contrast, sites at Death Valley NP, Sequoia-Kings Canyon NP, Lone Pine Wilderness Area (near Salt Lake City, UT), and South Lake Tahoe show strong evidence of aerosol pollution (Air Quality Group 1995).
Acid deposition harms aquatic and terrestrial life through direct contact and by changing the chemistry of surface water and soils. It can affect plants' seed germination and survival. Even dry acid deposition builds up on hairy surfaces of desert plants. Later dew or precipitation dissolves the deposition to form concentrated acid solutions that can harm foliage. Acid deposition also speeds the decay of buildings and other man-made structures. Acid deposition is often acocmpanied by nitrogen deposition, which is a artificial fertilization which can favor certain plants over other and change the plant community structure. In addition, sulfates and other components of acid deposition are among the leading contributors to reduced visibility in the U.S. (Mangis and others 1995).
Acid deposition occurs when sulfur dioxide (SO2) and nitrogen oxide (NOx) gases chemically change to sulfuric and nitric acid in the atmosphere and fall to the earth with rain and snow (wet deposition), or with dust and microscopic particles (dry deposition). Coal-fired power plants and smelters are the chief sources of SO2 emissions; automobiles and electric utilities are the chief source of NOx emissions (Mangis and others 1991).
Great Basin National Park has been monitoring acid deposition through the National Atmospheric Deposition Program (NADP) network since 1985. The data collected indicates that the acidity of Great Basin National Park rain and snow is at natural levels. Information from precipitation in this area provides a baseline to monitor the changes that have already taken place in other parts of the country.
From 1985 through 1994, the precipitation-weighted mean pH of rain and snow at Great Basin NP was 5.4--well within the "natural" pH range of 4.8 to 6.0, as suggested by Charlson and Rodhe (1982). During that 10-year period the lowest valid weekly sample value was 4.34 and the highest was 8.00. The average pH of rain and snow in Great Basin NP is generally similar to that at other sites in the intermountain West.
From 1985 through 1994, the average concentration of sulfate in precipitation was 0.8 mg/l, and the average annual deposition of sulfate was 2.5 kg per hectare. Nitrate concentrations averaged 1.1 mg/l, and the annual wet deposition of nitrate was 3.3 kg/ha during that period. In the arid West, dry deposition accounts for a significant portion of the total deposition of sulfates, nitrates, and other acidic pollutants (Mangis and others 1991).
The highest levels of sulfate, nitrate, and ammonium in the region in 1993 were recorded at Red Rock Canyon, a site which is strongly affected by emissions from nearby Las Vegas.
Although acid deposition does not appear to be a threat at the present time in Great Basin NP, it should be noted that all of the lakes in the park probably would be highly susceptible to acidification, should acid deposition occur. The granitic and quartzitic basins occupied by these lakes, combined with their high elevations, leave them with very little capacity to neutralize acidic pollutants.
The intermountain region has relatively clean air, compared to other areas of the United States. This is greatly due to low population density and few point sources of polution. Air quality in the desert southwest has been impaired by urban sources from southern California through Arizona.
Rural parks are not immune to pollution from urban areas. Visibility at the Grand Canyon has been reduced. Visibility at Big Bend National Park has dropped to less than 10 miles some days due to sulfate pollution from two coal-burning power plants near Piedras Negras, Mexico, 136 miles away (Huber 1996).
The greater Los Angeles urban area is the single largest source of regional air pollution in the southern half of the intermountain West (Malm 1989). It produces a broad plume of aerosol pollution that spreads across southern Nevada and the Grand Canyon to central Colorado. Great Basin NP lies along the northern edge of this plume and is affected by it, especially during the summer when winds are often southerly. Jarbridge Wilderness Area, located more than 200 miles north of Great Basin NP and farther from the plume, typically records sulfate concentrations that are 75% of those measured at Great Basin NP, despite the fact that average PM2.5 levels at the two sites are roughly equal (Air Quality Group 1994, 1995).
Great Basin National Park enjoys very good air quality most days due to its distance from major pollution sources and location in regards to prevailing winds from urban areas. However, just a small increase in pollution can greatly affect the park's visibility and natural resources. Air quality is a global concern. Therefore it is important to reduce pollution emissions in every way possible.
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Air Quality Group. 1995. Annual report of aerosol collection and compositional analysis for IMPROVE: July 1993-June 1994. Air Quality Group, Crocker Nuclear Laboratory; Davis, CA.
Air Quality Group. 1996. Annual report of aerosol collection and compositional analysis for IMPROVE: July 1994-June 1995. Air Quality Group, Crocker Nuclear Laboratory; Davis, CA.
Cahill, T.A. and others. 1987. Particulate monitoring and data analysis for the National Park Service, 1982-1985. Air Quality Group, Crocker Nuclear Laboratory; Davis, CA.
Copeland and others. 1995. Integrated report of optical and aerosol monitoring: Great Basin National Park--March 1993 through February 1994. Interagency Monitoring Protected Visual Environments Program.
Eldred, R.A. 1988. IMPROVE sampler manual, Version 2. Air Quality Group, Crocker Nuclear Laboratory; Davis, CA.
Ewell, D.M. 1996. Particulate matter standard. ARM: Forest Service/National Park Service Air Resource Management Quarterly Newsletter 25th Ed.
Malm, W.C. 1989. Atmospheric haze: Its sources and effects on visibility in rural areas of the continental United States. Environmental Monitoring and Assessment 12:203-225.
Mangis, D., and others. 1991. Acid rain and air pollution in desert park areas. Technical Report NPS/NRAQD/NRTR-91/02. National Park Service, Air Quality Division; Denver, CO.
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Last Modified: April 1996, Kurt Pfaff