Joshua Tree National Park Air Quality Information
Overview
Joshua Tree NP is located on the boundary of two air basins, Mojave Desert and Salton Sea, and is greatly affected by the heavily populated South Coast air basin to the west of the park. Mobile sources (e.g., autos) are the most significant contributors to high levels of ozone, nitrogen compounds, and other air pollutants in Joshua Tree NP. Other sources (e.g., agriculture and power plants) also contribute to air pollution in the area.
Air quality related values (AQRVs) of Joshua Tree NP are those resources that are potentially sensitive to air pollution, and include vegetation, wildlife, soils, and visibility.
Visibility in the park is often obscured by haze caused by high concentrations of small air pollution particles. As part of the Interagency Monitoring of Protected Visual Environments (IMPROVE) network, visual air quality in Joshua Tree NP has been monitored using an aerosol sampler (1992-1993; 2000-present) and 35mm camera (1982-1992). Information from the aerosol sampler confirms that visibility is often degraded. This finding is consistent with information from other monitors in southern California that indicates visibility in the area is often greatly reduced from natural conditions.
Soils and vegetation in Joshua Tree NP may be sensitive to atmospheric deposition of nitrogen compounds. In some areas of the country, elevated nitrogen deposition has been shown to alter soil nutrient cycling and vegetation species composition. Soil nitrogen levels are significantly elevated in the northern Coachella Valley, a few kilometers from the southwest corner of Joshua Tree NP, suggesting that soil nitrogen may also be elevated in the park and may have the potential to affect vegetation. Stable isotope studies have shown that soil nitrogen increases are due to increases in atmospheric deposition since the 1950’s. Over time, excess nitrogen deposition may cause desert plants that have adapted to nitrogen-poor conditions to be out-competed and replaced by nitrogen-loving nonnative grasses and other exotic species. A cooperative study (NPS, University of California, Riverside, and the USDA Forest Service, Pacific Southwest Research Station) is currently underway to investigate the effects of nitrogen fertilization on soils and vegetation in Joshua Tree NP.
Estimates of total nitrogen and sulfur can be made by adding wet and dry deposition. Dry deposition has been measured at Joshua Tree NP (site #JOT403), 1995-present, as part of the Clean Air Status and Trends Network (CASTNet). CASTNet has also estimated wet deposition in the park for that period, by interpolating data from area monitors. Dry deposition comprises approximately 70 percent of total nitrogen deposition and 50 percent of total sulfur deposition. Annual total nitrogen deposition for 1995-2001 was approximately 5 kg/ha/yr, an order of magnitude higher than natural (pre-1900’s) conditions, which have been estimated at 0.25-0.50 kg/ha/yr. Annual total sulfur deposition for 1995-2001 was approximately 1 kg/ha/yr, about double natural conditions, estimated at 0.50 kg/ha/yr.
In 2000, wet deposition monitoring was initiated in Joshua Tree NP (site CA67) as part of the National Atmospheric Deposition Program/National Trends Network (NADP/NTN). Data from this monitor will improve total deposition estimates.
Ozone concentrations have been monitored adjacent to the park from 1987-1993 and in the park from 1993-present (EPA site 060719002). Joshua Tree NP experiences very high ozone, with peak concentrations and cumulative doses that are some of the highest in the NPS. Ozone concentrations have frequently exceeded both the previous 1-hr primary National Ambient Air Quality Standard (NAAQS) of 0.12 ppm and the current NAAQS of 0.08 ppm (based on 4th highest 8-hr average), both set to protect human health. In addition to affecting human health, high periods of ozone may also affect wildlife. Laboratory studies have shown that respiratory systems of mammals and birds are adversely affected by ozone, but effects on free-living animals have not been studied.
Ozone peak and cumulative concentrations in the park exceed levels known to cause injury to vegetation. Several plant species that occur in Joshua Tree are sensitive to ozone (e.g., Rhus trilobata, i.e., skunkbush sumac). Limited assessments in the park have not documented ozone injury to vegetation growing naturally in the field; however, no assessment has been made of other ozone effects, e.g., growth effects. A study of Rhus trilobata conducted from 1987-1989 in a park biomonitoring plot demonstrated that under irrigated conditions the plants showed typical ozone injury symptoms, demonstrating that ozone levels are sufficiently high in Joshua Tree NP to induce foliar injury under certain conditions, and may also induce growth effects.
Sulfur dioxide concentrations were monitored from 1991-1992 and the measured values were far below any threshold of suggested sensitivity for plants or wildlife.
Additional information relative to air quality and air quality related values at Joshua Tree NP is available in T. Sullivan et al. 2001. Assessment of air quality and air pollutant impacts in Class I National Parks of California (April 2001, pdf 6.24mb). National Park Service. Denver, CO.