7.1. Setting the Context

The National Atmospheric Deposition Program/National Trends Network (NADP/NTN) is the premier long-term network for monitoring precipitation chemistry in the United States. Established in 1978, its standardized methods and comprehensive geographic coverage provide a robust dataset for analyzing the spatial and temporal trends of acid deposition. The network’s data serves as a real-world application of the scientific principles, monitoring techniques, and chemical models discussed previously.

7.2. Core Characteristics of the NADP/NTN

The network, which grew to approximately 200 sites, is defined by six noteworthy characteristics that ensure data consistency and quality:

1. Site Selection: Sites are generally located away from major local pollution sources to provide data that is representative of regional, rather than local, conditions.
2. Longevity: Sites are intended for long-term operation, ideally for many years, to enable the detection of meaningful environmental trends.
3. Standardized Equipment: Every site is equipped with the same core instruments, including an automatic wet-dry collector, a recording rain gage, and high-quality pH and conductivity meters.
4. Standardized Protocol: All sites follow a uniform protocol. A site operator services the equipment every Tuesday and ships the collected sample from the wet-side bucket to the central laboratory.
5. Single Central Laboratory: To ensure analytical consistency across the entire network, all samples are sent to and analyzed by a single lab: the Central Analytical Laboratory at the Illinois State Water Survey.
6. Measurement Focus: The network measures only the soluble portion of chemical constituents. Samples are filtered upon arrival at the lab, and this filtration step operationally defines which fraction is considered “soluble.”

7.3. Interpreting Network Data (Table 2 Analysis)

An analysis of NADP/NTN data from 1984 for four distinct sites illustrates how precipitation chemistry can vary dramatically by location (see Table 2 in source).

• New Hampshire & Ohio: At these northeastern sites, the precipitation is highly acidic (low pH). The dominant ions are sulfate, nitrate, and hydrogen, reflecting the impact of industrial emissions.
• Nebraska: This Midwestern site demonstrates that high levels of sulfate and nitrate do not automatically lead to high acidity. Here, the acidity from these anions is neutralized by base cations. The dominant positive ions are ammonium and calcium, likely from agricultural sources and dust, resulting in a much higher pH.
• Florida: As a coastal site, the chemistry is dominated by sodium and chloride from sea salt. Sulfate and nitrate levels are lower than at the other sites.

This comparison highlights three key takeaways:

1. Sulfate and nitrate concentrations are not always directly related to the acidity of precipitation.
2. To truly understand the acidity of a sample, all major ions—both acidic and basic—must be measured.
3. Precipitation is chemically much purer (i.e., has far lower ion concentrations) than treated drinking water.

7.4. Concluding Transition

While analyzing data from individual sites is revealing, the true power of the NADP/NTN network emerges when data from all sites are aggregated and mapped to uncover broad, continental-scale patterns.