“What goes up must come down.” Many air pollutants wash out in precipitation, or deposit as dusts or mists. This return to the ground may not end the problems caused by pollutants. Wet atmospheric deposition is often called “acid rain.” Most of this acid is composed of nitrogen and sulfur compounds related to nitric and sulfuric acid. Most of the nitrogen compounds come from high temperature and/or high pressure combustion, especially in motor vehicles and power plants. The sulfur compounds are a byproduct of burning anything containing sulfur, such as coal, and of smelting sulfur-containing mineral ores.
The NPS monitors precipitation chemistry in Grand Canyon through the National Trends Network / National Atmospheric Deposition Program (NTN/NADP). Normally, precipitation is slightly acidic (due to carbon dioxide) with a pH of 5.5 [1]. Grand Canyon precipitation pH is only slightly more acidic than natural levels. The alkaline soils present throughout the park are well-buffered against this slight increase [2]. While acidity has remained steady, concentrations of nitrogen compounds in rain and snow have risen, but the increase over the last 10 years is not statistically significant. Sulfate levels fell initially, but have begun creeping up recently.
Nitrogen is a common fertilizer, but it is not a welcome addition to the park’s arid soils. Extra nitrogen upsets nutrient balances and cycles, and favors the growth of non-native plant species. These increases do not directly affect human health, but have negative ecological effects.
Only about half of the nitrogen, and a third of the sulfate deposited in Grand Canyon ecosystems comes down in rain and snow, the rest is “dry deposition” of particles and droplets. Dry deposition is measured under the EPA CASTNet program. Although pollutant concentrations in precipitation increased, several years of drought have reduced total precipitation. Consequently, total wet and dry deposition levels in the Park have not increased dramatically (graph at left). Consequently, deposition rates in Grand Canyon are well below rates seen in the more polluted eastern U.S.
