Saguaro National Park Air Quality Information
Both local and distant air pollution sources affect air quality in Saguaro NP. Power plants, smelters, and other sources in Pima, Pinal, and Cochise counties in Arizona, and Hidalgo County in New Mexico, and mobile and area sources in the Tucson and Phoenix metropolitan areas all contribute pollutants to the park. In addition, pollutants from power plants, smelters, and other sources in Mexico affect the park.
The air quality related values (AQRVs) of Saguaro NP are those resources that are potentially sensitive to air pollution, and include vegetation, wildlife, water quality, soils, and visibility. At present, visibility has been identified as the most sensitive AQRV in the park; other AQRVs may also be very sensitive, but have not been sufficiently studied. Although visibility in the park is still superior to that in many parts of the country, visibility in the park is often impaired by light-scattering pollutants (haze).
The U.S. Environmental Protection Agency’s Regional Haze regulations require States to establish goals for each Class I air quality area to improve visibility on the haziest days and ensure no degradation occurs on the clearest days. As part of the Interagency Monitoring of Protected Visual Environments (IMPROVE) network, visual air quality in Saguaro NP has been monitored using an aerosol sampler (1988-1993; 1996-2000; 2001-present). At present, there are aerosol samplers operating in both the Saguaro East Unit and Saguaro West Unit. The data is insufficient for a detailed trend analysis; however, an analysis of 1990-1999 data from nearby Chiricahua National Monument (NM) indicates that visibility in the area is improving on the clearest days and degrading on the haziest days. Trends are similar at other Arizona monitoring sites, including Tonto NM, Petrified Forest NP, and Grand Canyon NP.
Surface waters in Saguaro NP are generally well-buffered and, therefore, not likely to be acidified by atmospheric deposition of sulfur and nitrogen compounds. Most soils are also likely to be well-buffered from acidification. However, there may be areas in the park where rock is resistant to weathering and soils and water (e.g., in potholes) may be sensitive to inputs of acidic deposition.
There is also concern that soils and vegetation in the park may be sensitive to nutrient enrichment from nitrogen deposition. In some parts of the country, nitrogen deposition has altered soil nutrient cycling and vegetation species composition; native plants that have evolved under nitrogen-poor conditions have been replaced by invasive species that are able to take advantage of increased nitrogen levels.
Estimates of total nitrogen and sulfur deposition can be made by adding wet and dry deposition. Deposition is not monitored in Saguaro NP. However, the NPS Air Resources Division has estimated deposition of sulfur and nitrogen in Saguaro NP, using interpolation techniques on data from deposition monitors throughout the Southwest. Although estimates of deposition are relatively low, they are significantly elevated above natural conditions. In many areas of the Southwest, sulfur deposition is decreasing somewhat while nitrogen deposition is increasing.
Several plant species that occur in Saguaro NP are known to be sensitive to ozone, including Pinus ponderosa (ponderosa pine), Populus tremuloides (quaking aspen), and Rhus trilobata (skunkbush). Tropospheric (ground-level) ozone concentrations have been monitored from 1982-present. In 2002, the highest daily maximum 8-hour average was 82 parts per billion (ppb); the national standard for the high daily maximum 8-hour average is 85 ppb, based on the 4th highest average. If ozone concentrations increase, the area may be in violation of the national standard. In addition, ozone cumulative doses over the growing season are within a range that can produce visible effects or growth effects on sensitive plant species under certain conditions. Vegetation surveys in the late 1980s found symptoms of ozone injury on ponderosa pine.