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 precipitation collector and a rain gauge.

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

Data

Results

Atmospheric deposition in the U.S. has changed significantly over the past two decades. Sulfate deposition has notably declined in the East since 1988, largely due to Title IV of the Clean Air Act which required SO2 emission reductions from power plants. Nitrate deposition has also declined in the Northeast, due to another regulatory program that reduced NOx emissions in the Northeast from motor vehicles.

Ammonium deposition, however, has increased across the U.S. 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.

Contacts