Wet Deposition Monitoring (including Mercury)

Overview

NADP site at Canyonlands National Park, Utah

Atmospheric deposition is the process by which airborne pollutants are deposited to the earth. These pollutants include, but are not limited to, sulfur dioxide, nitrogen oxides, ammonia, and mercury. Total deposition consists of both wet and dry components.

Wet deposition occurs when pollutants are deposited in combination with precipitation, predominantly by rain and snow, but also by clouds and fog. The NPS monitors wet deposition through the National Atmospheric Deposition Program (NADP).

Dry deposition of particles and gases occurs by complex processes such as settling, impaction, and adsorption. Dry deposition is monitored through the Clean Air Status and Trends Network (CASTNet).

Locations

The National Park Service maintains many deposition monitoring sites across the country. For a larger map and table of sites, please visit our Deposition Monitoring Locations web page.

Procedures

NTN and MDN sites at Mammoth Cave
National Park, Kentucky

The NADP National Trends Network (NTN) collects weekly precipitation samples that are analyzed by a central analytical laboratory at the Illinois State Water Survey for pH, conductivity, cations (hydrogen, calcium, sodium, magnesium, potassium, and ammonium) and anions (sulfate, nitrate, and chloride). Each site includes a precipitation collector and a rain gage.

The NADP Mercury Deposition Network (MDN) also collects weekly precipitation samples. These samples are analyzed by Frontier Geosciences for mercury. MDN sites include a modified NTN precipitation collector and a rain gauge.

Data for both networks is available on the NADP web site.

Data

Results

The following figure shows past and present wet deposition values, represented by years 1985-1989 and year 2000, respectively, for sulfate, nitrate, and ammonium ions across the U.S. Wet deposition values are calculated by multiplying the concentration of the pollutant by the precipitation amount, and are commonly reported in terms of kilograms per hectare per year (kg ha -1 yr -1 ).

In general, atmospheric deposition is higher in the eastern U.S. due to higher emissions of SO₂, NO_x, and NH₃. Deposition of ammonium is highest through the Midwest and over the Great Plains region, the result of high ammonia emissions associated with agricultural activities, such as fertilizer use and livestock production.

The figure also shows that atmospheric deposition in the U.S. has changed significantly since the 1980s. Sulfate deposition has declined significantly in the East, with the most dramatic decrease in the Northeast. Despite these declines, which resulted from the SO₂ emission reductions required by Title IV of the Clean Air Act, sulfate concentrations in surface waters have not decreased, showing that ecosystems may take several years to recover.

In contrast to the declines in sulfate deposition, nitrate deposition has not changed significantly, although areas of high deposition appear to be shifting west. However, ammonium deposition has increased dramatically in the past 10 to 15 years, affecting the east and the west.

Contacts