Acid Rain's Long Decline, Measured Every Tuesday

Rain and snow carry pollution out of the air and onto land and water. The National Atmospheric Deposition Program (NADP) measures that, at fixed sites across the United States, so that its effects on water supplies and ecosystems can be tracked over decades. It is run by a committee of federal and state agencies, universities, tribes and private companies, which meets each spring and each autumn; the autumn meeting adds a two-day symposium where scientists present research built on NADP data.

What forty years of samples show

The NADP began in 1978, growing out of efforts to monitor the effect of acid rain on crops. Its record since then shows:

How a sample is taken

The core network is the National Trends Network (NTN). Its sites sit mostly away from cities and point sources of pollution, so they record the background rather than one smokestack. Each has at least a rain gauge and a precipitation-chemistry collector, which opens automatically only while it rains or snows (wet-only sampling).

National Trends Network monitoring sites in 2024, those the USGS supports and those it does not. Map: U.S. Geological Survey.

1. Every Tuesday morning, the operator collects the week's sample, using only containers cleaned at the NADP Analytical Laboratory (Wisconsin State Laboratory of Hygiene).
2. The bucket is weighed to find the volume, and the water poured into a shipping bottle.
3. The laboratory measures acidity (pH), conductance, calcium, magnesium, sodium, potassium, sulfate (SO₄²⁻), nitrate (NO₃⁻), chloride and ammonium (NH₄⁺). It measures orthophosphate too, but only to catch contaminated samples.
4. Samples that were mishandled, hit by a collector failure or grossly contaminated are flagged; the NADP runs a final set of checks, and the data go on its website.

USGS–NADP site MN23 at Camp Ripley, Minnesota, with Mercury Deposition Network and National Trends Network equipment. Photo: Ben Torrison, U.S. Geological Survey, 2024.

The USGS part

The U.S. Geological Survey has taken part for more than 45 years and is the largest federal funder among the program's cooperators. It:

• supports 69 NTN sites, and runs their field operations;
• leads the NADP Budget Advisory Committee and sits on its Executive Committee;
• runs the independent Precipitation Chemistry Quality Assurance project for the NTN and the Mercury Deposition Network — field audits for contamination and stability, interlaboratory comparisons, and side-by-side collectors to estimate how much the measurements vary; for mercury, a system-blank program as well;
• supports a Mercury Deposition Network site in Puerto Rico and a Mercury Litterfall Network site in Tennessee.

Beyond acid rain

The NTN is the foundation of four more networks — for mercury in precipitation, atmospheric mercury, ammonia and mercury in fallen leaves. In 2023 the NADP approved a trial network to measure PFAS ("forever chemicals") in precipitation, which the USGS supports at site NY06. Next: telemetry for real-time checks on site conditions, and new methods for total reactive nitrogen, total phosphorus, microplastics and pollen.

A USGS site (NY06) measuring deposition of per- and polyfluoroalkyl substances (PFAS) under the forest canopy at Bronx Botanical Gardens, New York. Photo: Ashley Smith, U.S. Geological Survey, 2024.

Sources

Written from Exploring Atmospheric Deposition Chemistry Data Across the United States, U.S. Geological Survey Fact Sheet; a work of the United States government in the public domain. Its trend maps come from the National Atmospheric Deposition Program's annual NTN maps (1990 and 2021). The map and photographs are reproduced from the fact sheet.